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Unformatted text preview: cen58933_ch01.qxd 9/10/2002 8:29 AM Page 1 CHAPTER B A S I C S O F H E AT T R A N S F E R he science of thermodynamics deals with the amount of heat transfer as
a system undergoes a process from one equilibrium state to another, and
makes no reference to how long the process will take. But in engineering, we are often interested in the rate of heat transfer, which is the topic of
the science of heat transfer.
We start this chapter with a review of the fundamental concepts of thermodynamics that form the framework for heat transfer. We first present the
relation of heat to other forms of energy and review the first law of thermodynamics. We then present the three basic mechanisms of heat transfer, which
are conduction, convection, and radiation, and discuss thermal conductivity.
Conduction is the transfer of energy from the more energetic particles of a
substance to the adjacent, less energetic ones as a result of interactions between the particles. Convection is the mode of heat transfer between a solid
surface and the adjacent liquid or gas that is in motion, and it involves the
combined effects of conduction and fluid motion. Radiation is the energy
emitted by matter in the form of electromagnetic waves (or photons) as a result of the changes in the electronic configurations of the atoms or molecules.
We close this chapter with a discussion of simultaneous heat transfer. T 1
CONTENTS
1–1
Thermodynamics and
Heat Transfer 2
1–2
Engineering Heat Transfer 4
1–3
Heat and Other Forms
of Energy 6
1–4
The First Law of
Thermodynamics 11
1–5
Heat Transfer
Mechanisms 17
1–6
Conduction 17
1–7
Convection 25
1–8
Radiation 27
1–9
Simultaneous Heat Transfer
Mechanism 30
1–10 ProblemSolving Technique 35
Topic of Special Interest:
Thermal Comfort 40 1 cen58933_ch01.qxd 9/10/2002 8:29 AM Page 2 2
HEAT TRANSFER 1–1 Thermos
bottle Hot
coffee Insulation FIGURE 1–1
We are normally interested in how long
it takes for the hot coffee in a thermos to
cool to a certain temperature, which
cannot be determined from a
thermodynamic analysis alone. Hot
coffee
70°C Cool
environment
20°C
Heat FIGURE 1–2
Heat flows in the direction of
decreasing temperature. I THERMODYNAMICS AND HEAT TRANSFER We all know from experience that a cold canned drink left in a room warms up
and a warm canned drink left in a refrigerator cools down. This is accomplished by the transfer of energy from the warm medium to the cold one. The
energy transfer is always from the higher temperature medium to the lower
temperature one, and the energy transfer stops when the two mediums reach
the same temperature.
You will recall from thermodynamics that energy exists in various forms. In
this text we are primarily interested in heat, which is the form of energy that
can be transferred from one system to another as a result of temperature difference. The science that deals with the determination of the rates of such energy transfers is heat transfer.
You may be wondering why we need to undertake a detailed study on heat
transfer. After all, we can determine the amount of heat transfer for any system undergoing any process using a thermodynamic analysis alone. The reason is that thermodynamics is concerned with the amount of heat transfer as a
system undergoes a process from one equilibrium state to another, and it gives
no indication about how long the process will take. A thermodynamic analysis
simply tells us how much heat must be transferred to realize a specified
change of state to satisfy the conservation of energy principle.
In practice we are more concerned about the rate of heat transfer (heat transfer per unit time) than we are with the amount of it. For example, we can determine the amount of heat transferred from a thermos bottle as the hot coffee
inside cools from 90°C to 80°C by a thermodynamic analysis alone. But a typical user or designer of a thermos is primarily interested in how long it will be
before the hot coffee inside cools to 80°C, and a thermodynamic analysis cannot answer this question. Determining the rates of heat transfer to or from a
system and thus the times of cooling or heating, as well as the variation of the
temperature, is the subject of heat transfer (Fig. 1–1).
Thermodynamics deals with equilibrium states and changes from one equilibrium state to another. Heat transfer, on the other hand, deals with systems
that lack thermal equilibrium, and thus it is a nonequilibrium phenomenon.
Therefore, the study of heat transfer cannot be based on the principles of
thermodynamics alone. However, the laws of thermodynamics lay the framework for the science of heat transfer. The first law requires that the rate of
energy transfer into a system be equal to the rate of increase of the energy of
that system. The second law requires that heat be transferred in the direction
of decreasing temperature (Fig. 1–2). This is like a car parked on an inclined
road that must go downhill in the direction of decreasing elevation when its
brakes are released. It is also analogous to the electric current flowing in the
direction of decreasing voltage or the fluid flowing in the direction of decreasing total pressure.
The basic requirement for heat transfer is the presence of a temperature difference. There can be no net heat transfer between two mediums that are at the
same temperature. The temperature difference is the driving force for heat
transfer, just as the voltage difference is the driving force for electric current
flow and pressure difference is the driving force for fluid flow. The rate of heat
transfer in a certain direction depends on the magnitude of the temperature
gradient (the temperature difference per unit length or the rate of change of cen58933_ch01.qxd 9/10/2002 8:29 AM Page 3 3
CHAPTER 1 temperature) in that direction. The larger the temperature gradient, the higher
the rate of heat transfer. Application Areas of Heat Transfer
Heat transfer is commonly encountered in engineering systems and other aspects of life, and one does not need to go very far to see some application areas of heat transfer. In fact, one does not need to go anywhere. The human
body is constantly rejecting heat to its surroundings, and human comfort is
closely tied to the rate of this heat rejection. We try to control this heat transfer rate by adjusting our clothing to the environmental conditions.
Many ordinary household appliances are designed, in whole or in part, by
using the principles of heat transfer. Some examples include the electric or gas
range, the heating and airconditioning system, the refrigerator and freezer, the
water heater, the iron, and even the computer, the TV, and the VCR. Of course,
energyefficient homes are designed on the basis of minimizing heat loss in
winter and heat gain in summer. Heat transfer plays a major role in the design
of many other devices, such as car radiators, solar collectors, various components of power plants, and even spacecraft. The optimal insulation thickness
in the walls and roofs of the houses, on hot water or steam pipes, or on water
heaters is again determined on the basis of a heat transfer analysis with economic consideration (Fig. 1–3). Historical Background
Heat has always been perceived to be something that produces in us a sensation of warmth, and one would think that the nature of heat is one of the first
things understood by mankind. But it was only in the middle of the nineteenth The human body
Airconditioning
systems Circuit boards Water in Water out
Car radiators Power plants Refrigeration systems FIGURE 1–3
Some application areas of heat transfer. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 4 4
HEAT TRANSFER
Contact
surface
Hot
body Cold
body Caloric FIGURE 1–4
In the early nineteenth century, heat was
thought to be an invisible fluid called the
caloric that flowed from warmer bodies
to the cooler ones. century that we had a true physical understanding of the nature of heat, thanks
to the development at that time of the kinetic theory, which treats molecules
as tiny balls that are in motion and thus possess kinetic energy. Heat is then
defined as the energy associated with the random motion of atoms and molecules. Although it was suggested in the eighteenth and early nineteenth centuries that heat is the manifestation of motion at the molecular level (called the
live force), the prevailing view of heat until the middle of the nineteenth century was based on the caloric theory proposed by the French chemist Antoine
Lavoisier (1743–1794) in 1789. The caloric theory asserts that heat is a fluidlike substance called the caloric that is a massless, colorless, odorless, and
tasteless substance that can be poured from one body into another (Fig. 1–4).
When caloric was added to a body, its temperature increased; and when
caloric was removed from a body, its temperature decreased. When a body
could not contain any more caloric, much the same way as when a glass of
water could not dissolve any more salt or sugar, the body was said to be saturated with caloric. This interpretation gave rise to the terms saturated liquid
and saturated vapor that are still in use today.
The caloric theory came under attack soon after its introduction. It maintained that heat is a substance that could not be created or destroyed. Yet it
was known that heat can be generated indefinitely by rubbing one’s hands together or rubbing two pieces of wood together. In 1798, the American Benjamin Thompson (Count Rumford) (1753–1814) showed in his papers that
heat can be generated continuously through friction. The validity of the caloric
theory was also challenged by several others. But it was the careful experiments of the Englishman James P. Joule (1818–1889) published in 1843 that
finally convinced the skeptics that heat was not a substance after all, and thus
put the caloric theory to rest. Although the caloric theory was totally abandoned in the middle of the nineteenth century, it contributed greatly to the development of thermodynamics and heat transfer. 1–2 I ENGINEERING HEAT TRANSFER Heat transfer equipment such as heat exchangers, boilers, condensers, radiators, heaters, furnaces, refrigerators, and solar collectors are designed primarily on the basis of heat transfer analysis. The heat transfer problems
encountered in practice can be considered in two groups: (1) rating and
(2) sizing problems. The rating problems deal with the determination of the
heat transfer rate for an existing system at a specified temperature difference.
The sizing problems deal with the determination of the size of a system in
order to transfer heat at a specified rate for a specified temperature difference.
A heat transfer process or equipment can be studied either experimentally
(testing and taking measurements) or analytically (by analysis or calculations). The experimental approach has the advantage that we deal with the
actual physical system, and the desired quantity is determined by measurement, within the limits of experimental error. However, this approach is expensive, timeconsuming, and often impractical. Besides, the system we are
analyzing may not even exist. For example, the size of a heating system of
a building must usually be determined before the building is actually built
on the basis of the dimensions and specifications given. The analytical approach (including numerical approach) has the advantage that it is fast and cen58933_ch01.qxd 9/10/2002 8:29 AM Page 5 5
CHAPTER 1 inexpensive, but the results obtained are subject to the accuracy of the
assumptions and idealizations made in the analysis. In heat transfer studies,
often a good compromise is reached by reducing the choices to just a few by
analysis, and then verifying the findings experimentally. Modeling in Heat Transfer
The descriptions of most scientific problems involve expressions that relate
the changes in some key variables to each other. Usually the smaller the
increment chosen in the changing variables, the more general and accurate
the description. In the limiting case of infinitesimal or differential changes in
variables, we obtain differential equations that provide precise mathematical
formulations for the physical principles and laws by representing the rates of
changes as derivatives. Therefore, differential equations are used to investigate a wide variety of problems in sciences and engineering, including heat
transfer. However, most heat transfer problems encountered in practice can be
solved without resorting to differential equations and the complications associated with them.
The study of physical phenomena involves two important steps. In the first
step, all the variables that affect the phenomena are identified, reasonable assumptions and approximations are made, and the interdependence of these
variables is studied. The relevant physical laws and principles are invoked,
and the problem is formulated mathematically. The equation itself is very instructive as it shows the degree of dependence of some variables on others,
and the relative importance of various terms. In the second step, the problem
is solved using an appropriate approach, and the results are interpreted.
Many processes that seem to occur in nature randomly and without any order are, in fact, being governed by some visible or notsovisible physical
laws. Whether we notice them or not, these laws are there, governing consistently and predictably what seem to be ordinary events. Most of these laws are
well defined and well understood by scientists. This makes it possible to predict the course of an event before it actually occurs, or to study various aspects
of an event mathematically without actually running expensive and timeconsuming experiments. This is where the power of analysis lies. Very accurate results to meaningful practical problems can be obtained with relatively
little effort by using a suitable and realistic mathematical model. The preparation of such models requires an adequate knowledge of the natural phenomena
involved and the relevant laws, as well as a sound judgment. An unrealistic
model will obviously give inaccurate and thus unacceptable results.
An analyst working on an engineering problem often finds himself or herself in a position to make a choice between a very accurate but complex
model, and a simple but notsoaccurate model. The right choice depends on
the situation at hand. The right choice is usually the simplest model that yields
adequate results. For example, the process of baking potatoes or roasting a
round chunk of beef in an oven can be studied analytically in a simple way by
modeling the potato or the roast as a spherical solid ball that has the properties
of water (Fig. 1–5). The model is quite simple, but the results obtained are sufficiently accurate for most practical purposes. As another example, when we
analyze the heat losses from a building in order to select the right size for a
heater, we determine the heat losses under anticipated worst conditions and
select a furnace that will provide sufficient heat to make up for those losses. Oven
Potato Actual
175°C Water Ideal FIGURE 1–5
Modeling is a powerful engineering
tool that provides great insight and
simplicity at the expense of
some accuracy. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 6 6
HEAT TRANSFER Often we tend to choose a larger furnace in anticipation of some future expansion, or just to provide a factor of safety. A very simple analysis will be adequate in this case.
When selecting heat transfer equipment, it is important to consider the actual operating conditions. For example, when purchasing a heat exchanger
that will handle hard water, we must consider that some calcium deposits will
form on the heat transfer surfaces over time, causing fouling and thus a gradual decline in performance. The heat exchanger must be selected on the basis
of operation under these adverse conditions instead of under new conditions.
Preparing very accurate but complex models is usually not so difficult. But
such models are not much use to an analyst if they are very difficult and timeconsuming to solve. At the minimum, the model should reflect the essential
features of the physical problem it represents. There are many significant realworld problems that can be analyzed with a simple model. But it should always be kept in mind that the results obtained from an analysis are as accurate
as the assumptions made in simplifying the problem. Therefore, the solution
obtained should not be applied to situations for which the original assumptions do not hold.
A solution that is not quite consistent with the observed nature of the problem indicates that the mathematical model used is too crude. In that case, a
more realistic model should be prepared by eliminating one or more of the
questionable assumptions. This will result in a more complex problem that, of
course, is more difficult to solve. Thus any solution to a problem should be interpreted within the context of its formulation. 1–3 I HEAT AND OTHER FORMS OF ENERGY Energy can exist in numerous forms such as thermal, mechanical, kinetic, potential, electrical, magnetic, chemical, and nuclear, and their sum constitutes
the total energy E (or e on a unit mass basis) of a system. The forms of energy
related to the molecular structure of a system and the degree of the molecular
activity are referred to as the microscopic energy. The sum of all microscopic
forms of energy is called the internal energy of a system, and is denoted by
U (or u on a unit mass basis).
The international unit of energy is joule (J) or kilojoule (1 kJ 1000 J).
In the English system, the unit of energy is the British thermal unit (Btu),
which is defined as the energy needed to raise the temperature of 1 lbm of
water at 60°F by 1°F. The magnitudes of kJ and Btu are almost identical
(1 Btu
1.055056 kJ). Another wellknown unit of energy is the calorie
(1 cal 4.1868 J), which is defined as the energy needed to raise the temperature of 1 gram of water at 14.5°C by 1°C.
Internal energy may be viewed as the sum of the kinetic and potential energies of the molecules. The portion of the internal energy of a system associated with the kinetic energy of the molecules is called sensible energy or
sensible heat. The average velocity and the degree of activity of the molecules are proportional to the temperature. Thus, at higher temperatures the
molecules will possess higher kinetic energy, and as a result, the system will
have a higher internal energy.
The internal energy is also associated with the intermolecular forces between the molecules of a system. These are the forces that bind the molecules cen58933_ch01.qxd 9/10/2002 8:29 AM Page 7 7
CHAPTER 1 to each other, and, as one would expect, they are strongest in solids and weakest in gases. If sufficient energy is added to the molecules of a solid or liquid,
they will overcome these molecular forces and simply break away, turning the
system to a gas. This is a phase change process and because of this added energy, a system in the gas phase is at a higher internal energy level than it is in
the solid or the liquid phase. The internal energy associated with the phase of
a system is called latent energy or latent heat.
The changes mentioned above can occur without a change in the chemical
composition of a system. Most heat transfer problems fall into this category,
and one does not need to pay any attention to the forces binding the atoms in
a molecule together. The internal energy associated with the atomic bonds in
a molecule is called chemical (or bond) energy, whereas the internal energy
associated with the bonds within the nucleus of the atom itself is called nuclear energy. The chemical and nuclear energies are absorbed or released during chemical or nuclear reactions, respectively.
In the analysis of systems that involve fluid flow, we frequently encounter
the combination of properties u and Pv. For the sake of simplicity and convenience, this combination is defined as enthalpy h. That is, h u Pv where
the term Pv represents the flow energy of the fluid (also called the flow work),
which is the energy needed to push a fluid and to maintain flow. In the energy
analysis of flowing fluids, it is convenient to treat the flow energy as part of
the energy of the fluid and to represent the microscopic energy of a fluid
stream by enthalpy h (Fig. 1–6). Flowing
fluid Stationary
fluid Energy = h Energy = u FIGURE 1–6
The internal energy u represents the microscopic energy of a nonflowing fluid,
whereas enthalpy h represents the microscopic energy of a flowing fluid. Specific Heats of Gases, Liquids, and Solids
You may recall that an ideal gas is defined as a gas that obeys the relation
Pv RT or P RT (11) where P is the absolute pressure, v is the specific volume, T is the absolute
temperature, is the density, and R is the gas constant. It has been experimentally observed that the ideal gas relation given above closely approximates the PvT behavior of real gases at low densities. At low pressures and
high temperatures, the density of a gas decreases and the gas behaves like an
ideal gas. In the range of practical interest, many familiar gases such as air,
nitrogen, oxygen, hydrogen, helium, argon, neon, and krypton and even heavier gases such as carbon dioxide can be treated as ideal gases with negligible
error (often less than one percent). Dense gases such as water vapor in
steam power plants and refrigerant vapor in refrigerators, however, should not
always be treated as ideal gases since they usually exist at a state near
saturation.
You may also recall that specific heat is defined as the energy required to
raise the temperature of a unit mass of a substance by one degree (Fig. 1–7).
In general, this energy depends on how the process is executed. In thermodynamics, we are interested in two kinds of specific heats: specific heat at
constant volume Cv and specific heat at constant pressure Cp. The specific
heat at constant volume Cv can be viewed as the energy required to raise the
temperature of a unit mass of a substance by one degree as the volume is held
constant. The energy required to do the same as the pressure is held constant
is the specific heat at constant pressure Cp. The specific heat at constant m = 1 kg
∆T = 1°C
Specific heat = 5 kJ/kg·°C 5 kJ FIGURE 1–7
Specific heat is the energy required to
raise the temperature of a unit mass
of a substance by one degree in a
specified way. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 8 8
THERMODYNAMICS Air
m = 1 kg
300 → 301 K Air
m = 1 kg
1000 → 1001 K 0.718 kJ 0.855 kJ FIGURE 1–8
The specific heat of a substance changes
with temperature. pressure Cp is greater than Cv because at constant pressure the system is allowed to expand and the energy for this expansion work must also be supplied
to the system. For ideal gases, these two specific heats are related to each
other by Cp Cv R.
A common unit for specific heats is kJ/kg · °C or kJ/kg · K. Notice that these
two units are identical since ∆T(°C) ∆T(K), and 1°C change in temperature
is equivalent to a change of 1 K. Also,
1 kJ/kg · °C 1 J/g · °C 1 kJ/kg · K 1 J/g · K The specific heats of a substance, in general, depend on two independent
properties such as temperature and pressure. For an ideal gas, however, they
depend on temperature only (Fig. 1–8). At low pressures all real gases approach ideal gas behavior, and therefore their specific heats depend on temperature only.
The differential changes in the internal energy u and enthalpy h of an ideal
gas can be expressed in terms of the specific heats as
du Cv dT and dh Cp dT (12) The finite changes in the internal energy and enthalpy of an ideal gas during a
process can be expressed approximately by using specific heat values at the
average temperature as
u Cv, ave T and h Cp, ave T (J/g) (13) or
U
IRON
25°C
C = Cv = Cp
= 0.45 kJ/kg·°C FIGURE 1–9
The Cv and Cp values of incompressible
substances are identical and are
denoted by C. mCv, ave T and H mCp, ave T (J) (14) where m is the mass of the system.
A substance whose specific volume (or density) does not change with temperature or pressure is called an incompressible substance. The specific volumes of solids and liquids essentially remain constant during a process, and
thus they can be approximated as incompressible substances without sacrificing much in accuracy.
The constantvolume and constantpressure specific heats are identical for
incompressible substances (Fig. 1–9). Therefore, for solids and liquids the
subscripts on Cv and Cp can be dropped and both specific heats can be represented by a single symbol, C. That is, Cp Cv C. This result could also
be deduced from the physical definitions of constantvolume and constantpressure specific heats. Specific heats of several common gases, liquids, and
solids are given in the Appendix.
The specific heats of incompressible substances depend on temperature
only. Therefore, the change in the internal energy of solids and liquids can be
expressed as
U mCave T (J) (15) cen58933_ch01.qxd 9/10/2002 8:29 AM Page 9 9
CHAPTER 1 where Cave is the average specific heat evaluated at the average temperature.
Note that the internal energy change of the systems that remain in a single
phase (liquid, solid, or gas) during the process can be determined very easily
using average specific heats. Energy Transfer
Energy can be transferred to or from a given mass by two mechanisms: heat
Q and work W. An energy interaction is heat transfer if its driving force is a
temperature difference. Otherwise, it is work. A rising piston, a rotating shaft,
and an electrical wire crossing the system boundaries are all associated with
work interactions. Work done per unit time is called power, and is denoted
·
by W. The unit of power is W or hp (1 hp 746 W). Car engines and hydraulic, steam, and gas turbines produce work; compressors, pumps, and
mixers consume work. Notice that the energy of a system decreases as it does
work, and increases as work is done on it.
In daily life, we frequently refer to the sensible and latent forms of internal
energy as heat, and we talk about the heat content of bodies (Fig. 1–10). In
thermodynamics, however, those forms of energy are usually referred to as
thermal energy to prevent any confusion with heat transfer.
