Unformatted text preview: cen58933_ch02.qxd 9/10/2002 8:46 AM Page 61 CHAPTER H E AT C O N D U C T I O N
E Q U AT I O N
eat transfer has direction as well as magnitude. The rate of heat conduction in a specified direction is proportional to the temperature gradient, which is the change in temperature per unit length in that
direction. Heat conduction in a medium, in general, is threedimensional and
time dependent. That is, T T(x, y, z, t) and the temperature in a medium
varies with position as well as time. Heat conduction in a medium is said to be
steady when the temperature does not vary with time, and unsteady or transient when it does. Heat conduction in a medium is said to be onedimensional
when conduction is significant in one dimension only and negligible in the
other two dimensions, twodimensional when conduction in the third dimension is negligible, and threedimensional when conduction in all dimensions
is significant.
We start this chapter with a description of steady, unsteady, and multidimensional heat conduction. Then we derive the differential equation that
governs heat conduction in a large plane wall, a long cylinder, and a sphere,
and generalize the results to threedimensional cases in rectangular, cylindrical, and spherical coordinates. Following a discussion of the boundary conditions, we present the formulation of heat conduction problems and their
solutions. Finally, we consider heat conduction problems with variable thermal conductivity.
This chapter deals with the theoretical and mathematical aspects of heat
conduction, and it can be covered selectively, if desired, without causing a
significant loss in continuity. The more practical aspects of heat conduction
are covered in the following two chapters. H 2
CONTENTS
2–1
Introduction 62
2–2
OneDimensional Heat
Conduction Equation 68
2–3
General Heat
Conduction Equation 74
2–4
Boundary and
Initial Conditions 77
2–5
Solution of Steady
OneDimensional Heat
Conduction Problems 86
2–6
Heat Generation in a Solid 97
2–7
Variable Thermal
Conductivity k (T ) 104
Topic of Special Interest:
A Brief Review of
Differential Equations 107 61 cen58933_ch02.qxd 9/10/2002 8:46 AM Page 62 62
HEAT TRANSFER
Magnitude of
temperature
at a point A
(no direction)
50°C
Hot
baked
potato 80 W/ m2 A
Magnitude and
direction of heat
flux at the same
point FIGURE 2–1
Heat transfer has direction as well
as magnitude, and thus it is
a vector quantity. ·
Q = 500 W
Hot
medium
0 Cold
medium
x L ·
Q = –500 W
Cold
medium
0 Hot
medium
L x FIGURE 2–2
Indicating direction for heat transfer
(positive in the positive direction;
negative in the negative direction). 2–1 I INTRODUCTION In Chapter 1 heat conduction was defined as the transfer of thermal energy
from the more energetic particles of a medium to the adjacent less energetic
ones. It was stated that conduction can take place in liquids and gases as well
as solids provided that there is no bulk motion involved.
Although heat transfer and temperature are closely related, they are of a different nature. Unlike temperature, heat transfer has direction as well as magnitude, and thus it is a vector quantity (Fig. 2–1). Therefore, we must specify
both direction and magnitude in order to describe heat transfer completely at
a point. For example, saying that the temperature on the inner surface of a
wall is 18°C describes the temperature at that location fully. But saying that
the heat flux on that surface is 50 W/m2 immediately prompts the question “in
what direction?” We can answer this question by saying that heat conduction
is toward the inside (indicating heat gain) or toward the outside (indicating
heat loss).
To avoid such questions, we can work with a coordinate system and indicate
direction with plus or minus signs. The generally accepted convention is that
heat transfer in the positive direction of a coordinate axis is positive and in the
opposite direction it is negative. Therefore, a positive quantity indicates heat
transfer in the positive direction and a negative quantity indicates heat transfer in the negative direction (Fig. 2–2).
The driving force for any form of heat transfer is the temperature difference,
and the larger the temperature difference, the larger the rate of heat transfer.
Some heat transfer problems in engineering require the determination of the
temperature distribution (the variation of temperature) throughout the
medium in order to calculate some quantities of interest such as the local heat
transfer rate, thermal expansion, and thermal stress at some critical locations
at specified times. The specification of the temperature at a point in a medium
first requires the specification of the location of that point. This can be done
by choosing a suitable coordinate system such as the rectangular, cylindrical,
or spherical coordinates, depending on the geometry involved, and a convenient reference point (the origin).
The location of a point is specified as (x, y, z) in rectangular coordinates, as
(r, , z) in cylindrical coordinates, and as (r, , ) in spherical coordinates,
where the distances x, y, z, and r and the angles and are as shown in Figure 2–3. Then the temperature at a point (x, y, z) at time t in rectangular coordinates is expressed as T(x, y, z, t). The best coordinate system for a given
geometry is the one that describes the surfaces of the geometry best.
For example, a parallelepiped is best described in rectangular coordinates
since each surface can be described by a constant value of the x, y, or
zcoordinates. A cylinder is best suited for cylindrical coordinates since its lateral surface can be described by a constant value of the radius. Similarly, the
entire outer surface of a spherical body can best be described by a constant
value of the radius in spherical coordinates. For an arbitrarily shaped body, we
normally use rectangular coordinates since it is easier to deal with distances
than with angles.
The notation just described is also used to identify the variables involved
in a heat transfer problem. For example, the notation T(x, y, z, t) implies that
the temperature varies with the space variables x, y, and z as well as time. The cen58933_ch02.qxd 9/10/2002 8:46 AM Page 63 63
CHAPTER 2
z z z P(x, y, z) P(r, φ , z) z φ z
x y φ r y P(r, φ , θ ) θ
r y y
x x
(a) Rectangular coordinates x
(b) Cylindrical coordinates (c) Spherical coordinates FIGURE 2–3
The various distances
and angles involved when
describing the location of a point
in different coordinate systems. notation T(x), on the other hand, indicates that the temperature varies in the
xdirection only and there is no variation with the other two space coordinates
or time. Steady versus Transient Heat Transfer
Heat transfer problems are often classified as being steady (also called steadystate) or transient (also called unsteady). The term steady implies no change
with time at any point within the medium, while transient implies variation
with time or time dependence. Therefore, the temperature or heat flux remains
unchanged with time during steady heat transfer through a medium at any location, although both quantities may vary from one location to another
(Fig. 2–4). For example, heat transfer through the walls of a house will be
steady when the conditions inside the house and the outdoors remain constant
for several hours. But even in this case, the temperatures on the inner and
outer surfaces of the wall will be different unless the temperatures inside and
outside the house are the same. The cooling of an apple in a refrigerator, on
the other hand, is a transient heat transfer process since the temperature at any
fixed point within the apple will change with time during cooling. During
transient heat transfer, the temperature normally varies with time as well as
position. In the special case of variation with time but not with position, the
temperature of the medium changes uniformly with time. Such heat transfer
systems are called lumped systems. A small metal object such as a thermocouple junction or a thin copper wire, for example, can be analyzed as a
lumped system during a heating or cooling process.
Most heat transfer problems encountered in practice are transient in nature,
but they are usually analyzed under some presumed steady conditions since
steady processes are easier to analyze, and they provide the answers to our
questions. For example, heat transfer through the walls and ceiling of a typical house is never steady since the outdoor conditions such as the temperature,
the speed and direction of the wind, the location of the sun, and so on, change
constantly. The conditions in a typical house are not so steady either. Therefore, it is almost impossible to perform a heat transfer analysis of a house accurately. But then, do we really need an indepth heat transfer analysis? If the Time = 2 PM
15°C Time = 5 PM 7°C 12°C 5°C ·
Q1 ·
·
Q2 ≠ Q1 (a) Transient 15°C 7°C 15°C ·
Q1 7°C ·
·
Q2 = Q1 (b) Steadystate FIGURE 2–4
Steady and transient heat
conduction in a plane wall. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 64 64
HEAT TRANSFER purpose of a heat transfer analysis of a house is to determine the proper size of
a heater, which is usually the case, we need to know the maximum rate of heat
loss from the house, which is determined by considering the heat loss from the
house under worst conditions for an extended period of time, that is, during
steady operation under worst conditions. Therefore, we can get the answer to
our question by doing a heat transfer analysis under steady conditions. If the
heater is large enough to keep the house warm under the presumed worst conditions, it is large enough for all conditions. The approach described above is
a common practice in engineering. Multidimensional Heat Transfer 80°C 65°C
T(x, y) ·
Qy 70°C
65°C 80°C ·
Qx 70°C
65°C 80°C
70°C z
y x FIGURE 2–5
Twodimensional heat transfer
in a long rectangular bar. Negligible
heat transfer ·
Q
Primary
direction of
heat transfer FIGURE 2–6
Heat transfer through the window
of a house can be taken to be
onedimensional. Heat transfer problems are also classified as being onedimensional, twodimensional, or threedimensional, depending on the relative magnitudes of
heat transfer rates in different directions and the level of accuracy desired. In
the most general case, heat transfer through a medium is threedimensional.
That is, the temperature varies along all three primary directions within the
medium during the heat transfer process. The temperature distribution
throughout the medium at a specified time as well as the heat transfer rate at
any location in this general case can be described by a set of three coordinates
such as the x, y, and z in the rectangular (or Cartesian) coordinate system;
the r, , and z in the cylindrical coordinate system; and the r, , and in the
spherical (or polar) coordinate system. The temperature distribution in this
case is expressed as T(x, y, z, t), T(r, , z, t), and T(r, , , t) in the respective
coordinate systems.
The temperature in a medium, in some cases, varies mainly in two primary
directions, and the variation of temperature in the third direction (and thus
heat transfer in that direction) is negligible. A heat transfer problem in that
case is said to be twodimensional. For example, the steady temperature distribution in a long bar of rectangular cross section can be expressed as T(x, y)
if the temperature variation in the zdirection (along the bar) is negligible and
there is no change with time (Fig. 2–5).
A heat transfer problem is said to be onedimensional if the temperature in
the medium varies in one direction only and thus heat is transferred in one direction, and the variation of temperature and thus heat transfer in other directions are negligible or zero. For example, heat transfer through the glass of a
window can be considered to be onedimensional since heat transfer through
the glass will occur predominantly in one direction (the direction normal to
the surface of the glass) and heat transfer in other directions (from one side
edge to the other and from the top edge to the bottom) is negligible (Fig. 2–6).
Likewise, heat transfer through a hot water pipe can be considered to be onedimensional since heat transfer through the pipe occurs predominantly in the
radial direction from the hot water to the ambient, and heat transfer along the
pipe and along the circumference of a cross section (z and directions) is
typically negligible. Heat transfer to an egg dropped into boiling water is also
nearly onedimensional because of symmetry. Heat will be transferred to the
egg in this case in the radial direction, that is, along straight lines passing
through the midpoint of the egg.
We also mentioned in Chapter 1 that the rate of heat conduction through a
medium in a specified direction (say, in the xdirection) is proportional to the
temperature difference across the medium and the area normal to the direction cen58933_ch02.qxd 9/10/2002 8:46 AM Page 65 65
CHAPTER 2 of heat transfer, but is inversely proportional to the distance in that direction.
This was expressed in the differential form by Fourier’s law of heat conduction for onedimensional heat conduction as
·
Q cond kA dT
dx (W) kA T
n dT
slope — < 0
dx (21) where k is the thermal conductivity of the material, which is a measure of the
ability of a material to conduct heat, and dT/dx is the temperature gradient,
which is the slope of the temperature curve on a Tx diagram (Fig. 2–7). The
thermal conductivity of a material, in general, varies with temperature. But
sufficiently accurate results can be obtained by using a constant value for
thermal conductivity at the average temperature.
Heat is conducted in the direction of decreasing temperature, and thus
the temperature gradient is negative when heat is conducted in the positive
xdirection. The negative sign in Eq. 2–1 ensures that heat transfer in the positive xdirection is a positive quantity.
To obtain a general relation for Fourier ’s law of heat conduction, consider a
medium in which the temperature distribution is threedimensional. Figure
2–8 shows an isothermal surface in that medium. The heat flux vector at a
point P on this surface must be perpendicular to the surface, and it must point
in the direction of decreasing temperature. If n is the normal of the isothermal
surface at point P, the rate of heat conduction at that point can be expressed by
Fourier ’s law as
·
Qn T (W) T(x)
·
Q>0
Heat flow x FIGURE 2–7
The temperature gradient dT/dx is
simply the slope of the temperature
curve on a Tx diagram. (22) In rectangular coordinates, the heat conduction vector can be expressed in
terms of its components as
→
·
Qn ·→
Qx i ·→
Qy j ·→
Qz k (23) →→
→
··
·
where i, j, and k are the unit vectors, and Q x, Q y, and Q z are the magnitudes
of the heat transfer rates in the x, y, and zdirections, which again can be determined from Fourier ’s law as ·
Qx kAx T
,
x ·
Qy kAy T
,
y and ·
Qz kAz T
z (24) Here Ax, Ay and Az are heat conduction areas normal to the x, y, and
zdirections, respectively (Fig. 2–8).
Most engineering materials are isotropic in nature, and thus they have the
same properties in all directions. For such materials we do not need to be concerned about the variation of properties with direction. But in anisotropic materials such as the fibrous or composite materials, the properties may change
with direction. For example, some of the properties of wood along the grain
are different than those in the direction normal to the grain. In such cases the
thermal conductivity may need to be expressed as a tensor quantity to account
for the variation with direction. The treatment of such advanced topics is beyond the scope of this text, and we will assume the thermal conductivity of a
material to be independent of direction. z
Az
n
Ay ·
Qn ·
Qz Ax ·
Qy
P
y ·
Qx
An isotherm x FIGURE 2–8
The heat transfer vector is
always normal to an isothermal
surface and can be resolved into its
components like any other vector. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 66 66
HEAT TRANSFER Heat Generation FIGURE 2–9
Heat is generated in the heating coils
of an electric range as a result of the
conversion of electrical energy to heat. Sun
Solar
radiation
·
q
s x
Solar energy
absorbed by
water
Water
·
·
g(x) = qs, absorbed(x) A medium through which heat is conducted may involve the conversion of
electrical, nuclear, or chemical energy into heat (or thermal) energy. In heat
conduction analysis, such conversion processes are characterized as heat
generation.
For example, the temperature of a resistance wire rises rapidly when electric current passes through it as a result of the electrical energy being converted to heat at a rate of I2R, where I is the current and R is the electrical
resistance of the wire (Fig. 2–9). The safe and effective removal of this heat
away from the sites of heat generation (the electronic circuits) is the subject
of electronics cooling, which is one of the modern application areas of heat
transfer.
Likewise, a large amount of heat is generated in the fuel elements of nuclear
reactors as a result of nuclear fission that serves as the heat source for the nuclear power plants. The natural disintegration of radioactive elements in nuclear waste or other radioactive material also results in the generation of heat
throughout the body. The heat generated in the sun as a result of the fusion of
hydrogen into helium makes the sun a large nuclear reactor that supplies heat
to the earth.
Another source of heat generation in a medium is exothermic chemical reactions that may occur throughout the medium. The chemical reaction in this
case serves as a heat source for the medium. In the case of endothermic reactions, however, heat is absorbed instead of being released during reaction, and
thus the chemical reaction serves as a heat sink. The heat generation term becomes a negative quantity in this case.
Often it is also convenient to model the absorption of radiation such as solar energy or gamma rays as heat generation when these rays penetrate deep
into the body while being absorbed gradually. For example, the absorption of
solar energy in large bodies of water can be treated as heat generation
throughout the water at a rate equal to the rate of absorption, which varies
with depth (Fig. 2–10). But the absorption of solar energy by an opaque body
occurs within a few microns of the surface, and the solar energy that penetrates into the medium in this case can be treated as specified heat flux on the
surface.
Note that heat generation is a volumetric phenomenon. That is, it occurs
throughout the body of a medium. Therefore, the rate of heat generation in a
·
medium is usually specified per unit volume and is denoted by g, whose unit
3
3
is W/m or Btu/h · ft .
The rate of heat generation in a medium may vary with time as well as position within the medium. When the variation of heat generation with position
is known, the total rate of heat generation in a medium of volume V can be determined from
·
G ·
gdV (W) (25) V FIGURE 2–10
The absorption of solar radiation by
water can be treated as heat
generation. In the special case of uniform heat generation, as in the case of electric resistance heating throughout a homogeneous material, the relation in Eq. 2–5
·
·
·
reduces to G gV, where g is the constant rate of heat generation per unit
volume. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 67 67
CHAPTER 2
Heat transfer EXAMPLE 2–1 Heat Gain by a Refrigerator In order to size the compressor of a new refrigerator, it is desired to determine
the rate of heat transfer from the kitchen air into the refrigerated space through
the walls, door, and the top and bottom section of the refrigerator (Fig. 2–11).
In your analysis, would you treat this as a transient or steadystate heat transfer
problem? Also, would you consider the heat transfer to be onedimensional or
multidimensional? Explain. SOLUTION The heat transfer process from the kitchen air to the refrigerated
space is transient in nature since the thermal conditions in the kitchen and the
refrigerator, in general, change with time. However, we would analyze this problem as a steady heat transfer problem under the worst anticipated conditions
such as the lowest thermostat setting for the refrigerated space, and the anticipated highest temperature in the kitchen (the socalled design conditions). If
the compressor is large enough to keep the refrigerated space at the desired
temperature setting under the presumed worst conditions, then it is large
enough to do so under all conditions by cycling on and off.
Heat transfer into the refrigerated space is threedimensional in nature since
heat will be entering through all six sides of the refrigerator. However, heat
transfer through any wall or floor takes place in the direction normal to the surface, and thus it can be analyzed as being onedimensional. Therefore, this
problem can be simplified greatly by considering the heat transfer to be onedimensional at each of the four sides as well as the top and bottom sections,
and then by adding the calculated values of heat transfer at each surface. EXAMPLE 2–2 Heat Generation in a Hair Dryer The resistance wire of a 1200W hair dryer is 80 cm long and has a diameter of
D 0.3 cm (Fig. 2–12). Determine the rate of heat generation in the wire per
unit volume, in W/cm3, and the heat flux on the outer surface of the wire as a
result of this heat generation. SOLUTION The power consumed by the resistance wire of a hair dryer is given.
The heat generation and the heat flux are to be determined.
Assumptions Heat is generated uniformly in the resistance wire.
Analysis A 1200W hair dryer will convert electrical energy into heat in the
wire at a rate of 1200 W. Therefore, the rate of heat generation in a resistance
wire is equal to the power consumption of a resistance heater. Then the rate of
heat generation in the wire per unit volume is determined by dividing the total
rate of heat generation by the volume of the wire,
·
g ·
G
Vwire ·
G
2
( D /4)L 1200 W
[ (0.3 cm)2/4](80 cm) 212 W/cm3 Similarly, heat flux on the outer surface of the wire as a result of this heat generation is determined by dividing the total rate of heat generation by the surface
area of the wire, ·
q FIGURE 2–11
Schematic for Example 2–1. ·
G
Awire ·
G
DL 1200 W
(0.3 cm)(80 cm) 15.9 W/cm2 Hair dryer
1200 W FIGURE 2–12
Schematic for Example 2–2. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 68 68
HEAT TRANSFER Discussion Note that heat generation is expressed per unit volume in W/cm3 or
Btu/h · ft3, whereas heat flux is expressed per unit surface area in W/cm2 or
Btu/h · ft2. 2–2 ·
G Volume
element A
·
Qx L Consider heat conduction through a large plane wall such as the wall of a
house, the glass of a single pane window, the metal plate at the bottom of
a pressing iron, a cast iron steam pipe, a cylindrical nuclear fuel element,
an electrical resistance wire, the wall of a spherical container, or a spherical
metal ball that is being quenched or tempered. Heat conduction in these
and many other geometries can be approximated as being onedimensional
since heat conduction through these geometries will be dominant in one
direction and negligible in other directions. Below we will develop the onedimensional heat conduction equation in rectangular, cylindrical, and spherical coordinates. Consider a thin element of thickness x in a large plane wall, as shown in Figure 2–13. Assume the density of the wall is , the specific heat is C, and the
area of the wall normal to the direction of heat transfer is A. An energy balance on this thin element during a small time interval t can be expressed as 0
x + ∆x ONEDIMENSIONAL
HEAT CONDUCTION EQUATION Heat Conduction Equation in a Large Plane Wall ·
Qx + ∆ x x I x Rate of heat
conduction
at x Ax = Ax + ∆ x = A FIGURE 2–13
Onedimensional heat conduction
through a volume element
in a large plane wall. Rate of heat
generation
inside the
element Rate of heat
conduction
at x
x Rate of change
of the energy
content of the
element or
·
Qx ·
Qx x Eelement
t ·
Gelement
(26) But the change in the energy content of the element and the rate of heat generation within the element can be expressed as
Eelement
·
Gelement Et t Et mC(Tt
·
·
gVelement gA x t Tt) CA x(Tt t Tt) (27)
(28) Substituting into Equation 2–6, we get
·
Qx ·
Qx x ·
gA x x ·
Qx CA x Tt Tt t t (29) Dividing by A x gives
·
1 Qx
A x ·
g C Tt Tt t t (210) cen58933_ch02.qxd 9/10/2002 8:46 AM Page 69 69
CHAPTER 2 Taking the limit as x → 0 and t → 0 yields
T
1
kA
x
Ax ·
g C T
t (211) since, from the definition of the derivative and Fourier ’s law of heat conduction,
lim ·
Qx x→0 ·
Qx x ·
Q
x x kA x T
x (212) Noting that the area A is constant for a plane wall, the onedimensional transient heat conduction equation in a plane wall becomes
T
x k ·
g C T
t (213) The thermal conductivity k of a material, in general, depends on the temperature T (and therefore x), and thus it cannot be taken out of the derivative.
