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Unformatted text preview: cen58933_ch04.qxd 9/10/2002 9:12 AM Page 209 CHAPTER T R A N S I E N T H E AT CONDUCTION he temperature of a body, in general, varies with time as well as position. In rectangular coordinates, this variation is expressed as T(x, y, z, t), where (x, y, z) indicates variation in the x, y, and z directions, respectively, and t indicates variation with time. In the preceding chapter, we considered heat conduction under steady conditions, for which the temperature of a body at any point does not change with time. This certainly simplified the analysis, especially when the temperature varied in one direction only, and we were able to obtain analytical solutions. In this chapter, we consider the variation of temperature with time as well as position in one- and multidimensional systems. We start this chapter with the analysis of lumped systems in which the temperature of a solid varies with time but remains uniform throughout the solid at any time. Then we consider the variation of temperature with time as well as position for one-dimensional heat conduction problems such as those associated with a large plane wall, a long cylinder, a sphere, and a semi-infinite medium using transient temperature charts and analytical solutions. Finally, we consider transient heat conduction in multidimensional systems by utilizing the product solution. T 4 CONTENTS 4–1 Lumped Systems Analysis 210 4–2 Transient Heat Conduction in Large Plane Walls, Long Cylinders, and Spheres with Spatial Effects 216 4–3 Transient Heat Conduction in Semi-Infinite Solids 228 4–4 Transient Heat Conduction in Multidimensional Systems 231 Topic of Special Interest: Refrigeration and Freezing of Foods 239 209 cen58933_ch04.qxd 9/10/2002 9:12 AM Page 210 210 HEAT TRANSFER 4–1 70°C 70°C 70°C 70°C 70°C (a) Copper ball 110°C 90°C 40°C (b) Roast beef FIGURE 4–1 A small copper ball can be modeled as a lumped system, but a roast beef cannot. As SOLID BODY h T m = mass V = volume ρ = density Ti = initial temperature T = T(t) · Q = h As[T – T(t)] FIGURE 4–2 The geometry and parameters involved in the lumped system analysis. I LUMPED SYSTEM ANALYSIS In heat transfer analysis, some bodies are observed to behave like a “lump” whose interior temperature remains essentially uniform at all times during a heat transfer process. The temperature of such bodies can be taken to be a function of time only, T(t). Heat transfer analysis that utilizes this idealization is known as lumped system analysis, which provides great simplification in certain classes of heat transfer problems without much sacrifice from accuracy. Consider a small hot copper ball coming out of an oven (Fig. 4–1). Measurements indicate that the temperature of the copper ball changes with time, but it does not change much with position at any given time. Thus the temperature of the ball remains uniform at all times, and we can talk about the temperature of the ball with no reference to a specific location. Now let us go to the other extreme and consider a large roast in an oven. If you have done any roasting, you must have noticed that the temperature distribution within the roast is not even close to being uniform. You can easily verify this by taking the roast out before it is completely done and cutting it in half. You will see that the outer parts of the roast are well done while the center part is barely warm. Thus, lumped system analysis is not applicable in this case. Before presenting a criterion about applicability of lumped system analysis, we develop the formulation associated with it. Consider a body of arbitrary shape of mass m, volume V, surface area As, density , and specific heat Cp initially at a uniform temperature Ti (Fig. 4–2). At time t 0, the body is placed into a medium at temperature T , and heat transfer takes place between the body and its environment, with a heat transTi, but fer coefficient h. For the sake of discussion, we will assume that T the analysis is equally valid for the opposite case. We assume lumped system analysis to be applicable, so that the temperature remains uniform within the body at all times and changes with time only, T T(t). During a differential time interval dt, the temperature of the body rises by a differential amount dT. An energy balance of the solid for the time interval dt can be expressed as Heat transfer into the body during dt The increase in the energy of the body during dt or hAs(T Noting that m rearranged as V and dT d(T d(T T Integrating from t T) dt ln T ) since T T) T 0, at which T mCp dT (4-1) constant, Eq. 4–1 can be hAs dt VCp Ti, to any time t, at which T T(t) T Ti T hAs t VCp (4-2) T(t), gives (4-3) cen58933_ch04.qxd 9/10/2002 9:12 AM Page 211 211 CHAPTER 4 Taking the exponential of both sides and rearranging, we obtain T(t) T Ti T e bt T(t) (4-4) T b3 b2 where b hAs VCp (1/s) (4-5) is a positive quantity whose dimension is (time) 1. The reciprocal of b has time unit (usually s), and is called the time constant. Equation 4–4 is plotted in Fig. 4–3 for different values of b. There are two observations that can be made from this figure and the relation above: 1. Equation 4–4 enables us to determine the temperature T(t) of a body at time t, or alternatively, the time t required for the temperature to reach a specified value T(t). 2. The temperature of a body approaches the ambient temperature T exponentially. The temperature of the body changes rapidly at the beginning, but rather slowly later on. A large value of b indicates that the body will approach the environment temperature in a short time. The larger the value of the exponent b, the higher the rate of decay in temperature. Note that b is proportional to the surface area, but inversely proportional to the mass and the specific heat of the body. This is not surprising since it takes longer to heat or cool a larger mass, especially when it has a large specific heat. b1 b3 > b2 > b1 Ti t FIGURE 4–3 The temperature of a lumped system approaches the environment temperature as time gets larger. Once the temperature T(t) at time t is available from Eq. 4–4, the rate of convection heat transfer between the body and its environment at that time can be determined from Newton’s law of cooling as · Q (t) hAs[T(t) T] (W) (4-6) The total amount of heat transfer between the body and the surrounding medium over the time interval t 0 to t is simply the change in the energy content of the body: Q mCp[T(t) Ti] (kJ) (4-7) The amount of heat transfer reaches its upper limit when the body reaches the surrounding temperature T . Therefore, the maximum heat transfer between the body and its surroundings is (Fig. 4–4) Qmax mCp(T Ti) (kJ) (4-8) Ti Ti Criteria for Lumped System Analysis The lumped system analysis certainly provides great convenience in heat transfer analysis, and naturally we would like to know when it is appropriate t→ Ti Ti Ti We could also obtain this equation by substituting the T(t) relation from Eq. · 4–4 into the Q (t) relation in Eq. 4–6 and integrating it from t 0 to t → . h T t=0 T T Ti Ti T T T T T Q = Qmax = mCp (Ti – T ) FIGURE 4–4 Heat transfer to or from a body reaches its maximum value when the body reaches the environment temperature. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 212 212 HEAT TRANSFER to use it. The first step in establishing a criterion for the applicability of the lumped system analysis is to define a characteristic length as Convection h T SOLID BODY Lc V As Bi Conduction hLc k and a Biot number Bi as (4-9) It can also be expressed as (Fig. 4–5) heat convection Bi = ———————– heat conduction FIGURE 4–5 The Biot number can be viewed as the ratio of the convection at the surface to conduction within the body. Bi h T k /Lc T Convection at the surface of the body Conduction within the body or Bi Lc /k 1/h Conduction resistance within the body Convection resistance at the surface of the body When a solid body is being heated by the hotter fluid surrounding it (such as a potato being baked in an oven), heat is first convected to the body and subsequently conducted within the body. The Biot number is the ratio of the internal resistance of a body to heat conduction to its external resistance to heat convection. Therefore, a small Biot number represents small resistance to heat conduction, and thus small temperature gradients within the body. Lumped system analysis assumes a uniform temperature distribution throughout the body, which will be the case only when the thermal resistance of the body to heat conduction (the conduction resistance) is zero. Thus, lumped system analysis is exact when Bi 0 and approximate when Bi 0. Of course, the smaller the Bi number, the more accurate the lumped system analysis. Then the question we must answer is, How much accuracy are we willing to sacrifice for the convenience of the lumped system analysis? Before answering this question, we should mention that a 20 percent uncertainty in the convection heat transfer coefficient h in most cases is considered “normal” and “expected.” Assuming h to be constant and uniform is also an approximation of questionable validity, especially for irregular geometries. Therefore, in the absence of sufficient experimental data for the specific geometry under consideration, we cannot claim our results to be better than 20 percent, even when Bi 0. This being the case, introducing another source of uncertainty in the problem will hardly have any effect on the overall uncertainty, provided that it is minor. It is generally accepted that lumped system analysis is applicable if Bi 0.1 When this criterion is satisfied, the temperatures within the body relative to the surroundings (i.e., T T ) remain within 5 percent of each other even for well-rounded geometries such as a spherical ball. Thus, when Bi 0.1, the variation of temperature with location within the body will be slight and can reasonably be approximated as being uniform. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 213 213 CHAPTER 4 The first step in the application of lumped system analysis is the calculation of the Biot number, and the assessment of the applicability of this approach. One may still wish to use lumped system analysis even when the criterion Bi 0.1 is not satisfied, if high accuracy is not a major concern. Note that the Biot number is the ratio of the convection at the surface to conduction within the body, and this number should be as small as possible for lumped system analysis to be applicable. Therefore, small bodies with high thermal conductivity are good candidates for lumped system analysis, especially when they are in a medium that is a poor conductor of heat (such as air or another gas) and motionless. Thus, the hot small copper ball placed in quiescent air, discussed earlier, is most likely to satisfy the criterion for lumped system analysis (Fig. 4–6). Some Remarks on Heat Transfer in Lumped Systems To understand the heat transfer mechanism during the heating or cooling of a solid by the fluid surrounding it, and the criterion for lumped system analysis, consider this analogy (Fig. 4–7). People from the mainland are to go by boat to an island whose entire shore is a harbor, and from the harbor to their destinations on the island by bus. The overcrowding of people at the harbor depends on the boat traffic to the island and the ground transportation system on the island. If there is an excellent ground transportation system with plenty of buses, there will be no overcrowding at the harbor, especially when the boat traffic is light. But when the opposite is true, there will be a huge overcrowding at the harbor, creating a large difference between the populations at the harbor and inland. The chance of overcrowding is much lower in a small island with plenty of fast buses. In heat transfer, a poor ground transportation system corresponds to poor heat conduction in a body, and overcrowding at the harbor to the accumulation of heat and the subsequent rise in temperature near the surface of the body relative to its inner parts. Lumped system analysis is obviously not applicable when there is overcrowding at the surface. Of course, we have disregarded radiation in this analogy and thus the air traffic to the island. Like passengers at the harbor, heat changes vehicles at the surface from convection to conduction. Noting that a surface has zero thickness and thus cannot store any energy, heat reaching the surface of a body by convection must continue its journey within the body by conduction. Consider heat transfer from a hot body to its cooler surroundings. Heat will be transferred from the body to the surrounding fluid as a result of a temperature difference. But this energy will come from the region near the surface, and thus the temperature of the body near the surface will drop. This creates a temperature gradient between the inner and outer regions of the body and initiates heat flow by conduction from the interior of the body toward the outer surface. When the convection heat transfer coefficient h and thus convection heat transfer from the body are high, the temperature of the body near the surface will drop quickly (Fig. 4–8). This will create a larger temperature difference between the inner and outer regions unless the body is able to transfer heat from the inner to the outer regions just as fast. Thus, the magnitude of the maximum temperature difference within the body depends strongly on the ability of a body to conduct heat toward its surface relative to the ability of h = 15 W/m2 ·°C Spherical copper ball k = 401 W/ m·°C D = 12 cm 1 3 V – πD 1 6 Lc = — = —— = – D = 0.02 m 2 6 As π D hLc 15 × 0.02 Bi = —– = ———— = 0.00075 < 0.1 k 401 FIGURE 4–6 Small bodies with high thermal conductivities and low convection coefficients are most likely to satisfy the criterion for lumped system analysis. Boat Bus ISLAND FIGURE 4–7 Analogy between heat transfer to a solid and passenger traffic to an island. T = 20°C 50°C 70°C 85°C 110°C 130°C Convection h = 2000 W/ m2 ·°C FIGURE 4–8 When the convection coefficient h is high and k is low, large temperature differences occur between the inner and outer regions of a large solid. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 214 214 HEAT TRANSFER the surrounding medium to convect this heat away from the surface. The Biot number is a measure of the relative magnitudes of these two competing effects. Recall that heat conduction in a specified direction n per unit surface area is · expressed as q k T/ n, where T/ n is the temperature gradient and k is the thermal conductivity of the solid. Thus, the temperature distribution in the body will be uniform only when its thermal conductivity is infinite, and no such material is known to exist. Therefore, temperature gradients and thus temperature differences must exist within the body, no matter how small, in order for heat conduction to take place. Of course, the temperature gradient and the thermal conductivity are inversely proportional for a given heat flux. Therefore, the larger the thermal conductivity, the smaller the temperature gradient. EXAMPLE 4–1 Thermocouple wire Gas T ,h Junction D = 1 mm T(t) FIGURE 4–9 Schematic for Example 4–1. Temperature Measurement by Thermocouples The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a 1-mm-diameter sphere, as shown in Fig. 4–9. The properties of the junction are k 35 W/m · °C, 8500 kg/m3, and Cp 320 J/kg · °C, and the convection heat transfer coefficient between the junction and the gas is h 210 W/m2 · °C. Determine how long it will take for the thermocouple to read 99 percent of the initial temperature difference. SOLUTION The temperature of a gas stream is to be measured by a thermocouple. The time it takes to register 99 percent of the initial T is to be determined. Assumptions 1 The junction is spherical in shape with a diameter of D 0.001 m. 2 The thermal properties of the junction and the heat transfer coefficient are constant. 3 Radiation effects are negligible. Properties The properties of the junction are given in the problem statement. Analysis The characteristic length of the junction is Lc V As 1 6 D3 1 D 6 D2 1 (0.001 m) 6 1.67 10 4 m Then the Biot number becomes Bi hLc k (210 W/m2 · °C)(1.67 10 35 W/m · °C 4 m) 0.001 0.1 Therefore, lumped system analysis is applicable, and the error involved in this approximation is negligible. In order to read 99 percent of the initial temperature difference Ti T between the junction and the gas, we must have T (t ) T Ti T 0.01 For example, when Ti 0°C and T 100°C, a thermocouple is considered to have read 99 percent of this applied temperature difference when its reading indicates T (t ) 99°C. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 215 215 CHAPTER 4 The value of the exponent b is hAs CpV b h Cp Lc 210 W/m2 · °C (8500 kg/m )(320 J/kg · °C)(1.67 3 10 4 m) 0.462 s 1 We now substitute these values into Eq. 4–4 and obtain T (t ) T Ti T e → bt 0.01 e (0.462 s 1)t which yields t 10 s Therefore, we must wait at least 10 s for the temperature of the thermocouple junction to approach within 1 percent of the initial junction-gas temperature difference. Discussion Note that conduction through the wires and radiation exchange with the surrounding surfaces will affect the result, and should be considered in a more refined analysis. EXAMPLE 4–2 Predicting the Time of Death A person is found dead at 5 PM in a room whose temperature is 20°C. The temperature of the body is measured to be 25°C when found, and the heat transfer coefficient is estimated to be h 8 W/m2 · °C. Modeling the body as a 30-cm-diameter, 1.70-m-long cylinder, estimate the time of death of that person (Fig. 4–10). SOLUTION A body is found while still warm. The time of death is to be estimated. Assumptions 1 The body can be modeled as a 30-cm-diameter, 1.70-m-long cylinder. 2 The thermal properties of the body and the heat transfer coefficient are constant. 3 The radiation effects are negligible. 4 The person was healthy(!) when he or she died with a body temperature of 37°C. Properties The average human body is 72 percent water by mass, and thus we can assume the body to have the properties of water at the average temperature of (37 25)/2 31°C; k 0.617 W/m · °C, 996 kg/m3, and Cp 4178 J/kg · °C (Table A-9). Analysis The characteristic length of the body is Lc V As ro2 L 2 ro L 2 ro2 (0.15 m)2(1.7 m) 2 (0.15 m)(1.7 m) 2 (0.15 m)2 Then the Biot number becomes Bi hLc k (8 W/m2 · °C)(0.0689 m) 0.617 W/m · °C 0.89 0.1 0.0689 m FIGURE 4–10 Schematic for Example 4–2. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 216 216 HEAT TRANSFER Therefore, lumped system analysis is not applicable. However, we can still use it to get a “rough” estimate of the time of death. The exponent b in this case is b hAs CpV 2.79 8 W/m2 · °C (996 kg/m )(4178 J/kg · °C)(0.0689 m) h Cp Lc 10 5 3 1 s We now substitute these values into Eq. 4–4, T (t ) T Ti T 25 37 bt → t e 43,860 s 20 20 e (2.79 10 5 s 1)t which yields 12.2 h Therefore, as a rough estimate, the person died about 12 h before the body was found, and thus the time of death is 5 AM. This example demonstrates how to obtain “ball park” values using a simple analysis. 