The term heat and the associated phrases such as heat flow, heat addition,
heat rejection, heat absorption, heat gain, heat loss, heat storage, heat generation, electrical heating, latent heat, body heat, and heat source are in common use today, and the attempt to replace heat in these phrases by thermal
energy had only limited success. These phrases are deeply rooted in our vocabulary and they are used by both the ordinary people and scientists without
causing any misunderstanding. For example, the phrase body heat is understood to mean the thermal energy content of a body. Likewise, heat flow is
understood to mean the transfer of thermal energy, not the flow of a fluidlike
substance called heat, although the latter incorrect interpretation, based on the
caloric theory, is the origin of this phrase. Also, the transfer of heat into a system is frequently referred to as heat addition and the transfer of heat out of a
system as heat rejection.
Keeping in line with current practice, we will refer to the thermal energy as
heat and the transfer of thermal energy as heat transfer. The amount of heat
transferred during the process is denoted by Q. The amount of heat transferred
·
per unit time is called heat transfer rate, and is denoted by Q . The overdot
·
stands for the time derivative, or “per unit time.” The heat transfer rate Q has
the unit J/s, which is equivalent to W.
·
When the rate of heat transfer Q is available, then the total amount of heat
transfer Q during a time interval t can be determined from
t Q ·
Q dt (J) (16) 0 ·
provided that the variation of Q with time is known. For the special case of
·
Q constant, the equation above reduces to
Q ·
Qt (J) (17) Vapor
80°C Liquid
80°C Heat
transfer 25°C FIGURE 1–10
The sensible and latent forms of internal
energy can be transferred as a result of
a temperature difference, and they are
referred to as heat or thermal energy. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 10 10
HEAT TRANSFER . The rate of heat transfer per unit area normal to the direction of heat transfer
is called heat flux, and the average heat flux is expressed as (Fig. 1–11) Q = 24 W
= const.
3m A = 6 m2 q· ·
Q
A (W/m2) (18) where A is the heat transfer area. The unit of heat flux in English units is
Btu/h · ft2. Note that heat flux may vary with time as well as position on a
surface. 2m . . Q 24 W
q = — = –—— = 4 W/m2
A 6 m2 FIGURE 1–11
Heat flux is heat transfer per unit
time and per unit area, and is equal
·
·
to q· Q /A when Q is uniform over
the area A. EXAMPLE 1–1 Heating of a Copper Ball A 10cm diameter copper ball is to be heated from 100°C to an average temperature of 150°C in 30 minutes (Fig. 1–12). Taking the average density and
specific heat of copper in this temperature range to be
8950 kg/m3 and
Cp
0.395 kJ/kg · °C, respectively, determine (a) the total amount of heat
transfer to the copper ball, (b) the average rate of heat transfer to the ball, and
(c) the average heat flux. T2 = 150°C SOLUTION The copper ball is to be heated from 100°C to 150°C. The total
heat transfer, the average rate of heat transfer, and the average heat flux are to
be determined. T1 = 100°C
A = πD 2 Q FIGURE 1–12
Schematic for Example 1–1. Assumptions Constant properties can be used for copper at the average
temperature.
Properties The average density and specific heat of copper are given to be
8950 kg/m3 and Cp 0.395 kJ/kg · °C.
Analysis (a) The amount of heat transferred to the copper ball is simply the
change in its internal energy, and is determined from Energy transfer to the system
Q
U mCave (T2 T1) Energy increase of the system where m V 6 D3 6 (8950 kg/m3)(0.1 m)3 4.69 kg Substituting, Q (4.69 kg)(0.395 kJ/kg · °C)(150 100)°C 92.6 kJ Therefore, 92.6 kJ of heat needs to be transferred to the copper ball to heat it
from 100°C to 150°C.
(b) The rate of heat transfer normally changes during a process with time. However, we can determine the average rate of heat transfer by dividing the total
amount of heat transfer by the time interval. Therefore, ·
Q ave Q
t 92.6 kJ
1800 s 0.0514 kJ/s 51.4 W cen58933_ch01.qxd 9/10/2002 8:29 AM Page 11 11
CHAPTER 1 (c) Heat flux is defined as the heat transfer per unit time per unit area, or the
rate of heat transfer per unit area. Therefore, the average heat flux in this
case is ·
q ave ·
Qave
A ·
Qave
D2 51.4 W
(0.1 m)2 1636 W/m2 Discussion Note that heat flux may vary with location on a surface. The value
calculated above is the average heat flux over the entire surface of the ball. 1–4 I THE FIRST LAW OF THERMODYNAMICS The first law of thermodynamics, also known as the conservation of energy
principle, states that energy can neither be created nor destroyed; it can only
change forms. Therefore, every bit of energy must be accounted for during a
process. The conservation of energy principle (or the energy balance) for any
system undergoing any process may be expressed as follows: The net change
(increase or decrease) in the total energy of the system during a process is
equal to the difference between the total energy entering and the total energy
leaving the system during that process. That is,
Total energy
entering the
system Total energy
leaving the
system Change in the
total energy of
the system (19) Noting that energy can be transferred to or from a system by heat, work, and
mass flow, and that the total energy of a simple compressible system consists
of internal, kinetic, and potential energies, the energy balance for any system
undergoing any process can be expressed as
E
Eout
1in
424
3 Esystem
123 Net energy transfer
by heat, work, and mass (J) (110) Change in internal, kinetic,
potential, etc., energies (W) (111) or, in the rate form, as
·
·
E
Eout
1in
424
3 dE
/dt
1 system 3
424 Rate of net energy transfer
by heat, work, and mass Rate of change in internal
kinetic, potential, etc., energies Energy is a property, and the value of a property does not change unless the
state of the system changes. Therefore, the energy change of a system is zero
( Esystem 0) if the state of the system does not change during the process,
that is, the process is steady. The energy balance in this case reduces to
(Fig. 1–13)
·
Ein
123 ·
Eout
123 Rate of net energy transfer in
by heat, work, and mass Steady, rate form: Rate of net energy transfer out
by heat, work, and mass (112) In the absence of significant electric, magnetic, motion, gravity, and surface
tension effects (i.e., for stationary simple compressible systems), the change ·
Ein ·
Eout Heat
Work Heat
Steady
system Mass Work
Mass ·
·
Ein = Eout FIGURE 1–13
In steady operation, the rate of energy
transfer to a system is equal to the rate
of energy transfer from the system. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 12 12
HEAT TRANSFER in the total energy of a system during a process is simply the change in its inUsystem.
ternal energy. That is, Esystem
In heat transfer analysis, we are usually interested only in the forms of energy that can be transferred as a result of a temperature difference, that is, heat
or thermal energy. In such cases it is convenient to write a heat balance and
to treat the conversion of nuclear, chemical, and electrical energies into thermal energy as heat generation. The energy balance in that case can be expressed as
Q
Qout
1in
424
3 Egen
123 E
1 424 3
4thermal, system
4 Net heat
transfer Heat
generation (J) Change in thermal
energy of the system (113) Energy Balance for Closed Systems (Fixed Mass) Specific heat = Cv
Mass = m
Initial temp = T1
Final temp = T2 Q = mCv (T1 – T2 ) FIGURE 1–14
In the absence of any work interactions,
the change in the energy content of a
closed system is equal to the net
heat transfer. A closed system consists of a fixed mass. The total energy E for most systems
encountered in practice consists of the internal energy U. This is especially the
case for stationary systems since they don’t involve any changes in their velocity or elevation during a process. The energy balance relation in that case
reduces to
Stationary closed system: Ein Eout U mCv T (J) (114) where we expressed the internal energy change in terms of mass m, the specific heat at constant volume Cv, and the temperature change T of the system. When the system involves heat transfer only and no work interactions
across its boundary, the energy balance relation further reduces to (Fig. 1–14)
Stationary closed system, no work: Q mCv T (J) (115) where Q is the net amount of heat transfer to or from the system. This is the
form of the energy balance relation we will use most often when dealing with
a fixed mass. Energy Balance for SteadyFlow Systems
A large number of engineering devices such as water heaters and car radiators
involve mass flow in and out of a system, and are modeled as control volumes.
Most control volumes are analyzed under steady operating conditions. The
term steady means no change with time at a specified location. The opposite
of steady is unsteady or transient. Also, the term uniform implies no change
with position throughout a surface or region at a specified time. These meanings are consistent with their everyday usage (steady girlfriend, uniform
distribution, etc.). The total energy content of a control volume during a
steadyflow process remains constant (ECV constant). That is, the change
in the total energy of the control volume during such a process is zero
( ECV 0). Thus the amount of energy entering a control volume in all forms
(heat, work, mass transfer) for a steadyflow process must be equal to the
amount of energy leaving it.
The amount of mass flowing through a cross section of a flow device per
·
unit time is called the mass flow rate, and is denoted by m . A fluid may flow
in and out of a control volume through pipes or ducts. The mass flow rate of a
fluid flowing in a pipe or duct is proportional to the crosssectional area Ac of cen58933_ch01.qxd 9/10/2002 8:29 AM Page 13 13
CHAPTER 1 the pipe or duct, the density , and the velocity of the fluid. The mass flow
·
rate through a differential area dAc can be expressed as δm
n dAc where
n is the velocity component normal to dAc. The mass flow rate through the
entire crosssectional area is obtained by integration over Ac.
The flow of a fluid through a pipe or duct can often be approximated to be
onedimensional. That is, the properties can be assumed to vary in one direction only (the direction of flow). As a result, all properties are assumed to be
uniform at any cross section normal to the flow direction, and the properties
are assumed to have bulk average values over the entire cross section. Under
the onedimensional flow approximation, the mass flow rate of a fluid flowing in a pipe or duct can be expressed as (Fig. 1–15)
·
m Ac (kg/s) (116) where is the fluid density, is the average fluid velocity in the flow direction, and Ac is the crosssectional area of the pipe or duct.
The volume of a fluid flowing through a pipe or duct per unit time is called
·
the volume flow rate V, and is expressed as
·
V ·
m Ac (m3/s) ·
mh ·
m Cp T (kJ/s) (118) ·
where Q is the rate of net heat transfer into or out of the control volume. This
is the form of the energy balance relation that we will use most often for
steadyflow systems. Surface Energy Balance
As mentioned in the chapter opener, heat is transferred by the mechanisms of
conduction, convection, and radiation, and heat often changes vehicles as it is
transferred from one medium to another. For example, the heat conducted to
the outer surface of the wall of a house in winter is convected away by the
cold outdoor air while being radiated to the cold surroundings. In such cases,
it may be necessary to keep track of the energy interactions at the surface, and
this is done by applying the conservation of energy principle to the surface.
A surface contains no volume or mass, and thus no energy. Thereore, a surface can be viewed as a fictitious system whose energy content remains constant during a process (just like a steadystate or steadyflow system). Then
the energy balance for a surface can be expressed as
Surface energy balance: ·
Ein ·
Eout ·
m = ρ Ac FIGURE 1–15
The mass flow rate of a fluid at a cross
section is equal to the product of the
fluid density, average fluid velocity,
and the crosssectional area. (117) Note that the mass flow rate of a fluid through a pipe or duct remains constant
during steady flow. This is not the case for the volume flow rate, however, unless the density of the fluid remains constant.
For a steadyflow system with one inlet and one exit, the rate of mass flow
into the control volume must be equal to the rate of mass flow out of it. That
·
·
·
is, m in m out m. When the changes in kinetic and potential energies are
negligible, which is usually the case, and there is no work interaction, the energy balance for such a steadyflow system reduces to (Fig. 1–16)
·
Q Ac = π D 2/4
for a circular pipe (119) Control volume
·
m
T1 ·
m
T2
·
·
Etransfer = mCp(T2 – T1) FIGURE 1–16
Under steady conditions, the net rate of
energy transfer to a fluid in a control
volume is equal to the rate of increase in
the energy of the fluid stream flowing
through the control volume. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 14 14
HEAT TRANSFER WALL Control
surface This relation is valid for both steady and transient conditions, and the surface
energy balance does not involve heat generation since a surface does not have
a volume. The energy balance for the outer surface of the wall in Fig. 1–17,
for example, can be expressed as radiation ·
Q1 . Q3 conduction . Q1 . Q2
convection FIGURE 1–17
Energy interactions at the outer wall
surface of a house. ·
Q2 ·
Q3 (120) ·
·
where Q 1 is conduction through the wall to the surface, Q 2 is convection from
·
the surface to the outdoor air, and Q 3 is net radiation from the surface to the
surroundings.
When the directions of interactions are not known, all energy interactions
can be assumed to be towards the surface, and the surface energy balance can
·
be expressed as E in 0. Note that the interactions in opposite direction will
end up having negative values, and balance this equation. EXAMPLE 1–2 Heating of Water in an Electric Teapot 1.2 kg of liquid water initially at 15°C is to be heated to 95°C in a teapot
equipped with a 1200W electric heating element inside (Fig. 1–18). The
teapot is 0.5 kg and has an average specific heat of 0.7 kJ/kg · °C. Taking the
specific heat of water to be 4.18 kJ/kg · °C and disregarding any heat loss from
the teapot, determine how long it will take for the water to be heated. Electric
heating
element Water
15°C
1200 W FIGURE 1–18
Schematic for Example 1–2. SOLUTION Liquid water is to be heated in an electric teapot. The heating time
is to be determined.
Assumptions 1 Heat loss from the teapot is negligible. 2 Constant properties
can be used for both the teapot and the water.
Properties The average specific heats are given to be 0.7 kJ/kg · °C for the
teapot and 4.18 kJ/kg · °C for water.
Analysis We take the teapot and the water in it as the system, which is
a closed system (fixed mass). The energy balance in this case can be expressed as
Ein Eout
Ein Esystem
Usystem Uwater Uteapot Then the amount of energy needed to raise the temperature of water and the
teapot from 15°C to 95°C is Ein (mC T )water (mC T )teapot
(1.2 kg)(4.18 kJ/kg · °C)(95
(95 15)°C
429.3 kJ 15)°C (0.5 kg)(0.7 kJ/kg · °C) The 1200W electric heating unit will supply energy at a rate of 1.2 kW or
1.2 kJ per second. Therefore, the time needed for this heater to supply
429.3 kJ of heat is determined from t Total energy transferred
Rate of energy transfer Ein
·
E transfer 429.3 kJ
1.2 kJ/s 358 s 6.0 min cen58933_ch01.qxd 9/10/2002 8:29 AM Page 15 15
CHAPTER 1 Discussion In reality, it will take more than 6 minutes to accomplish this heating process since some heat loss is inevitable during heating. EXAMPLE 1–3 Heat Loss from Heating Ducts in a Basement A 5mlong section of an air heating system of a house passes through an unheated space in the basement (Fig. 1–19). The cross section of the rectangular
duct of the heating system is 20 cm
25 cm. Hot air enters the duct at
100 kPa and 60°C at an average velocity of 5 m/s. The temperature of the air
in the duct drops to 54°C as a result of heat loss to the cool space in the basement. Determine the rate of heat loss from the air in the duct to the basement
under steady conditions. Also, determine the cost of this heat loss per hour if
the house is heated by a natural gas furnace that has an efficiency of 80 percent, and the cost of the natural gas in that area is $0.60/therm (1 therm
100,000 Btu 105,500 kJ). SOLUTION The temperature of the air in the heating duct of a house drops as
a result of heat loss to the cool space in the basement. The rate of heat loss
from the hot air and its cost are to be determined.
Assumptions 1 Steady operating conditions exist. 2 Air can be treated as an
ideal gas with constant properties at room temperature.
Properties The constant pressure specific heat of air at the average temperature of (54 60)/2 57°C is 1.007 kJ/kg · °C (Table A15).
Analysis We take the basement section of the heating system as our system,
which is a steadyflow system. The rate of heat loss from the air in the duct can
be determined from ·
Q
·
where m is the mass flow rate and
air at the inlet conditions is P
RT ·
m Cp T
T is the temperature drop. The density of 100 kPa
(0.287 kPa · m3/kg · K)(60 1.046 kg/m3 273)K The crosssectional area of the duct is Ac (0.20 m)(0.25 m) 0.05 m2 Then the mass flow rate of air through the duct and the rate of heat loss
become ·
m Ac (1.046 kg/m3)(5 m/s)(0.05 m2) 0.2615 kg/s and ·
Q loss ·
m Cp(Tin Tout)
(0.2615 kg/s)(1.007 kJ/kg · °C)(60
1.580 kJ/s 54)°C 5m
20 cm
Hot air
100 kPa
60°C
5 m/s 25 cm 54°C ·
Qloss FIGURE 1–19
Schematic for Example 1–3. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 16 16
HEAT TRANSFER or 5688 kJ/h. The cost of this heat loss to the home owner is Cost of heat loss (Rate of heat loss)(Unit cost of energy input)
Furnace efficiency
(5688 kJ/h)($0.60/therm) 1 therm
0.80
105,500 kJ
$0.040/h Discussion The heat loss from the heating ducts in the basement is costing the
home owner 4 cents per hour. Assuming the heater operates 2000 hours during
a heating season, the annual cost of this heat loss adds up to $80. Most of this
money can be saved by insulating the heating ducts in the unheated areas. EXAMPLE 1–4 P = 12.2 psia
atm 50°F 9 ft 70°F
40 ft 50 ft FIGURE 1–20
Schematic for Example 1–4. Electric Heating of a House at High Elevation Consider a house that has a floor space of 2000 ft2 and an average height of 9
ft at 5000 ft elevation where the standard atmospheric pressure is 12.2 psia
(Fig. 1–20). Initially the house is at a uniform temperature of 50°F. Now the
electric heater is turned on, and the heater runs until the air temperature in the
house rises to an average value of 70°F. Determine the amount of energy transferred to the air assuming (a) the house is airtight and thus no air escapes during the heating process and (b) some air escapes through the cracks as the
heated air in the house expands at constant pressure. Also determine the cost
of this heat for each case if the cost of electricity in that area is $0.075/kWh. SOLUTION The air in the house is heated from 50°F to 70°F by an electric
heater. The amount and cost of the energy transferred to the air are to be determined for constantvolume and constantpressure cases.
Assumptions 1 Air can be treated as an ideal gas with constant properties at
room temperature. 2 Heat loss from the house during heating is negligible.
3 The volume occupied by the furniture and other things is negligible.
Properties The specific heats of air at the average temperature of (50 70)/2
60°F are Cp
0.240 Btu/lbm · °F and Cv Cp R
0.171 Btu/lbm · °F
(Tables A1E and A15E).
Analysis The volume and the mass of the air in the house are
V
m (Floor area)(Height) (2000 ft2)(9 ft) 18,000 ft3
(12.2 psia)(18,000 ft3)
PV
1162 lbm
RT (0.3704 psia · ft3/lbm · R)(50 460)R (a) The amount of energy transferred to air at constant volume is simply the
change in its internal energy, and is determined from Ein Eout
Ein, constant volume Esystem
Uair mCv T
(1162 lbm)(0.171 Btu/lbm · °F)(70
3974 Btu At a unit cost of $0.075/kWh, the total cost of this energy is 50)°F cen58933_ch01.qxd 9/10/2002 8:29 AM Page 17 17
CHAPTER 1 Cost of energy (Amount of energy)(Unit cost of energy)
1 kWh
(3974 Btu)($0.075/kWh)
3412 Btu
$0.087 (b) The amount of energy transferred to air at constant pressure is the change
in its enthalpy, and is determined from Ein, constant pressure Hair mCp T
(1162 lbm)(0.240 Btu/lbm · °F)(70
5578 Btu 50)°F At a unit cost of $0.075/kWh, the total cost of this energy is Cost of energy (Amount of energy)(Unit cost of energy)
1 kWh
(5578 Btu)($0.075/kWh)
3412 Btu
$0.123 Discussion It will cost about 12 cents to raise the temperature of the air in
this house from 50°F to 70°F. The second answer is more realistic since every
house has cracks, especially around the doors and windows, and the pressure in
the house remains essentially constant during a heating process. Therefore, the
second approach is used in practice. This conservative approach somewhat
overpredicts the amount of energy used, however, since some of the air will escape through the cracks before it is heated to 70°F. 1–5 I HEAT TRANSFER MECHANISMS In Section 1–1 we defined heat as the form of energy that can be transferred
from one system to another as a result of temperature difference. A thermodynamic analysis is concerned with the amount of heat transfer as a system
undergoes a process from one equilibrium state to another. The science that
deals with the determination of the rates of such energy transfers is the heat
transfer. The transfer of energy as heat is always from the highertemperature
medium to the lowertemperature one, and heat transfer stops when the two
mediums reach the same temperature.
Heat can be transferred in three different modes: conduction, convection,
and radiation. All modes of heat transfer require the existence of a temperature difference, and all modes are from the hightemperature medium to a
lowertemperature one. Below we give a brief description of each mode. A detailed study of these modes is given in later chapters of this text. 1–6 I CONDUCTION Conduction is the transfer of energy from the more energetic particles of a
substance to the adjacent less energetic ones as a result of interactions between the particles. Conduction can take place in solids, liquids, or gases. In
gases and liquids, conduction is due to the collisions and diffusion of the cen58933_ch01.qxd 9/10/2002 8:29 AM Page 18 18
HEAT TRANSFER T1
T2 . Q
A A ∆x
0 x FIGURE 1–21
Heat conduction through a large plane
wall of thickness x and area A. molecules during their random motion. In solids, it is due to the combination
of vibrations of the molecules in a lattice and the energy transport by free
electrons. A cold canned drink in a warm room, for example, eventually
warms up to the room temperature as a result of heat transfer from the room
to the drink through the aluminum can by conduction.