However, the thermal conductivity in most practical applications can be assumed to remain constant at some average value. The equation above in that
case reduces to
2 Constant conductivity: T
x2 ·
g
k 1T
t (214) where the property
k/ C is the thermal diffusivity of the material and
represents how fast heat propagates through a material. It reduces to the following forms under specified conditions (Fig. 2–14):
(1) Steadystate:
( / t 0)
(2) Transient, no heat generation:
·
(g 0)
(3) Steadystate, no heat generation:
·
( / t 0 and g 0) d 2T
dx2
2 T
x2 d 2T
dx2 ·
g
k 0 (215) 1T
t (216) 0 General, one dimensional:
No
Steadygeneration state
2 T
x2 ·0
g
k 0
1T
t → x → Variable conductivity: Steady, onedimensional:
d2T
dx2 0 FIGURE 2–14
The simplification of the onedimensional heat conduction equation
in a plane wall for the case of constant
conductivity for steady conduction
with no heat generation. (217)
L Note that we replaced the partial derivatives by ordinary derivatives in the
onedimensional steady heat conduction case since the partial and ordinary
derivatives of a function are identical when the function depends on a single
variable only [T T(x) in this case].
·
G Heat Conduction Equation in a Long Cylinder
Now consider a thin cylindrical shell element of thickness r in a long cylinder, as shown in Figure 2–15. Assume the density of the cylinder is , the specific heat is C, and the length is L. The area of the cylinder normal to the
direction of heat transfer at any location is A 2 rL where r is the value of
the radius at that location. Note that the heat transfer area A depends on r in
this case, and thus it varies with location. An energy balance on this thin
cylindrical shell element during a small time interval t can be expressed as 0
·
Qr r
·
Qr + ∆r r + ∆r
r
Volume element FIGURE 2–15
Onedimensional heat conduction
through a volume element
in a long cylinder. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 70 70
HEAT TRANSFER Rate of heat
conduction
at r Rate of heat
generation
inside the
element Rate of heat
conduction
at r
r Rate of change
of the energy
content of the
element or
·
Qr ·
Qr Eelement
t ·
Gelement r (218) The change in the energy content of the element and the rate of heat generation within the element can be expressed as
Eelement
·
Gelement Et t Et mC(Tt
·
·
gVelement gA r Tt) t CA r(Tt t Tt) (219)
(220) Substituting into Eq. 2–18, we get
·
Qr ·
Qr ·
gA r r Tt CA r Tt t t (221) where A 2 rL. You may be tempted to express the area at the middle of the
element using the average radius as A 2 (r
r/2)L. But there is nothing
we can gain from this complication since later in the analysis we will take the
limit as r → 0 and thus the term r/2 will drop out. Now dividing the equation above by A r gives
·
1 Qr
A ·
Qr r r ·
g C Tt Tt t (222) t Taking the limit as r → 0 and t → 0 yields
T
1
kA
r
Ar ·
g T
t C (223) since, from the definition of the derivative and Fourier ’s law of heat
conduction,
lim r→0 ·
Qr ·
Qr r r ·
Q
r kA r T
r (224) Noting that the heat transfer area in this case is A
2 rL, the onedimensional transient heat conduction equation in a cylinder becomes
Variable conductivity: T
1
r r rk r ·
g C T
t (225) For the case of constant thermal conductivity, the equation above reduces to
Constant conductivity: T
1
rrrr ·
g
k 1T
t (226) cen58933_ch02.qxd 9/10/2002 8:46 AM Page 71 71
CHAPTER 2 where again the property
k/ C is the thermal diffusivity of the material.
Equation 2–26 reduces to the following forms under specified conditions
(Fig. 2–16):
(1) Steadystate:
( / t 0)
(2) Transient, no heat generation:
·
(g 0)
(3) Steadystate, no heat generation:
·
( / t 0 and g 0) ·
g
k dT
1d
r dr r dr
T
1
rrrr
dT
d
r
dr dr 0 (227) 1T
t (228) 0 (229) Note that we again replaced the partial derivatives by ordinary derivatives in
the onedimensional steady heat conduction case since the partial and ordinary
derivatives of a function are identical when the function depends on a single
variable only [T T(r) in this case]. Now consider a sphere with density , specific heat C, and outer radius R. The
area of the sphere normal to the direction of heat transfer at any location is
A 4 r2, where r is the value of the radius at that location. Note that the heat
transfer area A depends on r in this case also, and thus it varies with location.
By considering a thin spherical shell element of thickness r and repeating
the approach described above for the cylinder by using A 4 r2 instead of
A
2 rL, the onedimensional transient heat conduction equation for a
sphere is determined to be (Fig. 2–17)
T
1
r2 k
r
r2 r ·
g C T
t (230) which, in the case of constant thermal conductivity, reduces to
Constant conductivity: ·
g
k T
1
r2
r
r2 r 1T
t (231) where again the property
k/ C is the thermal diffusivity of the material.
It reduces to the following forms under specified conditions:
(1) Steadystate:
( / t 0)
(2) Transient,
no heat generation:
·
(g 0)
(3) Steadystate,
no heat generation:
·
( / t 0 and g 0) 1 d 2 dT
r
dr
r 2 dr ·
g
k T
1
r2
r
r2 r 1T
t d 2 dT
r
dr
dr 0 0 or (232) (233) r d 2T
dr 2 dT
d
r
dr dr 2 dT
dr 0 (234) where again we replaced the partial derivatives by ordinary derivatives in the
onedimensional steady heat conduction case. 0 (b) The equivalent alternative form
r d 2T
dr 2 dT
dr 0 FIGURE 2–16
Two equivalent forms of the
differential equation for the onedimensional steady heat conduction in
a cylinder with no heat generation.
·
G Heat Conduction Equation in a Sphere Variable conductivity: (a) The form that is ready to integrate ·
Qr + ∆r
·
Qr
r + ∆r
0 r Rr
Volume
element FIGURE 2–17
Onedimensional heat conduction
through a volume element in a sphere. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 72 72
HEAT TRANSFER Combined OneDimensional
Heat Conduction Equation
An examination of the onedimensional transient heat conduction equations
for the plane wall, cylinder, and sphere reveals that all three equations can be
expressed in a compact form as
T
1
rn k
rn r
r ·
g C T
t (235) where n 0 for a plane wall, n 1 for a cylinder, and n 2 for a sphere. In
the case of a plane wall, it is customary to replace the variable r by x. This
equation can be simplified for steadystate or no heat generation cases as
described before.
EXAMPLE 2–3 Heat Conduction through the Bottom of a Pan Consider a steel pan placed on top of an electric range to cook spaghetti (Fig.
2–18). The bottom section of the pan is L 0.4 cm thick and has a diameter
of D 18 cm. The electric heating unit on the range top consumes 800 W of
power during cooking, and 80 percent of the heat generated in the heating element is transferred uniformly to the pan. Assuming constant thermal conductivity, obtain the differential equation that describes the variation of the
temperature in the bottom section of the pan during steady operation.
800 W FIGURE 2–18
Schematic for Example 2–3. SOLUTION The bottom section of the pan has a large surface area relative to
its thickness and can be approximated as a large plane wall. Heat flux is applied to the bottom surface of the pan uniformly, and the conditions on the
inner surface are also uniform. Therefore, we expect the heat transfer through
the bottom section of the pan to be from the bottom surface toward the top,
and heat transfer in this case can reasonably be approximated as being onedimensional. Taking the direction normal to the bottom surface of the pan to be
the xaxis, we will have T T (x) during steady operation since the temperature
in this case will depend on x only.
The thermal conductivity is given to be constant, and there is no heat generation in the medium (within the bottom section of the pan). Therefore, the differential equation governing the variation of temperature in the bottom section
of the pan in this case is simply Eq. 2–17, d 2T
dx2 0 which is the steady onedimensional heat conduction equation in rectangular
coordinates under the conditions of constant thermal conductivity and no heat
generation. Note that the conditions at the surface of the medium have no effect on the differential equation. EXAMPLE 2–4 Heat Conduction in a Resistance Heater A 2kW resistance heater wire with thermal conductivity k
15 W/m · °C, diameter D 0.4 cm, and length L 50 cm is used to boil water by immersing cen58933_ch02.qxd 9/10/2002 8:46 AM Page 73 73
CHAPTER 2 it in water (Fig. 2–19). Assuming the variation of the thermal conductivity of the
wire with temperature to be negligible, obtain the differential equation that describes the variation of the temperature in the wire during steady operation. SOLUTION The resistance wire can be considered to be a very long cylinder
since its length is more than 100 times its diameter. Also, heat is generated
uniformly in the wire and the conditions on the outer surface of the wire are uniform. Therefore, it is reasonable to expect the temperature in the wire to vary in
the radial r direction only and thus the heat transfer to be onedimensional.
Then we will have T
T (r ) during steady operation since the temperature in
this case will depend on r only.
The rate of heat generation in the wire per unit volume can be determined
from ·
g ·
G
Vwire ·
G
( D2/4)L 2000 W
[ (0.004 m)2/4](0.5 cm) 0.318 Water
Resistance
heater FIGURE 2–19
Schematic for Example 2–4. 109 W/m3 Noting that the thermal conductivity is given to be constant, the differential
equation that governs the variation of temperature in the wire is simply
Eq. 2–27, dT
1d
r dr r dr ·
g
k 0 which is the steady onedimensional heat conduction equation in cylindrical coordinates for the case of constant thermal conductivity. Note again that the conditions at the surface of the wire have no effect on the differential equation. EXAMPLE 2–5 Cooling of a Hot Metal Ball in Air A spherical metal ball of radius R is heated in an oven to a temperature of
600°F throughout and is then taken out of the oven and allowed to cool in ambient air at T
75°F by convection and radiation (Fig. 2–20). The thermal
conductivity of the ball material is known to vary linearly with temperature. Assuming the ball is cooled uniformly from the entire outer surface, obtain the differential equation that describes the variation of the temperature in the ball
during cooling. SOLUTION The ball is initially at a uniform temperature and is cooled uniformly from the entire outer surface. Also, the temperature at any point in the
ball will change with time during cooling. Therefore, this is a onedimensional
transient heat conduction problem since the temperature within the ball will
change with the radial distance r and the time t. That is, T T (r, t ).
The thermal conductivity is given to be variable, and there is no heat generation in the ball. Therefore, the differential equation that governs the variation of
temperature in the ball in this case is obtained from Eq. 2–30 by setting the
heat generation term equal to zero. We obtain
T
1
r2 k
r
r2 r C T
t 75°F
Metal ball
600°F ·
Q FIGURE 2–20
Schematic for Example 2–5. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 74 74
HEAT TRANSFER which is the onedimensional transient heat conduction equation in spherical
coordinates under the conditions of variable thermal conductivity and no heat
generation. Note again that the conditions at the outer surface of the ball have
no effect on the differential equation. 2–3 GENERAL HEAT CONDUCTION EQUATION I In the last section we considered onedimensional heat conduction and
assumed heat conduction in other directions to be negligible. Most heat transfer problems encountered in practice can be approximated as being onedimensional, and we will mostly deal with such problems in this text.
However, this is not always the case, and sometimes we need to consider heat
transfer in other directions as well. In such cases heat conduction is said to be
multidimensional, and in this section we will develop the governing differential equation in such systems in rectangular, cylindrical, and spherical coordinate systems.
·
Qz + ∆ z Rectangular Coordinates
·
Qy + ∆y Volume element
·
Qx ∆z ·
g∆ x∆y∆ z ·
Qx + ∆ x ·
Qy z ∆y ∆x y
x ·
Qz Consider a small rectangular element of length x, width y, and height z,
as shown in Figure 2–21. Assume the density of the body is and the specific
heat is C. An energy balance on this element during a small time interval t
can be expressed as
Rate of heat
conduction at
x, y, and z y Rate of heat
conduction
at x
x,
y, and z Rate of heat
generation
inside the
element z Rate of change
of the energy
content of
the element or FIGURE 2–21
Threedimensional heat conduction
through a rectangular volume element. ·
Qx ·
Qy ·
Qz ·
Qx x ·
Qy ·
Qz y z Eelement
t ·
Gelement (236) Noting that the volume of the element is Velement
x y z, the change in the
energy content of the element and the rate of heat generation within the element can be expressed as
Eelement
·
Gelement Et t Et mC(Tt
·
·
gVelement g x y z Tt) t C x y z(Tt Tt) t Substituting into Eq. 2–36, we get
·
Qx ·
Qy ·
Qz ·
Qx x ·
Qy y ·
Qz y ·
Qy z ·
gxyz Cxyz Tt Tt t t Dividing by x y z gives
·
1 Qx
yz ·
Qx x x ·
1 Qy
xz y ·
1 Qz
xy ·
Qz z z ·
g C Tt Tt t t
(237) cen58933_ch02.qxd 9/10/2002 8:46 AM Page 75 75
CHAPTER 2 Noting that the heat transfer areas of the element for heat conduction in the
y z, Ay
x z, and Az
x y, respectively,
x, y, and z directions are Ax
and taking the limit as x, y, z and t → 0 yields
x k T
x y T
y k k z T
z ·
g T
t C (238) since, from the definition of the derivative and Fourier ’s law of heat
conduction,
lim x→0 lim ·
1 Qx
yz
·
1 Qy y→0 y
·
Qz z z 1
yzx kyz T
x x 1
xzy kxz T
y y Qz
1
xy z ·
Qy y ·
1 Qz
xy z→0 Qx
1
yz x
Qy
1
xz y x xz lim ·
Qx x 1
xyz kxy T
z z k T
x k T
y k T
z Equation 2–38 is the general heat conduction equation in rectangular coordinates. In the case of constant thermal conductivity, it reduces to
2 T
x2 2 T
y2 ·
g
k 2 T
z2 1T
t 2 T
x2 2 (1) Steadystate:
(called the Poisson equation)
(2) Transient, no heat generation:
(called the diffusion equation)
(3) Steadystate, no heat generation:
(called the Laplace equation) T
x2
2
T
x2
2
T
x2 2 T
y2
2
T
y2
2
T
y2 2 T
z2
2
T
z2
2
T
z2 ·
g
k 0 T
y2 2 2 2 T
x2 T
z2
T
z2 T
y2 0 1T
t T
z2 0 FIGURE 2–22
The threedimensional heat
conduction equations reduce to the
onedimensional ones when the
temperature varies in one
dimension only. (241) 0 2 ·
g
k 2 2 (240) 1T
t T
y2 T
x2 where the property
k/ C is again the thermal diffusivity of the material.
Equation 2–39 is known as the FourierBiot equation, and it reduces to these
forms under specified conditions: 2 2 (239) (242) Note that in the special case of onedimensional heat transfer in the
xdirection, the derivatives with respect to y and z drop out and the equations
above reduce to the ones developed in the previous section for a plane wall
(Fig. 2–22). z
dz r
dr z Cylindrical Coordinates
The general heat conduction equation in cylindrical coordinates can be obtained from an energy balance on a volume element in cylindrical coordinates,
shown in Figure 2–23, by following the steps just outlined. It can also be obtained directly from Eq. 2–38 by coordinate transformation using the following relations between the coordinates of a point in rectangular and cylindrical
coordinate systems:
x r cos , y r sin , and z z y
x φ dφ FIGURE 2–23
A differential volume element in
cylindrical coordinates. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 76 76
HEAT TRANSFER After lengthy manipulations, we obtain z T
1
r r kr r dr θ 1
r2 kr T
z k T
z ·
g C T
t (243) Spherical Coordinates r
dθ
dφ φ y x FIGURE 2–24
A differential volume element in
spherical coordinates. The general heat conduction equations in spherical coordinates can be obtained from an energy balance on a volume element in spherical coordinates,
shown in Figure 2–24, by following the steps outlined above. It can also be
obtained directly from Eq. 2–38 by coordinate transformation using the following relations between the coordinates of a point in rectangular and spherical coordinate systems:
x r cos sin , y r sin sin , and z cos Again after lengthy manipulations, we obtain
T
1
kr 2
r
r2 r 1
r 2 sin2 k T 1
r 2 sin k sin T ·
g C T
t
(244) Obtaining analytical solutions to these differential equations requires a
knowledge of the solution techniques of partial differential equations, which
is beyond the scope of this introductory text. Here we limit our consideration
to onedimensional steadystate cases or lumped systems, since they result in
ordinary differential equations.
Heat
loss 600°F Metal
billet EXAMPLE 2–6 T = 65°F z Heat Conduction in a Short Cylinder A short cylindrical metal billet of radius R and height h is heated in an oven to
a temperature of 600°F throughout and is then taken out of the oven and allowed to cool in ambient air at T
65°F by convection and radiation. Assuming the billet is cooled uniformly from all outer surfaces and the variation of the
thermal conductivity of the material with temperature is negligible, obtain the
differential equation that describes the variation of the temperature in the billet during this cooling process. r
R φ FIGURE 2–25
Schematic for Example 2–6. SOLUTION The billet shown in Figure 2–25 is initially at a uniform temperature and is cooled uniformly from the top and bottom surfaces in the zdirection
as well as the lateral surface in the radial rdirection. Also, the temperature at
any point in the ball will change with time during cooling. Therefore, this is a
twodimensional transient heat conduction problem since the temperature
within the billet will change with the radial and axial distances r and z and with
time t. That is, T T (r, z, t ).
The thermal conductivity is given to be constant, and there is no heat generation in the billet. Therefore, the differential equation that governs the variation
of temperature in the billet in this case is obtained from Eq. 2–43 by setting
the heat generation term and the derivatives with respect to equal to zero. We
obtain T
1
r r kr r z k T
z C T
t cen58933_ch02.qxd 9/10/2002 8:46 AM Page 77 77
CHAPTER 2 In the case of constant thermal conductivity, it reduces to T
1
rrrr 2 T
z2 1T
t which is the desired equation. I BOUNDARY AND INITIAL CONDITIONS The heat conduction equations above were developed using an energy balance
on a differential element inside the medium, and they remain the same regardless of the thermal conditions on the surfaces of the medium. That is, the
differential equations do not incorporate any information related to the conditions on the surfaces such as the surface temperature or a specified heat flux.
Yet we know that the heat flux and the temperature distribution in a medium
depend on the conditions at the surfaces, and the description of a heat transfer
problem in a medium is not complete without a full description of the thermal
conditions at the bounding surfaces of the medium. The mathematical expressions of the thermal conditions at the boundaries are called the boundary
conditions.
From a mathematical point of view, solving a differential equation is essentially a process of removing derivatives, or an integration process, and thus
the solution of a differential equation typically involves arbitrary constants
(Fig. 2–26). It follows that to obtain a unique solution to a problem, we need
to specify more than just the governing differential equation. We need to specify some conditions (such as the value of the function or its derivatives at
some value of the independent variable) so that forcing the solution to satisfy
these conditions at specified points will result in unique values for the arbitrary constants and thus a unique solution. But since the differential equation
has no place for the additional information or conditions, we need to supply
them separately in the form of boundary or initial conditions.
Consider the variation of temperature along the wall of a brick house in
winter. The temperature at any point in the wall depends on, among other
things, the conditions at the two surfaces of the wall such as the air temperature of the house, the velocity and direction of the winds, and the solar energy
incident on the outer surface. That is, the temperature distribution in a medium
depends on the conditions at the boundaries of the medium as well as the heat
transfer mechanism inside the medium. To describe a heat transfer problem
completely, two boundary conditions must be given for each direction of
the coordinate system along which heat transfer is significant (Fig. 2–27).
Therefore, we need to specify two boundary conditions for onedimensional
problems, four boundary conditions for twodimensional problems, and six
boundary conditions for threedimensional problems. In the case of the wall
of a house, for example, we need to specify the conditions at two locations
(the inner and the outer surfaces) of the wall since heat transfer in this case is
onedimensional. But in the case of a parallelepiped, we need to specify six
boundary conditions (one at each face) when heat transfer in all three dimensions is significant. The differential equation:
d 2T
dx2 0 General solution:
T(x) C1x C2 → → 2–4 Arbitrary constants
Some specific solutions:
T(x)
T(x)
T(x)
T(x) 2x 5
x 12
3
6.2x
M FIGURE 2–26
The general solution of a typical
differential equation involves
arbitrary constants, and thus an
infinite number of solutions. T Some solutions of
d 2T
–— = 0
dx 2 50°C 0 15°C
The only solution
L x that satisfies
the conditions
T(0) = 50°C
and T(L) = 15°C. FIGURE 2–27
To describe a heat transfer problem
completely, two boundary conditions
must be given for each direction along
which heat transfer is significant. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 78 78
HEAT TRANSFER The physical argument presented above is consistent with the mathematical
nature of the problem since the heat conduction equation is second order (i.e.,
involves second derivatives with respect to the space variables) in all directions along which heat conduction is significant, and the general solution of a
secondorder linear differential equation involves two arbitrary constants for
each direction. That is, the number of boundary conditions that needs to be
specified in a direction is equal to the order of the differential equation in that
direction.
Reconsider the brick wall already discussed. The temperature at any point
on the wall at a specified time also depends on the condition of the wall at the
beginning of the heat conduction process. Such a condition, which is usually
specified at time t 0, is called the initial condition, which is a mathematical expression for the temperature distribution of the medium initially. Note
that we need only one initial condition for a heat conduction problem regardless of the dimension since the conduction equation is first order in time (it involves the first derivative of temperature with respect to time).