4–2 I TRANSIENT HEAT CONDUCTION IN LARGE PLANE WALLS, LONG CYLINDERS, AND SPHERES WITH SPATIAL EFFECTS In Section, 4–1, we considered bodies in which the variation of temperature within the body was negligible; that is, bodies that remain nearly isothermal during a process. Relatively small bodies of highly conductive materials approximate this behavior. In general, however, the temperature within a body will change from point to point as well as with time. In this section, we consider the variation of temperature with time and position in one-dimensional problems such as those associated with a large plane wall, a long cylinder, and a sphere. Consider a plane wall of thickness 2L, a long cylinder of radius ro, and a sphere of radius ro initially at a uniform temperature Ti, as shown in Fig. 4–11. At time t 0, each geometry is placed in a large medium that is at a constant temperature T and kept in that medium for t 0. Heat transfer takes place between these bodies and their environments by convection with a uniform and constant heat transfer coefficient h. Note that all three cases possess geometric and thermal symmetry: the plane wall is symmetric about its center plane (x 0), the cylinder is symmetric about its centerline (r 0), and the sphere is symmetric about its center point (r 0). We neglect radiation heat transfer between these bodies and their surrounding surfaces, or incorporate the radiation effect into the convection heat transfer coefficient h. The variation of the temperature profile with time in the plane wall is illustrated in Fig. 4–12. When the wall is first exposed to the surrounding Ti at t 0, the entire wall is at its initial temperature Ti. But medium at T the wall temperature at and near the surfaces starts to drop as a result of heat transfer from the wall to the surrounding medium. This creates a temperature cen58933_ch04.qxd 9/10/2002 9:12 AM Page 217 217 CHAPTER 4 T h Initially T = Ti 0 T h T h Lx (a) A large plane wall Initially T = Ti T h T Initially T = Ti 0 r 0 h ro ro r (b) A long cylinder (c) A sphere gradient in the wall and initiates heat conduction from the inner parts of the wall toward its outer surfaces. Note that the temperature at the center of the wall remains at Ti until t t2, and that the temperature profile within the wall remains symmetric at all times about the center plane. The temperature profile gets flatter and flatter as time passes as a result of heat transfer, and eventually becomes uniform at T T . That is, the wall reaches thermal equilibrium with its surroundings. At that point, the heat transfer stops since there is no longer a temperature difference. Similar discussions can be given for the long cylinder or sphere. The formulation of the problems for the determination of the onedimensional transient temperature distribution T(x, t) in a wall results in a partial differential equation, which can be solved using advanced mathematical techniques. The solution, however, normally involves infinite series, which are inconvenient and time-consuming to evaluate. Therefore, there is clear motivation to present the solution in tabular or graphical form. However, the solution involves the parameters x, L, t, k, , h, Ti, and T , which are too many to make any graphical presentation of the results practical. In order to reduce the number of parameters, we nondimensionalize the problem by defining the following dimensionless quantities: Dimensionless temperature: (x, t) Dimensionless distance from the center: X Dimensionless heat transfer coefficient: Bi Dimensionless time: T(x, t) T Ti T x L hL k t L2 (Biot number) (Fourier number) The nondimensionalization enables us to present the temperature in terms of three parameters only: X, Bi, and . This makes it practical to present the solution in graphical form. The dimensionless quantities defined above for a plane wall can also be used for a cylinder or sphere by replacing the space variable x by r and the half-thickness L by the outer radius ro. Note that the characteristic length in the definition of the Biot number is taken to be the FIGURE 4–11 Schematic of the simple geometries in which heat transfer is one-dimensional. Ti t = t1 t=0 t = t2 t = t3 T 0 h Initially T = Ti t→ Lx T h FIGURE 4–12 Transient temperature profiles in a plane wall exposed to convection from its surfaces for Ti T . cen58933_ch04.qxd 9/10/2002 9:12 AM Page 218 218 HEAT TRANSFER half-thickness L for the plane wall, and the radius ro for the long cylinder and sphere instead of V/A used in lumped system analysis. The one-dimensional transient heat conduction problem just described can be solved exactly for any of the three geometries, but the solution involves infinite series, which are difficult to deal with. However, the terms in the solu0.2, keeping the first tions converge rapidly with increasing time, and for term and neglecting all the remaining terms in the series results in an error under 2 percent. We are usually interested in the solution for times with 0.2, and thus it is very convenient to express the solution using this oneterm approximation, given as Plane wall: Cylinder: Sphere: T(x, t) T Ti T T(r, t) T Ti T T(r, t) T Ti T (x, t)wall (r, t)cyl (r, t)sph A1e A1e A1e 2 1 2 1 2 1 cos ( 1x/L), J0( 1r/ro), 0.2 0.2 sin( 1r /ro) , 1r /ro 0.2 (4-10) (4-11) (4-12) where the constants A1 and 1 are functions of the Bi number only, and their values are listed in Table 4–1 against the Bi number for all three geometries. The function J0 is the zeroth-order Bessel function of the first kind, whose value can be determined from Table 4–2. Noting that cos (0) J0(0) 1 and the limit of (sin x)/x is also 1, these relations simplify to the next ones at the center of a plane wall, cylinder, or sphere: Center of plane wall (x Center of cylinder (r Center of sphere (r 0): 0): 0): 0, wall 0, cyl 0, sph To Ti To Ti To Ti T T T T T T A1e A1e A1e 2 1 (4-13) 2 1 (4-14) 2 1 (4-15) Once the Bi number is known, the above relations can be used to determine the temperature anywhere in the medium. The determination of the constants A1 and 1 usually requires interpolation. For those who prefer reading charts to interpolating, the relations above are plotted and the one-term approximation solutions are presented in graphical form, known as the transient temperature charts. Note that the charts are sometimes difficult to read, and they are subject to reading errors. Therefore, the relations above should be preferred to the charts. The transient temperature charts in Figs. 4–13, 4–14, and 4–15 for a large plane wall, long cylinder, and sphere were presented by M. P. Heisler in 1947 and are called Heisler charts. They were supplemented in 1961 with transient heat transfer charts by H. Gröber. There are three charts associated with each geometry: the first chart is to determine the temperature To at the center of the geometry at a given time t. The second chart is to determine the temperature at other locations at the same time in terms of To. The third chart is to determine the total amount of heat transfer up to the time t. These plots are valid for 0.2. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 219 219 CHAPTER 4 TABLE 4–1 TABLE 4–2 Coefficients used in the one-term approximate solution of transient onedimensional heat conduction in plane walls, cylinders, and spheres (Bi hL/k for a plane wall of thickness 2L, and Bi hro /k for a cylinder or sphere of radius ro ) The zeroth- and first-order Bessel functions of the first kind Plane Wall A1 1 Bi 0.01 0.02 0.04 0.06 0.08 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 10.0 20.0 30.0 40.0 50.0 100.0 0.0998 0.1410 0.1987 0.2425 0.2791 0.3111 0.4328 0.5218 0.5932 0.6533 0.7051 0.7506 0.7910 0.8274 0.8603 1.0769 1.1925 1.2646 1.3138 1.3496 1.3766 1.3978 1.4149 1.4289 1.4961 1.5202 1.5325 1.5400 1.5552 1.5708 1.0017 1.0033 1.0066 1.0098 1.0130 1.0161 1.0311 1.0450 1.0580 1.0701 1.0814 1.0918 1.1016 1.1107 1.1191 1.1785 1.2102 1.2287 1.2403 1.2479 1.2532 1.2570 1.2598 1.2620 1.2699 1.2717 1.2723 1.2727 1.2731 1.2732 Cylinder A1 1 0.1412 0.1995 0.2814 0.3438 0.3960 0.4417 0.6170 0.7465 0.8516 0.9408 1.0184 1.0873 1.1490 1.2048 1.2558 1.5995 1.7887 1.9081 1.9898 2.0490 2.0937 2.1286 2.1566 2.1795 2.2880 2.3261 2.3455 2.3572 2.3809 2.4048 Sphere 1.0025 1.0050 1.0099 1.0148 1.0197 1.0246 1.0483 1.0712 1.0931 1.1143 1.1345 1.1539 1.1724 1.1902 1.2071 1.3384 1.4191 1.4698 1.5029 1.5253 1.5411 1.5526 1.5611 1.5677 1.5919 1.5973 1.5993 1.6002 1.6015 1.6021 1 0.1730 0.2445 0.3450 0.4217 0.4860 0.5423 0.7593 0.9208 1.0528 1.1656 1.2644 1.3525 1.4320 1.5044 1.5708 2.0288 2.2889 2.4556 2.5704 2.6537 2.7165 2.7654 2.8044 2.8363 2.9857 3.0372 3.0632 3.0788 3.1102 3.1416 A1 1.0030 1.0060 1.0120 1.0179 1.0239 1.0298 1.0592 1.0880 1.1164 1.1441 1.1713 1.1978 1.2236 1.2488 1.2732 1.4793 1.6227 1.7202 1.7870 1.8338 1.8673 1.8920 1.9106 1.9249 1.9781 1.9898 1.9942 1.9962 1.9990 2.0000 Note that the case 1/Bi k/hL 0 corresponds to h → , which corresponds to the case of specified surface temperature T . That is, the case in which the surfaces of the body are suddenly brought to the temperature T at t 0 and kept at T at all times can be handled by setting h to infinity (Fig. 4–16). The temperature of the body changes from the initial temperature Ti to the temperature of the surroundings T at the end of the transient heat conduction process. Thus, the maximum amount of heat that a body can gain (or lose if Ti T ) is simply the change in the energy content of the body. That is, Qmax mCp(T Ti ) VCp(T Ti ) (kJ) (4-16) Jo( ) J1( ) 0.0 0.1 0.2 0.3 0.4 1.0000 0.9975 0.9900 0.9776 0.9604 0.0000 0.0499 0.0995 0.1483 0.1960 0.5 0.6 0.7 0.8 0.9 0.9385 0.9120 0.8812 0.8463 0.8075 0.2423 0.2867 0.3290 0.3688 0.4059 1.0 1.1 1.2 1.3 1.4 0.7652 0.7196 0.6711 0.6201 0.5669 0.4400 0.4709 0.4983 0.5220 0.5419 1.5 1.6 1.7 1.8 1.9 0.5118 0.4554 0.3980 0.3400 0.2818 0.5579 0.5699 0.5778 0.5815 0.5812 2.0 2.1 2.2 2.3 2.4 0.2239 0.1666 0.1104 0.0555 0.0025 0.5767 0.5683 0.5560 0.5399 0.5202 2.6 2.8 3.0 3.2 0.0968 0.1850 0.2601 0.3202 0.4708 0.4097 0.3391 0.2613 cen58933_ch04.qxd 9/10/2002 9:12 AM Page 220 220 HEAT TRANSFER To – T Ti – T 1.0 0.7 0.5 0.4 0.3 0.2 θo = k hL = 1 Bi = 0.6 0.4 0.7 0.5 0.3 35 7 6 25 30 16 3 2 1.8 1.6 1.4 1.2 0.05 2.5 0 2 50 40 20 18 5 4 0.2 0.1 1 45 9 8 8 0 12 10 0. 0.01 0.007 0.005 0.004 0.003 0.002 3 4 6 8 10 14 18 22 26 30 50 τ = α t/L2 70 100 120 150 T h (a) Midplane temperature (from M. P. Heisler) 300 Initially T = Ti 0 T– T To – T x/L = 0.2 1.0 0.9 0.9 0.4 Bi = h L /k 0.2 50 10 5 2 1 0.5 0.05 0.1 0.2 0.3 0.9 0.1 1.0 0 0.01 0.1 Bi = 0.4 0.8 0.00 5 0.01 0.02 0.5 0.00 1 0.00 2 0.6 0.5 0.3 x L 0.7 0.6 0.6 0.4 T h 0.8 0.8 0.7 600 700 2L Q Qmax 1.0 θ= 400 500 20 0.001 100 80 90 60 70 14 1.0 0.1 0.07 0.05 0.04 0.03 0.02 Plate 0.2 Plate 1.0 10 100 0.1 0 10–5 Plate 10– 4 10–3 10–2 1 k = Bi hL (b) Temperature distribution (from M. P. Heisler) 10–1 1 Bi 2 τ = h2 α t /k 2 10 102 103 104 (c) Heat transfer (from H. Gröber et al.) FIGURE 4–13 Transient temperature and heat transfer charts for a plane wall of thickness 2L initially at a uniform temperature Ti subjected to convection from both sides to an environment at temperature T with a convection coefficient of h. where m is the mass, V is the volume, is the density, and Cp is the specific heat of the body. Thus, Qmax represents the amount of heat transfer for t → . The amount of heat transfer Q at a finite time t will obviously be less than this cen58933_ch04.qxd 9/10/2002 9:12 AM Page 221 221 CHAPTER 4 θo = To – T Ti – T 1.0 0.7 Cylinder 0.5 0.4 0.3 5 0.2 0.1 k 3 5 8 1. 16 90 18 70 14 12 1.6 10 0 80 60 9 1.2 50 10 7 0.8 0.6 8 45 35 30 0.3 0.1 0 0.5 6 40 0.4 0.2 0.01 0.007 0.005 0.004 0.003 25 20 2 1 .4 1.0 0.02 =1 Bi = o 4 2. 0.1 0.07 0.05 0.04 0.03 hr 0.002 0.001 0 1 2 3 4 6 8 10 14 18 22 26 2 τ = α t /ro 30 50 70 100 120 (a) Centerline temperature (from M. P. Heisler) 140 150 T h Q Qmax 1.0 0.9 0.4 0.4 0.8 50 20 10 5 2 0.3 0.2 0.9 0.1 1.0 0 0.1 0.01 1 0.5 0.4 0.2 0.00 1 0.00 2 0.00 5 0.01 0.02 0.6 0.5 0.6 0.05 0.1 0.2 0.7 0.5 0.3 Bi = hro /k 0.8 0.7 0.6 ro r Bi = 0.8 0.9 350 Initially T T = Ti h 0 T–T θ= To – T 1.0 r/ro = 0.2 250 Cylinder 1.0 10 100 1 k = Bi hro (b) Temperature distribution (from M. P. Heisler) 0.1 0 10–5 Cylinder 10– 4 10–3 10–2 10–1 Bi 2 τ = 1 10 102 103 104 h2 α t /k 2 (c) Heat transfer (from H. Gröber et al.) FIGURE 4–14 Transient temperature and heat transfer charts for a long cylinder of radius ro initially at a uniform temperature Ti subjected to convection from all sides to an environment at temperature T with a convection coefficient of h. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 222 222 HEAT TRANSFER To – T Ti – T θo = 1.0 0.7 0.5 0.4 0.3 0.2 12 14 2. 0.02 0 0.5 1.0 0.75 0.5 0.35 0.2 0.1 .05 0 0 0.01 0.007 0.005 0.004 0.003 0.002 4 3 .5 2.0 2.2 8 1.6 1. .2 1.4 1 1.0 6 2.8 2. 4 50 40 45 0 35 3 25 20 18 16 10 98 76 5 3.0 0.1 0.07 0.05 0.04 0.03 0.001 100 80 90 60 70 Sphere k hr = 1 o Bi = 1.5 2 2.5 3 4 5 6 7 8 9 10 20 2 τ = αt/ro 30 40 50 100 150 (a) Midpoint temperature (from M. P. Heisler) 200 250 T–T 1.0 0.9 0.9 0.4 r Bi = hro /k 0.8 0.7 0.3 0.8 0.9 1.0 50 20 10 0.2 0.1 5 0.3 0.2 2 0.4 0.5 1 0.5 0.4 0.05 0.1 0.2 0.5 0.00 1 0.00 2 0.6 0.6 0.00 5 0.01 0.02 0.7 Bi = 0.6 ro Q Qmax To – T r/ro = 0.2 1.0 0.8 T h 0 θ= Initially T = Ti T h 0 0.01 0.1 Sphere 0.1 1.0 10 100 0 10–5 Sphere 10– 4 10–3 10–2 1= k Bi hro (b) Temperature distribution (from M. P. Heisler) 10–1 1 Bi 2 τ = h2 α t /k 2 10 102 103 104 (c) Heat transfer (from H. Gröber et al.) FIGURE 4–15 Transient temperature and heat transfer charts for a sphere of radius ro initially at a uniform temperature Ti subjected to convection from all sides to an environment at temperature T with a convection coefficient of h. maximum. The ratio Q/Qmax is plotted in Figures 4–13c, 4–14c, and 4–15c against the variables Bi and h2 t/k2 for the large plane wall, long cylinder, and cen58933_ch04.qxd 9/10/2002 9:12 AM Page 223 223 CHAPTER 4 sphere, respectively. Note that once the fraction of heat transfer Q/Qmax has been determined from these charts for the given t, the actual amount of heat transfer by that time can be evaluated by multiplying this fraction by Qmax. A negative sign for Qmax indicates that heat is leaving the body (Fig. 4–17). The fraction of heat transfer can also be determined from these relations, which are based on the one-term approximations already discussed: Q Qmax Plane wall: Cylinder: Q Qmax Sphere: Q Qmax 1 sin 0, wall wall 1 2 1 sph 3 L2 kL2 (1/L) T Cp L3/ t T h T (a) Finite convection coefficient (4-18) 1 sin 1 cos 1 1 3 1 (4-19) The use of the Heisler/Gröber charts and the one-term solutions already discussed is limited to the conditions specified at the beginning of this section: the body is initially at a uniform temperature, the temperature of the medium surrounding the body and the convection heat transfer coefficient are constant and uniform, and there is no energy generation in the body. We discussed the physical significance of the Biot number earlier and indicated that it is a measure of the relative magnitudes of the two heat transfer mechanisms: convection at the surface and conduction through the solid. A small value of Bi indicates that the inner resistance of the body to heat conduction is small relative to the resistance to convection between the surface and the fluid. As a result, the temperature distribution within the solid becomes fairly uniform, and lumped system analysis becomes applicable. Recall that when Bi 0.1, the error in assuming the temperature within the body to be uniform is negligible. To understand the physical significance of the Fourier number , we express it as (Fig. 4–18) t Ts ≠ T h (4-17) J1( 1) 0, cyl 0, sph Ts 1 cyl 1 Ts T The rate at which heat is conducted across L of a body of volume L3 The rate at which heat is stored in a body of volume L3 (4-20) Therefore, the Fourier number is a measure of heat conducted through a body relative to heat stored. Thus, a large value of the Fourier number indicates faster propagation of heat through a body. Perhaps you are wondering about what constitutes an infinitely large plate or an infinitely long cylinder. After all, nothing in this world is infinite. A plate whose thickness is small relative to the other dimensions can be modeled as an infinitely large plate, except very near the outer edges. But the edge effects on large bodies are usually negligible, and thus a large plane wall such as the wall of a house can be modeled as an infinitely large wall for heat transfer purposes. Similarly, a long cylinder whose diameter is small relative to its length can be analyzed as an infinitely long cylinder. The use of the transient temperature charts and the one-term solutions is illustrated in the following examples. T h→ Ts Ts T h→ Ts = T (b) Infinite convection coefficient FIGURE 4–16 The specified surface temperature corresponds to the case of convection to an environment at T with a convection coefficient h that is infinite. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 224 224 HEAT TRANSFER . Qmax t=0 EXAMPLE 4–3 T = Ti T= T m, Cp h T (a) Maximum heat transfer (t → ) . An ordinary egg can be approximated as a 5-cm-diameter sphere (Fig. 4–19). The egg is initially at a uniform temperature of 5°C and is dropped into boiling water at 95°C. Taking the convection heat transfer coefficient to be h 1200 W/m2 · °C, determine how long it will take for the center of the egg to reach 70°C. SOLUTION An egg is cooked in boiling water. The cooking time of the egg is to be determined. Q t=0 T = Ti T = T (r, t) m, Cp h T Bi = . . . h2α t —— = Bi2τ = . . . k2 Q —— = . . . Qmax (Gröber chart) (b) Actual heat transfer for time t FIGURE 4–17 The fraction of total heat transfer Q/Qmax up to a specified time t is determined using the Gröber charts. L L Boiling Eggs L Assumptions 1 The egg is spherical in shape with a radius of r0 2.5 cm. 2 Heat conduction in the egg is one-dimensional because of thermal symmetry about the midpoint. 3 The thermal properties of the egg and the heat transfer coefficient are constant. 4 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. Properties The water content of eggs is about 74 percent, and thus the thermal conductivity and diffusivity of eggs can be approximated by those of water at the average temperature of (5 70)/2 37.5°C; k 0.627 W/m · °C and k/ Cp 0.151 10 6 m2/s (Table A-9). Analysis The temperature within the egg varies with radial distance as well as time, and the temperature at a specified location at a given time can be determined from the Heisler charts or the one-term solutions. Here we will use the latter to demonstrate their use. The Biot number for this problem is 47.8 which is much greater than 0.1, and thus the lumped system analysis is not applicable. The coefficients 1 and A1 for a sphere corresponding to this Bi are, from Table 4–1, · Qconducted · Q (1200 W/m2 · °C)(0.025 m) 0.627 W/m · °C hr0 k Bi 1 3.0753, A1 1.9958 Substituting these and other values into Eq. 4–15 and solving for · Qstored · Qconducted αt Fourier number: τ = —– = ———— · 2 L Qstored FIGURE 4–18 Fourier number at time t can be viewed as the ratio of the rate of heat conducted to the rate of heat stored at that time. To Ti T T A1e → 2 1 70 5 95 95 1.9958e (3.0753)2 → gives 0.209 which is greater than 0.2, and thus the one-term solution is applicable with an error of less than 2 percent. Then the cooking time is determined from the definition of the Fourier number to be t ro2 (0.209)(0.025 m)2 0.151 10 6 m2/s 865 s 14.4 min Therefore, it will take about 15 min for the center of the egg to be heated from 5°C to 70°C. Discussion Note that the Biot number in lumped system analysis was defined differently as Bi hLc /k h(r /3)/k. However, either definition can be used in determining the applicability of the lumped system analysis unless Bi 0.1. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 225 225 CHAPTER 4 EXAMPLE 4–4 Heating of Large Brass Plates in an Oven In a production facility, large brass plates of 4 cm thickness that are initially at a uniform temperature of 20°C are heated by passing them through an oven that is maintained at 500°C (Fig. 4–20). The plates remain in the oven for a period of 7 min. Taking the combined convection and radiation heat transfer coefficient to be h 120 W/m2 · °C, determine the surface temperature of the plates when they come out of the oven. SOLUTION Large brass plates are heated in an oven. The surface temperature of the plates leaving the oven is to be determined. Assumptions 1 Heat conduction in the plate is one-dimensional since the plate is large relative to its thickness and there is thermal symmetry about the center plane. 