The rate of heat conduction through a medium depends on the geometry of
the medium, its thickness, and the material of the medium, as well as the temperature difference across the medium. We know that wrapping a hot water
tank with glass wool (an insulating material) reduces the rate of heat loss from
the tank. The thicker the insulation, the smaller the heat loss. We also know
that a hot water tank will lose heat at a higher rate when the temperature of the
room housing the tank is lowered. Further, the larger the tank, the larger the
surface area and thus the rate of heat loss.
Consider steady heat conduction through a large plane wall of thickness
x L and area A, as shown in Fig. 1–21. The temperature difference across
the wall is T T2 T1. Experiments have shown that the rate of heat trans·
fer Q through the wall is doubled when the temperature difference T across
the wall or the area A normal to the direction of heat transfer is doubled, but is
halved when the wall thickness L is doubled. Thus we conclude that the rate
of heat conduction through a plane layer is proportional to the temperature
difference across the layer and the heat transfer area, but is inversely proportional to the thickness of the layer. That is,
Rate of heat conduction (Area)(Temperature difference)
Thickness or,
·
Q cond 30°C kA 20°C
.
q = 4010 W/m 2 1m (a) Copper (k = 401 W/m·°C) 30°C
20°C
.
q = 1480 W/m 2
1m (b) Silicon (k = 148 W/m ·°C) FIGURE 1–22
The rate of heat conduction through a
solid is directly proportional to
its thermal conductivity. T2 T1 kA x T
x (W) (121) where the constant of proportionality k is the thermal conductivity of the
material, which is a measure of the ability of a material to conduct heat
(Fig. 1–22). In the limiting case of x → 0, the equation above reduces to the
differential form
·
Q cond kA dT
dx (W) (122) which is called Fourier’s law of heat conduction after J. Fourier, who expressed it first in his heat transfer text in 1822. Here dT/dx is the temperature
gradient, which is the slope of the temperature curve on a Tx diagram (the
rate of change of T with x), at location x. The relation above indicates that the
rate of heat conduction in a direction is proportional to the temperature gradient in that direction. Heat is conducted in the direction of decreasing temperature, and the temperature gradient becomes negative when temperature
decreases with increasing x. The negative sign in Eq. 1–22 ensures that heat
transfer in the positive x direction is a positive quantity.
The heat transfer area A is always normal to the direction of heat transfer.
For heat loss through a 5mlong, 3mhigh, and 25cmthick wall, for example, the heat transfer area is A 15 m2. Note that the thickness of the wall has
no effect on A (Fig. 1–23). cen58933_ch01.qxd 9/10/2002 8:29 AM Page 19 19
CHAPTER 1 EXAMPLE 1–5 The Cost of Heat Loss through a Roof The roof of an electrically heated home is 6 m long, 8 m wide, and 0.25 m
thick, and is made of a flat layer of concrete whose thermal conductivity is
k 0.8 W/m · °C (Fig. 1–24). The temperatures of the inner and the outer surfaces of the roof one night are measured to be 15°C and 4°C, respectively, for a
period of 10 hours. Determine (a) the rate of heat loss through the roof that
night and (b) the cost of that heat loss to the home owner if the cost of electricity is $0.08/kWh. SOLUTION The inner and outer surfaces of the flat concrete roof of an electrically heated home are maintained at specified temperatures during a night. The
heat loss through the roof and its cost that night are to be determined.
Assumptions 1 Steady operating conditions exist during the entire night since
the surface temperatures of the roof remain constant at the specified values.
2 Constant properties can be used for the roof.
Properties The thermal conductivity of the roof is given to be k
0.8
W/m · °C.
Analysis (a) Noting that heat transfer through the roof is by conduction and
the area of the roof is A 6 m 8 m 48 m2, the steady rate of heat transfer through the roof is determined to be
·
Q kA T2 T1
L (0.8 W/m · °C)(48 m2) (15 4)°C
0.25 m 1690 W 1.69 kW H
A=W×H ·
Q W
L FIGURE 1–23
In heat conduction analysis, A represents
the area normal to the direction
of heat transfer.
Concrete roof 6m 0.25 m 8m
4°C
15°C (b) The amount of heat lost through the roof during a 10hour period and its
cost are determined from
Q Cost ·
Q t (1.69 kW)(10 h) 16.9 kWh
(Amount of energy)(Unit cost of energy)
(16.9 kWh)($0.08/kWh) $1.35 Discussion The cost to the home owner of the heat loss through the roof that
night was $1.35. The total heating bill of the house will be much larger since
the heat losses through the walls are not considered in these calculations. Thermal Conductivity
We have seen that different materials store heat differently, and we have defined the property specific heat Cp as a measure of a material’s ability to store
thermal energy. For example, Cp 4.18 kJ/kg · °C for water and Cp 0.45
kJ/kg · °C for iron at room temperature, which indicates that water can store
almost 10 times the energy that iron can per unit mass. Likewise, the thermal
conductivity k is a measure of a material’s ability to conduct heat. For example, k 0.608 W/m · °C for water and k 80.2 W/m · °C for iron at room
temperature, which indicates that iron conducts heat more than 100 times
faster than water can. Thus we say that water is a poor heat conductor relative
to iron, although water is an excellent medium to store thermal energy.
Equation 1–22 for the rate of conduction heat transfer under steady conditions can also be viewed as the defining equation for thermal conductivity.
Thus the thermal conductivity of a material can be defined as the rate of FIGURE 1–24
Schematic for Example 1–5. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 20 20
HEAT TRANSFER TABLE 1–1
The thermal conductivities of some
materials at room temperature
Material k, W/m · °C* Diamond
Silver
Copper
Gold
Aluminum
Iron
Mercury (l)
Glass
Brick
Water (l)
Human skin
Wood (oak)
Helium (g)
Soft rubber
Glass fiber
Air (g)
Urethane, rigid foam 2300
429
401
317
237
80.2
8.54
0.78
0.72
0.613
0.37
0.17
0.152
0.13
0.043
0.026
0.026 *Multiply by 0.5778 to convert to Btu/h · ft · °F. Electric
heater
Insulation Insulation T1 Sample
material k . . T2 Q = We
A . L We
Insulation .
L
k = ———— Q
A(T1 – T2)
FIGURE 1–25
A simple experimental setup to
determine the thermal conductivity
of a material. heat transfer through a unit thickness of the material per unit area per unit
temperature difference. The thermal conductivity of a material is a measure of
the ability of the material to conduct heat. A high value for thermal conductivity indicates that the material is a good heat conductor, and a low value
indicates that the material is a poor heat conductor or insulator. The thermal
conductivities of some common materials at room temperature are given in
Table 1–1. The thermal conductivity of pure copper at room temperature is
k 401 W/m · °C, which indicates that a 1mthick copper wall will conduct
heat at a rate of 401 W per m2 area per °C temperature difference across the
wall. Note that materials such as copper and silver that are good electric conductors are also good heat conductors, and have high values of thermal conductivity. Materials such as rubber, wood, and styrofoam are poor conductors
of heat and have low conductivity values.
A layer of material of known thickness and area can be heated from one side
by an electric resistance heater of known output. If the outer surfaces of the
heater are well insulated, all the heat generated by the resistance heater will be
transferred through the material whose conductivity is to be determined. Then
measuring the two surface temperatures of the material when steady heat
transfer is reached and substituting them into Eq. 1–22 together with other
known quantities give the thermal conductivity (Fig. 1–25).
The thermal conductivities of materials vary over a wide range, as shown in
Fig. 1–26. The thermal conductivities of gases such as air vary by a factor of
104 from those of pure metals such as copper. Note that pure crystals and metals have the highest thermal conductivities, and gases and insulating materials
the lowest.
Temperature is a measure of the kinetic energies of the particles such as the
molecules or atoms of a substance. In a liquid or gas, the kinetic energy of the
molecules is due to their random translational motion as well as their
vibrational and rotational motions. When two molecules possessing different kinetic energies collide, part of the kinetic energy of the more energetic
(highertemperature) molecule is transferred to the less energetic (lowertemperature) molecule, much the same as when two elastic balls of the same
mass at different velocities collide, part of the kinetic energy of the faster
ball is transferred to the slower one. The higher the temperature, the faster the
molecules move and the higher the number of such collisions, and the better
the heat transfer.
The kinetic theory of gases predicts and the experiments confirm that the
thermal conductivity of gases is proportional to the square root of the absolute temperature T, and inversely proportional to the square root of the molar
mass M. Therefore, the thermal conductivity of a gas increases with increasing temperature and decreasing molar mass. So it is not surprising that the
thermal conductivity of helium (M
4) is much higher than those of air
(M 29) and argon (M 40).
The thermal conductivities of gases at 1 atm pressure are listed in Table
A16. However, they can also be used at pressures other than 1 atm, since the
thermal conductivity of gases is independent of pressure in a wide range of
pressures encountered in practice.
The mechanism of heat conduction in a liquid is complicated by the fact that
the molecules are more closely spaced, and they exert a stronger intermolecular force field. The thermal conductivities of liquids usually lie between those cen58933_ch01.qxd 9/10/2002 8:29 AM Page 21 21
CHAPTER 1
NONMETALLIC
CRYSTALS
1000
PURE
METALS k,
W/m·°C METAL
ALLOYS Silver
Copper Aluminum
NONMETALLIC alloys
Iron
SOLIDS 100 Oxides
10 Bronze
Steel
Nichrome Diamond
Graphite
Silicon
carbide
Beryllium
oxide Manganese Quartz LIQUIDS
Mercury
Rock 1 INSULATORS Water
Food Fibers Wood Air 0.1 GASES
Hydrogen
Helium Foams Rubber
Oils Carbon
dioxide
0.01 of solids and gases. The thermal conductivity of a substance is normally highest in the solid phase and lowest in the gas phase. Unlike gases, the thermal
conductivities of most liquids decrease with increasing temperature, with water being a notable exception. Like gases, the conductivity of liquids decreases
with increasing molar mass. Liquid metals such as mercury and sodium have
high thermal conductivities and are very suitable for use in applications where
a high heat transfer rate to a liquid is desired, as in nuclear power plants.
In solids, heat conduction is due to two effects: the lattice vibrational waves
induced by the vibrational motions of the molecules positioned at relatively
fixed positions in a periodic manner called a lattice, and the energy transported via the free flow of electrons in the solid (Fig. 1–27). The thermal conductivity of a solid is obtained by adding the lattice and electronic
components. The relatively high thermal conductivities of pure metals are primarily due to the electronic component. The lattice component of thermal
conductivity strongly depends on the way the molecules are arranged. For example, diamond, which is a highly ordered crystalline solid, has the highest
known thermal conductivity at room temperature.
Unlike metals, which are good electrical and heat conductors, crystalline
solids such as diamond and semiconductors such as silicon are good heat conductors but poor electrical conductors. As a result, such materials find widespread use in the electronics industry. Despite their higher price, diamond heat
sinks are used in the cooling of sensitive electronic components because of the FIGURE 1–26
The range of thermal conductivity of
various materials at room temperature. GAS
* Molecular
collisions
* Molecular
diffusion LIQUID
* Molecular
collisions
* Molecular
diffusion electrons SOLID
* Lattice vibrations
* Flow of free
electrons FIGURE 1–27
The mechanisms of heat conduction in
different phases of a substance. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 22 22
HEAT TRANSFER TABLE 1–2
The thermal conductivity of an
alloy is usually much lower than
the thermal conductivity of either
metal of which it is composed
Pure metal or
alloy k, W/m · °C,
at 300 K Copper
Nickel
Constantan
(55% Cu, 45% Ni) 401
91 Copper
Aluminum
Commercial bronze
(90% Cu, 10% Al) 401
237 23 52 TABLE 1–3
Thermal conductivities of materials
vary with temperature
T, K Copper Aluminum 100
200
300
400
600
800 482
413
401
393
379
366 302
237
237
240
231
218 excellent thermal conductivity of diamond. Silicon oils and gaskets are commonly used in the packaging of electronic components because they provide
both good thermal contact and good electrical insulation.
Pure metals have high thermal conductivities, and one would think that
metal alloys should also have high conductivities. One would expect an alloy
made of two metals of thermal conductivities k1 and k2 to have a conductivity
k between k1 and k2. But this turns out not to be the case. The thermal conductivity of an alloy of two metals is usually much lower than that of either metal,
as shown in Table 1–2. Even small amounts in a pure metal of “foreign” molecules that are good conductors themselves seriously disrupt the flow of heat
in that metal. For example, the thermal conductivity of steel containing just
1 percent of chrome is 62 W/m · °C, while the thermal conductivities of iron
and chromium are 83 and 95 W/m · °C, respectively.
The thermal conductivities of materials vary with temperature (Table 1–3).
The variation of thermal conductivity over certain temperature ranges is negligible for some materials, but significant for others, as shown in Fig. 1–28.
The thermal conductivities of certain solids exhibit dramatic increases at temperatures near absolute zero, when these solids become superconductors. For
example, the conductivity of copper reaches a maximum value of about
20,000 W/m · °C at 20 K, which is about 50 times the conductivity at room
temperature. The thermal conductivities and other thermal properties of various materials are given in Tables A3 to A16.
10,000
k,
W/m·°C Solids
Liquids
Gases Diamonds 1000 Type IIa
Type IIb
Type I
Silver Copper 100 Gold Aluminum Tungsten Platinum
Iron
10 Aluminum oxide
Pyroceram glass
Clear fused quartz 1 Water 0.1 Helium Carbon tetrachloride FIGURE 1–28
The variation of the thermal
conductivity of various solids,
liquids, and gases with temperature
(from White, Ref. 10). Air Steam Argon
0.01
200 400 600 800
T, K 1000 1200 1400 cen58933_ch01.qxd 9/10/2002 8:29 AM Page 23 23
CHAPTER 1 The temperature dependence of thermal conductivity causes considerable
complexity in conduction analysis. Therefore, it is common practice to evaluate the thermal conductivity k at the average temperature and treat it as a constant in calculations.
In heat transfer analysis, a material is normally assumed to be isotropic; that
is, to have uniform properties in all directions. This assumption is realistic for
most materials, except those that exhibit different structural characteristics in
different directions, such as laminated composite materials and wood. The
thermal conductivity of wood across the grain, for example, is different than
that parallel to the grain. Thermal Diffusivity
The product Cp, which is frequently encountered in heat transfer analysis, is
called the heat capacity of a material. Both the specific heat Cp and the heat
capacity Cp represent the heat storage capability of a material. But Cp expresses it per unit mass whereas Cp expresses it per unit volume, as can be
noticed from their units J/kg · °C and J/m3 · °C, respectively.
Another material property that appears in the transient heat conduction
analysis is the thermal diffusivity, which represents how fast heat diffuses
through a material and is defined as
Heat conducted
Heat stored k
Cp (m2/s) (123) Note that the thermal conductivity k represents how well a material conducts heat, and the heat capacity Cp represents how much energy a material
stores per unit volume. Therefore, the thermal diffusivity of a material can be
viewed as the ratio of the heat conducted through the material to the heat
stored per unit volume. A material that has a high thermal conductivity or a
low heat capacity will obviously have a large thermal diffusivity. The larger
the thermal diffusivity, the faster the propagation of heat into the medium.
A small value of thermal diffusivity means that heat is mostly absorbed by the
material and a small amount of heat will be conducted further.
The thermal diffusivities of some common materials at 20°C are given in
Table 1–4. Note that the thermal diffusivity ranges from
0.14 10 6 m2/s
for water to 174 10 6 m2/s for silver, which is a difference of more than a
thousand times. Also note that the thermal diffusivities of beef and water are
the same. This is not surprising, since meat as well as fresh vegetables and
fruits are mostly water, and thus they possess the thermal properties of water.
EXAMPLE 1–6 Measuring the Thermal Conductivity of a Material A common way of measuring the thermal conductivity of a material is to sandwich an electric thermofoil heater between two identical samples of the material, as shown in Fig. 1–29. The thickness of the resistance heater, including
its cover, which is made of thin silicon rubber, is usually less than 0.5 mm.
A circulating fluid such as tap water keeps the exposed ends of the samples
at constant temperature. The lateral surfaces of the samples are well insulated
to ensure that heat transfer through the samples is onedimensional. Two
thermocouples are embedded into each sample some distance L apart, and a TABLE 1–4
The thermal diffusivities of some
materials at room temperature
, m2/s* Material
Silver
Gold
Copper
Aluminum
Iron
Mercury (l)
Marble
Ice
Concrete
Brick
Heavy soil (dry)
Glass
Glass wool
Water (l)
Beef
Wood (oak) 149
127
113
97.5
22.8
4.7
1.2
1.2
0.75
0.52
0.52
0.34
0.23
0.14
0.14
0.13 10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10 6
6
6
6
6
6
6
6
6
6
6
6
6
6
6
6 *Multiply by 10.76 to convert to ft2/s. Cooling
fluid
Sample
Insulation Thermocouple
L ∆T1 a Resistance
heater
Sample a
L ∆T1 Cooling
fluid FIGURE 1–29
Apparatus to measure the thermal
conductivity of a material using two
identical samples and a thin resistance
heater (Example 1–6). cen58933_ch01.qxd 9/10/2002 8:29 AM Page 24 24
HEAT TRANSFER differential thermometer reads the temperature drop T across this distance
along each sample. When steady operating conditions are reached, the total
rate of heat transfer through both samples becomes equal to the electric power
drawn by the heater, which is determined by multiplying the electric current by
the voltage.
In a certain experiment, cylindrical samples of diameter 5 cm and length
10 cm are used. The two thermocouples in each sample are placed 3 cm apart.
After initial transients, the electric heater is observed to draw 0.4 A at 110 V,
and both differential thermometers read a temperature difference of 15°C. Determine the thermal conductivity of the sample. SOLUTION The thermal conductivity of a material is to be determined by ensuring onedimensional heat conduction, and by measuring temperatures when
steady operating conditions are reached.
Assumptions 1 Steady operating conditions exist since the temperature
readings do not change with time. 2 Heat losses through the lateral surfaces
of the apparatus are negligible since those surfaces are well insulated, and
thus the entire heat generated by the heater is conducted through the samples.
3 The apparatus possesses thermal symmetry.
Analysis The electrical power consumed by the resistance heater and converted to heat is ·
We (110 V)(0.4 A) VI 44 W The rate of heat flow through each sample is ·
Q 1
2 ·
We 1
2 (44 W) 22 W since only half of the heat generated will flow through each sample because of
symmetry. Reading the same temperature difference across the same distance
in each sample also confirms that the apparatus possesses thermal symmetry.
The heat transfer area is the area normal to the direction of heat flow, which is
the crosssectional area of the cylinder in this case:
A 1
4 D2 1
4 (0.05 m)2 0.00196 m2 Noting that the temperature drops by 15°C within 3 cm in the direction of heat
flow, the thermal conductivity of the sample is determined to be ·
Q kA T
→k
L ·
QL
AT (22 W)(0.03 m)
(0.00196 m2)(15°C) 22.4 W/m · °C Discussion Perhaps you are wondering if we really need to use two samples in
the apparatus, since the measurements on the second sample do not give any
additional information. It seems like we can replace the second sample by insulation. Indeed, we do not need the second sample; however, it enables us to
verify the temperature measurements on the first sample and provides thermal
symmetry, which reduces experimental error. EXAMPLE 1–7 Conversion between SI and English Units An engineer who is working on the heat transfer analysis of a brick building in
English units needs the thermal conductivity of brick. But the only value he can cen58933_ch01.qxd 9/10/2002 8:29 AM Page 25 25
CHAPTER 1 find from his handbooks is 0.72 W/m · °C, which is in SI units. To make matters worse, the engineer does not have a direct conversion factor between the
two unit systems for thermal conductivity. Can you help him out? SOLUTION The situation this engineer is facing is not unique, and most engineers often find themselves in a similar position. A person must be very careful
during unit conversion not to fall into some common pitfalls and to avoid some
costly mistakes. Although unit conversion is a simple process, it requires utmost
care and careful reasoning.
The conversion factors for W and m are straightforward and are given in conversion tables to be
1W
1m 3.41214 Btu/h
3.2808 ft But the conversion of °C into °F is not so simple, and it can be a source of error if one is not careful. Perhaps the first thought that comes to mind is to replace °C by (°F
32)/1.8 since T(°C)
[T(°F)
32]/1.8. But this will be
wrong since the °C in the unit W/m · °C represents per °C change in temperature. Noting that 1°C change in temperature corresponds to 1.8°F, the proper
conversion factor to be used is 1°C 1.8°F Substituting, we get 1 W/m · °C 3.41214 Btu/h
(3.2808 ft)(1.8°F) 0.5778 Btu/h · ft · °F k = 0.72 W/m·°C
= 0.42 Btu/h·ft·°F which is the desired conversion factor. Therefore, the thermal conductivity of
the brick in English units is
kbrick 0.72 W/m · °C
0.72 (0.5778 Btu/h · ft · °F)
0.42 Btu/h · ft · °F Discussion Note that the thermal conductivity value of a material in English
units is about half that in SI units (Fig. 1–30). Also note that we rounded the
result to two significant digits (the same number in the original value) since expressing the result in more significant digits (such as 0.4160 instead of 0.42)
would falsely imply a more accurate value than the original one. 1–7 I CONVECTION Convection is the mode of energy transfer between a solid surface and the
adjacent liquid or gas that is in motion, and it involves the combined effects of
conduction and fluid motion. The faster the fluid motion, the greater the
convection heat transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. The
presence of bulk motion of the fluid enhances the heat transfer between the
solid surface and the fluid, but it also complicates the determination of heat
transfer rates. FIGURE 1–30
The thermal conductivity value in
English units is obtained by multiplying
the value in SI units by 0.5778. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 26 26
HEAT TRANSFER
Velocity
variation
of air T
T
Air
flow As Temperature
variation
of air ·
Qconv
Ts Hot Block FIGURE 1–31
Heat transfer from a hot
surface to air by convection. Forced
convection Natural
convection
Air Air
hot egg hot egg FIGURE 1–32 Consider the cooling of a hot block by blowing cool air over its top surface
(Fig. 1–31). Energy is first transferred to the air layer adjacent to the block by
conduction. This energy is then carried away from the surface by convection,
that is, by the combined effects of conduction within the air that is due to random motion of air molecules and the bulk or macroscopic motion of the air
that removes the heated air near the surface and replaces it by the cooler air.