In rectangular coordinates, the initial condition can be specified in the general form as
T(x, y, z, 0) f(x, y, z) (245) where the function f(x, y, z) represents the temperature distribution throughout
the medium at time t 0. When the medium is initially at a uniform temperature of Ti, the initial condition of Eq. 2–45 can be expressed as
T(x, y, z, 0) Ti. Note that under steady conditions, the heat conduction equation does not involve any time derivatives, and thus we do not need to specify
an initial condition.
The heat conduction equation is first order in time, and thus the initial condition cannot involve any derivatives (it is limited to a specified temperature).
However, the heat conduction equation is second order in space coordinates,
and thus a boundary condition may involve first derivatives at the boundaries
as well as specified values of temperature. Boundary conditions most commonly encountered in practice are the specified temperature, specified heat
flux, convection, and radiation boundary conditions. 150°C 70°C T(x, t) 1 Specified Temperature Boundary Condition
0 L x T(0, t) = 150°C
T(L, t) = 70°C FIGURE 2–28
Specified temperature boundary
conditions on both surfaces
of a plane wall. The temperature of an exposed surface can usually be measured directly and
easily. Therefore, one of the easiest ways to specify the thermal conditions on
a surface is to specify the temperature. For onedimensional heat transfer
through a plane wall of thickness L, for example, the specified temperature
boundary conditions can be expressed as (Fig. 2–28)
T(0, t)
T(L, t) T1
T2 (246) where T1 and T2 are the specified temperatures at surfaces at x 0 and x L,
respectively. The specified temperatures can be constant, which is the case for
steady heat conduction, or may vary with time. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 79 79
CHAPTER 2 2 Specified Heat Flux Boundary Condition
When there is sufficient information about energy interactions at a surface, it
·
may be possible to determine the rate of heat transfer and thus the heat flux q
2
(heat transfer rate per unit surface area, W/m ) on that surface, and this information can be used as one of the boundary conditions. The heat flux in the
positive xdirection anywhere in the medium, including the boundaries, can be
expressed by Fourier ’s law of heat conduction as
T
k
x ·
q Heat flux in the
positive xdirection (W/m2) (247) Then the boundary condition at a boundary is obtained by setting the specified
heat flux equal to k( T/ x) at that boundary. The sign of the specified heat
flux is determined by inspection: positive if the heat flux is in the positive direction of the coordinate axis, and negative if it is in the opposite direction.
Note that it is extremely important to have the correct sign for the specified
heat flux since the wrong sign will invert the direction of heat transfer and
cause the heat gain to be interpreted as heat loss (Fig. 2–29).
For a plate of thickness L subjected to heat flux of 50 W/m2 into the medium
from both sides, for example, the specified heat flux boundary conditions can
be expressed as
k T(0, t)
x 50 and Note that the heat flux at the surface at x
and thus it is 50 W/m2. k T(L, t)
x 50 Heat
flux Conduction ∂T(0, t)
q0 = – k ———
∂x Heat
flux Conduction ∂T(L, t)
– k ——— = qL
∂x
0 x L FIGURE 2–29
Specified heat flux
boundary conditions on both
surfaces of a plane wall. (248) L is in the negative xdirection, Special Case: Insulated Boundary
Some surfaces are commonly insulated in practice in order to minimize heat
loss (or heat gain) through them. Insulation reduces heat transfer but does not
totally eliminate it unless its thickness is infinity. However, heat transfer
through a properly insulated surface can be taken to be zero since adequate
insulation reduces heat transfer through a surface to negligible levels. Therefore, a wellinsulated surface can be modeled as a surface with a specified
heat flux of zero. Then the boundary condition on a perfectly insulated surface
(at x 0, for example) can be expressed as (Fig. 2–30)
k T(0, t)
x 0 or T(0, t)
x 0 (249) That is, on an insulated surface, the first derivative of temperature with respect to the space variable (the temperature gradient) in the direction normal
to the insulated surface is zero. This also means that the temperature function
must be perpendicular to an insulated surface since the slope of temperature at
the surface must be zero. Another Special Case: Thermal Symmetry
Some heat transfer problems possess thermal symmetry as a result of the
symmetry in imposed thermal conditions. For example, the two surfaces of a
large hot plate of thickness L suspended vertically in air will be subjected to Insulation 60°C T(x, t) 0 L x ∂T(0, t)
——— = 0
∂x
T(L, t) = 60°C FIGURE 2–30
A plane wall with insulation
and specified temperature
boundary conditions. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 80 80
HEAT TRANSFER
Center plane
Zero
slope
Temperature
distribution
(symmetric
about center
plane)
0 L
—
2
∂T(L /2, t)
———— = 0
∂x L the same thermal conditions, and thus the temperature distribution in one half
of the plate will be the same as that in the other half. That is, the heat transfer
problem in this plate will possess thermal symmetry about the center plane at
x
L/2. Also, the direction of heat flow at any point in the plate will be
toward the surface closer to the point, and there will be no heat flow across the
center plane. Therefore, the center plane can be viewed as an insulated surface, and the thermal condition at this plane of symmetry can be expressed as
(Fig. 2–31)
T(L/2, t)
x x FIGURE 2–31
Thermal symmetry boundary
condition at the center plane
of a plane wall. Water
110°C L
0
·
q0 FIGURE 2–32
Schematic for Example 2–7. (250) which resembles the insulation or zero heat flux boundary condition. This
result can also be deduced from a plot of temperature distribution with a
maximum, and thus zero slope, at the center plane.
In the case of cylindrical (or spherical) bodies having thermal symmetry
about the center line (or midpoint), the thermal symmetry boundary condition
requires that the first derivative of temperature with respect to r (the radial
variable) be zero at the centerline (or the midpoint). EXAMPLE 2–7 x 0 Heat Flux Boundary Condition Consider an aluminum pan used to cook beef stew on top of an electric range.
The bottom section of the pan is L 0.3 cm thick and has a diameter of D
20 cm. The electric heating unit on the range top consumes 800 W of power
during cooking, and 90 percent of the heat generated in the heating element is
transferred to the pan. During steady operation, the temperature of the inner
surface of the pan is measured to be 110°C. Express the boundary conditions
for the bottom section of the pan during this cooking process. SOLUTION The heat transfer through the bottom section of the pan is from the
bottom surface toward the top and can reasonably be approximated as being
onedimensional. We take the direction normal to the bottom surfaces of the
pan as the x axis with the origin at the outer surface, as shown in Figure 2–32.
Then the inner and outer surfaces of the bottom section of the pan can be represented by x
0 and x
L, respectively. During steady operation, the temperature will depend on x only and thus T T (x).
The boundary condition on the outer surface of the bottom of the pan at
x
0 can be approximated as being specified heat flux since it is stated that
90 percent of the 800 W (i.e., 720 W) is transferred to the pan at that surface.
Therefore,
k dT(0)
dx ·
q0 where q·0 Heat transfer rate
Bottom surface area 0.720 kW
(0.1 m)2 22.9 kW/m2 cen58933_ch02.qxd 9/10/2002 8:46 AM Page 81 81
CHAPTER 2 The temperature at the inner surface of the bottom of the pan is specified to be
110°C. Then the boundary condition on this surface can be expressed as T(L) 110°C where L
0.003 m. Note that the determination of the boundary conditions
may require some reasoning and approximations. 3 Convection Boundary Condition
Convection is probably the most common boundary condition encountered
in practice since most heat transfer surfaces are exposed to an environment at
a specified temperature. The convection boundary condition is based on a surface energy balance expressed as
Heat conduction
at the surface in a
selected direction Heat convection
at the surface in
the same direction For onedimensional heat transfer in the xdirection in a plate of thickness L,
the convection boundary conditions on both surfaces can be expressed as
T(0, t)
k
x h1[T 1 T(0, t)] Convection Conduction
h1[T (251a) 1 ∂T(0, t)
– T(0, t)] = – k ———
∂x Conduction Convection h1
T1 and
T(L, t)
k
x h2[T(L, t) T 2] (251b) where h1 and h2 are the convection heat transfer coefficients and T 1 and T 2
are the temperatures of the surrounding mediums on the two sides of the plate,
as shown in Figure 2–33.
In writing Eqs. 2–51 for convection boundary conditions, we have selected
the direction of heat transfer to be the positive xdirection at both surfaces. But
those expressions are equally applicable when heat transfer is in the opposite
direction at one or both surfaces since reversing the direction of heat transfer
at a surface simply reverses the signs of both conduction and convection terms
at that surface. This is equivalent to multiplying an equation by 1, which has
no effect on the equality (Fig. 2–34). Being able to select either direction as
the direction of heat transfer is certainly a relief since often we do not know
the surface temperature and thus the direction of heat transfer at a surface in
advance. This argument is also valid for other boundary conditions such as the
radiation and combined boundary conditions discussed shortly.
Note that a surface has zero thickness and thus no mass, and it cannot store
any energy. Therefore, the entire net heat entering the surface from one side
must leave the surface from the other side. The convection boundary condition simply states that heat continues to flow from a body to the surrounding
medium at the same rate, and it just changes vehicles at the surface from conduction to convection (or vice versa in the other direction). This is analogous
to people traveling on buses on land and transferring to the ships at the shore. h2
T2 ∂T(L, t)
– k ——— = h2[T(L, t) – T 2]
∂x
0 L x FIGURE 2–33
Convection boundary conditions on
the two surfaces of a plane wall. Convection Conduction
h1[T 1 ∂T(0, t)
– T(0, t)] = – k ———
∂x h1, T 1 Convection Conduction
∂T(0, t)
h1[T(0, t) – T 1] = k ———
∂x
0 L x FIGURE 2–34
The assumed direction of heat transfer
at a boundary has no effect on the
boundary condition expression. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 82 82
HEAT TRANSFER If the passengers are not allowed to wander around at the shore, then the rate
at which the people are unloaded at the shore from the buses must equal the
rate at which they board the ships. We may call this the conservation of “people” principle.
Also note that the surface temperatures T(0, t) and T(L, t) are not known
(if they were known, we would simply use them as the specified temperature
boundary condition and not bother with convection). But a surface temperature can be determined once the solution T(x, t) is obtained by substituting the
value of x at that surface into the solution.
EXAMPLE 2–8 Steam flows through a pipe shown in Figure 2–35 at an average temperature of
T
200°C. The inner and outer radii of the pipe are r1
8 cm and r2
8.5 cm, respectively, and the outer surface of the pipe is heavily insulated. If
the convection heat transfer coefficient on the inner surface of the pipe is
h 65 W/m2 · °C, express the boundary conditions on the inner and outer surfaces of the pipe during transient periods. Insulation h1
T Convection and Insulation Boundary Conditions r2
r1
SOLUTION During initial transient periods, heat transfer through the pipe maFIGURE 2–35
Schematic for Example 2–8. terial will predominantly be in the radial direction, and thus can be approximated as being onedimensional. Then the temperature within the pipe material
will change with the radial distance r and the time t. That is, T T (r, t ).
It is stated that heat transfer between the steam and the pipe at the inner
surface is by convection. Then taking the direction of heat transfer to be the
positive r direction, the boundary condition on that surface can be expressed as k T(r1, t)
r h[T T(r1)] The pipe is said to be well insulated on the outside, and thus heat loss through
the outer surface of the pipe can be assumed to be negligible. Then the boundary condition at the outer surface can be expressed as T(r2, t)
r 0 That is, the temperature gradient must be zero on the outer surface of the pipe
at all times. 4 Radiation Boundary Condition
In some cases, such as those encountered in space and cryogenic applications,
a heat transfer surface is surrounded by an evacuated space and thus there is
no convection heat transfer between a surface and the surrounding medium. In
such cases, radiation becomes the only mechanism of heat transfer between
the surface under consideration and the surroundings. Using an energy balance, the radiation boundary condition on a surface can be expressed as
Heat conduction
at the surface in a
selected direction Radiation exchange
at the surface in
the same direction cen58933_ch02.qxd 9/10/2002 8:46 AM Page 83 83
CHAPTER 2 For onedimensional heat transfer in the xdirection in a plate of thickness L,
the radiation boundary conditions on both surfaces can be expressed as
(Fig. 2–36)
T(0, t)
k
x 1 4
[T surr, 1 T(0, t)4] (252a) Radiation Conduction
∂T(0, t)
4
ε1σ [Tsurr, 1 – T(0, t)4] = – k ———
∂x
ε1
Tsurr, 1 and ε2
Tsurr, 2 Conduction Radiation k T(L, t)
x 2 [T(L, t)4 4
T surr, 2] ∂T(L, t)
4
– k ——— = ε 2σ [T(L, t)4 – Tsurr, 2]
∂x (252b) 0 where 1 and 2 are the emissivities of the boundary surfaces,
5.67
10 8 W/m2 · K4 is the Stefan–Boltzmann constant, and Tsurr, 1 and Tsurr, 2 are the
average temperatures of the surfaces surrounding the two sides of the plate,
respectively. Note that the temperatures in radiation calculations must be expressed in K or R (not in °C or °F).
The radiation boundary condition involves the fourth power of temperature,
and thus it is a nonlinear condition. As a result, the application of this boundary condition results in powers of the unknown coefficients, which makes it
difficult to determine them. Therefore, it is tempting to ignore radiation exchange at a surface during a heat transfer analysis in order to avoid the complications associated with nonlinearity. This is especially the case when heat
transfer at the surface is dominated by convection, and the role of radiation is
minor. L x FIGURE 2–36
Radiation boundary conditions on
both surfaces of a plane wall. Interface
Material
A 5 Interface Boundary Conditions TA(x0, t) = TB(x0, t) Some bodies are made up of layers of different materials, and the solution of
a heat transfer problem in such a medium requires the solution of the heat
transfer problem in each layer. This, in turn, requires the specification of the
boundary conditions at each interface.
The boundary conditions at an interface are based on the requirements that
(1) two bodies in contact must have the same temperature at the area of contact and (2) an interface (which is a surface) cannot store any energy, and thus
the heat flux on the two sides of an interface must be the same. The boundary
conditions at the interface of two bodies A and B in perfect contact at x x0
can be expressed as (Fig. 2–37)
TA(x0, t) TB(x0, t) (253) and
kA TA(x0, t)
x kB TB(x0, t)
x Material
B (254) where kA and kB are the thermal conductivities of the layers A and B, respectively. The case of imperfect contact results in thermal contact resistance,
which is considered in the next chapter. TA(x, t) TB(x, t) Conduction Conduction 0 ∂TA(x0, t)
∂TB(x0, t)
– kA ———— = – kB ————
∂x
∂x
x0 L x FIGURE 2–37
Boundary conditions at the interface
of two bodies in perfect contact. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 84 84
HEAT TRANSFER 6 Generalized Boundary Conditions
So far we have considered surfaces subjected to single mode heat transfer,
such as the specified heat flux, convection, or radiation for simplicity. In general, however, a surface may involve convection, radiation, and specified heat
flux simultaneously. The boundary condition in such cases is again obtained
from a surface energy balance, expressed as
Heat transfer
to the surface
in all modes Heat transfer
from the surface
in all modes (255) This is illustrated in Examples 2–9 and 2–10.
EXAMPLE 2–9 Radia tion Tsurr = 525 R n Convection o
cti u nd Metal
ball T = 78°F Co 0 r0 Ti = 600°F FIGURE 2–38
Schematic for Example 2–9. Combined Convection and Radiation Condition A spherical metal ball of radius r0 is heated in an oven to a temperature of
600°F throughout and is then taken out of the oven and allowed to cool in ambient air at T
78°F, as shown in Figure 2–38. The thermal conductivity of
the ball material is k
8.3 Btu/h · ft · °F, and the average convection heat
transfer coefficient on the outer surface of the ball is evaluated to be h
4.5
Btu/h · ft2 · °F. The emissivity of the outer surface of the ball is
0.6, and the
average temperature of the surrounding surfaces is Tsurr 525 R. Assuming the
ball is cooled uniformly from the entire outer surface, express the initial and
boundary conditions for the cooling process of the ball. r SOLUTION The ball is initially at a uniform temperature and is cooled uniformly from the entire outer surface. Therefore, this is a onedimensional transient heat transfer problem since the temperature within the ball will change
with the radial distance r and the time t. That is, T
T (r, t ). Taking the moment the ball is removed from the oven to be t 0, the initial condition can be
expressed as
T(r, 0) Ti 600°F The problem possesses symmetry about the midpoint (r
0) since the
isotherms in this case will be concentric spheres, and thus no heat will be
crossing the midpoint of the ball. Then the boundary condition at the midpoint
can be expressed as T(0, t)
r 0 The heat conducted to the outer surface of the ball is lost to the environment by
convection and radiation. Then taking the direction of heat transfer to be the
positive r direction, the boundary condition on the outer surface can be expressed as k T(r0, t)
r h[T(r0) T] [T(r0)4 4
T surr] cen58933_ch02.qxd 9/10/2002 8:46 AM Page 85 85
CHAPTER 2 All the quantities in the above relations are known except the temperatures
and their derivatives at r
0 and r0. Also, the radiation part of the boundary
condition is often ignored for simplicity by modifying the convection heat transfer coefficient to account for the contribution of radiation. The convection coefficient h in that case becomes the combined heat transfer coefficient. EXAMPLE 2–10 Combined Convection, Radiation, and Heat Flux Consider the south wall of a house that is L 0.2 m thick. The outer surface of
the wall is exposed to solar radiation and has an absorptivity of
0.5 for solar energy. The interior of the house is maintained at T 1 20°C, while the ambient air temperature outside remains at T 2
5°C. The sky, the ground, and
the surfaces of the surrounding structures at this location can be modeled as a
surface at an effective temperature of Tsky
255 K for radiation exchange on
the outer surface. The radiation exchange between the inner surface of the wall
and the surfaces of the walls, floor, and ceiling it faces is negligible. The convection heat transfer coefficients on the inner and the outer surfaces of the wall
are h1
6 W/m2 · °C and h2
25 W/m2 · °C, respectively. The thermal conductivity of the wall material is k
0.7 W/m · °C, and the emissivity of the
outer surface is 2
0.9. Assuming the heat transfer through the wall to be
steady and onedimensional, express the boundary conditions on the inner and
the outer surfaces of the wall. SOLUTION We take the direction normal to the wall surfaces as the xaxis with
the origin at the inner surface of the wall, as shown in Figure 2–39. The heat
transfer through the wall is given to be steady and onedimensional, and thus
the temperature depends on x only and not on time. That is, T T (x).
The boundary condition on the inner surface of the wall at x 0 is a typical
convection condition since it does not involve any radiation or specified heat
flux. Taking the direction of heat transfer to be the positive xdirection, the
boundary condition on the inner surface can be expressed as dT(0)
k
dx h1[T 1 Tsky Sun
South
wall Inner
surface R Conduction T(0)] The boundary condition on the outer surface at x 0 is quite general as it involves conduction, convection, radiation, and specified heat flux. Again taking
the direction of heat transfer to be the positive xdirection, the boundary condition on the outer surface can be expressed as Solar
Co h1
T1 nv
ec tio h2[T(L) T 2] 2 4 [T(L) 4
T sky] q·solar ·
where q solar is the incident solar heat flux. Assuming the opposite direction for
heat transfer would give the same result multiplied by 1, which is equivalent
to the relation here. All the quantities in these relations are known except the
temperatures and their derivatives at the two boundaries. n h2
T2
Convection Conduction dT(L)
k
dx n tio ia
ad 0 Outer
surface
L x FIGURE 2–39
Schematic for Example 2–10. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 86 86
HEAT TRANSFER Note that a heat transfer problem may involve different kinds of boundary
conditions on different surfaces. For example, a plate may be subject to heat
flux on one surface while losing or gaining heat by convection from the other
surface. Also, the two boundary conditions in a direction may be specified at
the same boundary, while no condition is imposed on the other boundary. For
example, specifying the temperature and heat flux at x 0 of a plate of thickness L will result in a unique solution for the onedimensional steady temperature distribution in the plate, including the value of temperature at the surface
x L. Although not necessary, there is nothing wrong with specifying more
than two boundary conditions in a specified direction, provided that there is
no contradiction. The extra conditions in this case can be used to verify the
results. 2–5 Heat transfer problem
Mathematical formulation
(Differential equation and
boundary conditions)
General solution of differential equation
Application of boundary conditions
Solution of the problem FIGURE 2–40
Basic steps involved in the solution of
heat transfer problems. I SOLUTION OF STEADY ONEDIMENSIONAL
HEAT CONDUCTION PROBLEMS So far we have derived the differential equations for heat conduction in
various coordinate systems and discussed the possible boundary conditions.
A heat conduction problem can be formulated by specifying the applicable
differential equation and a set of proper boundary conditions.
In this section we will solve a wide range of heat conduction problems in
rectangular, cylindrical, and spherical geometries. We will limit our attention
to problems that result in ordinary differential equations such as the steady
onedimensional heat conduction problems. We will also assume constant
thermal conductivity, but will consider variable conductivity later in this chapter. If you feel rusty on differential equations or haven’t taken differential
equations yet, no need to panic. Simple integration is all you need to solve the
steady onedimensional heat conduction problems.
The solution procedure for solving heat conduction problems can be summarized as (1) formulate the problem by obtaining the applicable differential
equation in its simplest form and specifying the boundary conditions, (2) obtain the general solution of the differential equation, and (3) apply the boundary conditions and determine the arbitrary constants in the general solution
(Fig. 2–40). This is demonstrated below with examples. EXAMPLE 2–11
Plane
wall
T1 T2 0 L x FIGURE 2–41
Schematic for Example 2–11. Heat Conduction in a Plane Wall Consider a large plane wall of thickness L
0.2 m, thermal conductivity k
1.2 W/m · °C, and surface area A 15 m2. The two sides of the wall are maintained at constant temperatures of T1 120°C and T2 50°C, respectively, as
shown in Figure 2–41. Determine (a) the variation of temperature within the
wall and the value of temperature at x
0.1 m and (b) the rate of heat conduction through the wall under steady conditions. SOLUTION A plane wall with specified surface temperatures is given. The variation of temperature and the rate of heat transfer are to be determined.