2 The thermal properties of the plate and the heat transfer coefficient are constant. 3 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. Properties The properties of brass at room temperature are k 110 W/m · °C, 8530 kg/m3, Cp 380 J/kg · °C, and 33.9 10 6 m2/s (Table A-3). More accurate results are obtained by using properties at average temperature. Analysis The temperature at a specified location at a given time can be determined from the Heisler charts or one-term solutions. Here we will use the charts to demonstrate their use. Noting that the half-thickness of the plate is L 0.02 m, from Fig. 4–13 we have 1 Bi k hL t L2 100 W/m · °C 45.8 (120 W/m2 · °C)(0.02 m) (33.9 10 6 m2/s)(7 60 s) 35.6 (0.02 m)2 To Ti T T 0.46 Also, 1 Bi x L k hL L L 45.8 T To 1 T T 0.99 0.46 0.99 Therefore, T To T Ti T T T 0.455(Ti T To T Ti T T T) 500 0.455 and T 0.455(20 500) 282°C Therefore, the surface temperature of the plates will be 282°C when they leave the oven. Discussion We notice that the Biot number in this case is Bi 1/45.8 0.022, which is much less than 0.1. Therefore, we expect the lumped system analysis to be applicable. This is also evident from (T T )/(To T ) 0.99, which indicates that the temperatures at the center and the surface of the plate relative to the surrounding temperature are within 1 percent of each other. Egg Ti = 5°C h = 1200 W/ m 2·°C T = 95°C FIGURE 4–19 Schematic for Example 4–3. T = 500°C h = 120 W/m 2·°C 2 L = 4 cm Brass plate Ti = 20°C FIGURE 4–20 Schematic for Example 4–4. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 226 226 HEAT TRANSFER Noting that the error involved in reading the Heisler charts is typically at least a few percent, the lumped system analysis in this case may yield just as accurate results with less effort. The heat transfer surface area of the plate is 2A, where A is the face area of the plate (the plate transfers heat through both of its surfaces), and the volume of the plate is V (2L)A, where L is the half-thickness of the plate. The exponent b used in the lumped system analysis is determined to be b hAs CpV h(2A) h Cp L Cp (2LA) 120 W/m2 · °C (8530 kg/m3)(380 J/kg · °C)(0.02 m) Then the temperature of the plate at t T (t ) T Ti T e bt → 7 min T (t ) 500 20 500 0.00185 s 1 420 s is determined from e (0.00185 s 1)(420 s) It yields T (t ) 279°C which is practically identical to the result obtained above using the Heisler charts. Therefore, we can use lumped system analysis with confidence when the Biot number is sufficiently small. EXAMPLE 4–5 T = 200°C h = 80 W/ m2 ·°C Stainless steel shaft Ti = 600°C D = 20 cm FIGURE 4–21 Schematic for Example 4–5. Cooling of a Long Stainless Steel Cylindrical Shaft A long 20-cm-diameter cylindrical shaft made of stainless steel 304 comes out of an oven at a uniform temperature of 600°C (Fig. 4–21). The shaft is then allowed to cool slowly in an environment chamber at 200°C with an average heat transfer coefficient of h 80 W/m2 · °C. Determine the temperature at the center of the shaft 45 min after the start of the cooling process. Also, determine the heat transfer per unit length of the shaft during this time period. SOLUTION A long cylindrical shaft at 600°C is allowed to cool slowly. The center temperature and the heat transfer per unit length are to be determined. Assumptions 1 Heat conduction in the shaft is one-dimensional since it is long and it has thermal symmetry about the centerline. 2 The thermal properties of the shaft and the heat transfer coefficient are constant. 3 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. Properties The properties of stainless steel 304 at room temperature are k 14.9 W/m · °C, 7900 kg/m3, Cp 477 J/kg · °C, and 6 2 3.95 10 m /s (Table A-3). More accurate results can be obtained by using properties at average temperature. Analysis The temperature within the shaft may vary with the radial distance r as well as time, and the temperature at a specified location at a given time can cen58933_ch04.qxd 9/10/2002 9:12 AM Page 227 227 CHAPTER 4 be determined from the Heisler charts. Noting that the radius of the shaft is ro 0.1 m, from Fig. 4–14 we have 1 Bi 14.9 W/m · °C (80 W/m2 · °C)(0.1 m) k hro (3.95 t 10 ro2 6 m2/s)(45 (0.1 m)2 1.86 60 s) 1.07 To Ti T T 0.40 and To T 0.4(Ti T) 200 0.4(600 200) 360°C Therefore, the center temperature of the shaft will drop from 600°C to 360°C in 45 min. To determine the actual heat transfer, we first need to calculate the maximum heat that can be transferred from the cylinder, which is the sensible energy of the cylinder relative to its environment. Taking L 1 m, V ro2 L mCp(T Ti) 47,354 kJ m Qmax (7900 kg/m3) (0.1 m)2(1 m) 248.2 kg (248.2 kg)(0.477 kJ/kg · °C)(600 200)°C The dimensionless heat transfer ratio is determined from Fig. 4–14c for a long cylinder to be 1 1/Bi Bi h2 t k2 1 1.86 Bi2 0.537 (0.537)2(1.07) 0.309 Q Qmax 0.62 Therefore, Q 0.62Qmax 0.62 (47,354 kJ) 29,360 kJ which is the total heat transfer from the shaft during the first 45 min of the cooling. ALTERNATIVE SOLUTION We could also solve this problem using the one-term solution relation instead of the transient charts. First we find the Biot number Bi (80 W/m2 · °C)(0.1 m) 14.9 W/m · °C hro k 0.537 The coefficients 1 and A1 for a cylinder corresponding to this Bi are determined from Table 4–1 to be 1 0.970, A1 1.122 Substituting these values into Eq. 4–14 gives 0 To Ti T T A1e 2 1 1.122e (0.970)2(1.07) 0.41 cen58933_ch04.qxd 9/10/2002 9:12 AM Page 228 228 HEAT TRANSFER and thus To T 0.41(Ti T) 200 0.41(600 200) 364°C The value of J1( 1) for 1 0.970 is determined from Table 4–2 to be 0.430. Then the fractional heat transfer is determined from Eq. 4–18 to be Q Qmax 1 2 J1( 1) 0 1 2 1 0.41 0.430 0.970 0.636 and thus Q 0.636Qmax 0.636 (47,354 kJ) 30,120 kJ Discussion The slight difference between the two results is due to the reading error of the charts. 4–3 Plane surface T h 0 x FIGURE 4–22 Schematic of a semi-infinite body. I TRANSIENT HEAT CONDUCTION IN SEMI-INFINITE SOLIDS A semi-infinite solid is an idealized body that has a single plane surface and extends to infinity in all directions, as shown in Fig. 4–22. This idealized body is used to indicate that the temperature change in the part of the body in which we are interested (the region close to the surface) is due to the thermal conditions on a single surface. The earth, for example, can be considered to be a semi-infinite medium in determining the variation of temperature near its surface. Also, a thick wall can be modeled as a semi-infinite medium if all we are interested in is the variation of temperature in the region near one of the surfaces, and the other surface is too far to have any impact on the region of interest during the time of observation. Consider a semi-infinite solid that is at a uniform temperature Ti. At time t 0, the surface of the solid at x 0 is exposed to convection by a fluid at a constant temperature T , with a heat transfer coefficient h. This problem can be formulated as a partial differential equation, which can be solved analytically for the transient temperature distribution T(x, t). The solution obtained is presented in Fig. 4–23 graphically for the nondimensionalized temperature defined as 1 (x, t) 1 T(x, t ) T Ti T T(x, t ) Ti T Ti (4-21) against the dimensionless variable x/(2 t) for various values of the parameter h t/k. Note that the values on the vertical axis correspond to x 0, and thus rept/k resent the surface temperature. The curve h corresponds to h → , which corresponds to the case of specified temperature T at the surface at x 0. That is, the case in which the surface of the semi-infinite body is suddenly brought to temperature T at t 0 and kept at T at all times can be handled by setting h to infinity. The specified surface temperature case is closely cen58933_ch04.qxd 9/10/2002 9:12 AM Page 229 229 CHAPTER 4 1.0 Ambient T(x, t) T ,h 0.5 0.4 T(x, t) – T 1 – ————— = 1 – θ(x, t) Ti – T 0.3 3 2 0.2 x 1 0.5 0.4 0.3 0.2 0.1 0.1 0.05 0.04 hα t k= 0.0 5 0.03 0.02 0.01 0 0.25 0.5 0.75 1.0 1.25 1.5 ξ = —x —– 2 αt FIGURE 4–23 Variation of temperature with position and time in a semi-infinite solid initially at Ti subjected to convection to an environment at T with a convection heat transfer coefficient of h (from P. J. Schneider, Ref. 10). approximated in practice when condensation or boiling takes place on the surface. For a finite heat transfer coefficient h, the surface temperature approaches the fluid temperature T as the time t approaches infinity. The exact solution of the transient one-dimensional heat conduction problem in a semi-infinite medium that is initially at a uniform temperature of Ti and is suddenly subjected to convection at time t 0 has been obtained, and is expressed as T(x, t) Ti T Ti erfc x 2 t hx k exp x h2 t erfc k2 2 t h t k (4-22) where the quantity erfc ( ) is the complementary error function, defined as erfc ( ) 1 2 e u2 du (4-23) 0 Despite its simple appearance, the integral that appears in the above relation cannot be performed analytically. Therefore, it is evaluated numerically for different values of , and the results are listed in Table 4–3. For the special case of h → , the surface temperature Ts becomes equal to the fluid temperature T , and Eq. 4–22 reduces to T(x, t) Ti Ts Ti erfc x 2 t (4-24) cen58933_ch04.qxd 9/10/2002 9:12 AM Page 230 230 HEAT TRANSFER TABLE 4–3 The complementary error function erfc ( ) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 0.22 0.24 0.26 0.28 0.30 0.32 0.34 0.36 erfc ( ) 1.00000 0.9774 0.9549 0.9324 0.9099 0.8875 0.8652 0.8431 0.8210 0.7991 0.7773 0.7557 0.7343 0.7131 0.6921 0.6714 0.6509 0.6306 0.6107 0.38 0.40 0.42 0.44 0.46 0.48 0.50 0.52 0.54 0.56 0.58 0.60 0.62 0.64 0.66 0.68 0.70 0.72 0.74 0.5910 0.5716 0.5525 0.5338 0.5153 0.4973 0.4795 0.4621 0.4451 0.4284 0.4121 0.3961 0.3806 0.3654 0.3506 0.3362 0.3222 0.3086 0.2953 erfc ( ) 0.76 0.78 0.80 0.82 0.84 0.86 0.88 0.90 0.92 0.94 0.96 0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 0.2825 0.2700 0.2579 0.2462 0.2349 0.2239 0.2133 0.2031 0.1932 0.1837 0.1746 0.1658 0.1573 0.1492 0.1413 0.1339 0.1267 0.1198 0.1132 erfc ( ) 1.14 1.16 1.18 1.20 1.22 1.24 1.26 1.28 1.30 1.32 1.34 1.36 1.38 1.40 1.42 1.44 1.46 1.48 1.50 0.1069 0.10090 0.09516 0.08969 0.08447 0.07950 0.07476 0.07027 0.06599 0.06194 0.05809 0.05444 0.05098 0.04772 0.04462 0.04170 0.03895 0.03635 0.03390 erfc ( ) 1.52 1.54 1.56 1.58 1.60 1.62 1.64 1.66 1.68 1.70 1.72 1.74 1.76 1.78 1.80 1.82 1.84 1.86 1.88 0.03159 0.02941 0.02737 0.02545 0.02365 0.02196 0.02038 0.01890 0.01751 0.01612 0.01500 0.01387 0.01281 0.01183 0.01091 0.01006 0.00926 0.00853 0.00784 erfc ( ) 1.90 1.92 1.94 1.96 1.98 2.00 2.10 2.20 2.30 2.40 2.50 2.60 2.70 2.80 2.90 3.00 3.20 3.40 3.60 0.00721 0.00662 0.00608 0.00557 0.00511 0.00468 0.00298 0.00186 0.00114 0.00069 0.00041 0.00024 0.00013 0.00008 0.00004 0.00002 0.00001 0.00000 0.00000 This solution corresponds to the case when the temperature of the exposed surface of the medium is suddenly raised (or lowered) to Ts at t 0 and is maintained at that value at all times. Although the graphical solution given in Fig. 4–23 is a plot of the exact analytical solution given by Eq. 4–23, it is subject to reading errors, and thus is of limited accuracy. EXAMPLE 4–6 Ts = –10°C Soil x Water pipe Ti = 15°C FIGURE 4–24 Schematic for Example 4–6. Minimum Burial Depth of Water Pipes to Avoid Freezing In areas where the air temperature remains below 0°C for prolonged periods of time, the freezing of water in underground pipes is a major concern. Fortunately, the soil remains relatively warm during those periods, and it takes weeks for the subfreezing temperatures to reach the water mains in the ground. Thus, the soil effectively serves as an insulation to protect the water from subfreezing temperatures in winter. The ground at a particular location is covered with snow pack at 10°C for a continuous period of three months, and the average soil properties at that location are k 0.4 W/m · °C and 0.15 10 6 m2/s (Fig. 4–24). Assuming an initial uniform temperature of 15°C for the ground, determine the minimum burial depth to prevent the water pipes from freezing. SOLUTION The water pipes are buried in the ground to prevent freezing. The minimum burial depth at a particular location is to be determined. Assumptions 1 The temperature in the soil is affected by the thermal conditions at one surface only, and thus the soil can be considered to be a semiinfinite medium with a specified surface temperature of 10°C. 2 The thermal properties of the soil are constant. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 231 231 CHAPTER 4 Properties The properties of the soil are as given in the problem statement. Analysis The temperature of the soil surrounding the pipes will be 0°C after three months in the case of minimum burial depth. Therefore, from Fig. 4–23, we have h t (since h → ) k 1 x T (x, t ) T Ti T 0 15 1 ( 10) ( 10) 2 0.6 0.36 t We note that t 7.78 106 s m2/s)(7.78 106 s) (90 days)(24 h/day)(3600 s/h) and thus x 2 t 2 0.36 (0.15 10 6 0.77 m Therefore, the water pipes must be buried to a depth of at least 77 cm to avoid freezing under the specified harsh winter conditions. ALTERNATIVE SOLUTION The solution of this problem could also be determined from Eq. 4–24: T (x, t ) Ti Ts Ti erfc x 2 t 0 15 10 15 → erfc x 2 t 0.60 The argument that corresponds to this value of the complementary error function is determined from Table 4–3 to be 0.37. Therefore, x 2 t 2 0.37 (0.15 10 6 m2/s)(7.78 106 s) T h T h T(r, t) 0.80 m Heat transfer Again, the slight difference is due to the reading error of the chart. (a) Long cylinder 4–4 I TRANSIENT HEAT CONDUCTION IN MULTIDIMENSIONAL SYSTEMS The transient temperature charts presented earlier can be used to determine the temperature distribution and heat transfer in one-dimensional heat conduction problems associated with a large plane wall, a long cylinder, a sphere, and a semi-infinite medium. Using a superposition approach called the product solution, these charts can also be used to construct solutions for the twodimensional transient heat conduction problems encountered in geometries such as a short cylinder, a long rectangular bar, or a semi-infinite cylinder or plate, and even three-dimensional problems associated with geometries such as a rectangular prism or a semi-infinite rectangular bar, provided that all surfaces of the solid are subjected to convection to the same fluid at temperature T h T(r, x, t) Heat transfer (b) Short cylinder (two-dimensional) FIGURE 4–25 The temperature in a short cylinder exposed to convection from all surfaces varies in both the radial and axial directions, and thus heat is transferred in both directions. cen58933_ch04.qxd 9/10/2002 9:12 AM Page 232 232 HEAT TRANSFER T h Plane wall a ro Long cylinder FIGURE 4–26 A short cylinder of radius ro and height a is the intersection of a long cylinder of radius ro and a plane wall of thickness a. Plane wall T h T , with the same heat transfer coefficient h, and the body involves no heat generation (Fig. 4–25). The solution in such multidimensional geometries can be expressed as the product of the solutions for the one-dimensional geometries whose intersection is the multidimensional geometry. Consider a short cylinder of height a and radius ro initially at a uniform tem0, the perature Ti. There is no heat generation in the cylinder. At time t cylinder is subjected to convection from all surfaces to a medium at temperature T with a heat transfer coefficient h. The temperature within the cylinder will change with x as well as r and time t since heat transfer will occur from the top and bottom of the cylinder as well as its side surfaces. That is, T T(r, x, t) and thus this is a two-dimensional transient heat conduction problem. When the properties are assumed to be constant, it can be shown that the solution of this two-dimensional problem can be expressed as T(r, x, t) T Ti T short cylinder T(x, t) T Ti T Plane wall FIGURE 4–27 A long solid bar of rectangular profile a b is the intersection of two plane walls of thicknesses a and b. t) cyl(r, a T(r, t) T Ti T infinite cylinder (4-25) That is, the solution for the two-dimensional short cylinder of height a and radius ro is equal to the product of the nondimensionalized solutions for the one-dimensional plane wall of thickness a and the long cylinder of radius ro, which are the two geometries whose intersection is the short cylinder, as shown in Fig. 4–26. We generalize this as follows: the solution for a multidimensional geometry is the product of the solutions of the one-dimensional geometries whose intersection is the multidimensional body. For convenience, the one-dimensional solutions are denoted by wall(x, b plane wall t) semi-inf(x, t) T(x, t) T Ti T T(r, t) T Ti T T(x, t) T Ti T plane wall infinite cylinder (4-26) semi-infinite solid For example, the solution for a long solid bar whose cross section is an a b rectangle is the intersection of the two infinite plane walls of thicknesses a and b, as shown in Fig. 4–27, and thus the transient temperature distribution for this rectangular bar can be expressed as T(x, y, t) T Ti T rectangular bar wall(x, t) wall(y, t) (4-27) The proper forms of the product solutions for some other geometries are given in Table 4–4. It is important to note that the x-coordinate is measured from the surface in a semi-infinite solid, and from the midplane in a plane wall. The radial distance r is always measured from the centerline. Note that the solution of a two-dimensional problem involves the product of two one-dimensional solutions, whereas the solution of a three-dimensional problem involves the product of three one-dimensional solutions. A modified form of the product solution can also be used to determine the total transient heat transfer to or from a multidimensional geometry by using the one-dimensional values, as shown by L. S. Langston in 1982. The cen58933_ch04.qxd 9/10/2002 9:13 AM Page 233 233 CHAPTER 4 TABLE 4–4 Multidimensional solutions expressed as products of one-dimensional solutions for bodies that are initially at a uniform temperature Ti and exposed to convection from all surfaces to a medium at T x 0 ro r r x θ (r, t) = θcyl(r, t) Infinite cylinder r θ (x,r, t) = θcyl (r, t) θwall (x,t) Short cylinder θ (x,r, t) = θcyl (r, t) θsemi-inf (x, t) Semi-infinite cylinder y x x y z θ (x, t) = θsemi-inf (x, t) Semi-infinite medium θ (x,y,t) = θsemi-inf (x, t) θsemi-inf (y, t) Quarter-infinite medium x θ (x, y, z, t) = θsemi-inf (x, t) θsemi-inf (y, t) θsemi-inf (z,t) Corner region of a large medium 2L 2L y 0 x Lx y z θ (x, t) = θwall(x, t) Infinite plate (or plane wall) θ (x, y, t) = θwall (x, t) θsemi-inf (y, t) Semi-infinite plate x θ (x,y,z, t) = θwall (x, t) θsemi-inf ( y, t) θsemi-inf (z , t) Quarter-infinite plate y x z y x z y x θ (x, y, t) = θwall (x, t) θwall ( y, t) Infinite rectangular bar θ (x,y,z, t) = θwall (x, t) θwall (y, t) θsemi-inf (z , t) Semi-infinite rectangular bar θ (x,y,z, t) = θwall (x, t) θwall (y, t) θwall (z , t) Rectangular parallelepiped cen58933_ch04.qxd 9/10/2002 9:13 AM Page 234 234 HEAT TRANSFER transient heat transfer for a two-dimensional geometry formed by the intersection of two one-dimensional geometries 1 and 2 is Q Qmax Q Qmax total, 2D Q Qmax 1 12 Q Qmax (4-28) 1 Transient heat transfer for a three-dimensional body formed by the intersection of three one-dimensional bodies 1, 2, and 3 is given by Q Qmax total, 3D Q Qmax Q Qmax 1 Q Qmax 3 12 Q 1Qmax Q Qmax 1- 1 1 Q Qmax (4-29) 2 The use of the product solution in transient two- and three-dimensional heat conduction problems is illustrated in the following examples. EXAMPLE 4–7 Cooling of a Short Brass Cylinder A short brass cylinder of diameter D 10 cm and height H 12 cm is initially at a uniform temperature Ti 120°C. The cylinder is now placed in atmospheric air at 25°C, where heat transfer takes place by convection, with a heat transfer coefficient of h 60 W/m2 · °C. Calculate the temperature at (a) the center of the cylinder and (b) the center of the top surface of the cylinder 15 min after the start of the cooling. SOLUTION A short cylinder is allowed to cool in atmospheric air. The temperatures at the centers of the cylinder and the top surface are to be determined. Assumptions 1 Heat conduction in the short cylinder is two-dimensional, and thus the temperature varies in both the axial x- and the radial r-directions. 