Convection is called forced convection if the fluid is forced to flow over
the surface by external means such as a fan, pump, or the wind. In contrast,
convection is called natural (or free) convection if the fluid motion is caused
by buoyancy forces that are induced by density differences due to the variation of temperature in the fluid (Fig. 1–32). For example, in the absence of a
fan, heat transfer from the surface of the hot block in Fig. 1–31 will be by natural convection since any motion in the air in this case will be due to the rise
of the warmer (and thus lighter) air near the surface and the fall of the cooler
(and thus heavier) air to fill its place. Heat transfer between the block and the
surrounding air will be by conduction if the temperature difference between
the air and the block is not large enough to overcome the resistance of air to
movement and thus to initiate natural convection currents.
Heat transfer processes that involve change of phase of a fluid are also considered to be convection because of the fluid motion induced during the
process, such as the rise of the vapor bubbles during boiling or the fall of the
liquid droplets during condensation.
Despite the complexity of convection, the rate of convection heat transfer is
observed to be proportional to the temperature difference, and is conveniently
expressed by Newton’s law of cooling as The cooling of a boiled egg
by forced and natural convection. TABLE 1–5
Typical values of convection heat
transfer coefficient
Type of
convection
Free convection of
gases
Free convection of
liquids
Forced convection
of gases
Forced convection
of liquids
Boiling and
condensation h, W/m2 · °C*
2–25
10–1000
25–250
50–20,000 ·
Q conv hAs (Ts T) (W) (124) where h is the convection heat transfer coefficient in W/m2 · °C or Btu/h · ft2 · °F,
As is the surface area through which convection heat transfer takes place, Ts is
the surface temperature, and T is the temperature of the fluid sufficiently far
from the surface. Note that at the surface, the fluid temperature equals the surface temperature of the solid.
The convection heat transfer coefficient h is not a property of the fluid. It is
an experimentally determined parameter whose value depends on all the variables influencing convection such as the surface geometry, the nature of fluid
motion, the properties of the fluid, and the bulk fluid velocity. Typical values
of h are given in Table 1–5.
Some people do not consider convection to be a fundamental mechanism of
heat transfer since it is essentially heat conduction in the presence of fluid motion. But we still need to give this combined phenomenon a name, unless we
are willing to keep referring to it as “conduction with fluid motion.” Thus, it
is practical to recognize convection as a separate heat transfer mechanism despite the valid arguments to the contrary. 2500–100,000 *Multiply by 0.176 to convert to Btu/h · ft2 · °F. EXAMPLE 1–8 Measuring Convection Heat Transfer Coefficient A 2mlong, 0.3cmdiameter electrical wire extends across a room at 15°C, as
shown in Fig. 1–33. Heat is generated in the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 152°C in steady cen58933_ch01.qxd 9/10/2002 8:29 AM Page 27 27
CHAPTER 1 operation. Also, the voltage drop and electric current through the wire are measured to be 60 V and 1.5 A, respectively. Disregarding any heat transfer by
radiation, determine the convection heat transfer coefficient for heat transfer
between the outer surface of the wire and the air in the room. T• = 15°C
1.5 A 152°C 60 V FIGURE 1–33
SOLUTION The convection heat transfer coefficient for heat transfer from an
electrically heated wire to air is to be determined by measuring temperatures
when steady operating conditions are reached and the electric power consumed.
Assumptions 1 Steady operating conditions exist since the temperature readings do not change with time. 2 Radiation heat transfer is negligible.
Analysis When steady operating conditions are reached, the rate of heat loss
from the wire will equal the rate of heat generation in the wire as a result of
resistance heating. That is, ·
Q ·
Egenerated VI (60 V)(1.5 A) 90 W The surface area of the wire is
As DL (0.003 m)(2 m) 0.01885 m2 Newton’s law of cooling for convection heat transfer is expressed as ·
Q conv hAs (Ts T) Disregarding any heat transfer by radiation and thus assuming all the heat loss
from the wire to occur by convection, the convection heat transfer coefficient is
determined to be
h ·
Q conv
As(Ts T ) 90 W
(0.01885 m2)(152 15)°C 34.9 W/m2 · °C Discussion Note that the simple setup described above can be used to determine the average heat transfer coefficients from a variety of surfaces in air.
Also, heat transfer by radiation can be eliminated by keeping the surrounding
surfaces at the temperature of the wire. 1–8 I RADIATION Radiation is the energy emitted by matter in the form of electromagnetic
waves (or photons) as a result of the changes in the electronic configurations
of the atoms or molecules. Unlike conduction and convection, the transfer of
energy by radiation does not require the presence of an intervening medium.
In fact, energy transfer by radiation is fastest (at the speed of light) and it
suffers no attenuation in a vacuum. This is how the energy of the sun reaches
the earth.
In heat transfer studies we are interested in thermal radiation, which is the
form of radiation emitted by bodies because of their temperature. It differs
from other forms of electromagnetic radiation such as xrays, gamma rays,
microwaves, radio waves, and television waves that are not related to temperature. All bodies at a temperature above absolute zero emit thermal radiation.
Radiation is a volumetric phenomenon, and all solids, liquids, and gases
emit, absorb, or transmit radiation to varying degrees. However, radiation is Schematic for Example 1–8. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 28 28
HEAT TRANSFER
·
q emit, max = σ Ts4
= 1452 W/ m 2 Ts = 400 K Blackbody (ε = 1) FIGURE 1–34
Blackbody radiation represents the
maximum amount of radiation that
can be emitted from a surface
at a specified temperature. TABLE 1–6
Emissivities of some materials
at 300 K
Material Emissivity Aluminum foil
Anodized aluminum
Polished copper
Polished gold
Polished silver
Polished stainless steel
Black paint
White paint
White paper
Asphalt pavement
Red brick
Human skin
Wood
Soil
Water
Vegetation 0.07
0.82
0.03
0.03
0.02
0.17
0.98
0.90
0.92–0.97
0.85–0.93
0.93–0.96
0.95
0.82–0.92
0.93–0.96
0.96
0.92–0.96 usually considered to be a surface phenomenon for solids that are opaque to
thermal radiation such as metals, wood, and rocks since the radiation emitted
by the interior regions of such material can never reach the surface, and the
radiation incident on such bodies is usually absorbed within a few microns
from the surface.
The maximum rate of radiation that can be emitted from a surface at an absolute temperature Ts (in K or R) is given by the Stefan–Boltzmann law as
·
Q emit, max ·
·
Qref = (1 – α ) Qincident ·
·
Qabs = α Qincident FIGURE 1–35
The absorption of radiation incident on
an opaque surface of absorptivity . (W) (125) where
5.67 10 8 W/m2 · K4 or 0.1714 10 8 Btu/h · ft2 · R4 is the
Stefan–Boltzmann constant. The idealized surface that emits radiation at this
maximum rate is called a blackbody, and the radiation emitted by a blackbody is called blackbody radiation (Fig. 1–34). The radiation emitted by all
real surfaces is less than the radiation emitted by a blackbody at the same temperature, and is expressed as
·
Q emit 4
AsT s (W) (126) where is the emissivity of the surface. The property emissivity, whose value
is in the range 0
1, is a measure of how closely a surface approximates
a blackbody for which
1. The emissivities of some surfaces are given in
Table 1–6.
Another important radiation property of a surface is its absorptivity ,
which is the fraction of the radiation energy incident on a surface that is absorbed by the surface. Like emissivity, its value is in the range 0
1.
A blackbody absorbs the entire radiation incident on it. That is, a blackbody is
a perfect absorber (
1) as it is a perfect emitter.
In general, both and of a surface depend on the temperature and the
wavelength of the radiation. Kirchhoff’s law of radiation states that the emissivity and the absorptivity of a surface at a given temperature and wavelength
are equal. In many practical applications, the surface temperature and the
temperature of the source of incident radiation are of the same order of magnitude, and the average absorptivity of a surface is taken to be equal to its average emissivity. The rate at which a surface absorbs radiation is determined
from (Fig. 1–35)
·
Q absorbed ·
Qincident 4
AsT s ·
Q incident (W) (127) ·
where Q incident is the rate at which radiation is incident on the surface and is
the absorptivity of the surface. For opaque (nontransparent) surfaces, the
portion of incident radiation not absorbed by the surface is reflected back.
The difference between the rates of radiation emitted by the surface and the
radiation absorbed is the net radiation heat transfer. If the rate of radiation absorption is greater than the rate of radiation emission, the surface is said to be
gaining energy by radiation. Otherwise, the surface is said to be losing energy
by radiation. In general, the determination of the net rate of heat transfer by radiation between two surfaces is a complicated matter since it depends on the
properties of the surfaces, their orientation relative to each other, and the interaction of the medium between the surfaces with radiation. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 29 29
CHAPTER 1 When a surface of emissivity and surface area As at an absolute temperature Ts is completely enclosed by a much larger (or black) surface at absolute
temperature Tsurr separated by a gas (such as air) that does not intervene with
radiation, the net rate of radiation heat transfer between these two surfaces is
given by (Fig. 1–36)
·
Q rad Air As (Ts4 4
Tsurr) (W) hcombined As (Ts T) (W) ·
Qemitted (128) In this special case, the emissivity and the surface area of the surrounding surface do not have any effect on the net radiation heat transfer.
Radiation heat transfer to or from a surface surrounded by a gas such as air
occurs parallel to conduction (or convection, if there is bulk gas motion) between the surface and the gas. Thus the total heat transfer is determined by
adding the contributions of both heat transfer mechanisms. For simplicity and
convenience, this is often done by defining a combined heat transfer coefficient hcombined that includes the effects of both convection and radiation.
Then the total heat transfer rate to or from a surface by convection and radiation is expressed as
·
Q total Surrounding
surfaces at
Tsurr ·
Qincident ε , As, Ts
·
Qrad = εσ As (T 4 – T 4 )
s
surr FIGURE 1–36
Radiation heat transfer between a
surface and the surfaces surrounding it. (129) Note that the combined heat transfer coefficient is essentially a convection
heat transfer coefficient modified to include the effects of radiation.
Radiation is usually significant relative to conduction or natural convection,
but negligible relative to forced convection. Thus radiation in forced convection applications is usually disregarded, especially when the surfaces involved
have low emissivities and low to moderate temperatures.
EXAMPLE 1–9 Radiation Effect on Thermal Comfort It is a common experience to feel “chilly” in winter and “warm” in summer in
our homes even when the thermostat setting is kept the same. This is due to the
so called “radiation effect” resulting from radiation heat exchange between our
bodies and the surrounding surfaces of the walls and the ceiling.
Consider a person standing in a room maintained at 22°C at all times. The
inner surfaces of the walls, floors, and the ceiling of the house are observed to
be at an average temperature of 10°C in winter and 25°C in summer. Determine
the rate of radiation heat transfer between this person and the surrounding surfaces if the exposed surface area and the average outer surface temperature of
the person are 1.4 m2 and 30°C, respectively (Fig. 1–37). SOLUTION The rates of radiation heat transfer between a person and the surrounding surfaces at specified temperatures are to be determined in summer
and winter.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer by convection
is not considered. 3 The person is completely surrounded by the interior surfaces of the room. 4 The surrounding surfaces are at a uniform temperature.
Properties The emissivity of a person is
0.95 (Table 1–6).
Analysis The net rates of radiation heat transfer from the body to the surrounding walls, ceiling, and floor in winter and summer are Room Tsurr 30°C
1.4 m2 ·
Qrad FIGURE 1–37
Schematic for Example 1–9. cen58933_ch01.qxd 9/10/2002 8:29 AM Page 30 30
HEAT TRANSFER ·
Q rad, winter As (Ts4 4
Tsurr, winter)
(0.95)(5.67 10 8 W/m2 · K4)(1.4 m2)
[(30 273)4 (10 273)4] K4
152 W and ·
Q rad, summer As (Ts4 4
Tsurr, summer) (0.95)(5.67 10 8 W/m2 · K4)(1.4 m2)
[(30 273)4 (25 273)4] K4
40.9 W
Discussion Note that we must use absolute temperatures in radiation calculations. Also note that the rate of heat loss from the person by radiation is almost
four times as large in winter than it is in summer, which explains the “chill” we
feel in winter even if the thermostat setting is kept the same. 1–9 T1 OPAQUE
SOLID T2
1 mode Conduction T1 GAS T2 Radiation
2 modes
Conduction or
convection T1 VACUUM Radiation T2
1 mode FIGURE 1–38
Although there are three mechanisms of
heat transfer, a medium may involve
only two of them simultaneously. I SIMULTANEOUS HEAT TRANSFER
MECHANISMS We mentioned that there are three mechanisms of heat transfer, but not all
three can exist simultaneously in a medium. For example, heat transfer is
only by conduction in opaque solids, but by conduction and radiation in
semitransparent solids. Thus, a solid may involve conduction and radiation
but not convection. However, a solid may involve heat transfer by convection
and/or radiation on its surfaces exposed to a fluid or other surfaces. For
example, the outer surfaces of a cold piece of rock will warm up in a warmer
environment as a result of heat gain by convection (from the air) and radiation
(from the sun or the warmer surrounding surfaces). But the inner parts of the
rock will warm up as this heat is transferred to the inner region of the rock by
conduction.
Heat transfer is by conduction and possibly by radiation in a still fluid (no
bulk fluid motion) and by convection and radiation in a flowing fluid. In the
absence of radiation, heat transfer through a fluid is either by conduction or
convection, depending on the presence of any bulk fluid motion. Convection
can be viewed as combined conduction and fluid motion, and conduction in a
fluid can be viewed as a special case of convection in the absence of any fluid
motion (Fig. 1–38).
Thus, when we deal with heat transfer through a fluid, we have either conduction or convection, but not both. Also, gases are practically transparent to
radiation, except that some gases are known to absorb radiation strongly at
certain wavelengths. Ozone, for example, strongly absorbs ultraviolet radiation. But in most cases, a gas between two solid surfaces does not interfere
with radiation and acts effectively as a vacuum. Liquids, on the other hand,
are usually strong absorbers of radiation.
Finally, heat transfer through a vacuum is by radiation only since conduction or convection requires the presence of a material medium. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 31 31
CHAPTER 1 EXAMPLE 1–10 Room
air Heat Loss from a Person 20°C Consider a person standing in a breezy room at 20°C. Determine the total rate
of heat transfer from this person if the exposed surface area and the average
outer surface temperature of the person are 1.6 m2 and 29°C, respectively, and
the convection heat transfer coefficient is 6 W/m2 · °C (Fig. 1–39). ·
Qconv
29°C ·
Qrad SOLUTION The total rate of heat transfer from a person by both convection
and radiation to the surrounding air and surfaces at specified temperatures is to
be determined.
Assumptions 1 Steady operating conditions exist. 2 The person is completely
surrounded by the interior surfaces of the room. 3 The surrounding surfaces are
at the same temperature as the air in the room. 4 Heat conduction to the floor
through the feet is negligible.
Properties The emissivity of a person is 0.95 (Table 1–6). Analysis The heat transfer between the person and the air in the room will be
by convection (instead of conduction) since it is conceivable that the air in the
vicinity of the skin or clothing will warm up and rise as a result of heat transfer
from the body, initiating natural convection currents. It appears that the experimentally determined value for the rate of convection heat transfer in this case
is 6 W per unit surface area (m2) per unit temperature difference (in K or °C)
between the person and the air away from the person. Thus, the rate of convection heat transfer from the person to the air in the room is ·
Q conv hAs (Ts T)
(6 W/m · °C)(1.6 m2)(29
86.4 W
2 20)°C The person will also lose heat by radiation to the surrounding wall surfaces.
We take the temperature of the surfaces of the walls, ceiling, and floor to be
equal to the air temperature in this case for simplicity, but we recognize that
this does not need to be the case. These surfaces may be at a higher or lower
temperature than the average temperature of the room air, depending on the
outdoor conditions and the structure of the walls. Considering that air does not
intervene with radiation and the person is completely enclosed by the surrounding surfaces, the net rate of radiation heat transfer from the person to the
surrounding walls, ceiling, and floor is ·
Q rad As (Ts4 4
Tsurr)
(0.95)(5.67 10 8 W/m2 · K4)(1.6 m2)
[(29 273)4 (20 273)4] K4
81.7 W Note that we must use absolute temperatures in radiation calculations. Also
note that we used the emissivity value for the skin and clothing at room temperature since the emissivity is not expected to change significantly at a slightly
higher temperature.
Then the rate of total heat transfer from the body is determined by adding
these two quantities: ·
Q total ·
Q conv ·
Q rad (86.4 81.7) W 168.1 W ·
Qcond FIGURE 1–39
Heat transfer from the person
described in Example 1–10. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 32 32
HEAT TRANSFER Discussion The heat transfer would be much higher if the person were not
dressed since the exposed surface temperature would be higher. Thus, an important function of the clothes is to serve as a barrier against heat transfer.
In these calculations, heat transfer through the feet to the floor by conduction, which is usually very small, is neglected. Heat transfer from the skin by
perspiration, which is the dominant mode of heat transfer in hot environments,
is not considered here. T1 = 300 K ·
Q T2 = 200 K L = 1 cm ε=1 FIGURE 1–40 EXAMPLE 1–11 Heat Transfer between Two Isothermal Plates Consider steady heat transfer between two large parallel plates at constant
temperatures of T1 300 K and T2 200 K that are L 1 cm apart, as shown
in Fig. 1–40. Assuming the surfaces to be black (emissivity
1), determine
the rate of heat transfer between the plates per unit surface area assuming the
gap between the plates is (a) filled with atmospheric air, (b) evacuated, (c) filled
with urethane insulation, and (d) filled with superinsulation that has an apparent thermal conductivity of 0.00002 W/m · °C. Schematic for Example 1–11. SOLUTION The total rate of heat transfer between two large parallel plates at
specified temperatures is to be determined for four different cases.
Assumptions 1 Steady operating conditions exist. 2 There are no natural convection currents in the air between the plates. 3 The surfaces are black and
thus
1.
Properties The thermal conductivity at the average temperature of 250 K is
k 0.0219 W/m · °C for air (Table A11), 0.026 W/m · °C for urethane insulation (Table A6), and 0.00002 W/m · °C for the superinsulation.
Analysis (a) The rates of conduction and radiation heat transfer between the
plates through the air layer are
·
Q cond kA T2 T1
L (0.0219 W/m · °C)(1 m2) (300 200)°C
0.01 m 219 W and ·
Q rad 4
A(T 1
(1)(5.67 4
T 2) 10 8 W/m2 · K4)(1 m2)[(300 K)4 (200 K)4] 368 W Therefore, ·
Q total ·
Q cond ·
Q rad 219 368 587 W The heat transfer rate in reality will be higher because of the natural convection
currents that are likely to occur in the air space between the plates.
(b) When the air space between the plates is evacuated, there will be no conduction or convection, and the only heat transfer between the plates will be by
radiation. Therefore, ·
Q total ·
Q rad 368 W (c) An opaque solid material placed between two plates blocks direct radiation
heat transfer between the plates. Also, the thermal conductivity of an insulating
material accounts for the radiation heat transfer that may be occurring through cen58933_ch01.qxd 9/10/2002 8:30 AM Page 33 33
CHAPTER 1
300 K 200 K 300 K 200 K 300 K 200 K 300 K 200 K ·
Q = 587 W ·
Q = 368 W ·
Q = 260 W ·
Q = 0.2 W 1 cm 1 cm 1 cm 1 cm (a) Air space (b) Vacuum (c) Insulation (d ) Superinsulation FIGURE 1–41
Different ways of reducing heat transfer between two isothermal plates, and their effectiveness. the voids in the insulating material. The rate of heat transfer through the urethane insulation is ·
Q total ·
Q cond kA T1 T2
L (300 200)°C
0.01 m (0.026 W/m · °C)(1 m2) 260 W Note that heat transfer through the urethane material is less than the heat
transfer through the air determined in (a), although the thermal conductivity of
the insulation is higher than that of air. This is because the insulation blocks
the radiation whereas air transmits it.
(d ) The layers of the superinsulation prevent any direct radiation heat transfer
between the plates. However, radiation heat transfer between the sheets of superinsulation does occur, and the apparent thermal conductivity of the superinsulation accounts for this effect. Therefore, ·
Q total kA T2 T1
L (300 200)°C
(0.00002 W/m · °C)(1 m2)
0.01 m 0.2 W 1
which is 1840 of the heat transfer through the vacuum. The results of this example are summarized in Fig. 1–41 to put them into perspective.