Assumptions 1 Heat conduction is steady. 2 Heat conduction is onedimensional since the wall is large relative to its thickness and the thermal cen58933_ch02.qxd 9/10/2002 8:46 AM Page 87 87
CHAPTER 2 conditions on both sides are uniform. 3 Thermal conductivity is constant.
4 There is no heat generation.
Properties The thermal conductivity is given to be k 1.2 W/m · °C. Analysis (a) Taking the direction normal to the surface of the wall to be the
xdirection, the differential equation for this problem can be expressed as d 2T
dx2 0
Differential equation: with boundary conditions T1
T2 d 2T
dx2 120°C
50°C C1 where C1 is an arbitrary constant. Notice that the order of the derivative went
down by one as a result of integration. As a check, if we take the derivative of
this equation, we will obtain the original differential equation. This equation is
not the solution yet since it involves a derivative.
Integrating one more time, we obtain T(x) C1x C1 0 C2 → C2 C1 Integrate again:
T(x) → General C1x constants FIGURE 2–42
Obtaining the general solution of a
simple second order differential
equation by integration. Boundary condition:
T(0) T1 C2 → Arbitrary solution C2 which is the general solution of the differential equation (Fig. 2–42). The general solution in this case resembles the general formula of a straight line whose
slope is C1 and whose value at x 0 is C2. This is not surprising since the second derivative represents the change in the slope of a function, and a zero second derivative indicates that the slope of the function remains constant.
Therefore, any straight line is a solution of this differential equation.
The general solution contains two unknown constants C1 and C2, and thus we
need two equations to determine them uniquely and obtain the specific solution. These equations are obtained by forcing the general solution to satisfy the
specified boundary conditions. The application of each condition yields one
equation, and thus we need to specify two conditions to determine the constants C1 and C2.
When applying a boundary condition to an equation, all occurrences of the
dependent and independent variables and any derivatives are replaced by the
specified values. Thus the only unknowns in the resulting equations are the arbitrary constants.
The first boundary condition can be interpreted as in the general solution, replace all the x’s by zero and T (x ) by T1. That is (Fig. 2–43), T(0) dT
dx Integrate: The differential equation is linear and second order, and a quick inspection of
it reveals that it has a single term involving derivatives and no terms involving
the unknown function T as a factor. Thus, it can be solved by direct integration.
Noting that an integration reduces the order of a derivative by one, the general
solution of the differential equation above can be obtained by two simple successive integrations, each of which introduces an integration constant.
Integrating the differential equation once with respect to x yields dT
dx 0 → T(0)
T(L) T1 General solution:
T(x) C1x C2 Applying the boundary condition:
T(x)
↑
0
{ C1x
↑
0 C2 C2 → C2 T1
Substituting:
T1 C1 0 T1 It cannot involve x or T(x) after the
boundary condition is applied. FIGURE 2–43
When applying a boundary condition
to the general solution at a specified
point, all occurrences of the dependent
and independent variables should be
replaced by their specified values
at that point. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 88 88
HEAT TRANSFER The second boundary condition can be interpreted as in the general solution, replace all the x’s by L and T (x ) by T2. That is, T(L) C1L → T2 C2 C1L T2 → C1 T1 T1
L Substituting the C1 and C2 expressions into the general solution, we obtain T(x) T2 T1
L x T1 (256) which is the desired solution since it satisfies not only the differential equation
but also the two specified boundary conditions. That is, differentiating Eq.
2–56 with respect to x twice will give d 2T /dx 2, which is the given differential
equation, and substituting x 0 and x L into Eq. 2–56 gives T (0) T1 and
T (L)
T2, respectively, which are the specified conditions at the boundaries.
Substituting the given information, the value of the temperature at x 0.1 m
is determined to be (50 T(0.1 m) 120)°C
(0.1 m)
0.2 m 120°C 85°C (b) The rate of heat conduction anywhere in the wall is determined from
Fourier’s law to be ·
Q wall kA dT
dx kAC1 kA T2 T1
L kA T1 T2 (257) L The numerical value of the rate of heat conduction through the wall is determined by substituting the given values to be ·
Q kA T2 T1
L (1.2 W/m · °C)(15 m2) (120 50)°C
0.2 m 6300 W Discussion Note that under steady conditions, the rate of heat conduction
through a plane wall is constant. EXAMPLE 2–12 A Wall with Various Sets of Boundary Conditions Consider steady onedimensional heat conduction in a large plane wall of thickness L and constant thermal conductivity k with no heat generation. Obtain expressions for the variation of temperature within the wall for the following pairs
of boundary conditions (Fig. 2–44): (a)
(b)
(c) dT(0)
dx
dT(0)
k
dx
dT(0)
k
dx
k q·0 40 W/cm2 and ·
q0 40 W/cm2 and ·
q0 40 W/cm2 and T(0) T0 dT(L)
dx
dT(L)
k
dx
k 15°C
q·L
q·0 25 W/cm2
40 W/cm2 cen58933_ch02.qxd 9/10/2002 8:46 AM Page 89 89
CHAPTER 2
15°C
Plane
wall
40 W/cm2 Plane
wall
40 W/cm2 T(x) Plane
wall
40 W/cm2 T(x) T(x) 25 W/cm2 0 0 x L (a) L 0 x (b) 40 W/cm2 L x (c) FIGURE 2–44
Schematic for Example 2–12. SOLUTION This is a steady onedimensional heat conduction problem with
constant thermal conductivity and no heat generation in the medium, and the
heat conduction equation in this case can be expressed as (Eq. 2–17)
d 2T
dx2 0 whose general solution was determined in the previous example by direct integration to be T(x) C1x C2 where C1 and C2 are two arbitrary integration constants. The specific solutions
corresponding to each specified pair of boundary conditions are determined as
follows.
(a) In this case, both boundary conditions are specified at the same boundary
at x 0, and no boundary condition is specified at the other boundary at x L.
Noting that dT
dx C1 the application of the boundary conditions gives k dT(0)
dx ·
q0 → q·0 kC1 → C1 ·
q0
k and T(0) T0 → T0 C1 0 C2 → C2 T0 Substituting, the specific solution in this case is determined to be T(x) ·
q0
k T0 Therefore, the two boundary conditions can be specified at the same boundary,
and it is not necessary to specify them at different locations. In fact, the fundamental theorem of linear ordinary differential equations guarantees that a cen58933_ch02.qxd 9/10/2002 8:46 AM Page 90 90
HEAT TRANSFER unique solution exists when both conditions are specified at the same location.
But no such guarantee exists when the two conditions are specified at different
boundaries, as you will see below. Differential equation:
T (x) 0 (b) In this case different heat fluxes are specified at the two boundaries. The
application of the boundary conditions gives General solution:
T(x) C1x C2 (a) Unique solution:
kT (0) q·0 T(x)
T(0) T0 ·
q0
x
k k dT(0)
dx ·
q0 → kC1 q·0 → C1 ·
q0
k k dT(L)
dx ·
qL → kC1 q·L → C1 ·
qL
k T0 and (b) No solution:
kT (0)
kT (L) q·0
q·L T(x) (c) Multiple solutions:
·
kT (0) q 0 T(x)
·
kT (L) q 0 None
·
q0
x
k C2
↑
Arbitrary FIGURE 2–45
A boundaryvalue problem may have a
unique solution, infinitely many
solutions, or no solutions at all. ·
·
Since q 0
q L and the constant C1 cannot be equal to two different things at
the same time, there is no solution in this case. This is not surprising since this
case corresponds to supplying heat to the plane wall from both sides and expecting the temperature of the wall to remain steady (not to change with time).
This is impossible.
(c) In this case, the same values for heat flux are specified at the two boundaries. The application of the boundary conditions gives k dT(0)
dx q·0 → kC1 q·0 → C1 ·
q0
k k dT(L)
dx ·
q0 → kC1 q·0 → C1 ·
q0
k and Thus, both conditions result in the same value for the constant C1, but no value
for C2. Substituting, the specific solution in this case is determined to be T(x) ·
q0
x
k C2 T = 20°C Resistance heater
1200 W which is not a unique solution since C2 is arbitrary. This solution represents a
·
family of straight lines whose slope is q 0/k. Physically, this problem corresponds to requiring the rate of heat supplied to the wall at x 0 be equal to the
rate of heat removal from the other side of the wall at x L. But this is a consequence of the heat conduction through the wall being steady, and thus the
second boundary condition does not provide any new information. So it is not
surprising that the solution of this problem is not unique. The three cases discussed above are summarized in Figure 2–45. x EXAMPLE 2–13 Base plate Insulation
300 cm2 h L FIGURE 2–46
Schematic for Example 2–13. Heat Conduction in the Base Plate of an Iron Consider the base plate of a 1200W household iron that has a thickness of
L
0.5 cm, base area of A
300 cm2, and thermal conductivity of k
15 W/m · °C. The inner surface of the base plate is subjected to uniform heat
flux generated by the resistance heaters inside, and the outer surface loses
heat to the surroundings at T
20°C by convection, as shown in Figure 2–46. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 91 91
CHAPTER 2 Taking the convection heat transfer coefficient to be h
80 W/m2 · °C and
disregarding heat loss by radiation, obtain an expression for the variation of
temperature in the base plate, and evaluate the temperatures at the inner and
the outer surfaces. SOLUTION The base plate of an iron is considered. The variation of temperature in the plate and the surface temperatures are to be determined.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since the surface area of the base plate is
large relative to its thickness, and the thermal conditions on both sides are uniform. 3 Thermal conductivity is constant. 4 There is no heat generation in the
medium. 5 Heat transfer by radiation is negligible. 6 The upper part of the iron
is well insulated so that the entire heat generated in the resistance wires is
transferred to the base plate through its inner surface.
Properties The thermal conductivity is given to be k 15 W/m · °C.
Analysis The inner surface of the base plate is subjected to uniform heat flux
at a rate of
·
q0 ·
Q0
Abase 1200 W
0.03 m2 0 with the boundary conditions k dT(0)
dx
k ·
q0 dT(L)
dx 40,000 W/m2
h[T(L) T] The general solution of the differential equation is again obtained by two successive integrations to be dT
dx C1 and T(x) C1x C2 (a) where C1 and C2 are arbitrary constants. Applying the first boundary condition, dT(0)
dx ·
q0 → kC1 Noting that dT /dx
C1 and T (L)
boundary condition gives C1L k Heat flux Conduction
h
T
dT(0)
· = – k –——
q0
dx
Conduction Convection 40,000 W/m2 The outer side of the plate is subjected to the convection condition. Taking the
direction normal to the surface of the wall as the xdirection with its origin on
the inner surface, the differential equation for this problem can be expressed as
(Fig. 2–47) d 2T
dx2 Base plate q·0 → C1 ·
q0
k C2, the application of the second dT(L)
– k –—— = h[T(L) – T ]
dx
0 L x FIGURE 2–47
The boundary conditions on the base
plate of the iron discussed
in Example 2–13. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 92 92
HEAT TRANSFER k dT(L)
dx h[T(L) T] → kC1 h[(C1L C2) T] ·
q 0/k and solving for C2, we obtain Substituting C1 C2 T ·
q0
L
k ·
q0
h Now substituting C1 and C2 into the general solution (a) gives T(x) T L
q·0 x
k 1
h (b) which is the solution for the variation of the temperature in the plate. The temperatures at the inner and outer surfaces of the plate are determined by substituting x 0 and x L, respectively, into the relation (b): T(0) T
20°C L
q·0
k 1
h (40,000 W/m2) 0.005 m
15 W/m · °C 1
80 W/m2 · °C 533°C and T(L) T q·0 0 1
h 20°C 40,000 W/m2
80 W/m2 · °C 520°C Discussion Note that the temperature of the inner surface of the base plate
will be 13°C higher than the temperature of the outer surface when steady operating conditions are reached. Also note that this heat transfer analysis enables
us to calculate the temperatures of surfaces that we cannot even reach. This example demonstrates how the heat flux and convection boundary conditions are
applied to heat transfer problems. EXAMPLE 2–14 So
l Plane wall ar Sun Conduction tio a
di Ra
n T1 ε
α
0 L Space
x FIGURE 2–48
Schematic for Example 2–14. Heat Conduction in a Solar Heated Wall Consider a large plane wall of thickness L 0.06 m and thermal conductivity
k
1.2 W/m · °C in space. The wall is covered with white porcelain tiles that
have an emissivity of
0.85 and a solar absorptivity of
0.26, as shown
in Figure 2–48. The inner surface of the wall is maintained at T1 300 K at all
times, while the outer surface is exposed to solar radiation that is incident at a
·
rate of q solar 800 W/m2. The outer surface is also losing heat by radiation to
deep space at 0 K. Determine the temperature of the outer surface of the wall
and the rate of heat transfer through the wall when steady operating conditions
are reached. What would your response be if no solar radiation was incident on
the surface? SOLUTION A plane wall in space is subjected to specified temperature on one
side and solar radiation on the other side. The outer surface temperature and
the rate of heat transfer are to be determined. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 93 93
CHAPTER 2 Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since the wall is large relative to its
thickness, and the thermal conditions on both sides are uniform. 3 Thermal
conductivity is constant. 4 There is no heat generation.
Properties The thermal conductivity is given to be k 1.2 W/m · °C.
Analysis Taking the direction normal to the surface of the wall as the
xdirection with its origin on the inner surface, the differential equation for this
problem can be expressed as d 2T
dx2 0 with boundary conditions T(0)
dT(L)
k
dx T1 300 K [T(L)4 q·solar 4
T space] where Tspace
0. The general solution of the differential equation is again obtained by two successive integrations to be T(x) C1x C2 (a) where C1 and C2 are arbitrary constants. Applying the first boundary condition
yields T(0) C1 0 → C2 C2 Noting that dT /dx
C1 and T (L)
C1L
the second boundary conditions gives k dT(L)
dx ·
qsolar T(L)4 C2 → T1 C1L kC1 T1, the application of (C1L T1)4 q·solar Although C1 is the only unknown in this equation, we cannot get an explicit expression for it because the equation is nonlinear, and thus we cannot get a
closedform expression for the temperature distribution. This should explain
why we do our best to avoid nonlinearities in the analysis, such as those associated with radiation.
Let us back up a little and denote the outer surface temperature by T (L) TL
instead of T (L) C1L T1. The application of the second boundary condition
in this case gives k dT(L)
dx T(L)4 ·
q solar → kC1 4
TL q·solar Solving for C1 gives C1 ·
q solar 4
TL (b) k Now substituting C1 and C2 into the general solution (a), we obtain T(x) ·
q solar 4
TL k x T1 (c) cen58933_ch02.qxd 9/10/2002 8:46 AM Page 94 94
HEAT TRANSFER (1) Rearrange the equation to be solved:
TL 310.4 0.240975 TL
100 4 The equation is in the proper form since the
left side consists of TL only.
(2) Guess the value of TL, say 300 K, and
substitute into the right side of the equation.
It gives
TL 290.2 K (3) Now substitute this value of TL into the
right side of the equation and get
TL TL ·
q solar TL 4
TL k L T1 292.6 K TL 0.26 (800 W/m2) 0.85 (5.67
1.2 W/m · K 8 10 4
W/m2 · K4) TL FIGURE 2–49
A simple method of solving a
nonlinear equation is to arrange the
equation such that the unknown is
alone on the left side while everything
else is on the right side, and to iterate
after an initial guess until
convergence. (0.06 m) 300 K which simplifies to TL 310.4 0.240975 TL
100 4 This equation can be solved by one of the several nonlinear equation solvers
available (or by the old fashioned trialanderror method) to give (Fig. 2–49) 292.7 K
292.7 K Therefore, the solution is TL 292.7 K. The
result is independent of the initial guess. (d ) which is an implicit relation for the outer surface temperature TL. Substituting
the given values, we get 293.1 K (4) Repeat step (3) until convergence to
desired accuracy is achieved. The
subsequent iterations give
TL
TL which is the solution for the variation of the temperature in the wall in terms of
the unknown outer surface temperature TL. At x L it becomes TL 292.7 K Knowing the outer surface temperature and knowing that it must remain constant under steady conditions, the temperature distribution in the wall can be
determined by substituting the TL value above into Eq. (c):
T(x) 0.26 (800 W/m 2) 0 .85
(5.67
10
1.2 W/m · K 8 W /m 2 · K 4)(292.7 K)4 x 300 K which simplifies to T(x) ( 121.5 K/m)x 300 K Note that the outer surface temperature turned out to be lower than the inner
surface temperature. Therefore, the heat transfer through the wall will be toward
the outside despite the absorption of solar radiation by the outer surface. Knowing both the inner and outer surface temperatures of the wall, the steady rate of
heat conduction through the wall can be determined from q· k T0 TL
L (1.2 W/m · K) (300 292.7) K
0.06 m 146 W/m2 Discussion In the case of no incident solar radiation, the outer surface tem·
perature, determined from Eq. (d ) by setting q solar 0, will be TL 284.3 K. It
is interesting to note that the solar energy incident on the surface causes the
surface temperature to increase by about 8 K only when the inner surface temperature of the wall is maintained at 300 K. L T2
T1
0 r1 EXAMPLE 2–15 r2
r FIGURE 2–50
Schematic for Example 2–15. Heat Loss through a Steam Pipe Consider a steam pipe of length L 20 m, inner radius r1 6 cm, outer radius
r2
8 cm, and thermal conductivity k
20 W/m · °C, as shown in Figure
2–50. The inner and outer surfaces of the pipe are maintained at average temperatures of T1 150°C and T2 60°C, respectively. Obtain a general relation cen58933_ch02.qxd 9/10/2002 8:46 AM Page 95 95
CHAPTER 2 for the temperature distribution inside the pipe under steady conditions, and
determine the rate of heat loss from the steam through the pipe. SOLUTION A steam pipe is subjected to specified temperatures on its
surfaces. The variation of temperature and the rate of heat transfer are to be
determined.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since there is thermal symmetry about the
centerline and no variation in the axial direction, and thus T T (r ). 3 Thermal
conductivity is constant. 4 There is no heat generation.
Properties The thermal conductivity is given to be k 20 W/m · °C.
Analysis The mathematical formulation of this problem can be expressed as
dT
d
r
dr dr 0 with boundary conditions T(r1)
T(r2) T1
T2 150°C
60°C Integrating the differential equation once with respect to r gives r dT
dr C1 where C1 is an arbitrary constant. We now divide both sides of this equation by
r to bring it to a readily integrable form, dT
dr C1
r Again integrating with respect to r gives (Fig. 2–51) T(r) C1 ln r Differential equation: C2 We now apply both boundary conditions by replacing all occurrences of r and
T (r ) in Eq. (a) with the specified values at the boundaries. We get T(r1)
T(r2) T1
T2 → C1 ln r1
→ C1 ln r2 C2
C2 T1
T2 T2 T1
ln(r2/r1) and C2 T1 ln(r/r1)
(T2
ln(r2/r1 T1) T1 dT
dr C1 0):
dT
dr C1
r Integrate again:
C1 ln r C2 which is the general solution. Substituting them into Eq. (a) and rearranging, the variation of temperature
within the pipe is determined to be T(r) r T(r) T2 T1
ln r1
ln(r2/r1) 0 Integrate: Divide by r (r which are two equations in two unknowns, C1 and C2. Solving them simultaneously gives C1 dT
d
r
dr dr (a) (258) The rate of heat loss from the steam is simply the total rate of heat conduction
through the pipe, and is determined from Fourier’s law to be FIGURE 2–51
Basic steps involved in the solution
of the steady onedimensional
heat conduction equation in
cylindrical coordinates. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 96 96
HEAT TRANSFER ·
Q cylinder kA dT
dr C1
k(2 rL) r 2 kLC1 2 kL T1 T2
ln(r2/r1) (259) The numerical value of the rate of heat conduction through the pipe is determined by substituting the given values ·
Q 2 (20 W/m · °C)(20 m) (150 60)°C
ln(0.08/0.06) 786 kW DISCUSSION Note that the total rate of heat transfer through a pipe is constant, but the heat flux is not since it decreases in the direction of heat trans·
·
fer with increasing radius since q
Q /(2 rL). T2 EXAMPLE 2–16
T1
0 r1 r2 r FIGURE 2–52
Schematic for Example 2–16. Heat Conduction through a Spherical Shell Consider a spherical container of inner radius r1
8 cm, outer radius r2
10 cm, and thermal conductivity k
45 W/m · °C, as shown in Figure 2–52.
The inner and outer surfaces of the container are maintained at constant temperatures of T1 200°C and T2 80°C, respectively, as a result of some chemical reactions occurring inside. Obtain a general relation for the temperature
distribution inside the shell under steady conditions, and determine the rate of
heat loss from the container. SOLUTION A spherical container is subjected to specified temperatures on its
surfaces. The variation of temperature and the rate of heat transfer are to be
determined.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since there is thermal symmetry about the
midpoint, and thus T T (r ). 3 Thermal conductivity is constant. 4 There is no
heat generation.
Properties The thermal conductivity is given to be k 45 W/m · °C.