2 The thermal properties of the cylinder and the heat transfer coefficient are constant. 3 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. Properties The properties of brass at room temperature are k 110 W/m · °C and 33.9 10 6 m2/s (Table A-3). More accurate results can be obtained by using properties at average temperature. T = 25°C h = 60 W/ m2 ·°C x 0 L r Ti = 120°C ro Analysis (a) This short cylinder can physically be formed by the intersection of a long cylinder of radius ro 5 cm and a plane wall of thickness 2L 12 cm, as shown in Fig. 4–28. The dimensionless temperature at the center of the plane wall is determined from Figure 4–13a to be L t 2 L FIGURE 4–28 Schematic for Example 4–7. 1 Bi k hL 10 5 m2/s)(900 s) (0.06 m)2 8.48 110 W/m · °C (60 W/m2 · °C)(0.06 m) 30.6 (3.39 wall(0, t) T (0, t ) T Ti T 0.8 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 235 235 CHAPTER 4 Similarly, at the center of the cylinder, we have 10 5 m2/s)(900 s) (0.05 m)2 12.2 110 W/m · °C (60 W/m2 · °C)(0.05 m) 36.7 (3.39 t ro2 1 Bi k hro cyl(0, T (0, t ) T Ti T t) 0.5 Therefore, T (0, 0, t ) T Ti T wall(0, short cylinder t) cyl(0, t) 0.8 0.5 0.4 and T (0, 0, t ) T 0.4(Ti T) 25 0.4(120 25) 63°C This is the temperature at the center of the short cylinder, which is also the center of both the long cylinder and the plate. (b) The center of the top surface of the cylinder is still at the center of the long cylinder (r 0), but at the outer surface of the plane wall (x L). Therefore, we first need to find the surface temperature of the wall. Noting that x L 0.06 m, x L 0.06 m 0.06 m 1 Bi k hL 1 110 W/m · °C (60 W/m2 · °C)(0.06 m) 30.6 T (L, t ) T To T 0.98 Then wall(L, t) T (L, t ) T Ti T T (L, t ) T To T To T Ti T 0.98 0.8 0.784 Therefore, T (L, 0, t ) T Ti T short cylinder wall(L, t) cyl(0, t) 0.784 0.5 0.392 25 0.392(120 25) 62.2°C and T(L, 0, t ) T 0.392(Ti T) which is the temperature at the center of the top surface of the cylinder. EXAMPLE 4–8 Heat Transfer from a Short Cylinder Determine the total heat transfer from the short brass cylinder ( kg/m3, Cp 0.380 kJ/kg · °C) discussed in Example 4–7. 8530 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 236 236 HEAT TRANSFER SOLUTION We first determine the maximum heat that can be transferred from the cylinder, which is the sensible energy content of the cylinder relative to its environment: m Qmax ro2 L V mCp(Ti T ) (8530 kg/m3) (0.05 m)2(0.06 m) 4.02 kg (4.02 kg)(0.380 kJ/kg · °C)(120 25)°C 145.1 kJ Then we determine the dimensionless heat transfer ratios for both geometries. For the plane wall, it is determined from Fig. 4–13c to be 1 1/Bi Bi h2 t k2 1 30.6 2 0.0327 (0.0327)2(8.48) Bi Q Qmax 0.0091 0.23 plane wall Similarly, for the cylinder, we have Bi 1 1/Bi h2 t k2 1 36.7 2 0.0272 2 Bi (0.0272) (12.2) Q Qmax 0.0090 0.47 infinite cylinder Then the heat transfer ratio for the short cylinder is, from Eq. 4–28, Q Qmax short cyl Q Qmax 0.23 Q Qmax 1 0.47(1 1 2 Q Qmax 0.23) 0.592 1 Therefore, the total heat transfer from the cylinder during the first 15 min of cooling is Q EXAMPLE 4–9 0.592Qmax 0.592 (145.1 kJ) 85.9 kJ Cooling of a Long Cylinder by Water A semi-infinite aluminum cylinder of diameter D 20 cm is initially at a uniform temperature Ti 200°C. The cylinder is now placed in water at 15°C where heat transfer takes place by convection, with a heat transfer coefficient of h 120 W/m2 · °C. Determine the temperature at the center of the cylinder 15 cm from the end surface 5 min after the start of the cooling. SOLUTION A semi-infinite aluminum cylinder is cooled by water. The temperature at the center of the cylinder 15 cm from the end surface is to be determined. Assumptions 1 Heat conduction in the semi-infinite cylinder is twodimensional, and thus the temperature varies in both the axial x- and the radial r-directions. 2 The thermal properties of the cylinder and the heat transfer coefficient are constant. 3 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 237 237 CHAPTER 4 Properties The properties of aluminum at room temperature are k 237 W/m · °C and 9.71 10 6 m2/s (Table A-3). More accurate results can be obtained by using properties at average temperature. Analysis This semi-infinite cylinder can physically be formed by the intersection of an infinite cylinder of radius ro 10 cm and a semi-infinite medium, as shown in Fig. 4–29. We will solve this problem using the one-term solution relation for the cylinder and the analytic solution for the semi-infinite medium. First we consider the infinitely long cylinder and evaluate the Biot number: 2 hro k Bi (120 W/m · °C)(0.1 m) 237 W/m · °C 0.05 The coefficients 1 and A1 for a cylinder corresponding to this Bi are determined from Table 4–1 to be 1 0.3126 and A1 1.0124. The Fourier number in this case is 10 5 m2/s)(5 (0.1 m)2 (9.71 t ro2 60 s) 2.91 0.2 and thus the one-term approximation is applicable. Substituting these values into Eq. 4–14 gives cyl(0, 0 t) A1e 2 1 (0.3126)2(2.91) 1.0124e 0.762 The solution for the semi-infinite solid can be determined from 1 semi-inf(x, t) erfc x 2 t hx k exp h2 t k2 erfc h x 2 t t k First we determine the various quantities in parentheses: x 2 t h t k hx k h2 t k2 0.15 m 0.44 2 (9.71 10 5 m2/s)(5 60 s) (120 W/m2 · °C) (9.71 10 5 m2/s)(300 s) 237 W/m · °C (120 W/m2 · °C)(0.15 m) 237 W/m · °C h t k 0.086 0.0759 2 (0.086)2 0.0074 Substituting and evaluating the complementary error functions from Table 4–3, semi-inf(x, t) 1 erfc (0.44) exp (0.0759 0.0074) erfc (0.44 1 0.5338 exp (0.0833) 0.457 0.963 0.086) Now we apply the product solution to get T (x, 0, t ) T Ti T semi-infinite cylinder semi-inf(x, t) cyl(0, t) 0.963 0.762 0.734 Ti = 200°C T = 15°C h = 120 W/ m2 ·°C D = 20 cm x x = 15 cm 0r FIGURE 4–29 Schematic for Example 4–9. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 238 238 HEAT TRANSFER and T (x, 0, t ) T 0.734(Ti T) 15 0.734(200 15) 151°C which is the temperature at the center of the cylinder 15 cm from the exposed bottom surface. EXAMPLE 4–10 5°F Steak 1 in. FIGURE 4–30 Schematic for Example 4–10. 35°F Refrigerating Steaks while Avoiding Frostbite In a meat processing plant, 1-in.-thick steaks initially at 75°F are to be cooled in the racks of a large refrigerator that is maintained at 5°F (Fig. 4–30). The steaks are placed close to each other, so that heat transfer from the 1-in.-thick edges is negligible. The entire steak is to be cooled below 45°F, but its temperature is not to drop below 35°F at any point during refrigeration to avoid “frostbite.” The convection heat transfer coefficient and thus the rate of heat transfer from the steak can be controlled by varying the speed of a circulating fan inside. Determine the heat transfer coefficient h that will enable us to meet both temperature constraints while keeping the refrigeration time to a minimum. The steak can be treated as a homogeneous layer having the properties 74.9 lbm/ft3, Cp 0.98 Btu/lbm · °F, k 0.26 Btu/h · ft · °F, and 0.0035 ft2/h. SOLUTION Steaks are to be cooled in a refrigerator maintained at 5°F. The heat transfer coefficient that will allow cooling the steaks below 45°F while avoiding frostbite is to be determined. Assumptions 1 Heat conduction through the steaks is one-dimensional since the steaks form a large layer relative to their thickness and there is thermal symmetry about the center plane. 2 The thermal properties of the steaks and the heat transfer coefficient are constant. 3 The Fourier number is 0.2 so that the one-term approximate solutions are applicable. Properties The properties of the steaks are as given in the problem statement. Analysis The lowest temperature in the steak will occur at the surfaces and the highest temperature at the center at a given time, since the inner part will be the last place to be cooled. In the limiting case, the surface temperature at x L 0.5 in. from the center will be 35°F, while the midplane temperature is 45°F in an environment at 5°F. Then, from Fig. 4–13b, we obtain 0.5 in. 1 0.5 in. T (L, t ) T 35 To T 45 x L 5 5 0.75 1 Bi k hL 1.5 which gives h 1k 1.5 L 0.26 Btu/h · ft · °F 1.5(0.5/12 ft) 4.16 Btu/h · ft2 · °F Discussion The convection heat transfer coefficient should be kept below this value to satisfy the constraints on the temperature of the steak during refrigeration. We can also meet the constraints by using a lower heat transfer coefficient, but doing so would extend the refrigeration time unnecessarily. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 239 239 CHAPTER 4 The restrictions that are inherent in the use of Heisler charts and the oneterm solutions (or any other analytical solutions) can be lifted by using the numerical methods discussed in Chapter 5. TOPIC OF SPECIAL INTEREST Refrigeration and Freezing of Foods Control of Microorganisms in Foods Microorganisms such as bacteria, yeasts, molds, and viruses are widely encountered in air, water, soil, living organisms, and unprocessed food items, and cause off-flavors and odors, slime production, changes in the texture and appearances, and the eventual spoilage of foods. Holding perishable foods at warm temperatures is the primary cause of spoilage, and the prevention of food spoilage and the premature degradation of quality due to microorganisms is the largest application area of refrigeration. The first step in controlling microorganisms is to understand what they are and the factors that affect their transmission, growth, and destruction. Of the various kinds of microorganisms, bacteria are the prime cause for the spoilage of foods, especially moist foods. Dry and acidic foods create an undesirable environment for the growth of bacteria, but not for the growth of yeasts and molds. Molds are also encountered on moist surfaces, cheese, and spoiled foods. Specific viruses are encountered in certain animals and humans, and poor sanitation practices such as keeping processed foods in the same area as the uncooked ones and being careless about handwashing can cause the contamination of food products. When contamination occurs, the microorganisms start to adapt to the new environmental conditions. This initial slow or no-growth period is called the lag phase, and the shelf life of a food item is directly proportional to the length of this phase (Fig. 4–31). The adaptation period is followed by an exponential growth period during which the population of microorganisms can double two or more times every hour under favorable conditions unless drastic sanitation measures are taken. The depletion of nutrients and the accumulation of toxins slow down the growth and start the death period. The rate of growth of microorganisms in a food item depends on the characteristics of the food itself such as the chemical structure, pH level, presence of inhibitors and competing microorganisms, and water activity as well as the environmental conditions such as the temperature and relative humidity of the environment and the air motion (Fig. 4–32). Microorganisms need food to grow and multiply, and their nutritional needs are readily provided by the carbohydrates, proteins, minerals, and vitamins in a food. Different types of microorganisms have different nutritional needs, and the types of nutrients in a food determine the types of microorganisms that may dwell on them. The preservatives added to the *This section can be skipped without a loss of continuity. Microorganism population Lag Exponential growth Death Time FIGURE 4–31 Typical growth curve of microorganisms. 50 ENVIRONMENT 0 100 % Temperature Oxygen level Relative humidity Air motion FOOD Water content Chemical composition Contamination level The use of inhibitors pH level FIGURE 4–32 The factors that affect the rate of growth of microorganisms. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 240 240 HEAT TRANSFER Rate of growth Temperature FIGURE 4–33 The rate of growth of microorganisms in a food product increases exponentially with increasing environmental temperature. food may also inhibit the growth of certain microorganisms. Different kinds of microorganisms that exist compete for the same food supply, and thus the composition of microorganisms in a food at any time depends on the initial make-up of the microorganisms. All living organisms need water to grow, and microorganisms cannot grow in foods that are not sufficiently moist. Microbiological growth in refrigerated foods such as fresh fruits, vegetables, and meats starts at the exposed surfaces where contamination is most likely to occur. Fresh meat in a package left in a room will spoil quickly, as you may have noticed. A meat carcass hung in a controlled environment, on the other hand, will age healthily as a result of dehydration on the outer surface, which inhibits microbiological growth there and protects the carcass. Microorganism growth in a food item is governed by the combined effects of the characteristics of the food and the environmental factors. We cannot do much about the characteristics of the food, but we certainly can alter the environmental conditions to more desirable levels through heating, cooling, ventilating, humidification, dehumidification, and control of the oxygen levels. The growth rate of microorganisms in foods is a strong function of temperature, and temperature control is the single most effective mechanism for controlling the growth rate. Microorganisms grow best at “warm” temperatures, usually between 20 and 60°C. The growth rate declines at high temperatures, and death occurs at still higher temperatures, usually above 70°C for most microorganisms. Cooling is an effective and practical way of reducing the growth rate of microorganisms and thus extending the shelf life of perishable foods. A temperature of 4°C or lower is considered to be a safe refrigeration temperature. Sometimes a small increase in refrigeration temperature may cause a large increase in the growth rate, and thus a considerable decrease in shelf life of the food (Fig. 4–33). The growth rate of some microorganisms, for example, doubles for each 3°C rise in temperature. Another factor that affects microbiological growth and transmission is the relative humidity of the environment, which is a measure of the water content of the air. High humidity in cold rooms should be avoided since condensation that forms on the walls and ceiling creates the proper environment for mold growth and buildups. The drip of contaminated condensate onto food products in the room poses a potential health hazard. Different microorganisms react differently to the presence of oxygen in the environment. Some microorganisms such as molds require oxygen for growth, while some others cannot grow in the presence of oxygen. Some grow best in low-oxygen environments, while others grow in environments regardless of the amount of oxygen. Therefore, the growth of certain microorganisms can be controlled by controlling the amount of oxygen in the environment. For example, vacuum packaging inhibits the growth of microorganisms that require oxygen. Also, the storage life of some fruits can be extended by reducing the oxygen level in the storage room. Microorganisms in food products can be controlled by (1) preventing contamination by following strict sanitation practices, (2) inhibiting growth by altering the environmental conditions, and (3) destroying the organisms by heat treatment or chemicals. The best way to minimize contamination cen58933_ch04.qxd 9/10/2002 9:13 AM Page 241 241 CHAPTER 4 in food processing areas is to use fine air filters in ventilation systems to capture the dust particles that transport the bacteria in the air. Of course, the filters must remain dry since microorganisms can grow in wet filters. Also, the ventilation system must maintain a positive pressure in the food processing areas to prevent any airborne contaminants from entering inside by infiltration. The elimination of condensation on the walls and the ceiling of the facility and the diversion of plumbing condensation drip pans of refrigerators to the drain system are two other preventive measures against contamination. Drip systems must be cleaned regularly to prevent microbiological growth in them. Also, any contact between raw and cooked food products should be minimized, and cooked products must be stored in rooms with positive pressures. Frozen foods must be kept at 18°C or below, and utmost care should be exercised when food products are packaged after they are frozen to avoid contamination during packaging. The growth of microorganisms is best controlled by keeping the temperature and relative humidity of the environment in the desirable range. Keeping the relative humidity below 60 percent, for example, prevents the growth of all microorganisms on the surfaces. Microorganisms can be destroyed by heating the food product to high temperatures (usually above 70°C), by treating them with chemicals, or by exposing them to ultraviolet light or solar radiation. Distinction should be made between survival and growth of microorganisms. A particular microorganism that may not grow at some low temperature may be able to survive at that temperature for a very long time (Fig. 4–34). Therefore, freezing is not an effective way of killing microorganisms. In fact, some microorganism cultures are preserved by freezing them at very low temperatures. The rate of freezing is also an important consideration in the refrigeration of foods since some microorganisms adapt to low temperatures and grow at those temperatures when the cooling rate is very low. Refrigeration and Freezing of Foods The storage life of fresh perishable foods such as meats, fish, vegetables, and fruits can be extended by several days by storing them at temperatures just above freezing, usually between 1 and 4°C. The storage life of foods can be extended by several months by freezing and storing them at subfreezing temperatures, usually between 18 and 35°C, depending on the particular food (Fig. 4–35). Refrigeration slows down the chemical and biological processes in foods, and the accompanying deterioration and loss of quality and nutrients. Sweet corn, for example, may lose half of its initial sugar content in one day at 21°C, but only 5 percent of it at 0°C. Fresh asparagus may lose 50 percent of its vitamin C content in one day at 20°C, but in 12 days at 0°C. Refrigeration also extends the shelf life of products. The first appearance of unsightly yellowing of broccoli, for example, may be delayed by three or more days by refrigeration. Early attempts to freeze food items resulted in poor-quality products because of the large ice crystals that formed. It was determined that the rate of freezing has a major effect on the size of ice crystals and the quality, texture, and nutritional and sensory properties of many foods. During slow Z Z Z Microorganisms Frozen food FIGURE 4–34 Freezing may stop the growth of microorganisms, but it may not necessarily kill them. Freezer –18 to –35°C Refrigerator 1 to 4°C Frozen foods Fresh foods FIGURE 4–35 Recommended refrigeration and freezing temperatures for most perishable foods. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 242 242 HEAT TRANSFER Temperature Cooling (above freezing) Beginning of freezing Freezing End of freezing Cooling (below freezing) Time FIGURE 4–36 Typical freezing curve of a food item. TABLE 4–5 Thermal properties of beef Quantity Typical value Average density 1070 kg/m3 Specific heat: Above freezing 3.14 kJ/kg · °C Below freezing 1.70 kJ/kg · °C Freezing point –2.7°C Latent heat of fusion 249 kJ/kg Thermal 0.41 W/m · °C conductivity (at 6°C) freezing, ice crystals can grow to a large size, whereas during fast freezing a large number of ice crystals start forming at once and are much smaller in size. Large ice crystals are not desirable since they can puncture the walls of the cells, causing a degradation of texture and a loss of natural juices during thawing. A crust forms rapidly on the outer layer of the product and seals in the juices, aromatics, and flavoring agents. The product quality is also affected adversely by temperature fluctuations of the storage room. The ordinary refrigeration of foods involves cooling only without any phase change. The freezing of foods, on the other hand, involves three stages: cooling to the freezing point (removing the sensible heat), freezing (removing the latent heat), and further cooling to the desired subfreezing temperature (removing the sensible heat of frozen food), as shown in Figure 4–36. Beef Products Meat carcasses in slaughterhouses should be cooled as fast as possible to a uniform temperature of about 1.7°C to reduce the growth rate of microorganisms that may be present on carcass surfaces, and thus minimize spoilage. The right level of temperature, humidity, and air motion should be selected to prevent excessive shrinkage, toughening, and discoloration. The deep body temperature of an animal is about 39°C, but this temperature tends to rise a couple of degrees in the midsections after slaughter as a result of the heat generated during the biological reactions that occur in the cells. The temperature of the exposed surfaces, on the other hand, tends to drop as a result of heat losses. The thickest part of the carcass is the round, and the center of the round is the last place to cool during chilling. Therefore, the cooling of the carcass can best be monitored by inserting a thermometer deep into the central part of the round. About 70 percent of the beef carcass is water, and the carcass is cooled mostly by evaporative cooling as a result of moisture migration toward the surface where evaporation occurs. But this shrinking translates into a loss of salable mass that can amount to 2 percent of the total mass during an overnight chilling. To prevent excessive loss of mass, carcasses are usually washed or sprayed with water prior to cooling. With adequate care, spray chilling can eliminate carcass cooling shrinkage almost entirely. The average total mass of dressed beef, which is normally split into two sides, is about 300 kg, and the average specific heat of the carcass is about 3.14 kJ/kg · °C (Table 4–5). The chilling room must have a capacity equal to the daily kill of the slaughterhouse, which may be several hundred. A beef carcass is washed before it enters the chilling room and absorbs a large amount of water (about 3.6 kg) at its surface during the washing process. This does not represent a net mass gain, however, since it is lost by dripping or evaporation in the chilling room during cooling. Ideally, the carcass does not lose or gain any net weight as it is cooled in the chilling room. However, it does lose about 0.5 percent of the total mass in the holding room as it continues to cool. The actual product loss is determined by first weighing the dry carcass before washing and then weighing it again after it is cooled. The refrigerated air temperature in the chilling room of beef carcasses must be sufficiently high to avoid freezing and discoloration on the outer cen58933_ch04.qxd 9/10/2002 9:13 AM Page 243 243 CHAPTER 4 40 Maximum carcass temp (deep round) Temperature, °C 30 Avg. carcass temp (all parts) 20 Minimum carcass temp (chuck surface) 10 0 Room air temp. –10 0 4 8 12 16 20 24 28 32 36 40 44 48 52 56 60 64 68 72 Time from start of chill, hours surfaces of the carcass. This means a long residence time for the massive beef carcasses in the chilling room to cool to the desired temperature. Beef carcasses are only partially cooled at the end of an overnight stay in the chilling room. The temperature of a beef carcass drops to 1.7 to 7°C at the surface and to about 15°C in mid parts of the round in 10 h. It takes another day or two in the holding room maintained at 1 to 2°C to complete chilling and temperature equalization. But hog carcasses are fully chilled during that period because of their smaller size. The air circulation in the holding room is kept at minimum levels to avoid excessive moisture loss and discoloration. The refrigeration load of the holding room is much smaller than that of the chilling room, and thus it requires a smaller refrigeration system. Beef carcasses intended for distant markets are shipped the day after slaughter in refrigerated trucks, where the rest of the cooling is done. This practice makes it possible to deliver fresh meat long distances in a timely manner. The variation in temperature of the beef carcass during cooling is given in Figure 4–37. Initially, the cooling process is dominated by sensible heat transfer. Note that the average temperature of the carcass is reduced by about 28°C (from 36 to 8°C) in 20 h. The cooling rate of the carcass could be increased by lowering the refrigerated air temperature and increasing the air velocity, but such measures also increase the risk of surface freezing. Most meats are judged on their tenderness, and the preservation of tenderness is an important consideration in the refrigeration and freezing of meats. Meat consists primarily of bundles of tiny muscle fibers bundled together inside long strings of connective tissues that hold it together. The tenderness of a certain cut of beef depends on the location of the cut, the age, and the activity level of the animal. Cuts from the relatively inactive mid-backbone section of the animal such as short loins, sirloin, and prime ribs are more tender than the cuts from the active parts such as the legs and the neck (Fig. 4–38). The more active the animal, the more the connective tissue, and the tougher the meat. The meat of an older animal is more flavorful, however, and is preferred for stewing since the toughness of the meat does not pose a problem for moist-heat cooking such as boiling. The FIGURE 4–37 Typical cooling curve of a beef carcass in the chilling and holding rooms at an average temperature of 0°C (from ASHRAE, Handbook: Refrigeration, Ref. 3, Chap. 11, Fig. 2). Chuck Rib Short loin Brisket Flank Foreshank Short plate Sirloin Round FIGURE 4–38 Various cuts of beef (from National Livestock and Meat Board). cen58933_ch04.qxd 9/10/2002 9:13 AM Page 244 244 HEAT TRANSFER Tenderness scale 10 5 0 5 Time in days 10 FIGURE 4–39 Variation of tenderness of meat stored at 2°C with time after slaughter. Meat freezer Air – 40 to –30°C 2.5 to 5 m/s Meat FIGURE 4–40 The freezing time of meat can be reduced considerably by using low temperature air at high velocity. TABLE 4–6 Storage life of frozen meat products at different storage temperatures (from ASHRAE Handbook: Refrigeration, Chap. 10, Table 7) Storage Life, Months Temperature Product Beef Lamb Veal Pork Chopped beef Cooked foods 12°C 4–12 3–8 3–4 2–6 3–4 2–3 18°C 23°C 6–18 12–24 6–16 12–18 4–14 8 4–12 8–15 4–6 8 2–4 protein collagen, which is the main component of the connective tissue, softens and dissolves in hot and moist environments and gradually transforms into gelatin, and tenderizes the meat. The old saying “one should either cook an animal immediately after slaughter or wait at least two days” has a lot of truth in it. The biomechanical reactions in the muscle continue after the slaughter until the energy supplied to the muscle to do work diminishes. The muscle then stiffens and goes into rigor mortis. This process begins several hours after the animal is slaughtered and continues for 12 to 36 h until an enzymatic action sets in and tenderizes the connective tissue, as shown in Figure 4–39. It takes about seven days to complete tenderization naturally in storage facilities maintained at 2°C. Electrical stimulation also causes the meat to be tender. To avoid toughness, fresh meat should not be frozen before rigor mortis has passed. You have probably noticed that steaks are tender and rather tasty when they are hot but toughen as they cool. This is because the gelatin that formed during cooking thickens as it cools, and meat loses its tenderness. So it is no surprise that first-class restaurants serve their steak on hot thick plates that keep the steaks warm for a long time. Also, cooking softens the connective tissue but toughens the tender muscle fibers. Therefore, barbecuing on low heat for a long time results in a tough steak. Variety meats intended for long-term storage must be frozen rapidly to reduce spoilage and preserve quality. Perhaps the first thought that comes to mind to freeze meat is to place the meat packages into the freezer and wait. But the freezing time is too long in this case, especially for large boxes. For example, the core temperature of a 4–cm-deep box containing 32 kg of variety meat can be as high as 16°C 24 h after it is placed into a 30°C freezer. The freezing time of large boxes can be shortened considerably by adding some dry ice into it. A more effective method of freezing, called quick chilling, involves the use of lower air temperatures, 40 to 30°C, with higher velocities of 2.5 m/s to 5 m/s over the product (Fig. 4–40). The internal temperature should be lowered to 4°C for products to be transferred to a storage freezer and to 18°C for products to be shipped immediately. The rate of freezing depends on the package material and its insulating properties, the thickness of the largest box, the type of meat, and the capacity of the refrigeration system. Note that the air temperature will rise excessively during initial stages of freezing and increase the freezing time if the capacity of the system is inadequate. A smaller refrigeration system will be adequate if dry ice is to be used in packages. Shrinkage during freezing varies from about 0.5 to 1 percent. Although the average freezing point of lean meat can be taken to be 2°C with a latent heat of 249 kJ/kg, it should be remembered that freezing occurs over a temperature range, with most freezing occurring between 1 and 4°C. Therefore, cooling the meat through this temperature range and removing the latent heat takes the most time during freezing. Meat can be kept at an internal temperature of 2 to 1°C for local use and storage for under a week. Meat must be frozen and stored at much lower temperatures for long-term storage. The lower the storage temperature, the longer the storage life of meat products, as shown in Table 4–6. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 245 245 CHAPTER 4 The internal temperature of carcasses entering the cooling sections varies from 38 to 41°C for hogs and from 37 to 39°C for lambs and calves. It takes about 15 h to cool the hogs and calves to the recommended temperature of 3 to 4°C. The cooling-room temperature is maintained at 1 to 0°C and the temperature difference between the refrigerant and the cooling air is kept at about 6°C. Air is circulated at a rate of about 7 to 12 air changes per hour. Lamb carcasses are cooled to an internal temperature of 1 to 2°C, which takes about 12 to 14 h, and are held at that temperature with 85 to 90 percent relative humidity until shipped or processed. The recommended rate of air circulation is 50 to 60 air changes per hour during the first 4 to 6 h, which is reduced to 10 to 12 changes per hour afterward. Freezing does not seem to affect the flavor of meat much, but it affects the quality in several ways. The rate and temperature of freezing may influence color, tenderness, and drip. Rapid freezing increases tenderness and reduces the tissue damage and the amount of drip after thawing. Storage at low freezing temperatures causes significant changes in animal fat. Frozen pork experiences more undesirable changes during storage because of its fat structure, and thus its acceptable storage period is shorter than that of beef, veal, or lamb. Meat storage facilities usually have a refrigerated shipping dock where the orders are assembled and shipped out. Such docks save valuable storage space from being used for shipping purposes and provide a more acceptable working environment for the employees. Packing plants that ship whole or half carcasses in bulk quantities may not need a shipping dock; a load-out door is often adequate for such cases. A refrigerated shipping dock, as shown in Figure 4–41, reduces the refrigeration load of freezers or coolers and prevents temperature fluctuations in the storage area. It is often adequate to maintain the shipping docks at 4 to 7°C for the coolers and about 1.5°C for the freezers. The dew point of the dock air should be below the product temperature to avoid condensation on the surface of the products and loss of quality. The rate of airflow through the loading doors and other openings is proportional to the square root of the temperature difference, and thus reducing the temperature difference at the opening by half by keeping the shipping dock at the average temperature reduces the rate of airflow into the dock and thus into the 0.5 freezer by 1 0.3, or 30 percent. Also, the air that flows into the freezer is already cooled to about 1.5°C by the refrigeration unit of the dock, which represents about 50 percent of the cooling load of the incoming air. Thus, the net effect of the refrigerated shipping dock is a reduction of the infiltration load of the freezer by about 65 percent since 1 0.7 0.5 0.65. The net gain is equal to the difference between the reduction of the infiltration load of the freezer and the refrigeration load of the shipping dock. Note that the dock refrigerators operate at much higher temperatures (1.5°C instead of about 23°C), and thus they consume much less power for the same amount of cooling. Poultry Products Poultry products can be preserved by ice-chilling to 1 to 2°C or deep chilling to about 2°C for short-term storage, or by freezing them to 18°C or Freezer –23°C Refrigerated dock 1.5°C Sliding door Refrigerated truck FIGURE 4–41 A refrigerated truck dock for loading frozen items to a refrigerated truck. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 246 246 HEAT TRANSFER Air chilling H 2O 1000 g 980 g Immersion chilling H 2O 1000 g 1050 g FIGURE 4–42 Air chilling causes dehydration and thus weight loss for poultry, whereas immersion chilling causes a weight gain as a result of water absorption. below for long-term storage. Poultry processing plants are completely automated, and the small size of the birds makes continuous conveyor line operation feasible. The birds are first electrically stunned before cutting to prevent struggling. Following a 90- to 120-s bleeding time, the birds are scalded by immersing them into a tank of warm water, usually at 51 to 55°C, for up to 120 s to loosen the feathers. Then the feathers are removed by featherpicking machines, and the eviscerated carcass is washed thoroughly before chilling. The internal temperature of the birds ranges from 24 to 35°C after washing, depending on the temperatures of the ambient air and the washing water as well as the extent of washing. To control the microbial growth, the USDA regulations require that poultry be chilled to 4°C or below in less than 4 h for carcasses of less than 1.8 kg, in less than 6 h for carcasses of 1.8 to 3.6 kg. and in less than 8 h for carcasses more than 3.6 kg. Meeting these requirements today is not difficult since the slow air chilling is largely replaced by the rapid immersion chilling in tanks of slush ice. Immersion chilling has the added benefit that it not only prevents dehydration, but it causes a net absorption of water and thus increases the mass of salable product. Cool air chilling of unpacked poultry can cause a moisture loss of 1 to 2 percent, while water immersion chilling can cause a moisture absorption of 4 to 15 percent (Fig. 4–42). Water spray chilling can cause a moisture absorption of up to 4 percent. Most water absorbed is held between the flesh and the skin and the connective tissues in the skin. In immersion chilling, some soluble solids are lost from the carcass to the water, but the loss has no significant effect on flavor. Many slush ice tank chillers today are replaced by continuous flow-type immersion slush ice chillers. Continuous slush ice-chillers can reduce the internal temperature of poultry from 32 to 4°C in about 30 minutes at a rate up to 10, 000 birds per hour. Ice requirements depend on the inlet and exit temperatures of the carcass and the water, but 0.25 kg of ice per kg of carcass is usually adequate. However, bacterial contamination such as salmonella remains a concern with this method, and it may be necessary to chloride the water to control contamination. Tenderness is an important consideration for poultry products just as it is for red meat, and preserving tenderness is an important consideration in the cooling and freezing of poultry. Birds cooked or frozen before passing through rigor mortis remain very tough. Natural tenderization begins soon after slaughter and is completed within 24 h when birds are held at 4°C. Tenderization is rapid during the first three hours and slows down thereafter. Immersion in hot water and cutting into the muscle adversely affect tenderization. Increasing the scalding temperature or the scalding time has been observed to increase toughness, and decreasing the scalding time has been observed to increase tenderness. The beating action of mechanical feather-picking machines causes considerable toughening. Therefore, it is recommended that any cutting be done after tenderization. Cutting up the bird into pieces before natural tenderization is completed reduces tenderness considerably. Therefore, it is recommended that any cutting be done after tenderization. Rapid chilling of poultry can also have a toughening cen58933_ch04.qxd 9/10/2002 9:13 AM Page 247 247 CHAPTER 4 Storage life (days) 12 10 8 6 4 2 0 –2 0 5 10 15 20 25 Storage temperature, °C FIGURE 4–43 The storage life of fresh poultry decreases exponentially with increasing storage temperature. 