Discussion This example demonstrates the effectiveness of superinsulations,
which are discussed in the next chapter, and explains why they are the insulation of choice in critical applications despite their high cost. EXAMPLE 1–12 Heat Transfer in Conventional
and Microwave Ovens The fast and efficient cooking of microwave ovens made them one of the essential appliances in modern kitchens (Fig. 1–42). Discuss the heat transfer
mechanisms associated with the cooking of a chicken in microwave and conventional ovens, and explain why cooking in a microwave oven is more efficient. FIGURE 1–42
SOLUTION Food is cooked in a microwave oven by absorbing the electromagnetic radiation energy generated by the microwave tube, called the magnetron. A chicken being cooked in a
microwave oven (Example 1–12). cen58933_ch01.qxd 9/10/2002 8:30 AM Page 34 34
HEAT TRANSFER The radiation emitted by the magnetron is not thermal radiation, since its emission is not due to the temperature of the magnetron; rather, it is due to the
conversion of electrical energy into electromagnetic radiation at a specified
wavelength. The wavelength of the microwave radiation is such that it is reflected by metal surfaces; transmitted by the cookware made of glass, ceramic,
or plastic; and absorbed and converted to internal energy by food (especially the
water, sugar, and fat) molecules.
In a microwave oven, the radiation that strikes the chicken is absorbed by
the skin of the chicken and the outer parts. As a result, the temperature of the
chicken at and near the skin rises. Heat is then conducted toward the inner
parts of the chicken from its outer parts. Of course, some of the heat absorbed
by the outer surface of the chicken is lost to the air in the oven by convection.
In a conventional oven, the air in the oven is first heated to the desired temperature by the electric or gas heating element. This preheating may take several minutes. The heat is then transferred from the air to the skin of the chicken
by natural convection in most ovens or by forced convection in the newer convection ovens that utilize a fan. The air motion in convection ovens increases
the convection heat transfer coefficient and thus decreases the cooking time.
Heat is then conducted toward the inner parts of the chicken from its outer
parts as in microwave ovens.
Microwave ovens replace the slow convection heat transfer process in conventional ovens by the instantaneous radiation heat transfer. As a result, microwave ovens transfer energy to the food at full capacity the moment they are
turned on, and thus they cook faster while consuming less energy. EXAMPLE 1–13 Heating of a Plate by Solar Energy
A thin metal plate is insulated on the back and exposed to solar radiation at the
front surface (Fig. 1–43). The exposed surface of the plate has an absorptivity
of 0.6 for solar radiation. If solar radiation is incident on the plate at a rate of
700 W/m2 and the surrounding air temperature is 25°C, determine the surface
temperature of the plate when the heat loss by convection and radiation equals
the solar energy absorbed by the plate. Assume the combined convection and
radiation heat transfer coefficient to be 50 W/m2 · °C. 700 W/ m2 α = 0.6 25°C FIGURE 1–43
Schematic for Example 1–13. SOLUTION The back side of the thin metal plate is insulated and the front
side is exposed to solar radiation. The surface temperature of the plate is to be
determined when it stabilizes.
Assumptions 1 Steady operating conditions exist. 2 Heat transfer through the
insulated side of the plate is negligible. 3 The heat transfer coefficient remains
constant.
Properties The solar absorptivity of the plate is given to be
0.6.
Analysis The absorptivity of the plate is 0.6, and thus 60 percent of the solar
radiation incident on the plate will be absorbed continuously. As a result, the
temperature of the plate will rise, and the temperature difference between the
plate and the surroundings will increase. This increasing temperature difference
will cause the rate of heat loss from the plate to the surroundings to increase.
At some point, the rate of heat loss from the plate will equal the rate of solar cen58933_ch01.qxd 9/10/2002 8:30 AM Page 35 35
CHAPTER 1 energy absorbed, and the temperature of the plate will no longer change. The
temperature of the plate when steady operation is established is determined from ·
Egained ·
Elost or ·
As q incident, solar hcombined As (Ts T) Solving for Ts and substituting, the plate surface temperature is determined
to be Ts T ·
q incident, solar
hcombined 25°C 0.6 (700 W/m2)
50 W/m2 · °C 33.4°C Discussion Note that the heat losses will prevent the plate temperature from
rising above 33.4°C. Also, the combined heat transfer coefficient accounts for
the effects of both convection and radiation, and thus it is very convenient
to use in heat transfer calculations when its value is known with reasonable
accuracy. SOLUTION PROBLEMSOLVING TECHNIQUE The first step in learning any science is to grasp the fundamentals, and to gain
a sound knowledge of it. The next step is to master the fundamentals by
putting this knowledge to test. This is done by solving significant realworld
problems. Solving such problems, especially complicated ones, requires a systematic approach. By using a stepbystep approach, an engineer can reduce
the solution of a complicated problem into the solution of a series of simple
problems (Fig. 1–44). When solving a problem, we recommend that you use
the following steps zealously as applicable. This will help you avoid some of
the common pitfalls associated with problem solving. Step 1: Problem Statement
In your own words, briefly state the problem, the key information given, and
the quantities to be found. This is to make sure that you understand the problem and the objectives before you attempt to solve the problem. Step 2: Schematic
Draw a realistic sketch of the physical system involved, and list the relevant
information on the figure. The sketch does not have to be something elaborate,
but it should resemble the actual system and show the key features. Indicate
any energy and mass interactions with the surroundings. Listing the given information on the sketch helps one to see the entire problem at once. Also,
check for properties that remain constant during a process (such as temperature during an isothermal process), and indicate them on the sketch. Step 3: Assumptions
State any appropriate assumptions made to simplify the problem to make it
possible to obtain a solution. Justify the questionable assumptions. Assume
reasonable values for missing quantities that are necessary. For example, in
the absence of specific data for atmospheric pressure, it can be taken to be AY SY W HARD WAY 1–10 I EA PROBLEM FIGURE 1–44
A stepbystep approach can greatly
simplify problem solving. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 36 36
HEAT TRANSFER Given: Air temperature in Denver 1 atm. However, it should be noted in the analysis that the atmospheric pressure decreases with increasing elevation. For example, it drops to 0.83 atm in
Denver (elevation 1610 m) (Fig. 1–45). To be found: Density of air
Missing information: Atmospheric
pressure
Assumption #1: Take P = 1 atm
(Inappropriate. Ignores effect of altitude.
Will cause more than 15% error.)
Assumption #2: Take P = 0.83 atm
(Appropriate. Ignores only minor effects
such as weather.) Step 4: Physical Laws
Apply all the relevant basic physical laws and principles (such as the conservation of energy), and reduce them to their simplest form by utilizing the assumptions made. However, the region to which a physical law is applied must
be clearly identified first. For example, the heating or cooling of a canned
drink is usually analyzed by applying the conservation of energy principle to
the entire can. Step 5: Properties
FIGURE 1–45
The assumptions made while solving
an engineering problem must be
reasonable and justifiable. Determine the unknown properties at known states necessary to solve the
problem from property relations or tables. List the properties separately, and
indicate their source, if applicable. Step 6: Calculations Energy use: $80/yr Substitute the known quantities into the simplified relations and perform the
calculations to determine the unknowns. Pay particular attention to the units
and unit cancellations, and remember that a dimensional quantity without a
unit is meaningless. Also, don’t give a false implication of high accuracy by
copying all the digits from the screen of the calculator—round the results to
an appropriate number of significant digits. Energy saved
by insulation: $200/yr Step 7: Reasoning, Verification, and Discussion IMPOSSIBLE! FIGURE 1–46
The results obtained from
an engineering analysis must
be checked for reasonableness. Check to make sure that the results obtained are reasonable and intuitive, and
verify the validity of the questionable assumptions. Repeat the calculations
that resulted in unreasonable values. For example, insulating a water heater
that uses $80 worth of natural gas a year cannot result in savings of $200 a
year (Fig. 1–46).
Also, point out the significance of the results, and discuss their implications.
State the conclusions that can be drawn from the results, and any recommendations that can be made from them. Emphasize the limitations under which
the results are applicable, and caution against any possible misunderstandings
and using the results in situations where the underlying assumptions do not
apply. For example, if you determined that wrapping a water heater with a
$20 insulation jacket will reduce the energy cost by $30 a year, indicate that
the insulation will pay for itself from the energy it saves in less than a year.
However, also indicate that the analysis does not consider labor costs, and that
this will be the case if you install the insulation yourself.
Keep in mind that you present the solutions to your instructors, and any engineering analysis presented to others is a form of communication. Therefore
neatness, organization, completeness, and visual appearance are of utmost importance for maximum effectiveness. Besides, neatness also serves as a great
checking tool since it is very easy to spot errors and inconsistencies in a neat
work. Carelessness and skipping steps to save time often ends up costing more
time and unnecessary anxiety. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 37 37
CHAPTER 1 The approach just described is used in the solved example problems without explicitly stating each step, as well as in the Solutions Manual of this text.
For some problems, some of the steps may not be applicable or necessary.
However, we cannot overemphasize the importance of a logical and orderly
approach to problem solving. Most difficulties encountered while solving a
problem are not due to a lack of knowledge; rather, they are due to a lack of
coordination. You are strongly encouraged to follow these steps in problem
solving until you develop your own approach that works best for you. A Remark on Significant Digits
In engineering calculations, the information given is not known to more than
a certain number of significant digits, usually three digits. Consequently, the
results obtained cannot possibly be accurate to more significant digits. Reporting results in more significant digits implies greater accuracy than exists,
and it should be avoided.
For example, consider a 3.75L container filled with gasoline whose density
is 0.845 kg/L, and try to determine its mass. Probably the first thought that
comes to your mind is to multiply the volume and density to obtain 3.16875
kg for the mass, which falsely implies that the mass determined is accurate to
six significant digits. In reality, however, the mass cannot be more accurate
than three significant digits since both the volume and the density are accurate
to three significant digits only. Therefore, the result should be rounded to three
significant digits, and the mass should be reported to be 3.17 kg instead of
what appears in the screen of the calculator. The result 3.16875 kg would be
correct only if the volume and density were given to be 3.75000 L and
0.845000 kg/L, respectively. The value 3.75 L implies that we are fairly confident that the volume is accurate within 0.01 L, and it cannot be 3.74 or
3.76 L. However, the volume can be 3.746, 3.750, 3.753, etc., since they all
round to 3.75 L (Fig. 1–47). It is more appropriate to retain all the digits during intermediate calculations, and to do the rounding in the final step since
this is what a computer will normally do.
When solving problems, we will assume the given information to be accurate to at least three significant digits. Therefore, if the length of a pipe is
given to be 40 m, we will assume it to be 40.0 m in order to justify using three
significant digits in the final results. You should also keep in mind that all experimentally determined values are subject to measurement errors, and such
errors will reflect in the results obtained. For example, if the density of a substance has an uncertainty of 2 percent, then the mass determined using this
density value will also have an uncertainty of 2 percent.
You should also be aware that we sometimes knowingly introduce small errors in order to avoid the trouble of searching for more accurate data. For example, when dealing with liquid water, we just use the value of 1000 kg/m3
for density, which is the density value of pure water at 0 C. Using this value
at 75 C will result in an error of 2.5 percent since the density at this temperature is 975 kg/m3. The minerals and impurities in the water will introduce additional error. This being the case, you should have no reservation in rounding
the final results to a reasonable number of significant digits. Besides, having
a few percent uncertainty in the results of engineering analysis is usually the
norm, not the exception. Given:
Volume:
Density: V = 3.75 L
ρ = 0.845 kg/L (3 significant digits)
Also,
Find: 3.75 × 0.845 = 3.16875
Mass: m = ρV = 3.16875 kg Rounding to 3 significant digits:
m = 3.17 kg FIGURE 1–47
A result with more significant digits than
that of given data falsely implies
more accuracy. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 38 38
HEAT TRANSFER Engineering Software Packages FIGURE 1–48
An excellent wordprocessing
program does not make a person a good
writer; it simply makes a good writer
a better and more efficient writer. Perhaps you are wondering why we are about to undertake a painstaking study
of the fundamentals of heat transfer. After all, almost all such problems we are
likely to encounter in practice can be solved using one of several sophisticated
software packages readily available in the market today. These software packages not only give the desired numerical results, but also supply the outputs in
colorful graphical form for impressive presentations. It is unthinkable to practice engineering today without using some of these packages. This tremendous computing power available to us at the touch of a button is both a
blessing and a curse. It certainly enables engineers to solve problems easily
and quickly, but it also opens the door for abuses and misinformation. In the
hands of poorly educated people, these software packages are as dangerous as
sophisticated powerful weapons in the hands of poorly trained soldiers.
Thinking that a person who can use the engineering software packages
without proper training on fundamentals can practice engineering is like
thinking that a person who can use a wrench can work as a car mechanic. If it
were true that the engineering students do not need all these fundamental
courses they are taking because practically everything can be done by computers quickly and easily, then it would also be true that the employers would
no longer need highsalaried engineers since any person who knows how to
use a wordprocessing program can also learn how to use those software packages. However, the statistics show that the need for engineers is on the rise,
not on the decline, despite the availability of these powerful packages.
We should always remember that all the computing power and the engineering software packages available today are just tools, and tools have meaning only in the hands of masters. Having the best wordprocessing program
does not make a person a good writer, but it certainly makes the job of a good
writer much easier and makes the writer more productive (Fig. 1–48). Hand
calculators did not eliminate the need to teach our children how to add or subtract, and the sophisticated medical software packages did not take the place
of medical school training. Neither will engineering software packages replace the traditional engineering education. They will simply cause a shift in
emphasis in the courses from mathematics to physics. That is, more time will
be spent in the classroom discussing the physical aspects of the problems in
greater detail, and less time on the mechanics of solution procedures.
All these marvelous and powerful tools available today put an extra burden
on today’s engineers. They must still have a thorough understanding of the
fundamentals, develop a “feel” of the physical phenomena, be able to put the
data into proper perspective, and make sound engineering judgments, just like
their predecessors. However, they must do it much better, and much faster, using more realistic models because of the powerful tools available today. The
engineers in the past had to rely on hand calculations, slide rules, and later
hand calculators and computers. Today they rely on software packages. The
easy access to such power and the possibility of a simple misunderstanding or
misinterpretation causing great damage make it more important today than
ever to have a solid training in the fundamentals of engineering. In this text we
make an extra effort to put the emphasis on developing an intuitive and physical understanding of natural phenomena instead of on the mathematical details of solution procedures. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 39 39
CHAPTER 1 Engineering Equation Solver (EES)
EES is a program that solves systems of linear or nonlinear algebraic or differential equations numerically. It has a large library of builtin thermodynamic property functions as well as mathematical functions, and allows the
user to supply additional property data. Unlike some software packages, EES
does not solve thermodynamic problems; it only solves the equations supplied
by the user. Therefore, the user must understand the problem and formulate it
by applying any relevant physical laws and relations. EES saves the user considerable time and effort by simply solving the resulting mathematical equations. This makes it possible to attempt significant engineering problems not
suitable for hand calculations, and to conduct parametric studies quickly and
conveniently. EES is a very powerful yet intuitive program that is very easy to
use, as shown in the examples below. The use and capabilities of EES are explained in Appendix 3. Heat Transfer Tools (HTT)
One software package specifically designed to help bridge the gap between
the textbook fundamentals and these powerful software packages is Heat
Transfer Tools, which may be ordered “bundled” with this text. The software
included in that package was developed for instructional use only and thus is
applicable only to fundamental problems in heat transfer. While it does not
have the power and functionality of the professional, commercial packages,
HTT uses researchgrade numerical algorithms behind the scenes and modern
graphical user interfaces. Each module is custom designed and applicable to a
single, fundamental topic in heat transfer to ensure that almost all time at the
computer is spent learning heat transfer. Nomenclature and all inputs and
outputs are consistent with those used in this and most other textbooks in
the field. In addition, with the capability of testing parameters so readily
available, one can quickly gain a physical feel for the effects of all the nondimensional numbers that arise in heat transfer. EXAMPLE 1–14 Solving a System of Equations with EES The difference of two numbers is 4, and the sum of the squares of these
two numbers is equal to the sum of the numbers plus 20. Determine these two
numbers. SOLUTION Relations are given for the difference and the sum of the squares
of two numbers. They are to be determined.
Analysis We start the EES program by doubleclicking on its icon, open a new
file, and type the following on the blank screen that appears:
xy=4
x^2+y^2=x+y+20
which is an exact mathematical expression of the problem statement with
x and y denoting the unknown numbers. The solution to this system of two cen58933_ch01.qxd 9/10/2002 8:30 AM Page 40 40
HEAT TRANSFER nonlinear equations with two unknowns is obtained by a single click on the
“calculator” symbol on the taskbar. It gives x=5 and y=1 Discussion Note that all we did is formulate the problem as we would on paper; EES took care of all the mathematical details of solution. Also note that
equations can be linear or nonlinear, and they can be entered in any order with
unknowns on either side. Friendly equation solvers such as EES allow the user
to concentrate on the physics of the problem without worrying about the mathematical complexities associated with the solution of the resulting system of
equations.
Throughout the text, problems that are unsuitable for hand calculations and
are intended to be solved using EES are indicated by a computer icon. TOPIC OF SPECIAL INTEREST* Thermal Comfort
Baby
Bird Fox FIGURE 1–49
Most animals come into this world with
builtin insulation, but human beings
come with a delicate skin. Unlike animals such as a fox or a bear that are born with builtin furs, human beings come into this world with little protection against the harsh environmental conditions (Fig. 1–49). Therefore, we can claim that the search
for thermal comfort dates back to the beginning of human history. It is believed that early human beings lived in caves that provided shelter as well
as protection from extreme thermal conditions. Probably the first form of
heating system used was open fire, followed by fire in dwellings through
the use of a chimney to vent out the combustion gases. The concept of central heating dates back to the times of the Romans, who heated homes by
utilizing doublefloor construction techniques and passing the fire’s fumes
through the opening between the two floor layers. The Romans were also
the first to use transparent windows made of mica or glass to keep the wind
and rain out while letting the light in. Wood and coal were the primary energy sources for heating, and oil and candles were used for lighting. The ruins of southfacing houses indicate that the value of solar heating was
recognized early in the history.
The term airconditioning is usually used in a restricted sense to imply
cooling, but in its broad sense it means to condition the air to the desired
level by heating, cooling, humidifying, dehumidifying, cleaning, and deodorizing. The purpose of the airconditioning system of a building is to
provide complete thermal comfort for its occupants. Therefore, we need to
understand the thermal aspects of the human body in order to design an effective airconditioning system.
The building blocks of living organisms are cells, which resemble miniature factories performing various functions necessary for the survival of
organisms. The human body contains about 100 trillion cells with an average diameter of 0.01 mm. In a typical cell, thousands of chemical reactions
*This section can be skipped without a loss in continuity. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 41 41
CHAPTER 1 occur every second during which some molecules are broken down and energy is released and some new molecules are formed. The high level of
chemical activity in the cells that maintain the human body temperature at
a temperature of 37.0°C (98.6°F) while performing the necessary bodily
functions is called the metabolism. In simple terms, metabolism refers to
the burning of foods such as carbohydrates, fat, and protein. The metabolizable energy content of foods is usually expressed by nutritionists in
terms of the capitalized Calorie. One Calorie is equivalent to 1 Cal 1
kcal 4.1868 kJ.
The rate of metabolism at the resting state is called the basal metabolic
rate, which is the rate of metabolism required to keep a body performing
the necessary bodily functions such as breathing and blood circulation at
zero external activity level. The metabolic rate can also be interpreted as
the energy consumption rate for a body. For an average man (30 years old,
70 kg, 1.73 m high, 1.8 m2 surface area), the basal metabolic rate is 84 W.
That is, the body is converting chemical energy of the food (or of the body
fat if the person had not eaten) into heat at a rate of 84 J/s, which is then
dissipated to the surroundings. The metabolic rate increases with the level
of activity, and it may exceed 10 times the basal metabolic rate when someone is doing strenuous exercise. That is, two people doing heavy exercising
in a room may be supplying more energy to the room than a 1kW resistance heater (Fig. 1–50). An average man generates heat at a rate of 108 W
while reading, writing, typing, or listening to a lecture in a classroom in a
seated position. The maximum metabolic rate of an average man is 1250 W
at age 20 and 730 at age 70. The corresponding rates for women are about
30 percent lower. Maximum metabolic rates of trained athletes can exceed
2000 W.
Metabolic rates during various activities are given in Table 1–7 per unit
body surface area. The surface area of a nude body was given by D.
DuBois in 1916 as
As 0.202m0.425 h0.725 (m2) (130) where m is the mass of the body in kg and h is the height in m. Clothing increases the exposed surface area of a person by up to about 50 percent. The
metabolic rates given in the table are sufficiently accurate for most purposes, but there is considerable uncertainty at high activity levels. More accurate values can be determined by measuring the rate of respiratory
oxygen consumption, which ranges from about 0.25 L/min for an average
resting man to more than 2 L/min during extremely heavy work. The entire
energy released during metabolism can be assumed to be released as heat
(in sensible or latent forms) since the external mechanical work done by the
muscles is very small. Besides, the work done during most activities such
as walking or riding an exercise bicycle is eventually converted to heat
through friction.
The comfort of the human body depends primarily on three environmental factors: the temperature, relative humidity, and air motion. The temperature of the environment is the single most important index of comfort.
Extensive research is done on human subjects to determine the “thermal
comfort zone” and to identify the conditions under which the body feels 1.2 kJ/s 1 kJ/s FIGURE1–50
Two fastdancing people supply
more heat to a room than a
1kW resistance heater. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 42 42
HEAT TRANSFER TABLE 1–7
Metabolic rates during various
activities (from ASHRAE
Handbook of Fundamentals,
Ref. 1, Chap. 8, Table 4).