Analysis The mathematical formulation of this problem can be expressed as d 2 dT
r
dr
dr 0 with boundary conditions T(r1)
T(r2) T1
T2 200°C
80°C Integrating the differential equation once with respect to r yields r2 dT
dr C1 where C1 is an arbitrary constant. We now divide both sides of this equation by
r 2 to bring it to a readily integrable form, dT
dr C1
r2 cen58933_ch02.qxd 9/10/2002 8:46 AM Page 97 97
CHAPTER 2 Again integrating with respect to r gives C1
r T(r) C2 (a) We now apply both boundary conditions by replacing all occurrences of r and
T (r ) in the relation above by the specified values at the boundaries. We get T(r1) T1 → T(r2) T2 → C1
r1
C1
r2 C2 T1 C2 T2
·
·
Q2 = Q1 which are two equations in two unknowns, C1 and C2 . Solving them simultaneously gives C1 r1r2
r2 r1 (T1 T2) and C2 r2T2
r2 ·
Q1 r1T1
r1 Substituting into Eq. (a), the variation of temperature within the spherical shell
is determined to be T(r) r1r2
(T
r (r2 r1) 1 T2) r2T2
r2 r1T1
r1 ·
Q sphere kA dT
dr k(4 r 2) C1
r2 4 kC1 T1
4 kr1r2 r
2 T2
r1 (261) The numerical value of the rate of heat conduction through the wall is determined by substituting the given values to be ·
Q 4 (45 W/m · °C)(0.08 m)(0.10 m) (200
(0.10 80)°C
0.08) m 27,140 W Discussion Note that the total rate of heat transfer through a spherical shell is
·
·
constant, but the heat flux, q
Q /4 r 2, is not since it decreases in the direction of heat transfer with increasing radius as shown in Figure 2–53. r2 r ·
q1 (260) The rate of heat loss from the container is simply the total rate of heat conduction through the container wall and is determined from Fourier’s law r1 0 ·
·
q2 < q1
·
Q1
27.14 kW
·
q1 = — = ————— = 337.5 kW/m2
A1 4π (0.08 m)2
·
Q2
27.14 kW
·
q2 = — = ————— = 216.0 kW/m2
A2 4π (0.10 m)2 FIGURE 2–53
During steady onedimensional
heat conduction in a spherical (or
cylindrical) container, the total rate
of heat transfer remains constant,
but the heat flux decreases with
increasing radius.
Chemical
reactions 2–6 HEAT GENERATION IN A SOLID Many practical heat transfer applications involve the conversion of some form
of energy into thermal energy in the medium. Such mediums are said to involve internal heat generation, which manifests itself as a rise in temperature
throughout the medium. Some examples of heat generation are resistance
heating in wires, exothermic chemical reactions in a solid, and nuclear reactions in nuclear fuel rods where electrical, chemical, and nuclear energies are
converted to heat, respectively (Fig. 2–54). The absorption of radiation
throughout the volume of a semitransparent medium such as water can also be
considered as heat generation within the medium, as explained earlier. Nuclear
fuel rods Electric
resistance
wires FIGURE 2–54
Heat generation in solids is
commonly encountered in practice. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 98 98
HEAT TRANSFER Heat generation is usually expressed per unit volume of the medium, and is
·
denoted by g, whose unit is W/m3. For example, heat generation in an electrical wire of outer radius r0 and length L can be expressed as
·
E g.electric
Vwire ·
g h, T
Ts
V
k ··
Q = Egen Heat generation
·
·
Egen = gV FIGURE 2–55
At steady conditions, the entire heat
generated in a solid must leave the
solid through its outer surface. I 2 Re
ro2L (W/m3) (262) where I is the electric current and Re is the electrical resistance of the wire.
The temperature of a medium rises during heat generation as a result of the
absorption of the generated heat by the medium during transient startup
period. As the temperature of the medium increases, so does the heat transfer
from the medium to its surroundings. This continues until steady operating
conditions are reached and the rate of heat generation equals the rate of heat
transfer to the surroundings. Once steady operation has been established, the
temperature of the medium at any point no longer changes.
The maximum temperature Tmax in a solid that involves uniform heat generation will occur at a location farthest away from the outer surface when the
outer surface of the solid is maintained at a constant temperature Ts. For example, the maximum temperature occurs at the midplane in a plane wall, at
the centerline in a long cylinder, and at the midpoint in a sphere. The temperature distribution within the solid in these cases will be symmetrical about the
center of symmetry.
The quantities of major interest in a medium with heat generation are the
surface temperature Ts and the maximum temperature Tmax that occurs in the
medium in steady operation. Below we develop expressions for these two
quantities for common geometries for the case of uniform heat generation
·
(g constant) within the medium.
Consider a solid medium of surface area As, volume V, and constant thermal
·
conductivity k, where heat is generated at a constant rate of g per unit volume.
Heat is transferred from the solid to the surrounding medium at T , with a
constant heat transfer coefficient of h. All the surfaces of the solid are maintained at a common temperature Ts. Under steady conditions, the energy balance for this solid can be expressed as (Fig. 2–55)
Rate of
heat transfer
from the solid Rate of
energy generation
within the solid (263) or
·
Q ·
gV (W) (264) Disregarding radiation (or incorporating it in the heat transfer coefficient h),
the heat transfer rate can also be expressed from Newton’s law of cooling as
·
Q hAs (Ts T) (W) (265) Combining Eqs. 2–64 and 2–65 and solving for the surface temperature
Ts gives
Ts T ·
gV
hAs (266) cen58933_ch02.qxd 9/10/2002 8:46 AM Page 99 99
CHAPTER 2 For a large plane wall of thickness 2L (As 2Awall and V 2LAwall), a long
2
r o L), and a solid sphere of
solid cylinder of radius ro (As 2 ro L and V
4 r 3 ), Eq. 2–66 reduces to
2
radius r0 (As 4 r o and V 3 o
Ts, plane wall
Ts, cylinder T Ts, sphere ·
gL
h
·
g ro
2h
·
g ro
3h T T (267)
(268)
(269) Note that the rise in surface temperature Ts is due to heat generation in the
solid.
Reconsider heat transfer from a long solid cylinder with heat generation.
We mentioned above that, under steady conditions, the entire heat generated
within the medium is conducted through the outer surface of the cylinder.
Now consider an imaginary inner cylinder of radius r within the cylinder
(Fig. 2–56). Again the heat generated within this inner cylinder must be equal
to the heat conducted through the outer surface of this inner cylinder. That is,
from Fourier ’s law of heat conduction,
kAr dT
dr ··
Q = Egen Ar ·
gVr Vr r
ro ·
·
Egen = gVr FIGURE 2–56
Heat conducted through a cylindrical
shell of radius r is equal to the heat
generated within a shell. (270) where Ar 2 rL and Vr
r 2 L at any location r. Substituting these expressions into Eq. 2–70 and separating the variables, we get
k(2 rL) Integrating from r dT
dr ·
g( r 2 L) → dT 0 where T(0) T0 to r Tmax, cylinder To ·
g
rdr
2k ro where T(ro)
·
gro2
4k Ts To = Tmax Ts yields
(271) To Ts Tmax (272) The approach outlined above can also be used to determine the maximum temperature rise in a plane wall of thickness 2L and a solid sphere of radius r0,
with these results:
Tmax, plane wall
Tmax, sphere ·
gL2
2k
·
gro2
6k ∆Tmax Ts T T
Heat generation where To is the centerline temperature of the cylinder, which is the maximum
temperature, and Tmax is the difference between the centerline and the surface temperatures of the cylinder, which is the maximum temperature rise in
the cylinder above the surface temperature. Once Tmax is available, the centerline temperature can easily be determined from (Fig. 2–57)
Tcenter Ts (273)
(274) Symmetry
line FIGURE 2–57
The maximum temperature in
a symmetrical solid with uniform
heat generation occurs at its center. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 100 100
HEAT TRANSFER Again the maximum temperature at the center can be determined from
Eq. 2–72 by adding the maximum temperature rise to the surface temperature
of the solid.
EXAMPLE 2–17 ·
Q
Water
·
q A 2kW resistance heater wire whose thermal conductivity is k
15 W/m · °C
has a diameter of D
4 mm and a length of L
0.5 m, and is used to boil
water (Fig. 2–58). If the outer surface temperature of the resistance wire is Ts
105°C, determine the temperature at the center of the wire. D = 4 m
m Ts = 105°C Centerline Temperature of a Resistance Heater T
o FIGURE 2–58
Schematic for Example 2–17. SOLUTION The surface temperature of a resistance heater submerged in water
is to be determined.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since there is thermal symmetry about the
centerline and no change in the axial direction. 3 Thermal conductivity is constant. 4 Heat generation in the heater is uniform.
Properties The thermal conductivity is given to be k 15 W/m · °C.
Analysis The 2kW resistance heater converts electric energy into heat at a rate
of 2 kW. The heat generation per unit volume of the wire is
·
g ·
Q gen
Vwire ·
Q gen
ro2L 2000 W
(0.002 m)2(0.5 m) 0.318 109 W/m3 Then the center temperature of the wire is determined from Eq. 2–71 to be To Ts ·
gro2
4k 105°C (0.318
4 109 W/m3)(0.002 m)2
(15 W/m · °C) 126°C Discussion Note that the temperature difference between the center and the
surface of the wire is 21°C. 226°F Water 0 r0 r ·
g FIGURE 2–59
Schematic for Example 2–18. We have developed these relations using the intuitive energy balance approach. However, we could have obtained the same relations by setting up the
appropriate differential equations and solving them, as illustrated in Examples
2–18 and 2–19.
EXAMPLE 2–18 Variation of Temperature in a Resistance Heater A long homogeneous resistance wire of radius r0
0.2 in. and thermal conductivity k 7.8 Btu/h · ft · °F is being used to boil water at atmospheric pressure by the passage of electric current, as shown in Figure 2–59. Heat is
·
generated in the wire uniformly as a result of resistance heating at a rate of g
3
2400 Btu/h · in . If the outer surface temperature of the wire is measured to be
Ts
226°F, obtain a relation for the temperature distribution, and determine
the temperature at the centerline of the wire when steady operating conditions
are reached. cen58933_ch02.qxd 9/10/2002 8:46 AM Page 101 101
CHAPTER 2 SOLUTION This heat transfer problem is similar to the problem in Example
2–17, except that we need to obtain a relation for the variation of temperature
within the wire with r. Differential equations are well suited for this purpose.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since there is no thermal symmetry about
the centerline and no change in the axial direction. 3 Thermal conductivity is
constant. 4 Heat generation in the wire is uniform.
Properties The thermal conductivity is given to be k 7.8 Btu/h · ft · °F.
Analysis The differential equation which governs the variation of temperature
in the wire is simply Eq. 2–27,
·
g
k dT
1d
r dr r dr 0 This is a secondorder linear ordinary differential equation, and thus its general
solution will contain two arbitrary constants. The determination of these constants requires the specification of two boundary conditions, which can be
taken to be T(r0) Ts dT(0)
–—— = 0
dr 226°F
T T(r) and dT(0)
dr 0 0 The first boundary condition simply states that the temperature of the outer
surface of the wire is 226°F. The second boundary condition is the symmetry
condition at the centerline, and states that the maximum temperature in the
wire will occur at the centerline, and thus the slope of the temperature at r 0
must be zero (Fig. 2–60). This completes the mathematical formulation of the
problem.
Although not immediately obvious, the differential equation is in a form that
can be solved by direct integration. Multiplying both sides of the equation by r
and rearranging, we obtain ·
g
r
k dT
d
r
dr dr
Integrating with respect to r gives r ·
g r2
k2 dT
dr C1 (a) since the heat generation is constant, and the integral of a derivative of a function is the function itself. That is, integration removes a derivative. It is convenient at this point to apply the second boundary condition, since it is related to
the first derivative of the temperature, by replacing all occurrences of r and
dT /dr in Eq. (a) by zero. It yields 0 dT(0)
dr ·
g
2k 0 C1 → C1 0 r0 r ·
g FIGURE 2–60
The thermal symmetry condition at the
centerline of a wire in which heat
is generated uniformly. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 102 102
HEAT TRANSFER Thus C1 cancels from the solution. We now divide Eq. (a) by r to bring it to a
readily integrable form, ·
g
r
2k dT
dr
Again integrating with respect to r gives ·
g2
r
4k T(r) C2 (b) We now apply the first boundary condition by replacing all occurrences of r by
r0 and all occurrences of T by Ts. We get Ts ·
g2
r
4k 0 C2 → C2 Ts ·
g2
r
4k 0 Substituting this C2 relation into Eq. (b) and rearranging give T(r) Ts ·
g
(r 2
4k 0 r 2) (c) which is the desired solution for the temperature distribution in the wire as a
function of r. The temperature at the centerline (r 0) is obtained by replacing
r in Eq. (c) by zero and substituting the known quantities, T(0) Ts ·
g2
r
4k 0 226°F 4 2400 Btu/h · in3
12 in.
(0.2 in.)2
(7.8 Btu/h · ft · °F) 1 ft 263°F Discussion The temperature of the centerline will be 37°F above the temperature of the outer surface of the wire. Note that the expression above for the centerline temperature is identical to Eq. 2–71, which was obtained using an
energy balance on a control volume. EXAMPLE 2–19 Heat Conduction in a TwoLayer Medium Consider a long resistance wire of radius r1 0.2 cm and thermal conductivity
kwire
15 W/m · °C in which heat is generated uniformly as a result of re·
sistance heating at a constant rate of g
50 W/cm3 (Fig. 2–61). The wire
is embedded in a 0.5cmthick layer of ceramic whose thermal conductivity is
kceramic
1.2 W/m · °C. If the outer surface temperature of the ceramic layer
is measured to be Ts 45°C, determine the temperatures at the center of the
resistance wire and the interface of the wire and the ceramic layer under steady
conditions. Interface Wire r1 r2 Ts = 45°C
r Ceramic layer FIGURE 2–61
Schematic for Example 2–19. SOLUTION The surface and interface temperatures of a resistance wire covered with a ceramic layer are to be determined.
Assumptions 1 Heat transfer is steady since there is no change with time.
2 Heat transfer is onedimensional since this twolayer heat transfer problem
possesses symmetry about the centerline and involves no change in the axial direction, and thus T T (r ). 3 Thermal conductivities are constant. 4 Heat generation in the wire is uniform.
Properties It is given that kwire
15 W/m · °C and kceramic
1.2 W/m · ° C. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 103 103
CHAPTER 2 Analysis Letting TI denote the unknown interface temperature, the heat transfer problem in the wire can be formulated as ·
g
k dTwire
1d
r dr r dr 0 with Twire(r1)
dTwire(0)
dr TI
0 This problem was solved in Example 2–18, and its solution was determined
to be Twire(r) TI ·
g
(r 2
4 kwire 1 r 2) (a) Noting that the ceramic layer does not involve any heat generation and its
outer surface temperature is specified, the heat conduction problem in that
layer can be expressed as dTceramic
d
r
dr
dr 0 with Tceramic (r1)
Tceramic (r2) TI
Ts 45°C This problem was solved in Example 2–15, and its solution was determined
to be Tceramic (r) ln(r/r1)
(T
ln(r2/r1) s TI) TI (b) We have already utilized the first interface condition by setting the wire and ceramic layer temperatures equal to TI at the interface r r1. The interface temperature TI is determined from the second interface condition that the heat flux
in the wire and the ceramic layer at r r1 must be the same: kwire dTwire (r1)
dr kceramic ·
dTceramic (r1)
gr1
→
2
dr kceramic Ts TI 1
ln(r2/r1) r1 Solving for TI and substituting the given values, the interface temperature is determined to be TI ·
gr12
r2
ln r
Ts
1
2kceramic
(50 106 W/m3)(0.002 m)2 0.007 m
ln
2(1.2 W/m · °C)
0.002 m 45° C 149.4°C Knowing the interface temperature, the temperature at the centerline (r
obtained by substituting the known quantities into Eq. (a), Twire (0) TI ·
gr12
4kwire 149.4°C (50 106 W/m3)(0.002 m)2
4 (15 W/m · °C) 0) is 152.7°C cen58933_ch02.qxd 9/10/2002 8:47 AM Page 104 104
HEAT TRANSFER Thus the temperature of the centerline will be slightly above the interface
temperature.
Discussion This example demonstrates how steady onedimensional heat conduction problems in composite media can be solved. We could also solve this
problem by determining the heat flux at the interface by dividing the total heat
generated in the wire by the surface area of the wire, and then using this value
as the specifed heat flux boundary condition for both the wire and the ceramic
layer. This way the two problems are decoupled and can be solved separately. 2–7
500
400
300 Silver
Copper
Gold
Aluminum Thermal conductivity (W/ m·K) 200
100 Tungsten
Platinum 50 Iron 20
10 Stainless steel,
AISI 304
Aluminum
oxide 5 I VARIABLE THERMAL CONDUCTIVITY, k (T ) You will recall from Chapter 1 that the thermal conductivity of a material, in
general, varies with temperature (Fig. 2–62). However, this variation is mild
for many materials in the range of practical interest and can be disregarded. In
such cases, we can use an average value for the thermal conductivity and treat
it as a constant, as we have been doing so far. This is also common practice for
other temperaturedependent properties such as the density and specific heat.
When the variation of thermal conductivity with temperature in a specified
temperature interval is large, however, it may be necessary to account for this
variation to minimize the error. Accounting for the variation of the thermal
conductivity with temperature, in general, complicates the analysis. But in the
case of simple onedimensional cases, we can obtain heat transfer relations in
a straightforward manner.
When the variation of thermal conductivity with temperature k(T) is known,
the average value of the thermal conductivity in the temperature range between T1 and T2 can be determined from
T2 Pyroceram kave 2 T2 Fused quartz
1
100 300 500 1000 2000 4000
Temperature (K) FIGURE 2–62
Variation of the thermal conductivity
of some solids with temperature. k(T)dT T1 (275) T1 This relation is based on the requirement that the rate of heat transfer through
a medium with constant average thermal conductivity kave equals the rate of
heat transfer through the same medium with variable conductivity k(T). Note
that in the case of constant thermal conductivity k(T) k, Eq. 2–75 reduces to
kave k, as expected.
Then the rate of steady heat transfer through a plane wall, cylindrical layer,
or spherical layer for the case of variable thermal conductivity can be determined by replacing the constant thermal conductivity k in Eqs. 2–57, 2–59,
and 2–61 by the kave expression (or value) from Eq. 2–75:
·
Q plane wall
·
Q cylinder
·
Q sphere kave A T1 T2
L T1 T2
ln(r2/r1)
T1 T2
4 kaver1r2 r
r1
2
2 kave L A
L T1 k(T)dT (276) T2 2L
ln(r2/r1)
4 r1r2
r2 r1 T1 k(T)dT (277) k(T)dT (278) T2
T1
T2 cen58933_ch02.qxd 9/10/2002 8:47 AM Page 105 105
CHAPTER 2 The variation in thermal conductivity of a material with temperature in the
temperature range of interest can often be approximated as a linear function
and expressed as
k(T) k0(1 Plane wall T) (279) where is called the temperature coefficient of thermal conductivity. The
average value of thermal conductivity in the temperature range T1 to T2 in this
case can be determined from
T2 kave T1 k0(1
T2 T)dT T2 k0 1 T1 T1
2 k(Tave) k(T) = k0(1 + β T) β>0 β=0 T1 T2 β<0
(280) Note that the average thermal conductivity in this case is equal to the thermal
conductivity value at the average temperature.
We have mentioned earlier that in a plane wall the temperature varies
linearly during steady onedimensional heat conduction when the thermal
conductivity is constant. But this is no longer the case when the thermal conductivity changes with temperature, even linearly, as shown in Figure 2–63.
EXAMPLE 2–20 T 0 L x FIGURE 2–63
The variation of temperature
in a plane wall during steady
onedimensional heat conduction
for the cases of constant and variable
thermal conductivity. Variation of Temperature in a Wall with k ( T ) Consider a plane wall of thickness L whose thermal conductivity varies linearly
in a specified temperature range as k (T ) k0(1
T ) where k0 and are constants. The wall surface at x 0 is maintained at a constant temperature of T1
while the surface at x
L is maintained at T2, as shown in Figure 2–64.
Assuming steady onedimensional heat transfer, obtain a relation for (a) the
heat transfer rate through the wall and (b) the temperature distribution T (x ) in
the wall. SOLUTION A plate with variable conductivity is subjected to specified temperatures on both sides. The variation of temperature and the rate of heat transfer are to be determined.
Assumptions 1 Heat transfer is given to be steady and onedimensional.
2 Thermal conductivity varies linearly. 3 There is no heat generation.
Properties The thermal conductivity is given to be k (T ) k0(1
T ).
Analysis (a) The rate of heat transfer through the wall can be determined from
·
Q kave A T1 k(Tave) L T1 T2 k0 1 2 is the average thermal conductivity (Eq. 2–80).
(b) To determine the temperature distribution in the wall, we begin with
Fourier’s law of heat conduction, expressed as ·
Q k(T) A dT
dx Plane
wall T1 T2
0 L x FIGURE 2–64
Schematic for Example 2–20. T2 where A is the heat conduction area of the wall and kave k(T) = k0(1 + β T) cen58933_ch02.qxd 9/10/2002 8:47 AM Page 106 106
HEAT TRANSFER ·
where the rate of conduction heat transfer Q and the area A are constant.