9 8 7 Freezing time, hours effect. It is found that the tenderization process can be speeded up considerably by a patented electrical stunning process. Poultry products are highly perishable, and thus they should be kept at the lowest possible temperature to maximize their shelf life. Studies have shown that the populations of certain bacteria double every 36 h at 2°C, 14 h at 0°C, 7 h at 5°C, and less than 1 h at 25°C (Fig. 4–43). Studies have also shown that the total bacterial counts on birds held at 2°C for 14 days are equivalent to those held at 10°C for 5 days or 24°C for 1 day. It has also been found that birds held at 1°C had 8 days of additional shelf life over those held at 4°C. The growth of microorganisms on the surfaces of the poultry causes the development of an off-odor and bacterial slime. The higher the initial amount of bacterial contamination, the faster the sliming occurs. Therefore, good sanitation practices during processing such as cleaning the equipment frequently and washing the carcasses are as important as the storage temperature in extending shelf life. Poultry must be frozen rapidly to ensure a light, pleasing appearance. Poultry that is frozen slowly appears dark and develops large ice crystals that damage the tissue. The ice crystals formed during rapid freezing are small. Delaying freezing of poultry causes the ice crystals to become larger. Rapid freezing can be accomplished by forced air at temperatures of 23 to 40°C and velocities of 1.5 to 5 m/s in air-blast tunnel freezers. Most poultry is frozen this way. Also, the packaged birds freeze much faster on open shelves than they do in boxes. If poultry packages must be frozen in boxes, then it is very desirable to leave the boxes open or to cut holes on the boxes in the direction of airflow during freezing. For best results, the blast tunnel should be fully loaded across its cross-section with even spacing between the products to assure uniform airflow around all sides of the packages. The freezing time of poultry as a function of refrigerated air temperature is given in Figure 4–44. Thermal properties of poultry are given in Table 4–7. Other freezing methods for poultry include sandwiching between cold plates, immersion into a refrigerated liquid such as glycol or calcium chloride brine, and cryogenic cooling with liquid nitrogen. Poultry can be frozen in several hours by cold plates. Very high freezing rates can be obtained by immersing the packaged birds into a low-temperature brine. The freezing time of birds in 29°C brine can be as low as 20 min, depending on the size of the bird (Fig. 4–45). Also, immersion freezing produces a very appealing light appearance, and the high rates of heat transfer make continuous line operation feasible. It also has lower initial and maintenance costs than forced air, but leaks into the packages through some small holes or cracks remain a concern. The convection heat transfer coefficient is 17 W/m2 · °C for air at 29°C and 2.5 m/s whereas it is 170 W/m2 · °C for sodium chloride brine at 18°C and a velocity of 0.02 m/s. Sometimes liquid nitrogen is used to crust freeze the poultry products to 73°C. The freezing is then completed with air in a holding room at 23°C. Properly packaged poultry products can be stored frozen for up to about a year at temperatures of 18°C or lower. The storage life drops considerably at higher (but still below-freezing) temperatures. Significant changes Giblets Inside surface 13 mm depth Under skin 6 5 4 3 2 1 0 – 84 –73 –62 –51 –40 –29 –18 Air temperature, degrees Celsius –7 Note: Freezing time is the time required for temperature to fall from 0 to –4°C. The values are for 2.3 to 3.6 kg chickens with initial temperature of 0 to 2°C and with air velocity of 2.3 to 2.8 m/s. FIGURE 4–44 The variation of freezing time of poultry with air temperature (from van der Berg and Lentz, Ref. 11). cen58933_ch04.qxd 9/10/2002 9:13 AM Page 248 248 HEAT TRANSFER 5 0 Giblets Temperature, °C –5 FIGURE 4–45 The variation of temperature of the breast of 6.8-kg turkeys initially at 1°C with depth during immersion cooling at 29°C (from van der Berg and Lentz, Ref. 11). TABLE 4–7 Thermal properties of poultry Quantity Typical value Average density: Muscle 1070 kg/m3 Skin 1030 kg/m3 Specific heat: Above freezing 2.94 kJ/kg · °C Below freezing 1.55 kJ/kg · °C Freezing point 2.8°C Latent heat of fusion 247 kJ/kg Thermal conductivity: (in W/m · °C) Breast muscle 0.502 at 20°C 1.384 at 20°C 1.506 at 40°C Dark muscle 1.557 at 40°C –10 –15 Inside surface –20 38 mm depth 25 mm depth 13 mm depth 6.5 mm depth Under skin Skin surface –25 –30 –35 0 25 50 75 100 150 125 Time, min. 175 200 225 250 occur in flavor and juiciness when poultry is frozen for too long, and a stale rancid odor develops. Frozen poultry may become dehydrated and experience freezer burn, which may reduce the eye appeal of the product and cause toughening of the affected area. Dehydration and thus freezer burn can be controlled by humidification, lowering the storage temperature, and packaging the product in essentially impermeable film. The storage life can be extended by packing the poultry in an oxygen-free environment. The bacterial counts in precooked frozen products must be kept at safe levels since bacteria may not be destroyed completely during the reheating process at home. Frozen poultry can be thawed in ambient air, water, refrigerator, or oven without any significant difference in taste. Big birds like turkey should be thawed safely by holding it in a refrigerator at 2 to 4°C for two to four days, depending on the size of the bird. They can also be thawed by immersing them into cool water in a large container for 4 to 6 h, or holding them in a paper bag. Care must be exercised to keep the bird’s surface cool to minimize microbiological growth when thawing in air or water. EXAMPLE 4–5 Chilling of Beef Carcasses in a Meat Plant The chilling room of a meat plant is 18 m 20 m 5.5 m in size and has a capacity of 450 beef carcasses. The power consumed by the fans and the lights of the chilling room are 26 and 3 kW, respectively, and the room gains heat through its envelope at a rate of 13 kW. The average mass of beef carcasses is 285 kg. The carcasses enter the chilling room at 36°C after they are washed to facilitate evaporative cooling and are cooled to 15°C in 10 h. The water is expected to evaporate at a rate of 0.080 kg/s. The air enters the evaporator section of the refrigeration system at 0.7°C and leaves at 2°C. The air side of the evaporator is heavily finned, and the overall heat transfer coefficient of the evaporator based on the air side is 20 W/m2 · °C. Also, the average temperature difference between the air and the refrigerant in the evaporator is 5.5°C. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 249 249 CHAPTER 4 Determine (a) the refrigeration load of the chilling room, (b) the volume flow rate of air, and (c) the heat transfer surface area of the evaporator on the air side, assuming all the vapor and the fog in the air freezes in the evaporator. SOLUTION The chilling room of a meat plant with a capacity of 450 beef carcasses is considered. The cooling load, the airflow rate, and the heat transfer area of the evaporator are to be determined. Assumptions 1 Water evaporates at a rate of 0.080 kg/s. 2 All the moisture in the air freezes in the evaporator. Properties The heat of fusion and the heat of vaporization of water at 0°C are 333.7 kJ/kg and 2501 kJ/kg (Table A-9). The density and specific heat of air at 0°C are 1.292 kg/m3 and 1.006 kJ/kg · °C (Table A-15). Also, the specific heat of beef carcass is determined from the relation in Table A-7b to be Cp 1.68 2.51 (water content) 1.68 2.51 0.58 3.14 kJ/kg · °C Analysis (a) A sketch of the chilling room is given in Figure 4–46. The amount of beef mass that needs to be cooled per unit time is Lights, 3 kW 13 kW mbeef (Total beef mass cooled)/(Cooling time) (450 carcasses)(285 kg/carcass)/(10 3600 s) Beef carcass 3.56 kg/s The product refrigeration load can be viewed as the energy that needs to be removed from the beef carcass as it is cooled from 36 to 15°C at a rate of 3.56 kg/s and is determined to be · Q beef (mC T)beef (3.56 kg/s)(3.14 kJ/kg · °C)(36 15)°C Evaporation 0.080 kg/s 36°C 285 kg Refrigerated air 235 kW Fans, 26 kW Then the total refrigeration load of the chilling room becomes · Q total, chillroom · Q beef · Q fan · Q lights · Q heat gain 235 26 3 13 277 kW 0.7°C Evaporator –2°C The amount of carcass cooling due to evaporative cooling of water is · Q beef, evaporative (mhfg)water (0.080 kg/s)(2490 kJ/kg) 199 kW which is 199/235 85 percent of the total product cooling load. The remaining 15 percent of the heat is transferred by convection and radiation. (b) Heat is transferred to air at the rate determined above, and the temperature of the air rises from 2°C to 0.7°C as a result. Therefore, the mass flow rate of air is · m air · Q air (Cp Tair) 277 kW (1.006 kJ/kg · °C)[0.7 ( 2)°C] 102.0 kg/s Then the volume flow rate of air becomes · Vair · m air air 102 kg/s 1.292 kg/m3 78.9 m3/s (c) Normally the heat transfer load of the evaporator is the same as the refrigeration load. But in this case the water that enters the evaporator as a liquid is · Qevap FIGURE 4–46 Schematic for Example 4–5. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 250 250 HEAT TRANSFER frozen as the temperature drops to 2°C, and the evaporator must also remove the latent heat of freezing, which is determined from · Q freezing · (m hlatent)water (0.080 kg/s)(334 kJ/kg) 27 kW Therefore, the total rate of heat removal at the evaporator is · Q evaporator · Q total, chill room · Q freezing 277 27 304 kW Then the heat transfer surface area of the evaporator on the air side is deter· mined from Q evaporator (UA)airside T, A · Q evaporator UT 304,000 W (20 W/m2 · °C)(5.5°C) 2764 m2 Obviously, a finned surface must be used to provide such a large surface area on the air side. SUMMARY In this chapter we considered the variation of temperature with time as well as position in one- or multidimensional systems. We first considered the lumped systems in which the temperature varies with time but remains uniform throughout the system at any time. The temperature of a lumped body of arbitrary shape of mass m, volume V, surface area As, density , and specific heat Cp initially at a uniform temperature Ti that is exposed to convection at time t 0 in a medium at temperature T with a heat transfer coefficient h is expressed as T(t) T Ti T e Q mCp[T(t) The amount of heat transfer reaches its upper limit when the body reaches the surrounding temperature T . Therefore, the maximum heat transfer between the body and its surroundings is Qmax mCp (T Bi hAs Cp V h Cp Lc (kJ) Ti ) (kJ) The error involved in lumped system analysis is negligible when bt where b Ti] (1/s) is a positive quantity whose dimension is (time) 1. This relation can be used to determine the temperature T(t) of a body at time t or, alternately, the time t required for the temperature to reach a specified value T(t). Once the temperature T(t) at time t is available, the rate of convection heat transfer between the body and its environment at that time can be determined from Newton’s law of cooling as · (W) Q (t) hAs [T(t) T ] The total amount of heat transfer between the body and the surrounding medium over the time interval t 0 to t is simply the change in the energy content of the body, hLc k 0.1 where Bi is the Biot number and Lc V/As is the characteristic length. When the lumped system analysis is not applicable, the variation of temperature with position as well as time can be determined using the transient temperature charts given in Figs. 4–13, 4–14, 4–15, and 4–23 for a large plane wall, a long cylinder, a sphere, and a semi-infinite medium, respectively. These charts are applicable for one-dimensional heat transfer in those geometries. Therefore, their use is limited to situations in which the body is initially at a uniform temperature, all surfaces are subjected to the same thermal conditions, and the body does not involve any heat generation. These charts can also be used to determine the total heat transfer from the body up to a specified time t. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 251 251 CHAPTER 4 Using a one-term approximation, the solutions of onedimensional transient heat conduction problems are expressed analytically as Plane wall: Cylinder: Sphere: (x, t)wall (r, t)cyl (r, t)sph T(x, t) T Ti T 2 A1e 1 cos ( 1x/L), T(r, t) T Ti T 2 A1e 1 J0( 1r/ro), T(r, t) T Ti T 2 sin( 1r /ro) , A1e 1 1r /ro 0.2 0.2 0.2 where the constants A1 and 1 are functions of the Bi number only, and their values are listed in Table 4–1 against the Bi number for all three geometries. The error involved in oneterm solutions is less than 2 percent when 0.2. Using the one-term solutions, the fractional heat transfers in different geometries are expressed as Plane wall: Cylinder: Sphere: Q Qmax wall Q Qmax cyl Q Qmax sin 1 0, wall 1 2 1 3 1 1 J1( 1) 0, cyl 1 cos 1 0, sph 1 3 1 sph erfc erfc x 2 exp t h x 2 t T(x, t) Ti Ts Ti erfc x 2 (Ts t constant) Using a clever superposition principle called the product solution these charts can also be used to construct solutions for the two-dimensional transient heat conduction problems encountered in geometries such as a short cylinder, a long rectangular bar, or a semi-infinite cylinder or plate, and even three-dimensional problems associated with geometries such as a rectangular prism or a semi-infinite rectangular bar, provided that all surfaces of the solid are subjected to convection to the same fluid at temperature T , with the same convection heat transfer coefficient h, and the body involves no heat generation. The solution in such multidimensional geometries can be expressed as the product of the solutions for the one-dimensional geometries whose intersection is the multidimensional geometry. The total heat transfer to or from a multidimensional geometry can also be determined by using the one-dimensional values. The transient heat transfer for a two-dimensional geometry formed by the intersection of two one-dimensional geometries 1 and 2 is 1 sin The analytic solution for one-dimensional transient heat conduction in a semi-infinite solid subjected to convection is given by T(x, t) Ti T Ti where the quantity erfc ( ) is the complementary error function. For the special case of h → , the surface temperature Ts becomes equal to the fluid temperature T , and the above equation reduces to hx k t h2 t k2 k Q Qmax total, 2D Q Qmax Q Qmax 1 12 Q Qmax 1 Transient heat transfer for a three-dimensional body formed by the intersection of three one-dimensional bodies 1, 2, and 3 is given by Q Qmax total, 3D Q Qmax 1 Q Qmax 3 Q Q 1Qmax 2 Qmax 1 Q Q 11Qmax 1 Qmax 2 REFERENCES AND SUGGESTED READING 1. ASHRAE. Handbook of Fundamentals. SI version. Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1993. 2. ASHRAE. Handbook of Fundamentals. SI version. Atlanta, GA: American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc., 1994. 3. H. S. Carslaw and J. C. Jaeger. Conduction of Heat in Solids. 2nd ed. London: Oxford University Press, 1959. 4. H. Gröber, S. Erk, and U. Grigull. Fundamentals of Heat Transfer. New York: McGraw-Hill, 1961. 5. M. P. Heisler. “Temperature Charts for Induction and Constant Temperature Heating.” ASME Transactions 69 (1947), pp. 227–36. 6. H. Hillman. Kitchen Science. Mount Vernon, NY: Consumers Union, 1981. 7. F. P. Incropera and D. P. DeWitt. Introduction to Heat Transfer. 4th ed. New York: John Wiley & Sons, 2002. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 252 252 HEAT TRANSFER 8. L. S. Langston. “Heat Transfer from Multidimensional Objects Using One-Dimensional Solutions for Heat Loss.” International Journal of Heat and Mass Transfer 25 (1982), pp. 149–50. 9. M. N. Özisik, Heat Transfer—A Basic Approach. New , York: McGraw-Hill, 1985. 10. P. J. Schneider. Conduction Heat Transfer. Reading, MA: Addison-Wesley, 1955. 11. L. van der Berg and C. P. Lentz. “Factors Affecting Freezing Rate and Appearance of Eviscerated Poultry Frozen in Air.” Food Technology 12 (1958). PROBLEMS* Lumped System Analysis 4–1C What is lumped system analysis? When is it applicable? 4–2C Consider heat transfer between two identical hot solid bodies and the air surrounding them. The first solid is being cooled by a fan while the second one is allowed to cool naturally. For which solid is the lumped system analysis more likely to be applicable? Why? 4–3C Consider heat transfer between two identical hot solid bodies and their environments. The first solid is dropped in a large container filled with water, while the second one is allowed to cool naturally in the air. For which solid is the lumped system analysis more likely to be applicable? Why? 4–4C Consider a hot baked potato on a plate. The temperature of the potato is observed to drop by 4°C during the first minute. Will the temperature drop during the second minute be less than, equal to, or more than 4°C? Why? Cool air Hot baked potato FIGURE P4–4C 4–5C Consider a potato being baked in an oven that is maintained at a constant temperature. The temperature of the potato is observed to rise by 5°C during the first minute. Will the temperature rise during the second minute be less than, equal to, or more than 5°C? Why? 4–6C What is the physical significance of the Biot number? Is the Biot number more likely to be larger for highly conducting solids or poorly conducting ones? *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 EES-CD icon are solved using EES, and complete solutions together with parametric studies are included on the enclosed CD. Problems with a computer-EES icon are comprehensive in nature, and are intended to be solved with a computer, preferably using the EES software that accompanies this text. 4–7C Consider two identical 4–kg pieces of roast beef. The first piece is baked as a whole, while the second is baked after being cut into two equal pieces in the same oven. Will there be any difference between the cooking times of the whole and cut roasts? Why? 4–8C Consider a sphere and a cylinder of equal volume made of copper. Both the sphere and the cylinder are initially at the same temperature and are exposed to convection in the same environment. Which do you think will cool faster, the cylinder or the sphere? Why? 4–9C In what medium is the lumped system analysis more likely to be applicable: in water or in air? Why? 4–10C For which solid is the lumped system analysis more likely to be applicable: an actual apple or a golden apple of the same size? Why? 4–11C For which kind of bodies made of the same material is the lumped system analysis more likely to be applicable: slender ones or well-rounded ones of the same volume? Why? 4–12 Obtain relations for the characteristic lengths of a large plane wall of thickness 2L, a very long cylinder of radius ro, and a sphere of radius ro. 4–13 Obtain a relation for the time required for a lumped T ), where system to reach the average temperature 1 (Ti 2 Ti is the initial temperature and T is the temperature of the environment. 4–14 The temperature of a gas stream is to be measured by a thermocouple whose junction can be approximated as a 1.2-mm-diameter sphere. The properties of the junction are k 35 W/m · °C, 8500 kg/m3, and Cp 320 J/kg · °C, and the heat transfer coefficient between the junction and the gas is h 65 W/m2 · °C. Determine how long it will take for the thermocouple to read 99 percent of the initial temperature Answer: 38.5 s difference. 4–15E In a manufacturing facility, 2-in.-diameter brass balls 0.092 (k 64.1 Btu/h · ft · °F, 532 lbm/ft3, and Cp Btu/lbm · °F) initially at 250°F are quenched in a water bath at 120°F for a period of 2 min at a rate of 120 balls per minute. If the convection heat transfer coefficient is 42 Btu/h · ft2 · °F, determine (a) the temperature of the balls after quenching and (b) the rate at which heat needs to be removed from the water in order to keep its temperature constant at 120°F. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 253 253 CHAPTER 4 250°F Brass balls 120°F Water bath FIGURE P4–15E 4–16E 4–20 Consider a 1000-W iron whose base plate is made of 0.5-cm-thick aluminum alloy 2024-T6 ( 2770 kg/m3, Cp 5 2 875 J/kg · °C, 7.3 10 m /s). The base plate has a surface area of 0.03 m2. Initially, the iron is in thermal equilibrium with the ambient air at 22°C. Taking the heat transfer coefficient at the surface of the base plate to be 12 W/m2 · °C and assuming 85 percent of the heat generated in the resistance wires is transferred to the plate, determine how long it will take for the plate temperature to reach 140°C. Is it realistic to assume the plate temperature to be uniform at all times? Repeat Problem 4–15E for aluminum balls. Air 22°C 4–17 To warm up some milk for a baby, a mother pours milk into a thin-walled glass whose diameter is 6 cm. The height of the milk in the glass is 7 cm. She then places the glass into a large pan filled with hot water at 60°C. The milk is stirred constantly, so that its temperature is uniform at all times. If the heat transfer coefficient between the water and the glass is 120 W/m2 · °C, determine how long it will take for the milk to warm up from 3°C to 38°C. Take the properties of the milk to be the same as those of water. Can the milk in this case be Answer: 5.8 min treated as a lumped system? Why? 4–18 Repeat Problem 4–17 for the case of water also being stirred, so that the heat transfer coefficient is doubled to 240 W/m2 · °C. 4–19E During a picnic on a hot summer day, all the cold drinks disappeared quickly, and the only available drinks were those at the ambient temperature of 80°F. In an effort to cool a 12-fluid-oz drink in a can, which is 5 in. high and has a diameter of 2.5 in., a person grabs the can and starts shaking it in the iced water of the chest at 32°F. The temperature of the drink can be assumed to be uniform at all times, and the heat transfer coefficient between the iced water and the aluminum can is 30 Btu/h · ft2 · °F. Using the properties of water for the drink, estimate how long it will take for the canned drink to cool to 45°F. 1000 W iron FIGURE P4–20 4–21 Reconsider Problem 4–20. Using EES (or other) software, investigate the effects of the heat transfer coefficient and the final plate temperature on the time it will take for the plate to reach this temperature. Let the heat transfer coefficient vary from 5 W/m2 · °C to 25 W/m2 · °C and the temperature from 30°C to 200°C. Plot the time as functions of the heat transfer coefficient and the temperature, and discuss the results. 4–22 Stainless steel ball bearings ( 8085 kg/m3, k 15.1 3.91 10 6 m2/s) W/m · °C, Cp 0.480 kJ/kg · °C, and having a diameter of 1.2 cm are to be quenched in water. The balls leave the oven at a uniform temperature of 900°C and are exposed to air at 30°C for a while before they are dropped into the water. If the temperature of the balls is not to fall below 850°C prior to quenching and the heat transfer coefficient in the air is 125 W/m2 · °C, determine how long they can stand in Answer: 3.7 s the air before being dropped into the water. 4–23 Carbon steel balls ( Cp 0.465 kJ/kg · °C, and Air, 35°C Furnace 900°C FIGURE P4–19E 7833 kg/m3, k 54 W/m · °C, 1.474 10 6 m2/s) 8 mm in FIGURE P4–23 Steel ball 100°C cen58933_ch04.qxd 9/10/2002 9:13 AM Page 254 254 HEAT TRANSFER diameter are annealed by heating them first to 900°C in a furnace and then allowing them to cool slowly to 100°C in ambient air at 35°C. If the average heat transfer coefficient is 75 W/m2 · °C, determine how long the annealing process will take. If 2500 balls are to be annealed per hour, determine the total rate of heat transfer from the balls to the ambient air. 4–24 Reconsider Problem 4–23. Using EES (or other) software, investigate the effect of the initial temperature of the balls on the annealing time and the total rate of heat transfer. Let the temperature vary from 500°C to 1000°C. Plot the time and the total rate of heat transfer as a function of the initial temperature, and discuss the results. 4–25 An electronic device dissipating 30 W has a mass of 20 g, a specific heat of 850 J/kg · °C, and a surface area of 5 cm2. The device is lightly used, and it is on for 5 min and then off for several hours, during which it cools to the ambient temperature of 25°C. Taking the heat transfer coefficient to be 12 W/m2 · °C, determine the temperature of the device at the end of the 5-min operating period. What would your answer be if the device were attached to an aluminum heat sink having a mass of 200 g and a surface area of 80 cm2? Assume the device and the heat sink to be nearly isothermal. Would you use lumped system analysis or the transient temperature charts when determining the midpoint temperature of the sphere? Why? 4–33 A student calculates that the total heat transfer from a spherical copper ball of diameter 15 cm initially at 200°C and its environment at a constant temperature of 25°C during the first 20 min of cooling is 4520 kJ. Is this result reasonable? Why? 4–34 An ordinary egg can be approximated as a 5.5-cmdiameter sphere whose properties are roughly k 0.6 W/m · °C and 0.14 10 6 m2/s. The egg is initially at a uniform temperature of 8°C and is dropped into boiling water at 97°C. Taking the convection heat transfer coefficient to be h 1400 W/m2 · °C, determine how long it will take for the center of the egg to reach 70°C. Egg Ti = 8°C Transient Heat Conduction in Large Plane Walls, Long Cylinders, and Spheres with Spatial Effects 4–26C What is an infinitely long cylinder? When is it proper to treat an actual cylinder as being infinitely long, and when is it not? For example, is it proper to use this model when finding the temperatures near the bottom or top surfaces of a cylinder? Explain. 4–27C Can the transient temperature charts in Fig. 4–13 for a plane wall exposed to convection on both sides be used for a plane wall with one side exposed to convection while the other side is insulated? Explain. 4–28C Why are the transient temperature charts prepared using nondimensionalized quantities such as the Biot and Fourier numbers instead of the actual variables such as thermal conductivity and time? 4–29C What is the physical significance of the Fourier number? Will the Fourier number for a specified heat transfer problem double when the time is doubled? 97°C Boiling water FIGURE P4–34 4–35 Reconsider Problem 4–34. Using EES (or other) software, investigate the effect of the final center temperature of the egg on the time it will take for the center to reach this temperature. Let the temperature vary from 50°C to 95°C. Plot the time versus the temperature, and discuss the results. 4–36 In a production facility, 3-cm-thick large brass plates (k 110 W/m · °C, 8530 kg/m3, Cp 380 J/kg · °C, and 6 2 33.9 10 m /s) that are initially at a uniform temperature of 25°C are heated by passing them through an oven maintained at 700°C. The plates remain in the oven for a period of 10 min. Taking the convection heat transfer coefficient to be h 80 W/m2 · °C, determine the surface temperature of the plates when they come out of the oven. Furnace, 700°C 4–30C How can we use the transient temperature charts when the surface temperature of the geometry is specified instead of the temperature of the surrounding medium and the convection heat transfer coefficient? 4–31C A body at an initial temperature of Ti is brought into a medium at a constant temperature of T . How can you determine the maximum possible amount of heat transfer between the body and the surrounding medium? 4–32C The Biot number during a heat transfer process between a sphere and its surroundings is determined to be 0.02. 3 cm Brass plate 25°C FIGURE P4–36 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 255 255 CHAPTER 4 4–37 Reconsider Problem 4–36. Using EES (or other) software, investigate the effects of the temperature of the oven and the heating time on the final surface temperature of the plates. Let the oven temperature vary from 500°C to 900°C and the time from 2 min to 30 min. Plot the surface temperature as the functions of the oven temperature and the time, and discuss the results. 4–38 A long 35-cm-diameter cylindrical shaft made of stainless steel 304 (k 14.9 W/m · °C, 7900 kg/m3, Cp 477 6 2 J/kg · °C, and 3.95 10 m /s) comes out of an oven at a uniform temperature of 400°C. The shaft is then allowed to cool slowly in a chamber at 150°C with an average convection heat transfer coefficient of h 60 W/m2 · °C. Determine the temperature at the center of the shaft 20 min after the start of the cooling process. Also, determine the heat transfer per unit length of the shaft during this time period. Answers: 390°C, 16,015 kJ/m 4–39 Reconsider Problem 4–38. Using EES (or other) software, investigate the effect of the cooling time on the final center temperature of the shaft and the amount of heat transfer. Let the time vary from 5 min to 60 min. Plot the center temperature and the heat transfer as a function of the time, and discuss the results. 4–40E Long cylindrical AISI stainless steel rods (k 7.74 Btu/h · ft · °F and 0.135 ft2/h) of 4-in. diameter are heattreated by drawing them at a velocity of 10 ft/min through a 30-ft-long oven maintained at 1700°F. The heat transfer coefficient in the oven is 20 Btu/h · ft2 · °F. If the rods enter the oven at 85°F, determine their centerline temperature when they leave. 500°C in a fireplace with a heat transfer coefficient of 13.6 W/m2 · °C on the surface. If the ignition temperature of the wood is 420°C, determine how long it will be before the log ignites. 4–43 In Betty Crocker ’s Cookbook, it is stated that it takes 2 h 45 min to roast a 3.2-kg rib initially at 4.5°C “rare” in an oven maintained at 163°C. It is recommended that a meat thermometer be used to monitor the cooking, and the rib is considered rare done when the thermometer inserted into the center of the thickest part of the meat registers 60°C. The rib can be treated as a homogeneous spherical object with the properties 1200 kg/m3, Cp 4.1 kJ/kg · °C, k 0.45 W/m · °C, and 0.91 10 7 m2/s. Determine (a) the heat transfer coefficient at the surface of the rib, (b) the temperature of the outer surface of the rib when it is done, and (c) the amount of heat transferred to the rib. (d) Using the values obtained, predict how long it will take to roast this rib to “medium” level, which occurs when the innermost temperature of the rib reaches 71°C. Compare your result to the listed value of 3 h 20 min. If the roast rib is to be set on the counter for about 15 min before it is sliced, it is recommended that the rib be taken out of the oven when the thermometer registers about 4°C below the indicated value because the rib will continue cooking even after it is taken out of the oven. Do you agree with this recommendation? Answers: (a) 156.9 W/m2 · °C, (b) 159.5°C, (c) 1629 kJ, (d) 3.0 h Oven, 163°C Oven 1700°F 10 ft/min 30 ft Stainless steel 85°F FIGURE P4–40E 4–41 In a meat processing plant, 2-cm-thick steaks (k 0.45 W/m · °C and 0.91 10 7 m2/s) that are initially at 25°C are to be cooled by passing them through a refrigeration room at 11°C. The heat transfer coefficient on both sides of the steaks is 9 W/m2 · °C. If both surfaces of the steaks are to be cooled to 2°C, determine how long the steaks should be kept in the refrigeration room. 4–42 A long cylindrical wood log (k 0.17 W/m · °C and 1.28 10 7 m2/s) is 10 cm in diameter and is initially at a uniform temperature of 10°C. It is exposed to hot gases at Rib Ti = 4.5°C FIGURE P4–43 4–44 Repeat Problem 4–43 for a roast rib that is to be “welldone” instead of “rare.” A rib is considered to be well-done when its center temperature reaches 77°C, and the roasting in this case takes about 4 h 15 min. 4–45 For heat transfer purposes, an egg can be considered to be a 5.5-cm-diameter sphere having the properties of water. An egg that is initially at 8°C is dropped into the boiling water at 100°C. The heat transfer coefficient at the surface of the egg is estimated to be 800 W/m2 · °C. If the egg is considered cooked when its center temperature reaches 60°C, determine how long the egg should be kept in the boiling water. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 256 256 HEAT TRANSFER 4–46 Repeat Problem 4–45 for a location at 1610-m elevation such as Denver, Colorado, where the boiling temperature of water is 94.4°C. 4–47 The author and his 6-year-old son have conducted the following experiment to determine the thermal conductivity of a hot dog. They first boiled water in a large pan and measured the temperature of the boiling water to be 94°C, which is not surprising, since they live at an elevation of about 1650 m in Reno, Nevada. They then took a hot dog that is 12.5 cm long and 2.2 cm in diameter and inserted a thermocouple into the midpoint of the hot dog and another thermocouple just under the skin. They waited until both thermocouples read 20°C, which is the ambient temperature. They then dropped the hot dog into boiling water and observed the changes in both temperatures. Exactly 2 min after the hot dog was dropped into the boiling water, they recorded the center and the surface temperatures to be 59°C and 88°C, respectively. The density of the hot dog can be taken to be 980 kg/m3, which is slightly less than the density of water, since the hot dog was observed to be floating in water while being almost completely immersed. The specific heat of a hot dog can be taken to be 3900 J/kg · °C, which is slightly less than that of water, since a hot dog is mostly water. Using transient temperature charts, determine (a) the thermal diffusivity of the hot dog, (b) the thermal conductivity of the hot dog, and (c) the convection heat transfer coefficient. Answers: (a) 2.02 (c) 467 W/m2 · °C. 10 Boiling water 94°C 7 m2/s, (b) 0.771 W/m · °C, Refrigerator 5°F Chicken Ti = 72°F FIGURE P4–49E minimum. The chicken can be treated as a homogeneous spherical object having the properties 74.9 lbm/ft3, Cp 0.98 Btu/lbm · °F, k 0.26 Btu/h · ft · °F, and 0.0035 ft2/h. 4–50 A person puts a few apples into the freezer at 15°C to cool them quickly for guests who are about to arrive. Initially, the apples are at a uniform temperature of 20°C, and the heat transfer coefficient on the surfaces is 8 W/m2 · °C. Treating the apples as 9-cm-diameter spheres and taking their properties to be 840 kg/m3, Cp 3.81 kJ/kg · °C, k 0.418 W/m · °C, and 1.3 10 7 m2/s, determine the center and surface temperatures of the apples in 1 h. Also, determine the amount of heat transfer from each apple. 4–51 Tsurface HOT DOG Tcenter FIGURE P4–47 4–48 Using the data and the answers given in Problem 4–47, determine the center and the surface temperatures of the hot dog 4 min after the start of the cooking. Also determine the amount of heat transferred to the hot dog. 4–49E In a chicken processing plant, whole chickens averaging 5 lb each and initially at 72°F are to be cooled in the racks of a large refrigerator that is maintained at 5°F. The entire chicken is to be cooled below 45°F, but the temperature of the chicken is not to drop below 35°F at any point during refrigeration. The convection heat transfer coefficient and thus the rate of heat transfer from the chicken can be controlled by varying the speed of a circulating fan inside. Determine the heat transfer coefficient that will enable us to meet both temperature constraints while keeping the refrigeration time to a Reconsider Problem 4–50. Using EES (or other) software, investigate the effect of the initial temperature of the apples on the final center and surface temperatures and the amount of heat transfer. Let the initial temperature vary from 2°C to 30°C. Plot the center temperature, the surface temperature, and the amount of heat transfer as a function of the initial temperature, and discuss the results. 4–52 Citrus fruits are very susceptible to cold weather, and extended exposure to subfreezing temperatures can destroy them. Consider an 8-cm-diameter orange that is initially at Ambient air –15°C Orange Ti = 15°C FIGURE P4–52 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 257 257 CHAPTER 4 15°C. A cold front moves in one night, and the ambient temperature suddenly drops to 6°C, with a heat transfer coefficient of 15 W/m2 · °C. Using the properties of water for the orange and assuming the ambient conditions to remain constant for 4 h before the cold front moves out, determine if any part of the orange will freeze that night. 4–53 An 8-cm-diameter potato ( 1100 kg/m3, Cp 3900 J/kg · °C, k 0.6 W/m · °C, and 1.4 10 7 m2/s) that is initially at a uniform temperature of 25°C is baked in an oven at 170°C until a temperature sensor inserted to the center of the potato indicates a reading of 70°C. The potato is then taken out of the oven and wrapped in thick towels so that almost no heat is lost from the baked potato. Assuming the heat transfer coefficient in the oven to be 25 W/m2 · °C, determine (a) how long the potato is baked in the oven and (b) the final equilibrium temperature of the potato after it is wrapped. 4–54 White potatoes (k 0.50 W/m · °C and 0.13 10 6 m2/s) that are initially at a uniform temperature of 25°C and have an average diameter of 6 cm are to be cooled by refrigerated air at 2°C flowing at a velocity of 4 m/s. The average heat transfer coefficient between the potatoes and the air is experimentally determined to be 19 W/m2 · °C. Determine how long it will take for the center temperature of the potatoes to drop to 6°C. Also, determine if any part of the potatoes will experience chilling injury during this process. Air 2°C 4 m/s Potato Ti = 25°C FIGURE P4–54 4–55E Oranges of 2.5-in. diameter (k 0.26 Btu/h · ft · °F and 1.4 10 6 ft2/s) initially at a uniform temperature of 78°F are to be cooled by refrigerated air at 25°F flowing at a velocity of 1 ft/s. The average heat transfer coefficient between the oranges and the air is experimentally determined to be 4.6 Btu/h · ft2 · °F. Determine how long it will take for the center temperature of the oranges to drop to 40°F. Also, determine if any part of the oranges will freeze during this process. 4–56 A 65-kg beef carcass (k 0.47 W/m · °C and 0.13 10 6 m2/s) initially at a uniform temperature of 37°C is to be cooled by refrigerated air at 6°C flowing at a velocity of 1.8 m/s. The average heat transfer coefficient between the carcass and the air is 22 W/m2 · °C. Treating the carcass as a cylinder of diameter 24 cm and height 1.4 m and disregarding heat transfer from the base and top surfaces, determine how long it will take for the center temperature of the carcass to drop to 4°C. Also, determine if any part of the carcass will Answer: 14.0 h freeze during this process. Air – 6°C 1.8 m/s Beef 37°C FIGURE P4–56 4–57 Layers of 23-cm-thick meat slabs (k 0.47 W/m · °C and 0.13 10 6 m2/s) initially at a uniform temperature of 7°C are to be frozen by refrigerated air at 30°C flowing at a velocity of 1.4 m/s. The average heat transfer coefficient between the meat and the air is 20 W/m2 · °C. Assuming the size of the meat slabs to be large relative to their thickness, determine how long it will take for the center temperature of the slabs to drop to 18°C. Also, determine the surface temperature of the meat slab at that time. 4–58E Layers of 6-in.-thick meat slabs (k 0.26 Btu/h · ft · °F and 1.4 10 6 ft2/s) initially at a uniform temperature of 50°F are cooled by refrigerated air at 23°F to a temperature of 36°F at their center in 12 h. Estimate the average heat transfer coefficient during this cooling process. Answer: 1.5 Btu/h · ft2 · °F 4–59 Chickens with an average mass of 1.7 kg (k 0.45 W/m · °C and 0.13 10 6 m2/s) initially at a uniform temperature of 15°C are to be chilled in agitated brine at 10°C. The average heat transfer coefficient between the chicken and the brine is determined experimentally to be 440 W/m2 · °C. Taking the average density of the chicken to be 0.95 g/cm3 and treating the chicken as a spherical lump, determine the center and the surface temperatures of the chicken in 2 h and 30 min. Also, determine if any part of the chicken will freeze during this process. Chicken 1.7 kg Brine –10°C FIGURE P4–59 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 258 258 HEAT TRANSFER Transient Heat Conduction in Semi-Infinite Solids 4–60C What is a semi-infinite medium? Give examples of solid bodies that can be treated as semi-infinite mediums for heat transfer purposes. 4–61C Under what conditions can a plane wall be treated as a semi-infinite medium? 