Metabolic
rate*
Activity W/m2 Resting:
Sleeping
Reclining
Seated, quiet
Standing, relaxed 40
45
60
70 Walking (on the level):
2 mph (0.89 m/s)
3 mph (1.34 m/s)
4 mph (1.79 m/s) 115
150
220 Office Activities:
Reading, seated
Writing
Typing
Filing, seated
Filing, standing
Walking about
Lifting/packing 55
60
65
70
80
100
120 Driving/Flying:
Car
Aircraft, routine
Heavy vehicle 60–115
70
185 Miscellaneous Occupational
Activities:
Cooking
Cleaning house
Machine work:
Light
Heavy
Handling 50kg bags
Pick and shovel work 95–115
115–140
115–140
235
235
235–280 Miscellaneous Leisure Activities:
Dancing, social
Calisthenics/exercise
Tennis, singles
Basketball
Wrestling, competitive 140–255
175–235
210–270
290–440
410–505 *Multiply by 1.8 m2 to obtain metabolic rates for
an average man. Multiply by 0.3171 to convert
to Btu/h · ft2. comfortable in an environment. It has been observed that most normally
clothed people resting or doing light work feel comfortable in the operative
temperature (roughly, the average temperature of air and surrounding surfaces) range of 23°C to 27°C or 73°C to 80°F (Fig. 1–51). For unclothed
people, this range is 29°C to 31°C. Relative humidity also has a considerable effect on comfort since it is a measure of air ’s ability to absorb
moisture and thus it affects the amount of heat a body can dissipate by
evaporation. High relative humidity slows down heat rejection by evaporation, especially at high temperatures, and low relative humidity speeds it
up. The desirable level of relative humidity is the broad range of 30 to
70 percent, with 50 percent being the most desirable level. Most people at
these conditions feel neither hot nor cold, and the body does not need to
activate any of the defense mechanisms to maintain the normal body temperature (Fig. 1–52).
Another factor that has a major effect on thermal comfort is excessive air
motion or draft, which causes undesired local cooling of the human body.
Draft is identified by many as a most annoying factor in work places, automobiles, and airplanes. Experiencing discomfort by draft is most common
among people wearing indoor clothing and doing light sedentary work, and
least common among people with high activity levels. The air velocity
should be kept below 9 m/min (30 ft/min) in winter and 15 m/min
(50 ft/min) in summer to minimize discomfort by draft, especially when the
air is cool. A low level of air motion is desirable as it removes the warm,
moist air that builds around the body and replaces it with fresh air. Therefore, air motion should be strong enough to remove heat and moisture from
the vicinity of the body, but gentle enough to be unnoticed. High speed air
motion causes discomfort outdoors as well. For example, an environment
at 10°C (50°F) with 48 km/h winds feels as cold as an environment at
7°C (20°F) with 3 km/h winds because of the chilling effect of the air
motion (the windchill factor).
A comfort system should provide uniform conditions throughout the
living space to avoid discomfort caused by nonuniformities such as drafts,
asymmetric thermal radiation, hot or cold floors, and vertical temperature
stratification. Asymmetric thermal radiation is caused by the cold surfaces of large windows, uninsulated walls, or cold products and the warm
surfaces of gas or electric radiant heating panels on the walls or ceiling,
solarheated masonry walls or ceilings, and warm machinery. Asymmetric
radiation causes discomfort by exposing different sides of the body to surfaces at different temperatures and thus to different heat loss or gain by
radiation. A person whose left side is exposed to a cold window, for example, will feel like heat is being drained from that side of his or her body
(Fig. 1–53). For thermal comfort, the radiant temperature asymmetry
should not exceed 5°C in the vertical direction and 10°C in the horizontal
direction. The unpleasant effect of radiation asymmetry can be minimized
by properly sizing and installing heating panels, using doublepane windows, and providing generous insulation at the walls and the roof.
Direct contact with cold or hot floor surfaces also causes localized discomfort in the feet. The temperature of the floor depends on the way it is
constructed (being directly on the ground or on top of a heated room, being
made of wood or concrete, the use of insulation, etc.) as well as the floor cen58933_ch01.qxd 9/10/2002 8:30 AM Page 43 43
CHAPTER 1
°C
2.0
Clothing insulation (clo) covering used such as pads, carpets, rugs, and linoleum. A floor temperature of 23 to 25°C is found to be comfortable to most people. The floor
asymmetry loses its significance for people with footwear. An effective and
economical way of raising the floor temperature is to use radiant heating
panels instead of turning the thermostat up. Another nonuniform condition
that causes discomfort is temperature stratification in a room that exposes the head and the feet to different temperatures. For thermal comfort,
the temperature difference between the head and foot levels should not exceed 3°C. This effect can be minimized by using destratification fans.
It should be noted that no thermal environment will please everyone. No
matter what we do, some people will express some discomfort. The thermal
comfort zone is based on a 90 percent acceptance rate. That is, an environment is deemed comfortable if only 10 percent of the people are dissatisfied with it. Metabolism decreases somewhat with age, but it has no effect
on the comfort zone. Research indicates that there is no appreciable difference between the environments preferred by old and young people. Experiments also show that men and women prefer almost the same environment.
The metabolism rate of women is somewhat lower, but this is compensated
by their slightly lower skin temperature and evaporative loss. Also, there is
no significant variation in the comfort zone from one part of the world to
another and from winter to summer. Therefore, the same thermal comfort
conditions can be used throughout the world in any season. Also, people
cannot acclimatize themselves to prefer different comfort conditions.
In a cold environment, the rate of heat loss from the body may exceed
the rate of metabolic heat generation. Average specific heat of the human
body is 3.49 kJ/kg · °C, and thus each 1°C drop in body temperature corresponds to a deficit of 244 kJ in body heat content for an average 70kg
man. A drop of 0.5°C in mean body temperature causes noticeable but acceptable discomfort. A drop of 2.6°C causes extreme discomfort. A sleeping person will wake up when his or her mean body temperature drops by
1.3°C (which normally shows up as a 0.5°C drop in the deep body and 3°C
in the skin area). The drop of deep body temperature below 35°C may damage the body temperature regulation mechanism, while a drop below 28°C
may be fatal. Sedentary people reported to feel comfortable at a mean skin
temperature of 33.3°C, uncomfortably cold at 31°C, shivering cold at
30°C, and extremely cold at 29°C. People doing heavy work reported to
feel comfortable at much lower temperatures, which shows that the activity
level affects human performance and comfort. The extremities of the body
such as hands and feet are most easily affected by cold weather, and their
temperature is a better indication of comfort and performance. A handskin
temperature of 20°C is perceived to be uncomfortably cold, 15°C to be
extremely cold, and 5°C to be painfully cold. Useful work can be performed by hands without difficulty as long as the skin temperature of fingers remains above 16°C (ASHRAE Handbook of Fundamentals, Ref. 1,
Chapter 8).
The first line of defense of the body against excessive heat loss in a cold
environment is to reduce the skin temperature and thus the rate of heat loss
from the skin by constricting the veins and decreasing the blood flow to the
skin. This measure decreases the temperature of the tissues subjacent to
the skin, but maintains the inner body temperature. The next preventive 20 25
Sedentary
50% RH
≤ 30 fpm
(0.15 m/s) 1.5 30 Heavy
clothing 1.0 Winter
clothing 0.5 Summer
clothing 0
64 68 72 76
80
°F
Operative temperature 84 Upper acceptability limit
Optimum
Lower acceptability limit FIGURE 1–51
The effect of clothing on
the environment temperature
that feels comfortable (1 clo
0.155 m2 · °C/W 0.880 ft2 · °F · h/Btu)
(from ASHRAE Standard 551981).
23°C
RH = 50%
Air motion
5 m/min FIGURE 1–52
A thermally comfortable environment. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 44 44
HEAT TRANSFER Cold
window Warm
wall Radiation
Radiation FIGURE 1–53
Cold surfaces cause excessive heat loss
from the body by radiation, and thus
discomfort on that side of the body.
Brrr!
Shivering FIGURE 1–54
The rate of metabolic heat generation
may go up by six times the resting
level during total body shivering
in cold weather. measure is increasing the rate of metabolic heat generation in the body by
shivering, unless the person does it voluntarily by increasing his or her
level of activity or puts on additional clothing. Shivering begins slowly in
small muscle groups and may double the rate of metabolic heat production
of the body at its initial stages. In the extreme case of total body shivering,
the rate of heat production may reach six times the resting levels (Fig.
1–54). If this measure also proves inadequate, the deep body temperature
starts falling. Body parts furthest away from the core such as the hands and
feet are at greatest danger for tissue damage.
In hot environments, the rate of heat loss from the body may drop below the metabolic heat generation rate. This time the body activates the opposite mechanisms. First the body increases the blood flow and thus heat
transport to the skin, causing the temperature of the skin and the subjacent
tissues to rise and approach the deep body temperature. Under extreme heat
conditions, the heart rate may reach 180 beats per minute in order to maintain adequate blood supply to the brain and the skin. At higher heart rates,
the volumetric efficiency of the heart drops because of the very short time
between the beats to fill the heart with blood, and the blood supply to the
skin and more importantly to the brain drops. This causes the person to
faint as a result of heat exhaustion. Dehydration makes the problem worse.
A similar thing happens when a person working very hard for a long time
stops suddenly. The blood that has flooded the skin has difficulty returning
to the heart in this case since the relaxed muscles no longer force the blood
back to the heart, and thus there is less blood available for pumping to the
brain.
The next line of defense is releasing water from sweat glands and resorting to evaporative cooling, unless the person removes some clothing and
reduces the activity level (Fig. 1–55). The body can maintain its core temperature at 37°C in this evaporative cooling mode indefinitely, even in environments at higher temperatures (as high as 200°C during military
endurance tests), if the person drinks plenty of liquids to replenish his or
her water reserves and the ambient air is sufficiently dry to allow the sweat
to evaporate instead of rolling down the skin. If this measure proves inadequate, the body will have to start absorbing the metabolic heat and the
deep body temperature will rise. A person can tolerate a temperature rise of
1.4°C without major discomfort but may collapse when the temperature
rise reaches 2.8°C. People feel sluggish and their efficiency drops considerably when the core body temperature rises above 39°C. A core temperature above 41°C may damage hypothalamic proteins, resulting in cessation cen58933_ch01.qxd 9/10/2002 8:30 AM Page 45 45
CHAPTER 1 of sweating, increased heat production by shivering, and a heat stroke with
irreversible and lifethreatening damage. Death can occur above 43°C.
A surface temperature of 46°C causes pain on the skin. Therefore, direct
contact with a metal block at this temperature or above is painful. However, a person can stay in a room at 100°C for up to 30 min without any
damage or pain on the skin because of the convective resistance at the skin
surface and evaporative cooling. We can even put our hands into an oven at
200°C for a short time without getting burned.
Another factor that affects thermal comfort, health, and productivity is
ventilation. Fresh outdoor air can be provided to a building naturally by
doing nothing, or forcefully by a mechanical ventilation system. In the first
case, which is the norm in residential buildings, the necessary ventilation is
provided by infiltration through cracks and leaks in the living space and by
the opening of the windows and doors. The additional ventilation needed in
the bathrooms and kitchens is provided by air vents with dampers or exhaust fans. With this kind of uncontrolled ventilation, however, the fresh
air supply will be either too high, wasting energy, or too low, causing poor
indoor air quality. But the current practice is not likely to change for residential buildings since there is not a public outcry for energy waste or air
quality, and thus it is difficult to justify the cost and complexity of mechanical ventilation systems.
Mechanical ventilation systems are part of any heating and air conditioning system in commercial buildings, providing the necessary amount of
fresh outdoor air and distributing it uniformly throughout the building. This
is not surprising since many rooms in large commercial buildings have no
windows and thus rely on mechanical ventilation. Even the rooms with
windows are in the same situation since the windows are tightly sealed and
cannot be opened in most buildings. It is not a good idea to oversize the
ventilation system just to be on the “safe side” since exhausting the heated
or cooled indoor air wastes energy. On the other hand, reducing the ventilation rates below the required minimum to conserve energy should also be
avoided so that the indoor air quality can be maintained at the required levels. The minimum fresh air ventilation requirements are listed in Table 1–8.
The values are based on controlling the CO2 and other contaminants with
an adequate margin of safety, which requires each person be supplied with
at least 7.5 L/s (15 ft3/min) of fresh air.
Another function of the mechanical ventilation system is to clean the air
by filtering it as it enters the building. Various types of filters are available
for this purpose, depending on the cleanliness requirements and the allowable pressure drop. Evaporation FIGURE 1–55
In hot environments, a body can
dissipate a large amount of metabolic
heat by sweating since the sweat absorbs
the body heat and evaporates. TABLE 1–8
Minimum fresh air requirements
in buildings (from ASHRAE
Standard 621989)
Requirement
(per person)
L/s ft3/min Classrooms,
libraries,
supermarkets 8 15 Dining rooms,
conference
rooms, offices 10 20 Hospital
rooms 13 25 Application Hotel rooms
Smoking
lounges 15
30
(per room) (per room)
30 Retail stores 1.0–1.5
(per m2) 60
0.2–0.3
(per ft2) Residential 0.35 air change per
buildings
hour, but not less than
7.5 L/s (or 15 ft3/min)
per person cen58933_ch01.qxd 9/10/2002 8:30 AM Page 46 46
HEAT TRANSFER SUMMARY
In this chapter, the basics of heat transfer are introduced and
discussed. The science of thermodynamics deals with the
amount of heat transfer as a system undergoes a process from
one equilibrium state to another, whereas the science of heat
transfer deals with the rate of heat transfer, which is the main
quantity of interest in the design and evaluation of heat transfer
equipment. The sum of all forms of energy of a system is called
total energy, and it includes the internal, kinetic, and potential
energies. The internal energy represents the molecular energy
of a system, and it consists of sensible, latent, chemical, and
nuclear forms. The sensible and latent forms of internal energy
can be transferred from one medium to another as a result of a
temperature difference, and are referred to as heat or thermal
energy. Thus, heat transfer is the exchange of the sensible and
latent forms of internal energy between two mediums as a result of a temperature difference. The amount of heat transferred
·
per unit time is called heat transfer rate and is denoted by Q .
·.
The rate of heat transfer per unit area is called heat flux, q
A system of fixed mass is called a closed system and a system that involves mass transfer across its boundaries is called
an open system or control volume. The first law of thermodynamics or the energy balance for any system undergoing any
process can be expressed as
Eout Ein Esystem When a stationary closed system involves heat transfer only
and no work interactions across its boundary, the energy balance relation reduces to
Q mCv T where Q is the amount of net heat transfer to or from the sys·
tem. When heat is transferred at a constant rate of Q , the
amount of heat transfer during a time interval t can be deter·
mined from Q Q t.
Under steady conditions and in the absence of any work interactions, the conservation of energy relation for a control volume with one inlet and one exit with negligible changes in
kinetic and potential energies can be expressed as
·
·
Q m Cp T
·
·
where m
Ac is the mass flow rate and Q is the rate of net
heat transfer into or out of the control volume.
Heat can be transferred in three different modes: conduction,
convection, and radiation. Conduction is the transfer of energy
from the more energetic particles of a substance to the adjacent
less energetic ones as a result of interactions between the particles, and is expressed by Fourier ’s law of heat conduction as ·
Q cond kA dT
dx where k is the thermal conductivity of the material, A is the
area normal to the direction of heat transfer, and dT/dx is the
temperature gradient. The magnitude of the rate of heat conduction across a plane layer of thickness L is given by
·
Q cond kA T
L where T is the temperature difference across the layer.
Convection is the mode of heat transfer between a solid surface and the adjacent liquid or gas that is in motion, and involves the combined effects of conduction and fluid motion.
The rate of convection heat transfer is expressed by Newton’s
law of cooling as
·
Q convection hAs (Ts T∞) where h is the convection heat transfer coefficient in W/m2 · °C
or Btu/h · ft2 · °F, As is the surface area through which convection heat transfer takes place, Ts is the surface temperature,
and T∞ is the temperature of the fluid sufficiently far from the
surface.
Radiation is the energy emitted by matter in the form of
electromagnetic waves (or photons) as a result of the changes
in the electronic configurations of the atoms or molecules. The
maximum rate of radiation that can be emitted from a surface
at an absolute temperature Ts is given by the Stefan–Boltzmann
·
AsTs4 where
,
5.67 10 8 W/m2 · K4
law as Q emit, max
8
2
4
or 0.1714
10 Btu/h · ft · R is the Stefan–Boltzmann
constant.
When a surface of emissivity and surface area As at an absolute temperature Ts is completely enclosed by a much larger
(or black) surface at absolute temperature Tsurr separated by a
gas (such as air) that does not intervene with radiation, the net
rate of radiation heat transfer between these two surfaces is
given by
·
4
As (Ts4 Tsurr)
Q rad
In this case, the emissivity and the surface area of the surrounding surface do not have any effect on the net radiation
heat transfer.
The rate at which a surface absorbs radiation is determined
·
·
·
Q incident where Q incident is the rate at which rafrom Q absorbed
diation is incident on the surface and is the absorptivity of
the surface. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 47 47
CHAPTER 1 REFERENCES AND SUGGESTED READING
1. American Society of Heating, Refrigeration, and AirConditioning Engineers, Handbook of Fundamentals.
Atlanta: ASHRAE, 1993.
2. Y. A. Çengel and R. H. Turner. Fundamentals of ThermalFluid Sciences. New York: McGrawHill, 2001.
3. Y. A. Çengel and M. A. Boles. Thermodynamics—An
Engineering Approach. 4th ed. New York: McGrawHill,
2002.
4. J. P. Holman. Heat Transfer. 9th ed. New York: McGrawHill, 2002. 6. F. Kreith and M. S. Bohn. Principles of Heat Transfer. 6th
ed. Pacific Grove, CA: Brooks/Cole, 2001.
7. A. F. Mills. Basic Heat and Mass Transfer. 2nd ed.
Upper Saddle River, NJ: PrenticeHall, 1999.
8. M. N. Ozisik. Heat Transfer—A Basic Approach. New
York: McGrawHill, 1985.
9. Robert J. Ribando. Heat Transfer Tools. New York:
McGrawHill, 2002.
10. F. M. White. Heat and Mass Transfer. Reading, MA:
AddisonWesley, 1988. 5. F. P. Incropera and D. P. DeWitt. Introduction to Heat
Transfer. 4th ed. New York: John Wiley & Sons, 2002. PROBLEMS* 1–1C How does the science of heat transfer differ from the
science of thermodynamics? 1–9C What are the mechanisms of energy transfer to a closed
system? How is heat transfer distinguished from the other
forms of energy transfer? 1–2C What is the driving force for (a) heat transfer, (b) electric current flow, and (c) fluid flow? 1–10C How are heat, internal energy, and thermal energy
related to each other? 1–3C What is the caloric theory? When and why was it
abandoned? 1–11C An ideal gas is heated from 50°C to 80°C (a) at constant volume and (b) at constant pressure. For which case do
you think the energy required will be greater? Why? Thermodynamics and Heat Transfer 1–4C How do rating problems in heat transfer differ from the
sizing problems?
1–5C What is the difference between the analytical and experimental approach to heat transfer? Discuss the advantages
and disadvantages of each approach.
1–6C What is the importance of modeling in engineering?
How are the mathematical models for engineering processes
prepared?
1–7C When modeling an engineering process, how is the
right choice made between a simple but crude and a complex
but accurate model? Is the complex model necessarily a better
choice since it is more accurate? Heat and Other Forms of Energy
1–8C What is heat flux? How is it related to the heat transfer rate?
*Problems designated by a “C” are concept questions, and
students are encouraged to answer them all. Problems designated
by an “E” are in English units, and the SI users can ignore them.
Problems with a CDEES icon
are solved using EES, and
complete solutions together with parametric studies are included
on the enclosed CD. Problems with a computerEES icon
are
comprehensive in nature, and are intended to be solved with a
computer, preferably using the EES software that accompanies
this text. 1–12 A cylindrical resistor element on a circuit board dissipates 0.6 W of power. The resistor is 1.5 cm long, and has a
diameter of 0.4 cm. Assuming heat to be transferred uniformly
from all surfaces, determine (a) the amount of heat this resistor
dissipates during a 24hour period, (b) the heat flux, and (c) the
fraction of heat dissipated from the top and bottom surfaces.
1–13E A logic chip used in a computer dissipates 3 W of
power in an environment at 120°F, and has a heat transfer surface area of 0.08 in2. Assuming the heat transfer from the surface to be uniform, determine (a) the amount of heat this chip
dissipates during an eighthour work day, in kWh, and (b) the
heat flux on the surface of the chip, in W/in2.
1–14 Consider a 150W incandescent lamp. The filament
of the lamp is 5 cm long and has a diameter of 0.5 mm. The
diameter of the glass bulb of the lamp is 8 cm. Determine the
heat flux, in W/m2, (a) on the surface of the filament and (b) on
the surface of the glass bulb, and (c) calculate how much it will
cost per year to keep that lamp on for eight hours a day every
day if the unit cost of electricity is $0.08/kWh.
Answers: (a) 1.91 106 W/m2, (b) 7500 W/m2, (c) $35.04/yr 1–15 A 1200W iron is left on the ironing board with its base
exposed to the air. About 90 percent of the heat generated
in the iron is dissipated through its base whose surface area is
150 cm2, and the remaining 10 percent through other surfaces.
Assuming the heat transfer from the surface to be uniform, cen58933_ch01.qxd 9/10/2002 8:30 AM Page 48 48
HEAT TRANSFER
D = 8 cm
Filament
d = 0.5 mm
L = 5 cm FIGURE P1–14
determine (a) the amount of heat the iron dissipates during a
2hour period, in kWh, (b) the heat flux on the surface of the
iron base, in W/m2, and (c) the total cost of the electrical energy consumed during this 2hour period. Take the unit cost of
electricity to be $0.07/kWh.