Separating variables and integrating from x 0 where T (0) T1 to any x where
T (x ) T, we get
x T ·
Q dx A k(T)dT 0 Substituting k (T ) k0(1 T1 T ) and performing the integrations we obtain ·
Qx Ak0[(T (T 2 T1) T12)/2] ·
Substituting the Q expression from part (a) and rearranging give T2 2 T 2kave x
(T
k0 L 1 T12 T2) 2 T1 0 which is a quadratic equation in the unknown temperature T. Using the quadratic formula, the temperature distribution T (x ) in the wall is determined to be 1 T(x) 1
2 2kave x
(T
k0 L 1 T2) T12 2 T1 The proper sign of the square root term ( or ) is determined from the requirement that the temperature at any point within the medium must remain
between T1 and T2. This result explains why the temperature distribution in a
plane wall is no longer a straight line when the thermal conductivity varies with
temperature. EXAMPLE 2–21 k(T) = k0(1 + β T)
Bronze
plate T1 = 600 K T2 = 400 K
·
Q
L FIGURE 2–65
Schematic for Example 2–21. Heat Conduction through a Wall with k ( T ) Consider a 2mhigh and 0.7mwide bronze plate whose thickness is 0.1 m.
One side of the plate is maintained at a constant temperature of 600 K while
the other side is maintained at 400 K, as shown in Figure 2–65. The thermal
conductivity of the bronze plate can be assumed to vary linearly in that temperature range as k (T )
k0(1
T ) where k0
38 W/m · K and
9.21
10 4 K 1. Disregarding the edge effects and assuming steady onedimensional
heat transfer, determine the rate of heat conduction through the plate. SOLUTION A plate with variable conductivity is subjected to specified temperatures on both sides. The rate of heat transfer is to be determined.
Assumptions 1 Heat transfer is given to be steady and onedimensional.
2 Thermal conductivity varies linearly. 3 There is no heat generation.
Properties The thermal conductivity is given to be k (T ) k0(1
T ).
Analysis The average thermal conductivity of the medium in this case is simply the value at the average temperature and is determined from
kave k(Tave) k0 1 (38 W/m · K) 1
55.5 W/m · K T2 T1
2 (9.21 10 4 K 1) (600 400) K
2 cen58933_ch02.qxd 9/10/2002 8:47 AM Page 107 107
CHAPTER 2 Then the rate of heat conduction through the plate can be determined from Eq.
2–76 to be ·
Q kave A T2 T1
L (55.5 W/m · K)(2 m 0.7 m) (600 400)K
0.1 m 155,400 W Discussion We would have obtained the same result by substituting the given
k (T ) relation into the second part of Eq. 2–76 and performing the indicated
integration. TOPIC OF SPECIAL INTEREST A Brief Review of Differential Equations*
As we mentioned in Chapter 1, the description of most scientific problems
involves relations that involve changes in some key variables with respect
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, which provide precise mathematical formulations for the
physical principles and laws by representing the rates of change as derivatives. Therefore, differential equations are used to investigate a wide variety of problems in science and engineering, including heat transfer.
Differential equations arise when relevant physical laws and principles
are applied to a problem by considering infinitesimal changes in the variables of interest. Therefore, obtaining the governing differential equation
for a specific problem requires an adequate knowledge of the nature of the
problem, the variables involved, appropriate simplifying assumptions, and
the applicable physical laws and principles involved, as well as a careful
analysis (Fig. 2–66).
An equation, in general, may involve one or more variables. As the name
implies, a variable is a quantity that may assume various values during a
study. A quantity whose value is fixed during a study is called a constant.
Constants are usually denoted by the earlier letters of the alphabet such as
a, b, c, and d, whereas variables are usually denoted by the later ones such
as t, x, y, and z. A variable whose value can be changed arbitrarily is called
an independent variable (or argument). A variable whose value depends
on the value of other variables and thus cannot be varied independently is
called a dependent variable (or a function).
A dependent variable y that depends on a variable x is usually denoted as
y(x) for clarity. However, this notation becomes very inconvenient and
cumbersome when y is repeated several times in an expression. In such
cases it is desirable to denote y(x) simply as y when it is clear that y is a
function of x. This shortcut in notation improves the appearance and the
*This section can be skipped if desired without a loss in continuity. Physical problem
Identify
important
variables Apply
relevant
physical laws Make
reasonable
assumptions and
approximations A differential equation
Apply
applicable
solution
technique Boundary
and initial
conditions Solution of the problem FIGURE 2–66
Mathematical modeling
of physical problems. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 108 108
HEAT TRANSFER
y readability of the equations. The value of y at a fixed number a is denoted
by y(a).
The derivative of a function y(x) at a point is equivalent to the slope of
the tangent line to the graph of the function at that point and is defined as
(Fig. 2–67) y(x) y(x + ∆ x)
∆y
y(x) y (x) ∆x
Tangent line
x x + ∆x x FIGURE 2–67
The derivative of a function at a point
represents the slope of the tangent
line of the function at that point. z ∂z
—
∂x y () y
x FIGURE 2–68
Graphical representation of
partial derivative z / x. dy(x)
dx lim x→0 y
x lim y(x x→0 x)
x y(x) (281) Here x represents a (small) change in the independent variable x and is
called an increment of x. The corresponding change in the function y is
called an increment of y and is denoted by y. Therefore, the derivative of
a function can be viewed as the ratio of the increment y of the function to
the increment x of the independent variable for very small x. Note that
y and thus y (x) will be zero if the function y does not change with x.
Most problems encountered in practice involve quantities that change
with time t, and their first derivatives with respect to time represent the rate
of change of those quantities with time. For example, if N(t) denotes the
population of a bacteria colony at time t, then the first derivative N
dN/dt represents the rate of change of the population, which is the amount
the population increases or decreases per unit time.
The derivative of the first derivative of a function y is called the second
derivative of y, and is denoted by y or d 2y/dx2. In general, the derivative of
the (n 1)st derivative of y is called the nth derivative of y and is denoted
by y(n) or d ny/dxn. Here, n is a positive integer and is called the order of the
derivative. The order n should not be confused with the degree of a derivative. For example, y is the thirdorder derivative of y, but (y )3 is the third
degree of the first derivative of y. Note that the first derivative of a function
represents the slope or the rate of change of the function with the independent variable, and the second derivative represents the rate of change of the
slope of the function with the independent variable.
When a function y depends on two or more independent variables such
as x and t, it is sometimes of interest to examine the dependence of the
function on one of the variables only. This is done by taking the derivative
of the function with respect to that variable while holding the other variables constant. Such derivatives are called partial derivatives. The first
partial derivatives of the function y(x, t) with respect to x and t are defined
as (Fig. 2–68)
y
x lim y
t lim y(x x→0 t→0 y(x, t x, t)
x
t)
t y(x, t)
y(x, t) (282)
(283) Note that when finding y/ x we treat t as a constant and differentiate y
with respect to x. Likewise, when finding y/ t we treat x as a constant and
differentiate y with respect to t.
Integration can be viewed as the inverse process of differentiation. Integration is commonly used in solving differential equations since solving a
differential equation is essentially a process of removing the derivatives cen58933_ch02.qxd 9/10/2002 8:47 AM Page 109 109
CHAPTER 2 from the equation. Differentiation is the process of finding y (x) when a
function y(x) is given, whereas integration is the process of finding the
function y(x) when its derivative y (x) is given. The integral of this derivative is expressed as
y (x)dx dy y(x) C (284) since y (x)dx dy and the integral of the differential of a function is the
function itself (plus a constant, of course). In Eq. 2–84, x is the integration
variable and C is an arbitrary constant called the integration constant.
The derivative of y(x) C is y (x) no matter what the value of the constant C is. Therefore, two functions that differ by a constant have the same
derivative, and we always add a constant C during integration to recover
this constant that is lost during differentiation. The integral in Eq. 2–84 is
called an indefinite integral since the value of the arbitrary constant C is
indefinite. The described procedure can be extended to higherorder derivatives (Fig. 2–69). For example,
y (x)dx y (x) C dy y C y dx y C y dx y C y dx y C y(n) dx y(n 1) C FIGURE 2–69
Some indefinite integrals
that involve derivatives. (285) This can be proved by defining a new variable u(x) y (x), differentiating
it to obtain u (x) y (x), and then applying Eq. 2–84. Therefore, the order
of a derivative decreases by one each time it is integrated. Classification of Differential Equations
A differential equation that involves only ordinary derivatives is called an
ordinary differential equation, and a differential equation that involves
partial derivatives is called a partial differential equation. Then it follows
that problems that involve a single independent variable result in ordinary
differential equations, and problems that involve two or more independent
variables result in partial differential equations. A differential equation may
involve several derivatives of various orders of an unknown function. The
order of the highest derivative in a differential equation is the order of the
equation. For example, the order of y
(y )4 7x5 is 3 since it contains
no fourth or higher order derivatives.
You will remember from algebra that the equation 3x 5 0 is much
easier to solve than the equation x4 3x 5 0 because the first equation
is linear whereas the second one is nonlinear. This is also true for differential equations. Therefore, before we start solving a differential equation, we
usually check for linearity. A differential equation is said to be linear if the
dependent variable and all of its derivatives are of the first degree and their
coefficients depend on the independent variable only. In other words, a differential equation is linear if it can be written in a form that does not involve (1) any powers of the dependent variable or its derivatives such as y3
or (y )2, (2) any products of the dependent variable or its derivatives such
as yy or y y , and (3) any other nonlinear functions of the dependent variable such as sin y or ey. If any of these conditions apply, it is nonlinear
(Fig. 2–70). (a) A nonlinear equation:
3(y′′)2 – 4yy′ + e2xy = 6x2
Power Product Other
nonlinear
functions (b) A linear equation:
3x2y′′ – 4xy′ + e2xy = 6x2 FIGURE 2–70
A differential equation that is
(a) nonlinear and (b) linear. When
checking for linearity, we examine the
dependent variable only. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 110 110
HEAT TRANSFER A linear differential equation, however, may contain (1) powers or nonlinear functions of the independent variable, such as x2 and cos x and
(2) products of the dependent variable (or its derivatives) and functions of
the independent variable, such as x3y , x2y, and e 2xy . A linear differential
equation of order n can be expressed in the most general form as
y(n)
(a) With constant coefficients:
6y 2y xe → → y 2x Constant
(b) With variable coefficients:
2 6x2y x y 1 → → y xe 2x Variable FIGURE 2–71
A differential equation with
(a) constant coefficients and
(b) variable coefficients. f1(x)y(n 1) ··· fn–1(x)y fn(x)y R(x) (286) A differential equation that cannot be put into this form is nonlinear. A
linear differential equation in y is said to be homogeneous as well if
R(x) 0. Otherwise, it is nonhomogeneous. That is, each term in a linear
homogeneous equation contains the dependent variable or one of its derivatives after the equation is cleared of any common factors. The term R(x) is
called the nonhomogeneous term.
Differential equations are also classified by the nature of the coefficients
of the dependent variable and its derivatives. A differential equation is said
to have constant coefficients if the coefficients of all the terms that involve
the dependent variable or its derivatives are constants. If, after clearing any
common factors, any of the terms with the dependent variable or its derivatives involve the independent variable as a coefficient, that equation is
said to have variable coefficients (Fig. 2–71). Differential equations with
constant coefficients are usually much easier to solve than those with variable coefficients. Solutions of Differential Equations (a) An algebraic equation:
y2 7y Solution: y 10 0 2 and y 5 (b) A differential equation:
y 7y Solution: y 0
e7x FIGURE 2–72
Unlike those of algebraic equations,
the solutions of differential equations
are typically functions instead of
discrete values. Solving a differential equation can be as easy as performing one or more
integrations; but such simple differential equations are usually the exception rather than the rule. There is no single general solution method applicable to all differential equations. There are different solution techniques,
each being applicable to different classes of differential equations. Sometimes solving a differential equation requires the use of two or more techniques as well as ingenuity and mastery of solution methods. Some
differential equations can be solved only by using some very clever tricks.
Some cannot be solved analytically at all.
In algebra, we usually seek discrete values that satisfy an algebraic equation such as x2 7x 10 0. When dealing with differential equations,
however, we seek functions that satisfy the equation in a specified interval.
For example, the algebraic equation x2 7x 10 0 is satisfied by two
numbers only: 2 and 5. But the differential equation y
7y 0 is satisfied by the function e7x for any value of x (Fig. 2–72).
6x2
11x
6
0. Obviously,
Consider the algebraic equation x3
x 1 satisfies this equation, and thus it is a solution. However, it is not the
only solution of this equation. We can easily show by direct substitution
that x 2 and x 3 also satisfy this equation, and thus they are solutions
as well. But there are no other solutions to this equation. Therefore, we
say that the set 1, 2, and 3 forms the complete solution to this algebraic
equation.
The same line of reasoning also applies to differential equations. Typically, differential equations have multiple solutions that contain at least one
arbitrary constant. Any function that satisfies the differential equation on an cen58933_ch02.qxd 9/10/2002 8:47 AM Page 111 111
CHAPTER 2 interval is called a solution of that differential equation in that interval.
A solution that involves one or more arbitrary constants represents a family of functions that satisfy the differential equation and is called a general
solution of that equation. Not surprisingly, a differential equation may
have more than one general solution. A general solution is usually referred
to as the general solution or the complete solution if every solution of the
equation can be obtained from it as a special case. A solution that can be
obtained from a general solution by assigning particular values to the arbitrary constants is called a specific solution.
You will recall from algebra that a number is a solution of an algebraic
equation if it satisfies the equation. For example, 2 is a solution of the equation x3 8 0 because the substitution of 2 for x yields identically zero.
Likewise, a function is a solution of a differential equation if that function
satisfies the differential equation. In other words, a solution function yields
identity when substituted into the differential equation. For example, it
can be shown by direct substitution that the function 3e 2x is a solution of
y
4y 0 (Fig. 2–73). Function: f 3e 2x Differential equation: y 4y 0 Derivatives of f:
6e 2x
12e 2x f
f
Substituting into y
2x 0: f
12e 4y 4f 4 3e 2x 0
Therefore, the function 3e
the differential equation y 2x 0
0
0 is a solution of
4y 0. FIGURE 2–73
Verifying that a given function is a
solution of a differential equation. SUMMARY
In this chapter we have studied the heat conduction equation
and its solutions. Heat conduction in a medium is said to be
steady when the temperature does not vary with time and unsteady or transient when it does. Heat conduction in a medium
is said to be onedimensional when conduction is significant
in one dimension only and negligible in the other two dimensions. It is said to be twodimensional when conduction in
the third dimension is negligible and threedimensional when
conduction in all dimensions is significant. In heat transfer
analysis, the conversion of electrical, chemical, or nuclear
energy into heat (or thermal) energy is characterized as heat
generation.
The heat conduction equation can be derived by performing
an energy balance on a differential volume element. The onedimensional heat conduction equation in rectangular, cylindrical, and spherical coordinate systems for the case of constant
thermal conductivities are expressed as
2 T
x2
T
1
rrrr
T
1
r2
r
r2 r ·
g
k
·
g
k
·
g
k 1T
t
1T
t
1T
t boundary conditions. The solution of transient heat conduction
problems also depends on the condition of the medium at the
beginning of the heat conduction process. Such a condition,
which is usually specified at time t
0, is called the initial
condition, which is a mathematical expression for the temperature distribution of the medium initially. Complete mathematical description of a heat conduction problem requires the
specification of two boundary conditions for each dimension
along which heat conduction is significant, and an initial condition when the problem is transient. The most common
boundary conditions are the specified temperature, specified
heat flux, convection, and radiation boundary conditions. A
boundary surface, in general, may involve specified heat flux,
convection, and radiation at the same time.
For steady onedimensional heat transfer through a plate of
thickness L, the various types of boundary conditions at the
surfaces at x 0 and x L can be expressed as
Specified temperature:
T(0) T1 and T(L) T2 where T1 and T2 are the specified temperatures at surfaces at
x 0 and x L.
Specified heat flux: where the property
k/ C is the thermal diffusivity of the
material.
The solution of a heat conduction problem depends on the
conditions at the surfaces, and the mathematical expressions
for the thermal conditions at the boundaries are called the k dT(0)
dx q·0 and k dT(L)
dx q·L where q·0 and q·L are the specified heat fluxes at surfaces at
x 0 and x L. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 112 112
HEAT TRANSFER where h is the convection heat transfer coefficient. The maximum temperature rise between the surface and the midsection
of a medium is given by Insulation or thermal symmetry:
dT(0)
dx 0 dT(L)
dx and 0 Tmax, plane wall Convection:
k dT(0)
dx h1[T 1 T(0)] and k dT(L)
dx h2[T(L) where h1 and h2 are the convection heat transfer coefficients
and T 1 and T 2 are the temperatures of the surrounding mediums on the two sides of the plate.
Radiation:
k dT(0)
dx dT(L)
k
dx 1 2 [T 4 1
surr, T(0)4] 4 kA dTA (x0)
dx kB x0: dTB (x0)
dx where kA and kB are the thermal conductivities of the layers
A and B.
Heat generation is usually expressed per unit volume of the
·
medium and is denoted by g, whose unit is W/m3. Under steady
conditions, the surface temperature Ts of a plane wall of thickness 2L, a cylinder of outer radius ro, and a sphere of radius ro
·
in which heat is generated at a constant rate of g per unit volume in a surrounding medium at T can be expressed as
Ts, plane wall T Ts, cylinder T Ts, sphere T k(T)dT T1 kave T 4 2]
surr, [T(L) and When the variation of thermal conductivity with temperature
k(T) is known, the average value of the thermal conductivity in
the temperature range between T1 and T2 can be determined
from
T2 Interface of two bodies A and B in perfect contact at x
TB (x0) Tmax, sphere and where 1 and 2 are the emissivities of the boundary surfaces,
5.67 10 8 W/m2 · K4 is the Stefan–Boltzmann constant,
and Tsurr, 1 and Tsurr, 2 are the average temperatures of the surfaces surrounding the two sides of the plate. In radiation calculations, the temperatures must be in K or R. TA (x0) Tmax, cylinder T 2] ·
g L2
2k
·
g ro2
4k
·
g ro2
6k T2 T1 Then the rate of steady heat transfer through a plane wall,
cylindrical layer, or spherical layer can be expressed as
·
Q plane wall
·
Q cylinder
·
Q sphere kaveA T1 T2
L A
L T1 k(T)dT T2 T1
T1 T2
2L
k(T)dT
ln(r2/r1) ln(r2/r1) T2
T1 T2 4 r1r2 T1
4 kaver1r2 r
r1 r2 r1 T k(T)dT
2 2 kaveL 2 The variation of thermal conductivity of a material with
temperature can often be approximated as a linear function and
expressed as
k(T) k0(1 T) where
is called the temperature coefficient of thermal
conductivity. ·
gL
h
· ro
g
2h
·
g ro
3h REFERENCES AND SUGGESTED READING
1. W. E. Boyce and R. C. Diprima. Elementary Differential
Equations and Boundary Value Problems. 4th ed.
New York: John Wiley & Sons, 1986. 2. J. P. Holman. Heat Transfer. 9th ed. New York:
McGrawHill, 2002. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 113 113
CHAPTER 1 3. F. P. Incropera and D. P. DeWitt. Introduction to Heat
Transfer. 4th ed. New York: John Wiley & Sons, 2002. 5. M. N. Ozisik. Heat Transfer—A Basic Approach.
New York: McGrawHill, 1985. 4. S. S. Kutateladze. Fundamentals of Heat Transfer.
New York: Academic Press, 1963. 6. F. M. White. Heat and Mass Transfer. Reading, MA:
AddisonWesley, 1988. PROBLEMS*
Introduction
2–1C Is heat transfer a scalar or vector quantity? Explain.
Answer the same question for temperature.
2–2C How does transient heat transfer differ from steady
heat transfer? How does onedimensional heat transfer differ
from twodimensional heat transfer?
2–3C Consider a cold canned drink left on a dinner table.
Would you model the heat transfer to the drink as one, two, or
threedimensional? Would the heat transfer be steady or transient? Also, which coordinate system would you use to analyze
this heat transfer problem, and where would you place the origin? Explain. 2–5C Consider an egg being cooked in boiling water in a
pan. Would you model the heat transfer to the egg as one,
two, or threedimensional? Would the heat transfer be steady
or transient? Also, which coordinate system would you use to
solve this problem, and where would you place the origin?
Explain.
2–6C Consider a hot dog being cooked in boiling water in a
pan. Would you model the heat transfer to the hot dog as one,
two, or threedimensional? Would the heat transfer be steady
or transient? Also, which coordinate system would you use to
solve this problem, and where would you place the origin?
Explain. 2–4C Consider a round potato being baked in an oven.
Would you model the heat transfer to the potato as one, two,
or threedimensional? Would the heat transfer be steady or
transient? Also, which coordinate system would you use to
solve this problem, and where would you place the origin?
Explain. Boiling water
Hot dog FIGURE P2–6
2–7C Consider the cooking process of a roast beef in an
oven. Would you consider this to be a steady or transient heat
transfer problem? Also, would you consider this to be one,
two, or threedimensional? Explain.
2–8C Consider heat loss from a 200L cylindrical hot water
tank in a house to the surrounding medium. Would you consider this to be a steady or transient heat transfer problem?
Also, would you consider this heat transfer problem to be one,
two, or threedimensional? Explain.
2–9C Does a heat flux vector at a point P on an isothermal
surface of a medium have to be perpendicular to the surface at
that point? Explain. FIGURE P2–4
*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 an EESCD 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. 2–10C From a heat transfer point of view, what is the difference between isotropic and unisotropic materials?
2–11C What is heat generation in a solid? Give examples. 2–12C Heat generation is also referred to as energy generation or thermal energy generation. What do you think of these
phrases?