1–16 A 15cm 20cm circuit board houses on its surface
120 closely spaced logic chips, each dissipating 0.12 W. If the
heat transfer from the back surface of the board is negligible,
determine (a) the amount of heat this circuit board dissipates
during a 10hour period, in kWh, and (b) the heat flux on the
surface of the circuit board, in W/m2. Chips
15 cm 1–18 The average specific heat of the human body is 3.6
kJ/kg · °C. If the body temperature of a 70kg man rises from
37°C to 39°C during strenuous exercise, determine the increase
in the thermal energy content of the body as a result of this rise
in body temperature.
1–19 Infiltration of cold air into a warm house during winter
through the cracks around doors, windows, and other openings
is a major source of energy loss since the cold air that enters
needs to be heated to the room temperature. The infiltration is
often expressed in terms of ACH (air changes per hour). An
ACH of 2 indicates that the entire air in the house is replaced
twice every hour by the cold air outside.
Consider an electrically heated house that has a floor space
of 200 m2 and an average height of 3 m at 1000 m elevation,
where the standard atmospheric pressure is 89.6 kPa. The
house is maintained at a temperature of 22°C, and the infiltration losses are estimated to amount to 0.7 ACH. Assuming the
pressure and the temperature in the house remain constant, determine the amount of energy loss from the house due to infiltration for a day during which the average outdoor temperature
is 5°C. Also, determine the cost of this energy loss for that day
if the unit cost of electricity in that area is $0.082/kWh.
Answers: 53.8 kWh/day, $4.41/day 1–20 Consider a house with a floor space of 200 m2 and an
average height of 3 m at sea level, where the standard atmospheric pressure is 101.3 kPa. Initially the house is at a uniform
temperature of 10°C. Now the electric heater is turned on, and
the heater runs until the air temperature in the house rises to an
average value of 22°C. Determine how much heat is absorbed
by the air assuming some air escapes through the cracks as the
heated air in the house expands at constant pressure. Also, determine the cost of this heat if the unit cost of electricity in that
area is $0.075/kWh.
1–21E Consider a 60gallon water heater that is initially
filled with water at 45°F. Determine how much energy needs to
be transferred to the water to raise its temperature to 140°F.
Take the density and specific heat of water to be 62 lbm/ft3 and
1.0 Btu/lbm · °F, respectively. The First Law of Thermodynamics
20 cm FIGURE P1–16
1–17 A 15cmdiameter aluminum ball is to be heated from
80°C to an average temperature of 200°C. Taking the average
density and specific heat of aluminum in this temperature
range to be
2700 kg/m3 and Cp 0.90 kJ/kg · °C, respectively, determine the amount of energy that needs to be transAnswer: 515 kJ
ferred to the aluminum ball. 1–22C On a hot summer day, a student turns his fan on when
he leaves his room in the morning. When he returns in the
evening, will his room be warmer or cooler than the neighboring rooms? Why? Assume all the doors and windows are kept
closed.
1–23C Consider two identical rooms, one with a refrigerator
in it and the other without one. If all the doors and windows are
closed, will the room that contains the refrigerator be cooler or
warmer than the other room? Why?
1–24C Define mass and volume flow rates. How are they related to each other? cen58933_ch01.qxd 9/10/2002 8:30 AM Page 49 49
CHAPTER 1 1–25 Two 800kg cars moving at a velocity of 90 km/h have
a headon collision on a road. Both cars come to a complete
rest after the crash. Assuming all the kinetic energy of cars is
converted to thermal energy, determine the average temperature rise of the remains of the cars immediately after the crash.
Take the average specific heat of the cars to be 0.45 kJ/kg · °C.
1–26 A classroom that normally contains 40 people is to be
airconditioned using window airconditioning units of 5kW
cooling capacity. A person at rest may be assumed to dissipate
heat at a rate of 360 kJ/h. There are 10 lightbulbs in the room,
each with a rating of 100 W. The rate of heat transfer to the
classroom through the walls and the windows is estimated to
be 15,000 kJ/h. If the room air is to be maintained at a constant
temperature of 21°C, determine the number of window airAnswer: two units
conditioning units required.
1–27E A rigid tank contains 20 lbm of air at 50 psia and
80°F. The air is now heated until its pressure is doubled. Determine (a) the volume of the tank and (b) the amount of heat
Answers: (a) 80 ft3, (b) 2035 Btu
transfer.
1–28 A 1m3 rigid tank contains hydrogen at 250 kPa and
420 K. The gas is now cooled until its temperature drops to 300
K. Determine (a) the final pressure in the tank and (b) the
amount of heat transfer from the tank.
1–29 A 4m 5m 6m room is to be heated by a baseboard resistance heater. It is desired that the resistance heater
be able to raise the air temperature in the room from 7°C to
25°C within 15 minutes. Assuming no heat losses from the
room and an atmospheric pressure of 100 kPa, determine the
required power rating of the resistance heater. Assume constant
specific heats at room temperature.
Answer: 3.01 kW
1–30 A 4m 5m 7m room is heated by the radiator of
a steam heating system. The steam radiator transfers heat at a
rate of 10,000 kJ/h and a 100W fan is used to distribute the
warm air in the room. The heat losses from the room are estimated to be at a rate of about 5000 kJ/h. If the initial temperature of the room air is 10°C, determine how long it will take for
the air temperature to rise to 20°C. Assume constant specific
heats at room temperature. 5000 kJ/h Room
4m×6m×6m Fan FIGURE P1–31
1–31 A student living in a 4m
6m
6m dormitory
room turns his 150W fan on before she leaves her room on a
summer day hoping that the room will be cooler when she
comes back in the evening. Assuming all the doors and windows are tightly closed and disregarding any heat transfer
through the walls and the windows, determine the temperature
in the room when she comes back 10 hours later. Use specific
heat values at room temperature and assume the room to be at
100 kPa and 15°C in the morning when she leaves.
Answer: 58.1°C 1–32E A 10ft3 tank contains oxygen initially at 14.7 psia
and 80°F. A paddle wheel within the tank is rotated until the
pressure inside rises to 20 psia. During the process 20 Btu of
heat is lost to the surroundings. Neglecting the energy stored in
the paddle wheel, determine the work done by the paddle
wheel.
1–33 A room is heated by a baseboard resistance heater.
When the heat losses from the room on a winter day amount to
7000 kJ/h, it is observed that the air temperature in the room
remains constant even though the heater operates continuously.
Determine the power rating of the heater, in kW.
1–34 A 50kg mass of copper at 70°C is dropped into an insulated tank containing 80 kg of water at 25°C. Determine the
final equilibrium temperature in the tank.
1–35 A 20kg mass of iron at 100°C is brought into contact
with 20 kg of aluminum at 200°C in an insulated enclosure.
Determine the final equilibrium temperature of the combined
system.
Answer: 168°C
1–36 An unknown mass of iron at 90°C is dropped into an
insulated tank that contains 80 L of water at 20°C. At the same Room
4m×5m×7m
Steam
·
Wpw FIGURE P1–30 Water Iron
Wpw 10,000 kJ/ h FIGURE P1–36 cen58933_ch01.qxd 9/10/2002 8:30 AM Page 50 50
HEAT TRANSFER time, a paddle wheel driven by a 200W motor is activated to
stir the water. Thermal equilibrium is established after 25 minutes with a final temperature of 27°C. Determine the mass of
the iron. Neglect the energy stored in the paddle wheel, and
Answer: 72.1 kg
take the density of water to be 1000 kg/m3.
1–37E A 90lbm mass of copper at 160°F and a 50lbm mass
of iron at 200°F are dropped into a tank containing 180 lbm of
water at 70°F. If 600 Btu of heat is lost to the surroundings during the process, determine the final equilibrium temperature.
1–38 A 5m 6m 8m room is to be heated by an electrical resistance heater placed in a short duct in the room. Initially, the room is at 15°C, and the local atmospheric pressure
is 98 kPa. The room is losing heat steadily to the outside at a
rate of 200 kJ/min. A 200W fan circulates the air steadily
through the duct and the electric heater at an average mass flow
rate of 50 kg/min. The duct can be assumed to be adiabatic, and
there is no air leaking in or out of the room. If it takes 15 minutes for the room air to reach an average temperature of 25°C,
find (a) the power rating of the electric heater and (b) the temperature rise that the air experiences each time it passes
through the heater.
1–39 A house has an electric heating system that consists of
a 300W fan and an electric resistance heating element placed
in a duct. Air flows steadily through the duct at a rate of 0.6
kg/s and experiences a temperature rise of 5°C. The rate of heat
loss from the air in the duct is estimated to be 250 W. Determine the power rating of the electric resistance heating
element.
1–40 A hair dryer is basically a duct in which a few layers of
electric resistors are placed. A small fan pulls the air in and
forces it to flow over the resistors where it is heated. Air enters
a 1200W hair dryer at 100 kPa and 22°C, and leaves at 47°C.
The crosssectional area of the hair dryer at the exit is 60 cm2.
Neglecting the power consumed by the fan and the heat losses
through the walls of the hair dryer, determine (a) the volume
flow rate of air at the inlet and (b) the velocity of the air at the
Answers: (a) 0.0404 m3/s, (b) 7.30 m/s
exit. T2 = 47°C
A 2 = 60 cm 2 P1 = 100 kPa
T1 = 22°C ·
We = 1200 W FIGURE P1–40
1–41 The ducts of an air heating system pass through an unheated area. As a result of heat losses, the temperature of the air
in the duct drops by 3°C. If the mass flow rate of air is 120
kg/min, determine the rate of heat loss from the air to the cold
environment. 1–42E Air enters the duct of an airconditioning system at 15
psia and 50°F at a volume flow rate of 450 ft3/min. The diameter of the duct is 10 inches and heat is transferred to the air in
the duct from the surroundings at a rate of 2 Btu/s. Determine
(a) the velocity of the air at the duct inlet and (b) the temperaAnswers: (a) 825 ft/min, (b) 64°F
ture of the air at the exit.
1–43 Water is heated in an insulated, constant diameter tube
by a 7kW electric resistance heater. If the water enters the
heater steadily at 15°C and leaves at 70°C, determine the mass
flow rate of water.
70°C Water
15°C
Resistance
heater, 7 kW FIGURE P1–43 Heat Transfer Mechanisms
1–44C Define thermal conductivity and explain its significance in heat transfer.
1–45C What are the mechanisms of heat transfer? How are
they distinguished from each other?
1–46C What is the physical mechanism of heat conduction in
a solid, a liquid, and a gas?
1–47C Consider heat transfer through a windowless wall of
a house in a winter day. Discuss the parameters that affect the
rate of heat conduction through the wall.
1–48C Write down the expressions for the physical laws that
govern each mode of heat transfer, and identify the variables
involved in each relation.
1–49C How does heat conduction differ from convection? 1–50C Does any of the energy of the sun reach the earth by
conduction or convection?
1–51C How does forced convection differ from natural
convection?
1–52C Define emissivity and absorptivity. What is Kirchhoff ’s law of radiation?
1–53C What is a blackbody? How do real bodies differ from
blackbodies?
1–54C Judging from its unit W/m · °C, can we define thermal conductivity of a material as the rate of heat transfer
through the material per unit thickness per unit temperature
difference? Explain.
1–55C Consider heat loss through the two walls of a house
on a winter night. The walls are identical, except that one of
them has a tightly fit glass window. Through which wall will
the house lose more heat? Explain.
1–56C Which is a better heat conductor, diamond or silver? cen58933_ch01.qxd 9/10/2002 8:30 AM Page 51 51
CHAPTER 1 1–57C Consider two walls of a house that are identical except that one is made of 10cmthick wood, while the other is
made of 25cmthick brick. Through which wall will the house
lose more heat in winter?
1–58C How do the thermal conductivity of gases and liquids
vary with temperature? 105°C 1–59C Why is the thermal conductivity of superinsulation
orders of magnitude lower than the thermal conductivity of
ordinary insulation?
1–60C Why do we characterize the heat conduction ability
of insulators in terms of their apparent thermal conductivity
instead of the ordinary thermal conductivity?
1–61C Consider an alloy of two metals whose thermal conductivities are k1 and k2. Will the thermal conductivity of the
alloy be less than k1, greater than k2, or between k1 and k2?
1–62 The inner and outer surfaces of a 5m 6m brick wall
of thickness 30 cm and thermal conductivity 0.69 W/m · °C are
maintained at temperatures of 20°C and 5°C, respectively.
Determine the rate of heat transfer through the wall, in W.
Answer: 1035 W 800 W FIGURE P1–65
1–66E The north wall of an electrically heated home is 20 ft
long, 10 ft high, and 1 ft thick, and is made of brick whose
thermal conductivity is k 0.42 Btu/h · ft · °F. On a certain
winter night, the temperatures of the inner and the outer surfaces of the wall are measured to be at about 62°F and 25°F,
respectively, for a period of 8 hours. Determine (a) the rate of
heat loss through the wall that night and (b) the cost of that heat
loss to the home owner if the cost of electricity is $0.07/kWh.
1–67 In a certain experiment, cylindrical samples of diameter
4 cm and length 7 cm are used (see Fig. 1–29). The two
thermocouples in each sample are placed 3 cm apart. After initial transients, the electric heater is observed to draw 0.6 A at
110 V, and both differential thermometers read a temperature
difference of 10°C. Determine the thermal conductivity of the
Answer: 78.8 W/m · °C
sample. Brick
wall 20°C 0.4 cm 5°C
30 cm FIGURE P1–62
1–63 The inner and outer surfaces of a 0.5cmthick 2m
2m window glass in winter are 10°C and 3°C, respectively. If
the thermal conductivity of the glass is 0.78 W/m · °C, determine the amount of heat loss, in kJ, through the glass over a
period of 5 hours. What would your answer be if the glass were
Answers: 78,624 kJ, 39,312 kJ
1 cm thick?
Reconsider Problem 1–63. Using EES (or other)
software, plot the amount of heat loss through the
glass as a function of the window glass thickness in the range
of 0.1 cm to 1.0 cm. Discuss the results. 1–68 One way of measuring the thermal conductivity of a
material is to sandwich an electric thermofoil heater between
two identical rectangular samples of the material and to heavily
insulate the four outer edges, as shown in the figure. Thermocouples attached to the inner and outer surfaces of the samples
record the temperatures.
During an experiment, two 0.5cmthick samples 10 cm
10 cm in size are used. When steady operation is reached, the
heater is observed to draw 35 W of electric power, and the temperature of each sample is observed to drop from 82°C at the
inner surface to 74°C at the outer surface. Determine the thermal conductivity of the material at the average temperature.
Samples Insulation Wattmeter 1–64 1–65 An aluminum pan whose thermal conductivity is
237 W/m · °C has a flat bottom with diameter 20 cm and thickness 0.4 cm. Heat is transferred steadily to boiling water in the
pan through its bottom at a rate of 800 W. If the inner surface
of the bottom of the pan is at 105°C, determine the temperature
of the outer surface of the bottom of the pan. Insulation ~
Source 0.5 cm
Resistance
heater FIGURE P1–68
1–69 Repeat Problem 1–68 for an electric power consumption of 28 W. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 52 52
HEAT TRANSFER 1–70 A heat flux meter attached to the inner surface of a
3cmthick refrigerator door indicates a heat flux of 25 W/m2
through the door. Also, the temperatures of the inner and the
outer surfaces of the door are measured to be 7°C and 15°C,
respectively. Determine the average thermal conductivity of
Answer: 0.0938 W/m · °C
the refrigerator door. 1–77 A 50cmlong, 800W electric resistance heating element with diameter 0.5 cm and surface temperature 120°C is
immersed in 60 kg of water initially at 20°C. Determine how
long it will take for this heater to raise the water temperature to
80°C. Also, determine the convection heat transfer coefficients
at the beginning and at the end of the heating process. 1–71 Consider a person standing in a room maintained at
20°C at all times. The inner surfaces of the walls, floors, and
ceiling of the house are observed to be at an average temperature of 12°C in winter and 23°C in summer. Determine the
rates of radiation heat transfer between this person and the surrounding surfaces in both summer and winter if the exposed
surface area, emissivity, and the average outer surface temperature of the person are 1.6 m2, 0.95, and 32°C, respectively. 1–78 A 5cmexternaldiameter, 10mlong hot water pipe at
80°C is losing heat to the surrounding air at 5°C by natural
convection with a heat transfer coefficient of 25 W/m2 · °C.
Determine the rate of heat loss from the pipe by natural conAnswer: 2945 W
vection, in W. 1–72 Reconsider Problem 1–71. Using EES (or other)
software, plot the rate of radiation heat transfer in
winter as a function of the temperature of the inner surface of
the room in the range of 8°C to 18°C. Discuss the results. 1–79 A hollow spherical iron container with outer diameter
20 cm and thickness 0.4 cm is filled with iced water at 0°C. If
the outer surface temperature is 5°C, determine the approximate rate of heat loss from the sphere, in kW, and the rate at
which ice melts in the container. The heat from fusion of water
is 333.7 kJ/kg.
5°C 1–73 For heat transfer purposes, a standing man can be modeled as a 30cmdiameter, 170cmlong vertical cylinder with
both the top and bottom surfaces insulated and with the side
surface at an average temperature of 34°C. For a convection
heat transfer coefficient of 15 W/m2 · °C, determine the rate of
heat loss from this man by convection in an environment at
Answer: 336 W
20°C. Iced
water
0.4 cm 1–74 Hot air at 80°C is blown over a 2m 4m flat surface
at 30°C. If the average convection heat transfer coefficient is
55 W/m2 · °C, determine the rate of heat transfer from the air to
Answer: 22 kW
the plate, in kW. FIGURE P1–79 1–75 Reconsider Problem 1–74. Using EES (or other)
software, plot the rate of heat transfer as a function of the heat transfer coefficient in the range of 20 W/m2 · °C
to 100 W/m2 · °C. Discuss the results. Reconsider Problem 1–79. Using EES (or other)
software, plot the rate at which ice melts as a
function of the container thickness in the range of 0.2 cm to
2.0 cm. Discuss the results. 1–76 The heat generated in the circuitry on the surface of a
silicon chip (k 130 W/m · °C) is conducted to the ceramic
substrate to which it is attached. The chip is 6 mm 6 mm in
size and 0.5 mm thick and dissipates 3 W of power. Disregarding any heat transfer through the 0.5mmhigh side surfaces,
determine the temperature difference between the front and
back surfaces of the chip in steady operation. 1–81E The inner and outer glasses of a 6ft 6ft doublepane window are at 60°F and 42°F, respectively. If the 0.25in.
space between the two glasses is filled with still air, determine
the rate of heat transfer through the window. Silicon
chip 0.5 mm Ceramic
substrate FIGURE P1–76 6 6 mm m m 3W 1–80 Answer: 439 Btu/h 1–82 Two surfaces of a 2cmthick plate are maintained at
0°C and 80°C, respectively. If it is determined that heat is
transferred through the plate at a rate of 500 W/m2, determine
its thermal conductivity.
1–83 Four power transistors, each dissipating 15 W, are
mounted on a thin vertical aluminum plate 22 cm 22 cm in
size. The heat generated by the transistors is to be dissipated by
both surfaces of the plate to the surrounding air at 25°C, which
is blown over the plate by a fan. The entire plate can be assumed to be nearly isothermal, and the exposed surface area of
the transistor can be taken to be equal to its base area. If the
average convection heat transfer coefficient is 25 W/m2 · °C,
determine the temperature of the aluminum plate. Disregard
any radiation effects. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 53 53
CHAPTER 1 1–84 An ice chest whose outer dimensions are 30 cm
40 cm 40 cm is made of 3cmthick Styrofoam (k 0.033
W/m · °C). Initially, the chest is filled with 40 kg of ice at 0°C,
and the inner surface temperature of the ice chest can be taken
to be 0°C at all times. The heat of fusion of ice at 0°C is 333.7
kJ/kg, and the surrounding ambient air is at 30°C. Disregarding
any heat transfer from the 40cm
40cm base of the ice
chest, determine how long it will take for the ice in the chest to
melt completely if the outer surfaces of the ice chest are at 8°C. 1–87E A 200ftlong section of a steam pipe whose outer diameter is 4 inches passes through an open space at 50°F. The
average temperature of the outer surface of the pipe is measured to be 280°F, and the average heat transfer coefficient on
that surface is determined to be 6 Btu/h · ft2 · °F. Determine
(a) the rate of heat loss from the steam pipe and (b) the annual
cost of this energy loss if steam is generated in a natural gas
furnace having an efficiency of 86 percent, and the price of natural gas is $0.58/therm (1 therm 100,000 Btu).
Answers: (a) 289,000 Btu/h, (b) $17,074/yr Answer: 32.7 days
Tair = 30°C Ice chest
0°C 3 cm
0°C Styrofoam FIGURE P1–84
1–85 A transistor with a height of 0.4 cm and a diameter of
0.6 cm is mounted on a circuit board. The transistor is cooled
by air flowing over it with an average heat transfer coefficient
of 30 W/m2 · °C. If the air temperature is 55°C and the transistor case temperature is not to exceed 70°C, determine the
amount of power this transistor can dissipate safely. Disregard
any heat transfer from the transistor base. 1–88 The boiling temperature of nitrogen at atmospheric
pressure at sea level (1 atm) is 196°C. Therefore, nitrogen is
commonly used in low temperature scientific studies since the
temperature of liquid nitrogen in a tank open to the atmosphere
will remain constant at 196°C until the liquid nitrogen in
the tank is depleted. Any heat transfer to the tank will result
in the evaporation of some liquid nitrogen, which has a heat of
vaporization of 198 kJ/kg and a density of 810 kg/m3 at 1 atm.