2–13C In order to determine the size of the heating element
of a new oven, it is desired to determine the rate of heat transfer through the walls, door, and the top and bottom section of
the oven. In your analysis, would you consider this to be a cen58933_ch02.qxd 9/10/2002 8:47 AM Page 114 114
HEAT TRANSFER steady or transient heat transfer problem? Also, would you consider the heat transfer to be onedimensional or multidimensional? Explain.
2–14E The resistance wire of a 1000W iron is 15 in. long
and has a diameter of D 0.08 in. Determine the rate of heat
generation in the wire per unit volume, in Btu/h · ft3, and the
heat flux on the outer surface of the wire, in Btu/h · ft2, as a result of this heat generation.
q
D ity and heat generation in its simplest form, and indicate what
each variable represents.
2–20 Write down the onedimensional transient heat conduction equation for a long cylinder with constant thermal conductivity and heat generation, and indicate what each variable
represents.
2–21 Starting with an energy balance on a rectangular volume element, derive the onedimensional transient heat conduction equation for a plane wall with constant thermal
conductivity and no heat generation.
2–22 Starting with an energy balance on a cylindrical shell
volume element, derive the steady onedimensional heat conduction equation for a long cylinder with constant thermal con·
ductivity in which heat is generated at a rate of g. ·
g FIGURE P2–14E
2–15E Reconsider Problem 2–14E. Using EES (or
other) software, evaluate and plot the surface
heat flux as a function of wire diameter as the diameter varies
from 0.02 to 0.20 in. Discuss the results. L
2–16 In a nuclear reactor, heat is generated uniformly in the
5cmdiameter cylindrical uranium rods at a rate of 7
107
W/m3. If the length of the rods is 1 m, determine the rate of
Answer: 137.4 kW
heat generation in each rod.
2–17 In a solar pond, the absorption of solar energy can be
·
modeled as heat generation and can be approximated by g
·
g0e bx, where g0 is the rate of heat absorption at the top surface
per unit volume and b is a constant. Obtain a relation for the total rate of heat generation in a water layer of surface area A and
thickness L at the top of the pond. 0 0
r r + ∆r
r FIGURE P2–22
2–23 Starting with an energy balance on a spherical shell
volume element, derive the onedimensional transient heat
conduction equation for a sphere with constant thermal conductivity and no heat generation. Radiation Solar
beam being energy
absorbed L
x Solar
pond
r + ∆r FIGURE P2–17
2–18 Consider a large 3cmthick stainless steel plate in
which heat is generated uniformly at a rate of 5 106 W/m3.
Assuming the plate is losing heat from both sides, determine
the heat flux on the surface of the plate during steady operaAnswer: 75,000 W/m2
tion. Heat Conduction Equation
2–19 Write down the onedimensional transient heat conduction equation for a plane wall with constant thermal conductiv 0 r Rr FIGURE P2–23
2–24 Consider a medium in which the heat conduction equation is given in its simplest form as
2 T
x2 1T
t cen58933_ch02.qxd 9/10/2002 8:47 AM Page 115 115
CHAPTER 1 (a)
(b)
(c)
(d) Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? ∆z dT
1d
r dr rk dr
(a)
(b)
(c)
(d) ·
g 0 FIGURE P2–29 Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? Disk (a)
(b)
(c)
(d) A = constant
0 2–27 Consider a medium in which the heat conduction equation is given in its simplest form as
r
(a)
(b)
(c)
(d) d 2T
dr 2 dT
dr z 1T
t Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? 2–28 Starting with an energy balance on a volume element,
derive the twodimensional transient heat conduction equation
in rectangular coordinates for T(x, y, t) for the case of constant
thermal conductivity and no heat generation.
2–29 Starting with an energy balance on a ringshaped volume element, derive the twodimensional steady heat conduction equation in cylindrical coordinates for T(r, z) for the case
of constant thermal conductivity and no heat generation.
2–30 Starting with an energy balance on a disk volume element, derive the onedimensional transient heat conduction
equation for T(z, t) in a cylinder of diameter D with an insulated side surface for the case of constant thermal conductivity
with heat generation.
2–31 Consider a medium in which the heat conduction equation is given in its simplest form as z + ∆z
z FIGURE P2–30
2 T
x2 (a)
(b)
(c)
(d) 0 Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? Insulation ·
g 2–26 Consider a medium in which the heat conduction equation is given in its simplest form as
T
1
r2
r
r2 r r r + ∆r r 2–25 Consider a medium in which the heat conduction equation is given in its simplest form as 2 T
y2 1T
t Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? 2–32 Consider a medium in which the heat conduction equation is given in its simplest form as
T
1
r r kr r
(a)
(b)
(c)
(d) z k T
z ·
g 0 Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? 2–33 Consider a medium in which the heat conduction equation is given in its simplest form as
T
1
r2
t
r2 r
(a)
(b)
(c)
(d) 1
r 2 sin2 2 T
2 1T
t Is heat transfer steady or transient?
Is heat transfer one, two, or threedimensional?
Is there heat generation in the medium?
Is the thermal conductivity of the medium constant or
variable? cen58933_ch02.qxd 9/10/2002 8:47 AM Page 116 116
HEAT TRANSFER Boundary and Initial Conditions;
Formulation of Heat Conduction Problems Express the radiation boundary condition on the outer surface
of the shell. 2–34C What is a boundary condition? How many boundary
conditions do we need to specify for a twodimensional heat
transfer problem? 2–44 A container consists of two spherical layers, A and B,
that are in perfect contact. If the radius of the interface is r0,
express the boundary conditions at the interface. 2–35C What is an initial condition? How many initial conditions do we need to specify for a twodimensional heat transfer
problem? 2–45 Consider a steel pan used to boil water on top of an
electric range. The bottom section of the pan is L
0.5 cm
thick and has a diameter of D 20 cm. The electric heating
unit on the range top consumes 1000 W of power during cooking, and 85 percent of the heat generated in the heating element
is transferred uniformly to the pan. Heat transfer from the top
surface of the bottom section to the water is by convection with
a heat transfer coefficient of h. Assuming constant thermal
conductivity and onedimensional heat transfer, express the
mathematical formulation (the differential equation and the
boundary conditions) of this heat conduction problem during
steady operation. Do not solve. 2–36C What is a thermal symmetry boundary condition?
How is it expressed mathematically?
2–37C How is the boundary condition on an insulated surface expressed mathematically?
2–38C It is claimed that the temperature profile in a medium
must be perpendicular to an insulated surface. Is this a valid
claim? Explain.
2–39C Why do we try to avoid the radiation boundary conditions in heat transfer analysis? Steel pan 2–40 Consider a spherical container of inner radius r1, outer
radius r2, and thermal conductivity k. Express the boundary
condition on the inner surface of the container for steady onedimensional conduction for the following cases: (a) specified
temperature of 50°C, (b) specified heat flux of 30 W/m2 toward
the center, (c) convection to a medium at T with a heat transfer coefficient of h. Water
x
L
0 Spherical container r1 r2 r FIGURE P2–40
2–41 Heat is generated in a long wire of radius r0 at a con·
stant rate of g0 per unit volume. The wire is covered with a
plastic insulation layer. Express the heat flux boundary condition at the interface in terms of the heat generated.
2–42 Consider a long pipe of inner radius r1, outer radius r2,
and thermal conductivity k. The outer surface of the pipe is
subjected to convection to a medium at T with a heat transfer
coefficient of h, but the direction of heat transfer is not known.
Express the convection boundary condition on the outer surface of the pipe.
2–43 Consider a spherical shell of inner radius r1, outer radius r2, thermal conductivity k, and emissivity . The outer surface of the shell is subjected to radiation to surrounding
surfaces at Tsurr, but the direction of heat transfer is not known. FIGURE P2–45
2–46E A 2kW resistance heater wire whose thermal conductivity is k 10.4 Btu/h · ft · °F has a radius of r0 0.06 in.
and a length of L 15 in., and is used for space heating. Assuming constant thermal conductivity and onedimensional
heat transfer, express the mathematical formulation (the differential equation and the boundary conditions) of this heat conduction problem during steady operation. Do not solve.
2–47 Consider an aluminum pan used to cook stew on top of
an electric range. The bottom section of the pan is L 0.25 cm
thick and has a diameter of D 18 cm. The electric heating
unit on the range top consumes 900 W of power during cooking, and 90 percent of the heat generated in the heating element
Aluminum pan Stew
x 108°C L
0 FIGURE P2–47 cen58933_ch02.qxd 9/10/2002 8:47 AM Page 117 117
CHAPTER 1 is transferred to the pan. During steady operation, the temperature of the inner surface of the pan is measured to be 108°C.
Assuming temperaturedependent thermal conductivity and
onedimensional heat transfer, express the mathematical formulation (the differential equation and the boundary conditions) of this heat conduction problem during steady operation.
Do not solve.
2–48 Water flows through a pipe at an average temperature
of T
50°C. The inner and outer radii of the pipe are r1
6.5 cm, respectively. The outer surface of
6 cm and r2
the pipe is wrapped with a thin electric heater that consumes
300 W per m length of the pipe. The exposed surface of the
heater is heavily insulated so that the entire heat generated in
the heater is transferred to the pipe. Heat is transferred from the
inner surface of the pipe to the water by convection with a heat
transfer coefficient of h 55 W/m2 · °C. Assuming constant
thermal conductivity and onedimensional heat transfer, express the mathematical formulation (the differential equation
and the boundary conditions) of the heat conduction in the pipe
during steady operation. Do not solve. Insulation h T∞
r1
0
Water r2
r
Electric heater FIGURE P2–48 2–49 A spherical metal ball of radius r0 is heated in an oven
to a temperature of Ti throughout and is then taken out of the
oven and dropped into a large body of water at T where it is
cooled by convection with an average convection heat transfer
coefficient of h. Assuming constant thermal conductivity and
transient onedimensional heat transfer, express the mathematical formulation (the differential equation and the boundary
and initial conditions) of this heat conduction problem. Do not
solve.
2–50 A spherical metal ball of radius r0 is heated in an oven
to a temperature of Ti throughout and is then taken out of the
oven and allowed to cool in ambient air at T by convection
and radiation. The emissivity of the outer surface of the cylinder is , and the temperature of the surrounding surfaces is
Tsurr. The average convection heat transfer coefficient is estimated to be h. Assuming variable thermal conductivity and
transient onedimensional heat transfer, express the mathematical formulation (the differential equation and the boundary Tsurr
Radiation Convection
T∞
h Metal
ball
r0 0 r Ti FIGURE P2–50
and initial conditions) of this heat conduction problem. Do not
solve.
2–51 Consider the north wall of a house of thickness L. The
outer surface of the wall exchanges heat by both convection
and radiation. The interior of the house is maintained at T 1,
while the ambient air temperature outside remains at T 2. The
sky, the ground, and the surfaces of the surrounding structures
at this location can be modeled as a surface at an effective temperature of Tsky for radiation exchange on the outer surface.
The radiation exchange between the inner surface of the wall
and the surfaces of the walls, floor, and ceiling it faces is negligible. The convection heat transfer coefficients on the inner
and outer surfaces of the wall are h1 and h2, respectively. The
thermal conductivity of the wall material is k and the emissivity of the outer surface is 2. Assuming the heat transfer
through the wall to be steady and onedimensional, express the
mathematical formulation (the differential equation and the
boundary and initial conditions) of this heat conduction problem. Do not solve. Tsky
Wall h1
T∞1 h2
T∞2 0 FIGURE P2–51 L x cen58933_ch02.qxd 9/10/2002 8:47 AM Page 118 118
HEAT TRANSFER Solution of Steady OneDimensional
Heat Conduction Problems Base
plate 2–52C Consider onedimensional heat conduction through a
large plane wall with no heat generation that is perfectly insulated on one side and is subjected to convection and radiation
on the other side. It is claimed that under steady conditions, the
temperature in a plane wall must be uniform (the same everywhere). Do you agree with this claim? Why? 85°C 2–53C It is stated that the temperature in a plane wall with
constant thermal conductivity and no heat generation varies
linearly during steady onedimensional heat conduction. Will
this still be the case when the wall loses heat by radiation from
its surfaces?
2–54C Consider a solid cylindrical rod whose ends are maintained at constant but different temperatures while the side surface is perfectly insulated. There is no heat generation. It is
claimed that the temperature along the axis of the rod varies
linearly during steady heat conduction. Do you agree with this
claim? Why?
2–55C Consider a solid cylindrical rod whose side surface is
maintained at a constant temperature while the end surfaces are
perfectly insulated. The thermal conductivity of the rod material is constant and there is no heat generation. It is claimed that
the temperature in the radial direction within the rod will not
vary during steady heat conduction. Do you agree with this
claim? Why?
2–56 Consider a large plane wall of thickness L
0.4 m,
thermal conductivity k 2.3 W/m · °C, and surface area A
20 m2. The left side of the wall is maintained at a constant temperature of T1 80°C while the right side loses heat by con15°C with a heat transfer
vection to the surrounding air at T
coefficient of h 24 W/m2 · °C. Assuming constant thermal
conductivity and no heat generation in the wall, (a) express the
differential equation and the boundary conditions for steady
onedimensional heat conduction through the wall, (b) obtain a
relation for the variation of temperature in the wall by solving
the differential equation, and (c) evaluate the rate of heat transAnswer: (c) 6030 W
fer through the wall. 0 FIGURE P2–59
mal conductivity of k 20 W/m · °C. The inner surface of the
base plate is subjected to uniform heat flux generated by the resistance heaters inside. When steady operating conditions are
reached, the outer surface temperature of the plate is measured
to be 85°C. Disregarding any heat loss through the upper part
of the iron, (a) express the differential equation and the boundary conditions for steady onedimensional heat conduction
through the plate, (b) obtain a relation for the variation of temperature in the base plate by solving the differential equation,
and (c) evaluate the inner surface temperature.
Answer: (c) 100°C 2–60 Repeat Problem 2–59 for a 1200W iron. 2–61 Reconsider Problem 2–59. Using the relation obtained for the variation of temperature in the base
plate, plot the temperature as a function of the distance x in the
range of x 0 to x L, and discuss the results. Use the EES
(or other) software.
2–62E Consider a steam pipe of length L 15 ft, inner radius r1 2 in., outer radius r2 2.4 in., and thermal conductivity k 7.2 Btu/h · ft · °F. Steam is flowing through the pipe
at an average temperature of 250°F, and the average convection
heat transfer coefficient on the inner surface is given to be h
1.25 Btu/h · ft2 · °F . If the average temperature on the outer 2–57 Consider a solid cylindrical rod of length 0.15 m and
diameter 0.05 m. The top and bottom surfaces of the rod are
maintained at constant temperatures of 20°C and 95°C, respectively, while the side surface is perfectly insulated. Determine the rate of heat transfer through the rod if it is made of
(a) copper, k 380 W/m · °C, (b) steel, k 18 W/m · °C, and
(c) granite, k 1.2 W/m · °C.
2–58 Reconsider Problem 2–57. Using EES (or other)
software, plot the rate of heat transfer as a function of the thermal conductivity of the rod in the range of
1 W/m · °C to 400 W/m · °C. Discuss the results.
2–59 Consider the base plate of a 800W household iron with
a thickness of L 0.6 cm, base area of A 160 cm2, and ther x L L T2 = 160°F Steam
250°F h
0
h r1 r2 FIGURE P2–62E r cen58933_ch02.qxd 9/10/2002 8:47 AM Page 119 119
CHAPTER 1 surfaces of the pipe is T2 160°F, (a) express the differential
equation and the boundary conditions for steady onedimensional heat conduction through the pipe, (b) obtain a relation for the variation of temperature in the pipe by solving the
differential equation, and (c) evaluate the rate of heat loss from
Answer: (c) 16,800 Btu/h
the steam through the pipe.
2–63 A spherical container of inner radius r1 2 m, outer radius r2 2.1 m, and thermal conductivity k 30 W/m · °C is
filled with iced water at 0°C. The container is gaining heat by
25°C with a heat
convection from the surrounding air at T
transfer coefficient of h 18 W/m2 · °C. Assuming the inner
surface temperature of the container to be 0°C, (a) express the
differential equation and the boundary conditions for steady
onedimensional heat conduction through the container, (b) obtain a relation for the variation of temperature in the container
by solving the differential equation, and (c) evaluate the rate of
heat gain to the iced water.
2–64 Consider a large plane wall of thickness L
0.3 m,
thermal conductivity k 2.5 W/m · °C, and surface area A
12 m2. The left side of the wall at x 0 is subjected to a net
heat flux of q·0 700 W/m2 while the temperature at that surface is measured to be T1 80°C. Assuming constant thermal
conductivity and no heat generation in the wall, (a) express the
differential equation and the boundary conditions for steady
onedimensional heat conduction through the wall, (b) obtain a
relation for the variation of temperature in the wall by solving
the differential equation, and (c) evaluate the temperature of
Answer: (c) 4°C
the right surface of the wall at x L.
·
q0
T1
0 L x Tsky
Radiation
x h, T∞
Convection 75°F ε L Plate
0 Ground FIGURE P2–66E
the differential equation, and (c) determine the value of the
lower surface temperature of the plate at x 0.
2–67E Repeat Problem 2–66E by disregarding radiation heat
transfer.
2–68 When a long section of a compressed air line passes
through the outdoors, it is observed that the moisture in the
compressed air freezes in cold weather, disrupting and even
completely blocking the air flow in the pipe. To avoid this
problem, the outer surface of the pipe is wrapped with electric
strip heaters and then insulated.
Consider a compressed air pipe of length L 6 m, inner radius r1 3.7 cm, outer radius r2 4.0 cm, and thermal conductivity k 14 W/m · °C equipped with a 300W strip heater.
Air is flowing through the pipe at an average temperature of
10°C, and the average convection heat transfer coefficient on
the inner surface is h 30 W/m2 · °C. Assuming 15 percent of
the heat generated in the strip heater is lost through the insulation, (a) express the differential equation and the boundary
conditions for steady onedimensional heat conduction through
the pipe, (b) obtain a relation for the variation of temperature in
the pipe material by solving the differential equation, and
(c) evaluate the inner and outer surface temperatures of the
Answers: (c) 3.91°C, 3.87°C
pipe.
r FIGURE P2–64 r2 2–65 Repeat Problem 2–64 for a heat flux of 950 W/m2 and
a surface temperature of 85°C at the left surface at x 0.
2–66E A large steel plate having a thickness of L
4 in.,
thermal conductivity of k 7.2 Btu/h · ft · °F, and an emissivity of
0.6 is lying on the ground. The exposed surface of
the plate at x
L is known to exchange heat by convection
with the ambient air at T
90°F with an average heat transfer
coefficient of h 12 Btu/h · ft2 · °F as well as by radiation with
the open sky with an equivalent sky temperature of Tsky
510 R. Also, the temperature of the upper surface of the plate is
measured to be 75°F. Assuming steady onedimensional heat
transfer, (a) express the differential equation and the boundary
conditions for heat conduction through the plate, (b) obtain a
relation for the variation of temperature in the plate by solving Electric heater r1
0
Compressed air – 10°C Insulation FIGURE P2–68
2–69 Reconsider Problem 2–68. Using the relation obtained for the variation of temperature in the pipe
material, plot the temperature as a function of the radius r in cen58933_ch02.qxd 9/10/2002 8:47 AM Page 120 120
HEAT TRANSFER the range of r r1 to r
EES (or other) software. r2, and discuss the results. Use the 2–70 In a food processing facility, a spherical container of
inner radius r1 40 cm, outer radius r2 41 cm, and thermal
conductivity k 1.5 W/m · °C is used to store hot water and to
keep it at 100°C at all times. To accomplish this, the outer surface of the container is wrapped with a 500W electric strip
heater and then insulated. The temperature of the inner surface
of the container is observed to be nearly 100°C at all times. Assuming 10 percent of the heat generated in the heater is lost
through the insulation, (a) express the differential equation and
the boundary conditions for steady onedimensional heat conduction through the container, (b) obtain a relation for the variation of temperature in the container material by solving the
differential equation, and (c) evaluate the outer surface temperature of the container. Also determine how much water at
100°C this tank can supply steadily if the cold water enters
at 20°C. 2–76C Consider uniform heat generation in a cylinder and a
sphere of equal radius made of the same material in the same
environment. Which geometry will have a higher temperature
at its center? Why?
2–77 A 2kW resistance heater wire with thermal conductivity of k 20 W/m · °C, a diameter of D 5 mm, and a length
of L 0.7 m is used to boil water. If the outer surface temperature of the resistance wire is Ts 110°C, determine the temperature at the center of the wire. 110°C 0 r
D Insulation
Electric
heater Hot
water
0 Resistance
heater r1 r2 FIGURE P2–77
r 100°C 2–78 Consider a long solid cylinder of radius r0 4 cm and
thermal conductivity k 25 W/m · °C. Heat is generated in the
·
cylinder uniformly at a rate of g0 35 W/cm3. The side surface
of the cylinder is maintained at a constant temperature of Ts
80°C. The variation of temperature in the cylinder is given by Spherical
container T(r) FIGURE P2–70
2–71 Reconsider Problem 2–70. Using the relation obtained for the variation of temperature in the container material, plot the temperature as a function of the radius
r in the range of r r1 to r r2, and discuss the results. Use
the EES (or other) software. Heat Generation in a Solid
2–72C Does heat generation in a solid violate the first law of
thermodynamics, which states that energy cannot be created or
destroyed? Explain.
2–73C What is heat generation? Give some examples.
2–74C An iron is left unattended and its base temperature
rises as a result of resistance heating inside. When will the rate
of heat generation inside the iron be equal to the rate of heat
loss from the iron?