Consider a 4mdiameter spherical tank initially filled
with liquid nitrogen at 1 atm and 196°C. The tank is exposed to 20°C ambient air with a heat transfer coefficient of
25 W/m2 · °C. The temperature of the thinshelled spherical
tank is observed to be almost the same as the temperature of
the nitrogen inside. Disregarding any radiation heat exchange,
determine the rate of evaporation of the liquid nitrogen in the
tank as a result of the heat transfer from the ambient air.
N2 vapor
Tair = 20°C Air
55°C
1 atm
Liquid N2
–196°C
Power
transistor
Ts ≤ 70°C 0.6 cm 0.4 cm FIGURE P1–85
1–86 Reconsider Problem 1–85. Using EES (or other)
software, plot the amount of power the transistor
can dissipate safely as a function of the maximum case temperature in the range of 60°C to 90°C. Discuss the results. FIGURE P1–88
1–89 Repeat Problem 1–88 for liquid oxygen, which has
a boiling temperature of 183°C, a heat of vaporization of
213 kJ/kg, and a density of 1140 kg/m3 at 1 atm pressure.
1–90 Reconsider Problem 1–88. Using EES (or other)
software, plot the rate of evaporation of liquid
nitrogen as a function of the ambient air temperature in the
range of 0°C to 35°C. Discuss the results.
1–91 Consider a person whose exposed surface area is
1.7 m2, emissivity is 0.7, and surface temperature is 32°C. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 54 54
HEAT TRANSFER Determine the rate of heat loss from that person by radiation in
a large room having walls at a temperature of (a) 300 K and
Answers: (a) 37.4 W, (b) 169.2 W
(b) 280 K.
1–92 A 0.3cmthick, 12cmhigh, and 18cmlong circuit
board houses 80 closely spaced logic chips on one side, each
dissipating 0.06 W. The board is impregnated with copper fillings and has an effective thermal conductivity of 16 W/m · °C.
All the heat generated in the chips is conducted across the circuit board and is dissipated from the back side of the board to
the ambient air. Determine the temperature difference between
Answer: 0.042°C
the two sides of the circuit board.
1–93 Consider a sealed 20cmhigh electronic box whose
base dimensions are 40 cm 40 cm placed in a vacuum chamber. The emissivity of the outer surface of the box is 0.95. If the
electronic components in the box dissipate a total of 100 W of
power and the outer surface temperature of the box is not to exceed 55°C, determine the temperature at which the surrounding
surfaces must be kept if this box is to be cooled by radiation
alone. Assume the heat transfer from the bottom surface of the
box to the stand to be negligible.
40 cm 40 cm
100 W
ε = 0.95
Ts = 60°C Electronic
box 20 cm Stand FIGURE P1–93
1–94 Using the conversion factors between W and Btu/h, m
and ft, and K and R, express the Stefan–Boltzmann constant
5.67 10 8 W/m2 · K4 in the English unit Btu/h · ft2 · R4.
1–95 An engineer who is working on the heat transfer analysis of a house in English units needs the convection heat transfer coefficient on the outer surface of the house. But the only
value he can find from his handbooks is 20 W/m2 · °C, which
is in SI units. The engineer does not have a direct conversion
factor between the two unit systems for the convection heat
transfer coefficient. Using the conversion factors between
W and Btu/h, m and ft, and °C and °F, express the given convection heat transfer coefficient in Btu/h · ft2 · °F.
Answer: 3.52 Btu/h · ft2 · °F Simultaneous Heat Transfer Mechanisms
1–96C Can all three modes of heat transfer occur simultaneously (in parallel) in a medium?
1–97C Can a medium involve (a) conduction and convection, (b) conduction and radiation, or (c) convection and radiation simultaneously? Give examples for the “yes” answers. 1–98C The deep human body temperature of a healthy
person remains constant at 37°C while the temperature and
the humidity of the environment change with time. Discuss the
heat transfer mechanisms between the human body and the environment both in summer and winter, and explain how a person can keep cooler in summer and warmer in winter.
1–99C We often turn the fan on in summer to help us cool.
Explain how a fan makes us feel cooler in the summer. Also
explain why some people use ceiling fans also in winter.
1–100 Consider a person standing in a room at 23°C. Determine the total rate of heat transfer from this person if the exposed surface area and the skin temperature of the person are
1.7 m2 and 32°C, respectively, and the convection heat transfer
coefficient is 5 W/m2 · °C. Take the emissivity of the skin and
the clothes to be 0.9, and assume the temperature of the inner
surfaces of the room to be the same as the air temperature.
Answer: 161 W 1–101 Consider steady heat transfer between two large
290 K and
parallel plates at constant temperatures of T1
T2 150 K that are L 2 cm apart. Assuming the surfaces to
be black (emissivity
1), determine the rate of heat transfer
between the plates per unit surface area assuming the gap
between the plates is (a) filled with atmospheric air, (b) evacuated, (c) filled with fiberglass insulation, and (d) filled with
superinsulation having an apparent thermal conductivity of
0.00015 W/m · °C.
1–102 A 1.4mlong, 0.2cmdiameter electrical wire extends
across a room that is maintained at 20°C. Heat is generated in
the wire as a result of resistance heating, and the surface temperature of the wire is measured to be 240°C in steady operation. Also, the voltage drop and electric current through
the wire are measured to be 110 V and 3 A, respectively. Disregarding any heat transfer by radiation, determine the convection heat transfer coefficient for heat transfer between the
outer surface of the wire and the air in the room.
Answer: 170.5 W/m2 · °C
Room
20°C
240°C
Electric resistance heater FIGURE P1–102
1–103 Reconsider Problem 1–102. Using EES (or
other) software, plot the convection heat transfer coefficient as a function of the wire surface temperature in
the range of 100°C to 300°C. Discuss the results. 1–104E A 2indiameter spherical ball whose surface is
maintained at a temperature of 170°F is suspended in the middle of a room at 70°F. If the convection heat transfer coefficient
is 12 Btu/h · ft2 · °F and the emissivity of the surface is 0.8, determine the total rate of heat transfer from the ball. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 55 55
CHAPTER 1 1–105 A 1000W iron is left on the iron board with its
base exposed to the air at 20°C. The convection
heat transfer coefficient between the base surface and the surrounding air is 35 W/m2 · °C. If the base has an emissivity of
0.6 and a surface area of 0.02 m2, determine the temperature of
Answer: 674°C
the base of the iron.
Iron
1000 W of the collector is 100°F. The emissivity of the exposed surface of the collector is 0.9. Determine the rate of heat loss from
the collector by convection and radiation during a calm day
when the ambient air temperature is 70°F and the effective
sky temperature for radiation exchange is 50°F. Take the convection heat transfer coefficient on the exposed surface to be
2.5 Btu/h · ft2 · °F. 20°C
Tsky = 50°F
Solar collector 70°F FIGURE P1–105
1–106 The outer surface of a spacecraft in space has an emissivity of 0.8 and a solar absorptivity of 0.3. If solar radiation is
incident on the spacecraft at a rate of 950 W/m2, determine the
surface temperature of the spacecraft when the radiation emitted equals the solar energy absorbed.
1–107 A 3minternaldiameter spherical tank made of 1cmthick stainless steel is used to store iced water at 0°C. The tank
is located outdoors at 25°C. Assuming the entire steel tank to
be at 0°C and thus the thermal resistance of the tank to be negligible, determine (a) the rate of heat transfer to the iced water
in the tank and (b) the amount of ice at 0°C that melts during a
24hour period. The heat of fusion of water at atmospheric pressure is hif 333.7 kJ/kg. The emissivity of the outer surface of
the tank is 0.6, and the convection heat transfer coefficient on
the outer surface can be taken to be 30 W/m2 · °C. Assume the
average surrounding surface temperature for radiation exAnswer: 5898 kg
change to be 15°C.
The roof of a house consists of a 15cmthick
concrete slab (k
2 W/m · °C) that is 15 m
wide and 20 m long. The emissivity of the outer surface of the
roof is 0.9, and the convection heat transfer coefficient on that
surface is estimated to be 15 W/m2 · °C. The inner surface of
the roof is maintained at 15°C. On a clear winter night, the ambient air is reported to be at 10°C while the night sky temperature for radiation heat transfer is 255 K. Considering both
radiation and convection heat transfer, determine the outer surface temperature and the rate of heat transfer through the roof.
If the house is heated by a furnace burning natural gas with
an efficiency of 85 percent, and the unit cost of natural gas is
$0.60/therm (1 therm
105,500 kJ of energy content), determine the money lost through the roof that night during a
14hour period. FIGURE P1–109E
Problem Solving Technique and EES
1–110C What is the value of the engineering software packages in (a) engineering education and (b) engineering practice?
1–111 Determine a positive real root of the following
equation using EES:
2x3 1–112 10x0.5 3x 3 Solve the following system of two equations
with two unknowns using EES:
x3
3xy y2
y 7.75
3.5 1–108 1–109E Consider a flat plate solar collector placed horizontally on the flat roof of a house. The collector is 5 ft wide and
15 ft long, and the average temperature of the exposed surface 1–113 Solve the following system of three equations
with three unknowns using EES:
2x y z
3x2 2y
xy 2z 1–114 5
z
8 2 Solve the following system of three equations
with three unknowns using EES:
x x2y z
3y0.5 xz
xyz 1
2
2 Special Topic: Thermal Comfort
1–115C What is metabolism? What is the range of metabolic
rate for an average man? Why are we interested in metabolic cen58933_ch01.qxd 9/10/2002 8:30 AM Page 56 56
HEAT TRANSFER rate of the occupants of a building when we deal with heating
and air conditioning?
1–116C Why is the metabolic rate of women, in general,
lower than that of men? What is the effect of clothing on the
environmental temperature that feels comfortable?
1–117C What is asymmetric thermal radiation? How does it
cause thermal discomfort in the occupants of a room?
1–118C How do (a) draft and (b) cold floor surfaces cause
discomfort for a room’s occupants? Resistance
heater 1–119C What is stratification? Is it likely to occur at places
with low or high ceilings? How does it cause thermal discomfort for a room’s occupants? How can stratification be prevented?
1–120C Why is it necessary to ventilate buildings? What is
the effect of ventilation on energy consumption for heating in
winter and for cooling in summer? Is it a good idea to keep the
bathroom fans on all the time? Explain. Review Problems
1–121 2.5 kg of liquid water initially at 18°C is to be heated
to 96°C in a teapot equipped with a 1200W electric heating
element inside. The teapot is 0.8 kg and has an average specific
heat of 0.6 kJ/kg · °C. Taking the specific heat of water to be
4.18 kJ/kg · °C and disregarding any heat loss from the teapot,
determine how long it will take for the water to be heated.
1–122 A 4mlong section of an air heating system of a house
passes through an unheated space in the attic. The inner diameter of the circular duct of the heating system is 20 cm. Hot air
enters the duct at 100 kPa and 65°C at an average velocity of
3 m/s. The temperature of the air in the duct drops to 60°C as a
result of heat loss to the cool space in the attic. Determine the
rate of heat loss from the air in the duct to the attic under steady
conditions. Also, determine the cost of this heat loss per hour if
the house is heated by a natural gas furnace having an efficiency of 82 percent, and the cost of the natural gas in that area
is $0.58/therm (1 therm 105,500 kJ).
Answers: 0.488 kJ/s, $0.012/h
4m
65°C
3 m/s Hot air 60°C FIGURE P1–122
1–123 Reconsider Problem 1–122. Using EES (or
other) software, plot the cost of the heat loss per
hour as a function of the average air velocity in the range of
1 m/s to 10 m/s. Discuss the results.
1–124 Water flows through a shower head steadily at a rate
of 10 L/min. An electric resistance heater placed in the water
pipe heats the water from 16°C to 43°C. Taking the density of FIGURE P1–124
water to be 1 kg/L, determine the electric power input to the
heater, in kW.
In an effort to conserve energy, it is proposed to pass the
drained warm water at a temperature of 39°C through a heat
exchanger to preheat the incoming cold water. If the heat exchanger has an effectiveness of 0.50 (that is, it recovers only
half of the energy that can possibly be transferred from the
drained water to incoming cold water), determine the electric
power input required in this case. If the price of the electric energy is 8.5 ¢/kWh, determine how much money is saved during
a 10minute shower as a result of installing this heat exchanger.
Answers: 18.8 kW, 10.8 kW, $0.0113 1–125 It is proposed to have a water heater that consists of an
insulated pipe of 5 cm diameter and an electrical resistor inside. Cold water at 15°C enters the heating section steadily at a
rate of 18 L/min. If water is to be heated to 50°C, determine
(a) the power rating of the resistance heater and (b) the average
velocity of the water in the pipe.
1–126 A passive solar house that is losing heat to the outdoors at an average rate of 50,000 kJ/h is maintained at 22°C at
all times during a winter night for 10 hours. The house is to be
heated by 50 glass containers each containing 20 L of water
heated to 80°C during the day by absorbing solar energy.
A thermostatcontrolled 15kW backup electric resistance
heater turns on whenever necessary to keep the house at 22°C.
(a) How long did the electric heating system run that night?
(b) How long would the electric heater have run that night if
the house incorporated no solar heating?
Answers: (a) 4.77 h, (b) 9.26 h 1–127 It is well known that wind makes the cold air feel
much colder as a result of the windchill effect that is due to the
increase in the convection heat transfer coefficient with increasing air velocity. The windchill effect is usually expressed
in terms of the windchill factor, which is the difference between the actual air temperature and the equivalent calmair cen58933_ch01.qxd 9/10/2002 8:30 AM Page 57 57
CHAPTER 1
50,000 kJ/h and the surrounding air temperature is 10°C, determine the surface temperature of the plate when the heat loss by convection
equals the solar energy absorbed by the plate. Take the convection heat transfer coefficient to be 30 W/m2 · °C, and disregard
any heat loss by radiation.
1–129 A 4m 5m 6m room is to be heated by one ton
(1000 kg) of liquid water contained in a tank placed in the
room. The room is losing heat to the outside at an average rate
of 10,000 kJ/h. The room is initially at 20°C and 100 kPa, and
is maintained at an average temperature of 20°C at all times. If
the hot water is to meet the heating requirements of this room
for a 24hour period, determine the minimum temperature of
the water when it is first brought into the room. Assume constant specific heats for both air and water at room temperature. 22°C Water
80°C Answer: 77.4°C FIGURE P1–126
temperature. For example, a windchill factor of 20°C for an
actual air temperature of 5°C means that the windy air at 5°C
feels as cold as the still air at 15°C. In other words, a person
will lose as much heat to air at 5°C with a windchill factor of
20°C as he or she would in calm air at 15°C.
For heat transfer purposes, a standing man can be modeled
as a 30cmdiameter, 170cmlong vertical cylinder with both
the top and bottom surfaces insulated and with the side surface
at an average temperature of 34°C. For a convection heat transfer coefficient of 15 W/m2 · °C, determine the rate of heat loss
from this man by convection in still air at 20°C. What would
your answer be if the convection heat transfer coefficient is increased to 50 W/m2 · °C as a result of winds? What is the windAnswers: 336 W, 1120 W, 32.7°C
chill factor in this case? 1–130 Consider a 3m 3m 3m cubical furnace whose
top and side surfaces closely approximate black surfaces at a
temperature of 1200 K. The base surface has an emissivity of
0.7, and is maintained at 800 K. Determine the net rate
of radiation heat transfer to the base surface from the top and
Answer: 594,400 W
side surfaces.
1–131 Consider a refrigerator whose dimensions are 1.8 m
1.2 m 0.8 m and whose walls are 3 cm thick. The refrigerator consumes 600 W of power when operating and has a COP
of 2.5. It is observed that the motor of the refrigerator remains
on for 5 minutes and then is off for 15 minutes periodically. If
the average temperatures at the inner and outer surfaces of the
refrigerator are 6°C and 17°C, respectively, determine the average thermal conductivity of the refrigerator walls. Also, determine the annual cost of operating this refrigerator if the unit
cost of electricity is $0.08/kWh. 1–128 A thin metal plate is insulated on the back and exposed to solar radiation on the front surface. The exposed surface of the plate has an absorptivity of 0.7 for solar radiation. If
solar radiation is incident on the plate at a rate of 700 W/m2 17°C
6°C
700 W/ m2 α = 0.7 FIGURE P1–128 10°C FIGURE P1–131
1–132 A 0.2L glass of water at 20°C is to be cooled with
ice to 5°C. Determine how much ice needs to be added to
the water, in grams, if the ice is at 0°C. Also, determine how
much water would be needed if the cooling is to be done with
cold water at 0°C. The melting temperature and the heat of
fusion of ice at atmospheric pressure are 0°C and 333.7 kJ/kg,
respectively, and the density of water is 1 kg/L. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 58 58
HEAT TRANSFER
Ice, 0°C Water
0.2 L
20°C FIGURE P1–132 0.6 cm and a thermal conductivity of 0.7 W/m · C. Heat is lost
from the outer surface of the cover by convection and radiation
with a convection heat transfer coefficient of 10 W/m2 · °C and
an ambient temperature of 15°C. Determine the fraction of heat
lost from the glass cover by radiation.
1–138 The rate of heat loss through a unit surface area of
a window per unit temperature difference between the indoors and the outdoors is called the Ufactor. The value of
the Ufactor ranges from about 1.25 W/m2 · °C (or 0.22
Btu/h · ft2 · °F) for lowe coated, argonfilled, quadruplepane
windows to 6.25 W/m2 · °C (or 1.1 Btu/h · ft2 · °F) for a singlepane window with aluminum frames. Determine the range for
the rate of heat loss through a 1.2m
1.8m window of a
house that is maintained at 20°C when the outdoor air temperature is 8°C.
1.2 m 1–133 Reconsider Problem 1–132. Using EES (or
other) software, plot the amount of ice that
needs to be added to the water as a function of the ice temperature in the range of 24°C to 0°C. Discuss the results. Indoors
20°C 1–134E In order to cool 1 short ton (2000 lbm) of water at
70°F in a tank, a person pours 160 lbm of ice at 25°F into the
water. Determine the final equilibrium temperature in the tank.
The melting temperature and the heat of fusion of ice at atmospheric pressure are 32°F and 143.5 Btu/lbm, respectively. . Q Answer: 56.3°F 1–135 Engine valves (Cp
440 J/kg · °C and
7840
kg/m3) are to be heated from 40°C to 800°C in 5 minutes in the
heat treatment section of a valve manufacturing facility. The
valves have a cylindrical stem with a diameter of 8 mm and a
length of 10 cm. The valve head and the stem may be assumed
to be of equal surface area, with a total mass of 0.0788 kg. For
a single valve, determine (a) the amount of heat transfer,
(b) the average rate of heat transfer, and (c) the average heat
flux, (d) the number of valves that can be heat treated per day if
the heating section can hold 25 valves, and it is used 10 hours
per day.
1–136 The hot water needs of a household are met by an
electric 60L hot water tank equipped with a 1.6kW heating
element. The tank is initially filled with hot water at 80°C, and
the cold water temperature is 20°C. Someone takes a shower
by mixing constant flow rates of hot and cold waters. After a
showering period of 8 minutes, the average water temperature
in the tank is measured to be 60°C. The heater is kept on during
the shower and hot water is replaced by cold water. If the cold
water is mixed with the hot water stream at a rate of 0.06 kg/s,
determine the flow rate of hot water and the average temperature of mixed water used during the shower.
1–137 Consider a flat plate solar collector placed at the roof
of a house. The temperatures at the inner and outer surfaces of
glass cover are measured to be 28°C and 25°C, respectively.
The glass cover has a surface area of 2.2. m2 and a thickness of Outdoors
–8°C 1.8 m FIGURE P1–138
1–139 Reconsider Problem 1–138. Using EES (or
other) software, plot the rate of heat loss
through the window as a function of the Ufactor. Discuss
the results. Design and Essay Problems
1–140 Write an essay on how microwave ovens work, and
explain how they cook much faster than conventional ovens.
Discuss whether conventional electric or microwave ovens
consume more electricity for the same task.
1–141 Using information from the utility bills for the coldest
month last year, estimate the average rate of heat loss from
your house for that month. In your analysis, consider the contribution of the internal heat sources such as people, lights, and
appliances. Identify the primary sources of heat loss from your
house and propose ways of improving the energy efficiency of
your house.
1–142 Design a 1200W electric hair dryer such that the air
temperature and velocity in the dryer will not exceed 50°C and
3/ms, respectively.
1–143 Design an electric hot water heater for a family of
four in your area. The maximum water temperature in the tank cen58933_ch01.qxd 9/10/2002 8:30 AM Page 59 59
CHAPTER 1 and the power consumption are not to exceed 60°C and 4 kW,
respectively. There are two showers in the house, and the
flow rate of water through each of the shower heads is about
10 L/min. Each family member takes a 5minute shower every
morning. Explain why a hot water tank is necessary, and determine the proper size of the tank for this family.
1–144 Conduct this experiment to determine the heat transfer
coefficient between an incandescent lightbulb and the surrounding air using a 60W lightbulb. You will need an indoor–
outdoor thermometer, which can be purchased for about $10 in a hardware store, and a metal glue. You will also need a piece
of string and a ruler to calculate the surface area of the lightbulb. First, measure the air temperature in the room, and then
glue the tip of the thermocouple wire of the thermometer to the
glass of the lightbulb. Turn the light on and wait until the temperature reading stabilizes. The temperature reading will give
the surface temperature of the lightbulb. Assuming 10 percent
of the rated power of the bulb is converted to light, calculate
the heat transfer coefficient from Newton’s law of cooling. cen58933_ch01.qxd 9/10/2002 8:30 AM Page 60 ...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.
 Spring '10
 Ghaz

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