2–75C Consider the uniform heating of a plate in an environment at a constant temperature. Is it possible for part of the
heat generated in the left half of the plate to leave the plate
through the right surface? Explain. ·
g r02
1
k r
r0 2 Ts Based on this relation, determine (a) if the heat conduction is
steady or transient, (b) if it is one, two, or threedimensional,
and (c) the value of heat flux on the side surface of the cylinder
at r r0.
2–79 Reconsider Problem 2–78. Using the relation
obtained for the variation of temperature in the
cylinder, plot the temperature as a function of the radius r in
the range of r 0 to r r0, and discuss the results. Use the
EES (or other) software.
2–80E A long homogeneous resistance wire of radius r0
0.25 in. and thermal conductivity k 8.6 Btu/h · ft · °F is being
used to boil water at atmospheric pressure by the passage of
r
Water r0
0
Resistance
heater FIGURE P2–80E T∞
h cen58933_ch02.qxd 9/10/2002 8:47 AM Page 121 121
CHAPTER 1 electric current. Heat is generated in the wire uniformly as a
·
result of resistance heating at a rate of g 1800 Btu/h · in3.
The heat generated is transferred to water at 212°F by convection with an average heat transfer coefficient of h
820
Btu/h · ft2 · °F. Assuming steady onedimensional heat transfer,
(a) express the differential equation and the boundary conditions for heat conduction through the wire, (b) obtain a relation
for the variation of temperature in the wire by solving the differential equation, and (c) determine the temperature at the
Answer: (c) 290.8°F
centerline of the wire.
2–81E Reconsider Problem 2–80E. Using the relation
obtained for the variation of temperature in the
wire, plot the temperature at the centerline of the wire as a
·
function of the heat generation g in the range of 400 Btu/h · in3
3
to 2400 Btu/h · in , and discuss the results. Use the EES (or
other) software.
2–82 In a nuclear reactor, 1cmdiameter cylindrical uranium
rods cooled by water from outside serve as the fuel. Heat is
generated uniformly in the rods (k 29.5 W/m · °C) at a rate
of 7 107 W/m3. If the outer surface temperature of rods is
175°C, determine the temperature at their center.
175°C
·
g Uranium rod FIGURE P2–82
2–83 Consider a large 3cmthick stainless steel plate (k
15.1 W/m · °C) in which heat is generated uniformly at a rate
of 5 105 W/m3. Both sides of the plate are exposed to an environment at 30°C with a heat transfer coefficient of 60 W/m2
· °C. Explain where in the plate the highest and the lowest temperatures will occur, and determine their values.
2–84 Consider a large 5cmthick brass plate (k
111
W/m · °C) in which heat is generated uniformly at a rate of
2 105 W/m3. One side of the plate is insulated while the other
side is exposed to an environment at 25°C with a heat transfer coefficient of 44 W/m2 · °C. Explain where in the plate the
highest and the lowest temperatures will occur, and determine
their values.
2–85 Reconsider Problem 2–84. Using EES (or other)
software, investigate the effect of the heat transfer coefficient on the highest and lowest temperatures in the
plate. Let the heat transfer coefficient vary from 20 W/m2 · °C
to 100 W/m2 · °C. Plot the highest and lowest temperatures as
a function of the heat transfer coefficient, and discuss the
results.
2–86 A 6mlong 2kW electrical resistance wire is made of
0.2cmdiameter stainless steel (k 15.1 W/m · °C). The resistance wire operates in an environment at 30°C with a heat
transfer coefficient of 140 W/m2 · °C at the outer surface. Determine the surface temperature of the wire (a) by using the applicable relation and (b) by setting up the proper differential
Answers: (a) 409°C, (b) 409°C
equation and solving it.
2–87E Heat is generated uniformly at a rate of 3 kW per ft
length in a 0.08in.diameter electric resistance wire made of
nickel steel (k 5.8 Btu/h · ft · °F). Determine the temperature
difference between the centerline and the surface of the wire.
2–88E Repeat Problem 2–87E for a manganese wire (k
4.5 Btu/h · ft · °F).
2–89 Consider a homogeneous spherical piece of radioactive
material of radius r0 0.04 m that is generating heat at a con·
stant rate of g 4 107 W/m3. The heat generated is dissipated to the environment steadily. The outer surface of the
sphere is maintained at a uniform temperature of 80°C and
the thermal conductivity of the sphere is k 15 W/m · °C. Assuming steady onedimensional heat transfer, (a) express the
differential equation and the boundary conditions for heat conduction through the sphere, (b) obtain a relation for the variation of temperature in the sphere by solving the differential
equation, and (c) determine the temperature at the center of the
sphere.
80°C
·
g 0 Brass
plate r0 r ·
g
h
T∞ 0 L FIGURE P2–84 FIGURE P2–89
2–90 x Reconsider Problem 2–89. Using the relation obtained for the variation of temperature in the
sphere, plot the temperature as a function of the radius r in the
range of r 0 to r r0. Also, plot the center temperature of
the sphere as a function of the thermal conductivity in the
range of 10 W/m · °C to 400 W/m · C. Discuss the results. Use
the EES (or other) software. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 122 122
HEAT TRANSFER 2–91 A long homogeneous resistance wire of radius r0
5 mm is being used to heat the air in a room by the passage of
electric current. Heat is generated in the wire uniformly at a
·
rate of g 5 107 W/m3 as a result of resistance heating. If
the temperature of the outer surface of the wire remains at
180°C, determine the temperature at r 2 mm after steady operation conditions are reached. Take the thermal conductivity
Answer: 212.8°C
of the wire to be k 8 W/m · °C.
r
180°C
·
g r0
0 FIGURE P2–91
2–92 Consider a large plane wall of thickness L 0.05 m.
The wall surface at x 0 is insulated, while the surface at x
L is maintained at a temperature of 30°C. The thermal conductivity of the wall is k 30 W/m · °C, and heat is generated in
·
·
the wall at a rate of g g0e 0.5x/L W/m3 where g0 8 106
W/m3. Assuming steady onedimensional heat transfer, (a) express the differential equation and the boundary conditions for
heat conduction through the wall, (b) obtain a relation for the
variation of temperature in the wall by solving the differential
equation, and (c) determine the temperature of the insulated
Answer: (c) 314°C
surface of the wall.
2–93 Reconsider Problem 2–92. Using the relation
given for the heat generation in the wall, plot the
heat generation as a function of the distance x in the range of
x 0 to x L, and discuss the results. Use the EES (or other)
software. always equivalent to the conductivity value at the average temperature?
2–99 Consider a plane wall of thickness L whose thermal
conductivity varies in a specified temperature range as k(T)
k0(1
T2) where k0 and are two specified constants. The
wall surface at x 0 is maintained at a constant temperature of
T1, while the surface at x L is maintained at T2. Assuming
steady onedimensional heat transfer, obtain a relation for the
heat transfer rate through the wall.
2–100 Consider a cylindrical shell of length L, inner radius
r1, and outer radius r2 whose thermal conductivity varies
T)
linearly in a specified temperature range as k(T) k0(1
where k0 and are two specified constants. The inner surface
of the shell is maintained at a constant temperature of T1, while
the outer surface is maintained at T2. Assuming steady onedimensional heat transfer, obtain a relation for (a) the heat
transfer rate through the wall and (b) the temperature distribution T(r) in the shell. Cylindrical
shell
T2
T1
0 k(T) h
r1 r2 r FIGURE P2–100 Variable Thermal Conductivity, k (T )
2–94C Consider steady onedimensional heat conduction in
a plane wall, long cylinder, and sphere with constant thermal
conductivity and no heat generation. Will the temperature in
any of these mediums vary linearly? Explain.
2–95C Is the thermal conductivity of a medium, in general,
constant or does it vary with temperature?
2–96C Consider steady onedimensional heat conduction in
a plane wall in which the thermal conductivity varies linearly.
The error involved in heat transfer calculations by assuming
constant thermal conductivity at the average temperature is
(a) none, (b) small, or (c) significant.
2–97C The temperature of a plane wall during steady onedimensional heat conduction varies linearly when the thermal
conductivity is constant. Is this still the case when the thermal conductivity varies linearly with temperature?
2–98C When the thermal conductivity of a medium varies
linearly with temperature, is the average thermal conductivity 2–101 Consider a spherical shell of inner radius r1 and outer
radius r2 whose thermal conductivity varies linearly in a speciT) where k0 and
fied temperature range as k(T) k0(1
are two specified constants. The inner surface of the shell is
maintained at a constant temperature of T1 while the outer surface is maintained at T2. Assuming steady onedimensional
heat transfer, obtain a relation for (a) the heat transfer rate
through the shell and (b) the temperature distribution T(r) in
the shell.
2–102 Consider a 1.5mhigh and 0.6mwide plate whose
thickness is 0.15 m. One side of the plate is maintained at a
constant temperature of 500 K while the other side is maintained at 350 K. The thermal conductivity of the plate can be
assumed to vary linearly in that temperature range as k(T)
k0(1
T) where k0 25 W/m · K and
8.7 10 4 K 1.
Disregarding the edge effects and assuming steady onedimensional heat transfer, determine the rate of heat conducAnswer: 30,800 W
tion through the plate. cen58933_ch02.qxd 9/10/2002 8:47 AM Page 123 123
CHAPTER 1 2–103 Reconsider Problem 2–102. Using EES (or
other) software, plot the rate of heat conduction
through the plate as a function of the temperature of the hot
side of the plate in the range of 400 K to 700 K. Discuss the
results. A
h
T∞ m, C, Ti
T = T(t) Special Topic: Review of Differential Equations
2–104C Why do we often utilize simplifying assumptions
when we derive differential equations?
2–105C What is a variable? How do you distinguish a dependent variable from an independent one in a problem?
2–106C Can a differential equation involve more than one
independent variable? Can it involve more than one dependent
variable? Give examples.
2–107C What is the geometrical interpretation of a derivative? What is the difference between partial derivatives and ordinary derivatives?
2–108C What is the difference between the degree and the
order of a derivative?
2–109C Consider a function f(x, y) and its partial derivative
f/ x. Under what conditions will this partial derivative be
equal to the ordinary derivative df/dx?
2–110C Consider a function f(x) and its derivative df/dx.
Does this derivative have to be a function of x?
2–111C How is integration related to derivation? FIGURE P2–120
with a heat transfer coefficient of h. The temperature of the
metal object is observed to vary uniformly with time during
cooling. Writing an energy balance on the entire metal object,
derive the differential equation that describes the variation of
temperature of the ball with time, T(t). Assume constant thermal conductivity and no heat generation in the object. Do not
solve.
2–121 Consider a long rectangular bar of length a in the
xdirection and width b in the ydirection that is initially at a
uniform temperature of Ti. The surfaces of the bar at x 0 and
y 0 are insulated, while heat is lost from the other two surfaces by convection to the surrounding medium at temperature
T with a heat transfer coefficient of h. Assuming constant
thermal conductivity and transient twodimensional heat transfer with no heat generation, express the mathematical formulation (the differential equation and the boundary and initial
conditions) of this heat conduction problem. Do not solve. 2–112C What is the difference between an algebraic equation and a differential equation? y 2–113C What is the difference between an ordinary differential equation and a partial differential equation? b 2–114C
mined? Ti How is the order of a differential equation deter 2–115C How do you distinguish a linear differential equation
from a nonlinear one?
2–116C How do you recognize a linear homogeneous differential equation? Give an example and explain why it is linear
and homogeneous.
2–117C How do differential equations with constant coefficients differ from those with variable coefficients? Give an example for each type.
2–118C What kind of differential equations can be solved by
direct integration?
2–119C Consider a third order linear and homogeneous differential equation. How many arbitrary constants will its general solution involve? Review Problems
2–120 Consider a small hot metal object of mass m and specific heat C that is initially at a temperature of Ti. Now the object is allowed to cool in an environment at T by convection T∞ h 0 h
a x FIGURE P2–121
2–122 Consider a short cylinder of radius r0 and height H in
·
which heat is generated at a constant rate of g0. Heat is lost
from the cylindrical surface at r r0 by convection to the surrounding medium at temperature T with a heat transfer coefficient of h. The bottom surface of the cylinder at z
0 is
insulated, while the top surface at z H is subjected to uni·
form heat flux q h. Assuming constant thermal conductivity and
steady twodimensional heat transfer, express the mathematical
formulation (the differential equation and the boundary conditions) of this heat conduction problem. Do not solve.
2–123E Consider a large plane wall of thickness L 0.5 ft
and thermal conductivity k
1.2 Btu/h · ft · °F. The wall
is covered with a material that has an emissivity of
0.80
and a solar absorptivity of
0.45. The inner surface of the
wall is maintained at T1 520 R at all times, while the outer
surface is exposed to solar radiation that is incident at a rate of
q·solar 300 Btu/h · ft2. The outer surface is also losing heat by cen58933_ch02.qxd 9/10/2002 8:47 AM Page 124 124
HEAT TRANSFER Sun
Plate
·
qsolar
520 R 0 x L FIGURE P2–123E
radiation to deep space at 0 K. Determine the temperature of
the outer surface of the wall and the rate of heat transfer
through the wall when steady operating conditions are reached.
Answers: 530.9 R, 26.2 Btu/h · ft2 2–124E Repeat Problem 2–123E for the case of no solar
radiation incident on the surface.
2–125 Consider a steam pipe of length L, inner radius r1,
outer radius r2, and constant thermal conductivity k. Steam
flows inside the pipe at an average temperature of Ti with a
convection heat transfer coefficient of hi. The outer surface of
the pipe is exposed to convection to the surrounding air at a
temperature of T0 with a heat transfer coefficient of ho. Assuming steady onedimensional heat conduction through the pipe,
(a) express the differential equation and the boundary conditions for heat conduction through the pipe material, (b) obtain
a relation for the variation of temperature in the pipe material
by solving the differential equation, and (c) obtain a relation
for the temperature of the outer surface of the pipe. L 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 thickwalled spherical tank of inner radius r1
2 m, outer radius r2 2.1 m , and constant thermal conductivity k
18 W/m · °C. The tank is initially filled with liquid
nitrogen at 1 atm and 196°C, and is exposed to ambient air
20°C with a heat transfer coefficient of h
25
at T
W/m2 · °C. The inner surface temperature of the spherical tank
is observed to be almost the same as the temperature of the nitrogen inside. Assuming steady onedimensional heat transfer,
(a) express the differential equation and the boundary conditions for heat conduction through the tank, (b) obtain a relation
for the variation of temperature in the tank material by solving
the differential equation, and (c) determine the rate of evaporation of the liquid nitrogen in the tank as a result of the heat
Answer: (c) 1.32 kg/s
transfer from the ambient air.
2–127 Repeat Problem 2–126 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.
2–128 Consider a large plane wall of thickness L
0.4 m
and thermal conductivity k 8.4 W/m · °C. There is no access
to the inner side of the wall at x 0 and thus the thermal conditions on that surface are not known. However, the outer surface of the wall at x L, whose emissivity is
0.7, is known
to exchange heat by convection with ambient air at T
25°C
with an average heat transfer coefficient of h 14 W/m2 · °C
as well as by radiation with the surrounding surfaces at an average temperature of Tsurr 290 K. Further, the temperature of
45°C. Assuming
the outer surface is measured to be T2
steady onedimensional heat transfer, (a) express the differential equation and the boundary conditions for heat conduction
through the plate, (b) obtain a relation for the temperature of
the outer surface of the plate by solving the differential equation, and (c) evaluate the inner surface temperature of the wall
Answer: (c) 64.3°C
at x 0. Plane
wall
hi
Ti
0 h
r1 ho
r2 Tsurr
45°C T0 h
T∞ r FIGURE P2–125
0 2–126 The boiling temperature of nitrogen at atmospheric
pressure at sea level (1 atm pressure) is 196°C. Therefore, nitrogen is commonly used in low temperature scientific studies L FIGURE P2–128 x cen58933_ch02.qxd 9/10/2002 8:47 AM Page 125 125
CHAPTER 1 2–129 A 1000W iron is left on the iron board with its base
exposed to ambient air at 20°C. The base plate of the iron has
a thickness of L 0.5 cm, base area of A 150 cm2, and thermal conductivity of k 18 W/m · °C. The inner surface of the
base plate is subjected to uniform heat flux generated by the resistance heaters inside. The outer surface of the base plate
whose emissivity is
0.7, loses heat by convection to ambi22° C with an average heat transfer coefficient
ent air at T
of h 30 W/m2 · °C as well as by radiation to the surrounding
290 K. Dissurfaces at an average temperature of Tsurr
regarding any heat loss through the upper part of the iron,
(a) express the differential equation and the boundary conditions for steady onedimensional heat conduction through
the plate, (b) obtain a relation for the temperature of the outer
surface of the plate by solving the differential equation, and
(c) evaluate the outer surface temperature. Iron
base
plate 62°F, determine the
surface temperature of the roof is T1
outer surface temperature of the roof and the rate of heat loss
through the roof when steady operating conditions are reached.
2–132 Consider a long resistance wire of radius r1 0.3 cm
and thermal conductivity kwire 18 W/m · °C in which heat is
·
generated uniformly at a constant rate of g 1.5 W/cm3 as a
result of resistance heating. The wire is embedded in a 0.4cmthick layer of plastic whose thermal conductivity is kplastic 1.8
W/m · °C. The outer surface of the plastic cover loses heat by
25°C with an average
convection to the ambient air at T
combined heat transfer coefficient of h 14 W/m2 · °C. Assuming onedimensional heat transfer, determine the temperatures at the center of the resistance wire and the wireplastic
layer interface under steady conditions.
Answers: 97.1°C, 97.3°C T∞
h Tsurr Wire
·
g
r1 h
T∞ r2
r
Plastic cover 0 x L FIGURE P2–129
2–130 Repeat Problem 2–129 for a 1500W iron. 2–131E The roof of a house consists of a 0.8ftthick concrete slab (k 1.1 Btu/h · ft · °F) that is 25 ft wide and 35 ft
long. The emissivity of the outer surface of the roof is 0.8, and
the convection heat transfer coefficient on that surface is estimated to be 3.2 Btu/h · ft2 · °F. On a clear winter night, the ambient air is reported to be at 50°F, while the night sky
temperature for radiation heat transfer is 310 R. If the inner FIGURE P2–132
2–133 Consider a cylindrical shell of length L, inner radius
r1, and outer radius r2 whose thermal conductivity varies in
T 2) where
a specified temperature range as k(T)
k0(1
k0 and are two specified constants. The inner surface of the
shell is maintained at a constant temperature of T1 while
the outer surface is maintained at T2. Assuming steady onedimensional heat transfer, obtain a relation for the heat transfer
rate through the shell.
2–134 In a nuclear reactor, heat is generated in 1cmdiameter cylindrical uranium fuel rods at a rate of 4
107 W/m3. Determine the temperature difference between the
Answer: 9.0°C
center and the surface of the fuel rod. Tsky Ts
D y
T∞
h L Concrete
0 FIGURE P2–131E T0 Fuel rod ·
g FIGURE P2–134
2–135 Consider a 20cmthick large concrete plane wall
(k 0.77 W/m · °C) subjected to convection on both sides with
T 1 27°C and h1 5 W/m2 · °C on the inside, and T 2 8°C
12 W/m2 · °C on the outside. Assuming constant
and h2
thermal conductivity with no heat generation and negligible cen58933_ch02.qxd 9/10/2002 8:47 AM Page 126 126
HEAT TRANSFER radiation, (a) express the differential equations and the boundary conditions for steady onedimensional heat conduction
through the wall, (b) obtain a relation for the variation of temperature in the wall by solving the differential equation, and
(c) evaluate the temperatures at the inner and outer surfaces of
the wall.
2–136 Consider a water pipe of length L 12 m, inner radius r1 15 cm, outer radius r2 20 cm, and thermal conductivity k 20 W/m · °C. Heat is generated in the pipe material
uniformly by a 25kW electric resistance heater. The inner and
outer surfaces of the pipe are at T1 60°C and T2 80°C, respectively. Obtain a general relation for temperature distribution inside the pipe under steady conditions and determine the
temperature at the center plane of the pipe.
2–137 Heat is generated uniformly at a rate of 2.6
106
W/m3 in a spherical ball (k 45 W/m · °C) of diameter 30 cm.
The ball is exposed to icedwater at 0°C with a heat transfer coefficient of 1200 W/m2 · °C. Determine the temperatures at the
center and the surface of the ball. Computer, Design, and Essay Problems
2–138 Write an essay on heat generation in nuclear fuel rods.
Obtain information on the ranges of heat generation, the variation of heat generation with position in the rods, and the absorption of emitted radiation by the cooling medium. 2–139 Write an interactive computer program to calculate the heat transfer rate and the value of temperature anywhere in the medium for steady onedimensional
heat conduction in a long cylindrical shell for any combination
of specified temperature, specified heat flux, and convection
boundary conditions. Run the program for five different sets of
specified boundary conditions.
2–140 Write an interactive computer program to calculate the
heat transfer rate and the value of temperature anywhere in
the medium for steady onedimensional heat conduction in
a spherical shell for any combination of specified temperature, specified heat flux, and convection boundary conditions.
Run the program for five different sets of specified boundary
conditions.
2–141 Write an interactive computer program to calculate the
heat transfer rate and the value of temperature anywhere in the
medium for steady onedimensional heat conduction in a plane
wall whose thermal conductivity varies linearly as k(T)
k0(1
T) where the constants k0 and are specified by the
user for specified temperature boundary conditions. ...
View
Full Document
 Spring '10
 Ghaz
 Heat, Heat Transfer, heat sink

Click to edit the document details