4–62C Consider a hot semi-infinite solid at an initial temperature of Ti that is exposed to convection to a cooler medium at a constant temperature of T , with a heat transfer coefficient of h. Explain how you can determine the total amount of heat transfer from the solid up to a specified time to. 4–63 In areas where the air temperature remains below 0°C for prolonged periods of time, the freezing of water in underground pipes is a major concern. Fortunately, the soil remains relatively warm during those periods, and it takes weeks for the subfreezing temperatures to reach the water mains in the ground. Thus, the soil effectively serves as an insulation to protect the water from the freezing atmospheric temperatures in winter. The ground at a particular location is covered with snow pack at 8°C for a continuous period of 60 days, and the average soil properties at that location are k 0.35 W/m · °C and 0.15 80 6 m2/s. Assuming an initial uniform temperature of 8°C for the ground, determine the minimum burial depth to prevent the water pipes from freezing. 4–64 The soil temperature in the upper layers of the earth varies with the variations in the atmospheric conditions. Before a cold front moves in, the earth at a location is initially at a uniform temperature of 10°C. Then the area is subjected to a temperature of 10°C and high winds that resulted in a convection heat transfer coefficient of 40 W/m2 · °C on the earth’s surface for a period of 10 h. Taking the properties of the soil at that location to be k 0.9 W/m · °C and 1.6 10 5 m2/s, determine the soil temperature at distances 0, 10, 20, and 50 cm from the earth’s surface at the end of this 10-h period. Winds, –10°C furnace and the surrounding air are in thermal equilibrium at 70°F. The furnace is then fired, and the inner surfaces of the furnace are subjected to hot gases at 1800°F with a very large heat transfer coefficient. Determine how long it will take for the temperature of the outer surface of the furnace walls to rise to 70.1°F. Answer: 181 min 4–67 A thick wood slab (k 0.17 W/m · °C and 1.28 10 7 m2/s) that is initially at a uniform temperature of 25°C is exposed to hot gases at 550°C for a period of 5 minutes. The heat transfer coefficient between the gases and the wood slab is 35 W/m2 · °C. If the ignition temperature of the wood is 450°C, determine if the wood will ignite. 4–68 A large cast iron container (k 52 W/m · °C and 1.70 10 5 m2/s) with 5-cm-thick walls is initially at a uniform temperature of 0°C and is filled with ice at 0°C. Now the outer surfaces of the container are exposed to hot water at 60°C with a very large heat transfer coefficient. Determine how long it will be before the ice inside the container starts melting. Also, taking the heat transfer coefficient on the inner surface of the container to be 250 W/m2 · °C, determine the rate of heat transfer to the ice through a 1.2-m-wide and 2-m-high section of the wall when steady operating conditions are reached. Assume the ice starts melting when its inner surface temperature rises to 0.1°C. Hot water 60°C Cast iron chest Ice 0°C 5 cm FIGURE P4–68 Transient Heat Conduction in Multidimensional Systems Soil 10°C FIGURE P4–64 4–65 Reconsider Problem 4–64. Using EES (or other) software, plot the soil temperature as a function of the distance from the earth’s surface as the distance varies from 0 m to 1m, and discuss the results. 4–66E The walls of a furnace are made of 1.5-ft-thick concrete (k 0.64 Btu/h · ft · °F and 0.023 ft2/h). Initially, the 4–69C What is the product solution method? How is it used to determine the transient temperature distribution in a twodimensional system? 4–70C How is the product solution used to determine the variation of temperature with time and position in threedimensional systems? 4–71C A short cylinder initially at a uniform temperature Ti is subjected to convection from all of its surfaces to a medium at temperature T . Explain how you can determine the temperature of the midpoint of the cylinder at a specified time t. 4–72C Consider a short cylinder whose top and bottom surfaces are insulated. The cylinder is initially at a uniform temperature Ti and is subjected to convection from its side surface cen58933_ch04.qxd 9/10/2002 9:13 AM Page 259 259 CHAPTER 4 to a medium at temperature T with a heat transfer coefficient of h. Is the heat transfer in this short cylinder one- or twodimensional? Explain. 4–77E Repeat Problem 4–76E for a location at 5300 ft elevation such as Denver, Colorado, where the boiling temperature of water is 202°F. 4–73 A short brass cylinder ( 8530 kg/m3, Cp 0.389 kJ/kg · °C, k 110 W/m · °C, and 3.39 10 5 m2/s) of diameter D 8 cm and height H 15 cm is initially at a 150°C. The cylinder is now uniform temperature of Ti placed in atmospheric air at 20°C, where heat transfer takes place by convection with a heat transfer coefficient of h 40 W/m2 · °C. Calculate (a) the center temperature of the cylinder, (b) the center temperature of the top surface of the cylinder, and (c) the total heat transfer from the cylinder 15 min after the start of the cooling. 4–78 A 5-cm-high rectangular ice block (k 2.22 W/m · °C and 0.124 10 7 m2/s) initially at 20°C is placed on a table on its square base 4 cm 4 cm in size in a room at 18°C. The heat transfer coefficient on the exposed surfaces of the ice block is 12 W/m2 · °C. Disregarding any heat transfer from the base to the table, determine how long it will be before the ice block starts melting. Where on the ice block will the first liquid droplets appear? Brass cylinder Room air 18°C Ambient air 20°C Ice block –20°C 15 cm 8 cm Ti = 150°C FIGURE P4–78 FIGURE P4–73 4–74 Reconsider Problem 4–73. Using EES (or other) software, investigate the effect of the cooling time on the center temperature of the cylinder, the center temperature of the top surface of the cylinder, and the total heat transfer. Let the time vary from 5 min to 60 min. Plot the center temperature of the cylinder, the center temperature of the top surface, and the total heat transfer as a function of the time, and discuss the results. 4–75 A semi-infinite aluminum cylinder (k 237 W/m · °C, 9.71 10 5 m2/s) of diameter D 15 cm is initially at a 150°C. The cylinder is now uniform temperature of Ti placed in water at 10°C, where heat transfer takes place by convection with a heat transfer coefficient of h 140 W/m2 · °C. Determine the temperature at the center of the cylinder 5 cm from the end surface 8 min after the start of cooling. 4–76E A hot dog can be considered to be a cylinder 5 in. long and 0.8 in. in diameter whose properties are 61.2 lbm/ft3, Cp 0.93 Btu/lbm · °F, k 0.44 Btu/h · ft · °F, and 0.0077 ft2/h. A hot dog initially at 40°F is dropped into boiling water at 212°F. If the heat transfer coefficient at the surface of the hot dog is estimated to be 120 Btu/h · ft2 · °F, determine the center temperature of the hot dog after 5, 10, and 15 min by treating the hot dog as (a) a finite cylinder and (b) an infinitely long cylinder. 4–79 Reconsider Problem 4–78. Using EES (or other) software, investigate the effect of the initial temperature of the ice block on the time period before the ice block starts melting. Let the initial temperature vary from 26°C to 4°C. Plot the time versus the initial temperature, and discuss the results. 4–80 A 2-cm-high cylindrical ice block (k 2.22 W/m · °C and 0.124 10 7 m2/s) is placed on a table on its base of diameter 2 cm in a room at 20°C. The heat transfer coefficient on the exposed surfaces of the ice block is 13 W/m2 · °C, and heat transfer from the base of the ice block to the table is negligible. If the ice block is not to start melting at any point for at least 2 h, determine what the initial temperature of the ice block should be. 4–81 Consider a cubic block whose sides are 5 cm long and a cylindrical block whose height and diameter are also 5 cm. Both blocks are initially at 20°C and are made of granite (k 2.5 W/m · °C and 1.15 10 6 m2/s). Now both blocks are exposed to hot gases at 500°C in a furnace on all of their surfaces with a heat transfer coefficient of 40 W/m2 · °C. Determine the center temperature of each geometry after 10, 20, and 60 min. 4–82 Repeat Problem 4–81 with the heat transfer coefficient at the top and the bottom surfaces of each block being doubled to 80 W/m2 · °C. cen58933_ch04.qxd 9/10/2002 9:13 AM Page 260 260 HEAT TRANSFER 5 cm To reduce the cooling time, it is proposed to cool the carcass with refrigerated air at –10°C. How would you evaluate this proposal? 5 cm 5 cm Ti = 20°C 5 cm 5 cm Ti = 20°C Hot gases, 500°C FIGURE P4–81 4–83 A 20-cm-long cylindrical aluminum block ( 2702 0.896 kJ/kg · °C, k 236 W/m · °C, and kg/m3, Cp 9.75 10 5 m2/s), 15 cm in diameter, is initially at a uniform temperature of 20°C. The block is to be heated in a furnace at 1200°C until its center temperature rises to 300°C. If the heat transfer coefficient on all surfaces of the block is 80 W/m2 · °C, determine how long the block should be kept in the furnace. Also, determine the amount of heat transfer from the aluminum block if it is allowed to cool in the room until its temperature drops to 20°C throughout. 4–84 Repeat Problem 4–83 for the case where the aluminum block is inserted into the furnace on a low-conductivity material so that the heat transfer to or from the bottom surface of the block is negligible. 4–85 Reconsider Problem 4–83. Using EES (or other) software, investigate the effect of the final center temperature of the block on the heating time and the amount of heat transfer. Let the final center temperature vary from 50°C to 1000°C. Plot the time and the heat transfer as a function of the final center temperature, and discuss the results. Special Topic: Refrigeration and Freezing of Foods 4–86C What are the common kinds of microorganisms? What undesirable changes do microorganisms cause in foods? 4–87C How does refrigeration prevent or delay the spoilage of foods? Why does freezing extend the storage life of foods for months? 4–88C What are the environmental factors that affect the growth rate of microorganisms in foods? 4–89C What is the effect of cooking on the microorganisms in foods? Why is it important that the internal temperature of a roast in an oven be raised above 70°C? 4–90C How can the contamination of foods with microorganisms be prevented or minimized? How can the growth of microorganisms in foods be retarded? How can the microorganisms in foods be destroyed? 4–93C Consider the freezing of packaged meat in boxes with refrigerated air. How do (a) the temperature of air, (b) the velocity of air, (c) the capacity of the refrigeration system, and (d) the size of the meat boxes affect the freezing time? 4–94C How does the rate of freezing affect the tenderness, color, and the drip of meat during thawing? 4–95C It is claimed that beef can be stored for up to two years at –23°C but no more than one year at –12°C. Is this claim reasonable? Explain. 4–96C What is a refrigerated shipping dock? How does it reduce the refrigeration load of the cold storage rooms? 4–97C How does immersion chilling of poultry compare to forced-air chilling with respect to (a) cooling time, (b) moisture loss of poultry, and (c) microbial growth. 4–98C What is the proper storage temperature of frozen poultry? What are the primary methods of freezing for poultry? 4–99C What are the factors that affect the quality of frozen fish? 4–100 The chilling room of a meat plant is 15 m 18 m 5.5 m in size and has a capacity of 350 beef carcasses. The power consumed by the fans and the lights in the chilling room are 22 and 2 kW, respectively, and the room gains heat through its envelope at a rate of 11 kW. The average mass of beef carcasses is 280 kg. The carcasses enter the chilling room at 35°C, after they are washed to facilitate evaporative cooling, and are cooled to 16°C in 12 h. The air enters the chilling room at 2.2°C and leaves at 0.5°C. Determine (a) the refrigeration load of the chilling room and (b) the volume flow rate of air. The average specific heats of beef carcasses and air are 3.14 and 1.0 kJ/kg · °C, respectively, and the density of air can be taken to be 1.28 kg/m3. 4–101 Turkeys with a water content of 64 percent that are initially at 1°C and have a mass of about 7 kg are to be frozen by submerging them into brine at 29°C. Using Figure 4–45, determine how long it will take to reduce the temperature of the turkey breast at a depth of 3.8 cm to 18°C. If the temperature at a depth of 3.8 cm in the breast represents the average Brine –29°C Turkey 7 kg 1°C 4–91C How does (a) the air motion and (b) the relative humidity of the environment affect the growth of microorganisms in foods? 4–92C The cooling of a beef carcass from 37°C to 5°C with refrigerated air at 0°C in a chilling room takes about 48 h. FIGURE P4–101 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 261 261 CHAPTER 4 temperature of the turkey, determine the amount of heat transfer per turkey assuming (a) the entire water content of the turkey is frozen and (b) only 90 percent of the water content of the turkey is frozen at 18°C. Take the specific heats of turkey to be 2.98 and 1.65 kJ/kg · °C above and below the freezing point of 2.8°C, respectively, and the latent heat of fusion of Answers: (a) 1753 kJ, (b) 1617 kJ turkey to be 214 kJ/kg. 4–102E Chickens with a water content of 74 percent, an initial temperature of 32°F, and a mass of about 6 lbm are to be frozen by refrigerated air at 40°F. Using Figure 4–44, determine how long it will take to reduce the inner surface temperature of chickens to 25°F. What would your answer be if the air temperature were 80°F? 4–103 Chickens with an average mass of 2.2 kg and average specific heat of 3.54 kJ/kg · °C are to be cooled by chilled water that enters a continuous-flow-type immersion chiller at 0.5°C. Chickens are dropped into the chiller at a uniform temperature of 15°C at a rate of 500 chickens per hour and are cooled to an average temperature of 3°C before they are taken out. The chiller gains heat from the surroundings at a rate of 210 kJ/min. Determine (a) the rate of heat removal from the chicken, in kW, and (b) the mass flow rate of water, in kg/s, if the temperature rise of water is not to exceed 2°C. 4–104 In a meat processing plant, 10-cm-thick beef slabs (ρ 1090 kg/m3, Cp 3.54 kJ/kg · °C, k 0.47 W/m · °C, and α 0.13 10–6 m2/s) initially at 15°C are to be cooled in the racks of a large freezer that is maintained at 12°C. The meat slabs are placed close to each other so that heat transfer from the 10-cm-thick edges is negligible. The entire slab is to be cooled below 5°C, but the temperature of the steak is not to drop below 1°C anywhere during refrigeration to avoid “frost bite.” The convection heat transfer coefficient and thus the rate of heat transfer from the steak can be controlled by varying the speed of a circulating fan inside. Determine the heat transfer coefficient h that will enable us to meet both temperature constraints while keeping the refrigeration time to a Answer: 9.9 W/m2 · °C. minimum. Air –12°C Meat 10 cm FIGURE P4–104 between the two plates has bonded them together. In an effort to melt the ice between the plates and separate them, the worker takes a large hairdryer and blows hot air at 50°C all over the exposed surface of the plate on the top. The convection heat transfer coefficient at the top surface is estimated to be 40 W/m2 · °C. Determine how long the worker must keep blowing hot air before the two plates separate. Answer: 482 s 4–106 Consider a curing kiln whose walls are made of 30-cm-thick concrete whose properties are k 0.9 W/m · °C and 0.23 10 5 m2/s. Initially, the kiln and its walls are in equilibrium with the surroundings at 2°C. Then all the doors are closed and the kiln is heated by steam so that the temperature of the inner surface of the walls is raised to 42°C and is maintained at that level for 3 h. The curing kiln is then opened and exposed to the atmospheric air after the stream flow is turned off. If the outer surfaces of the walls of the kiln were insulated, would it save any energy that day during the period the kiln was used for curing for 3 h only, or would it make no difference? Base your answer on calculations. 2°C 42°C 30 cm FIGURE P4–106 4–107 The water main in the cities must be placed at sufficient depth below the earth’s surface to avoid freezing during extended periods of subfreezing temperatures. Determine the minimum depth at which the water main must be placed at a location where the soil is initially at 15°C and the earth’s surface temperature under the worst conditions is expected to remain at 10°C for a period of 75 days. Take the properties of soil at that location to be k 0.7 W/m · °C and Answer: 7.05 m 1.4 10 5 m2/s. 4–108 A hot dog can be considered to be a 12-cm-long cylinder whose diameter is 2 cm and whose properties are 980 kg/m3, Cp 3.9 kJ/kg · °C, k 0.76 W/m · °C, and Review Problems 4–105 Consider two 2-cm-thick large steel plates (k 43 W/m · °C and 1.17 10 5 m2/s) that were put on top of each other while wet and left outside during a cold winter night at 15°C. The next day, a worker needs one of the plates, but the plates are stuck together because the freezing of the water Hot dog Water, 100°C FIGURE P4–108 cen58933_ch04.qxd 9/10/2002 9:13 AM Page 262 262 HEAT TRANSFER 2 10 7 m2/s. A hot dog initially at 5°C is dropped into boiling water at 100°C. The heat transfer coefficient at the surface of the hot dog is estimated to be 600 W/m2 · °C. If the hot dog is considered cooked when its center temperature reaches 80°C, determine how long it will take to cook it in the boiling water. 4–109 A long roll of 2-m-wide and 0.5-cm-thick 1-Mn manganese steel plate coming off a furnace at 820°C is to be 2.0 kJ/kg · °C) at 45°C. The quenched in an oil bath (Cp metal sheet is moving at a steady velocity of 10 m/min, and the oil bath is 5 m long. Taking the convection heat transfer coefficient on both sides of the plate to be 860 W/m2 · °C, determine the temperature of the sheet metal when it leaves the oil bath. Also, determine the required rate of heat removal from the oil to keep its temperature constant at 45°C. During a fire, the trunks of some dry oak trees (k 0.17 W/m · °C and 1.28 10 7 m2/s) that are initially at a uniform temperature of 30°C are exposed to hot gases at 520°C for a period of 5 h, with a heat transfer coefficient of 65 W/m2 · °C on the surface. The ignition temperature of the trees is 410°C. Treating the trunks of the trees as long cylindrical rods of diameter 20 cm, determine if these dry trees will ignite as the fire sweeps through them. Hot gases 520°C 30°C 20 cm Furnace 10 m/min 4–111 Steel plate FIGURE P4–111 Oil bath, 45°C FIGURE P4–109 4–110E In Betty Crocker ’s Cookbook, it is stated that it takes 5 h to roast a 14-lb stuffed turkey initially at 40°F in an oven maintained at 325°F. It is recommended that a meat thermometer be used to monitor the cooking, and the turkey is considered done when the thermometer inserted deep into the thickest part of the breast or thigh without touching the bone registers 185°F. The turkey can be treated as a homogeneous spheri0.98 cal object with the properties 75 lbm/ft3, Cp Btu/lbm · °F, k 0.26 Btu/h · ft · °F, and 0.0035 ft2/h. Assuming the tip of the thermometer is at one-third radial distance from the center of the turkey, determine (a) the average heat transfer coefficient at the surface of the turkey, (b) the temperature of the skin of the turkey when it is done, and (c) the total amount of heat transferred to the turkey in the oven. Will the reading of the thermometer be more or less than 185°F 5 min after the turkey is taken out of the oven? 4–112 We often cut a watermelon in half and put it into the freezer to cool it quickly. But usually we forget to check on it and end up having a watermelon with a frozen layer on the top. To avoid this potential problem a person wants to set the timer such that it will go off when the temperature of the exposed surface of the watermelon drops to 3°C. Consider a 30-cm-diameter spherical watermelon that is cut into two equal parts and put into a freezer at 12°C. Initially, the entire watermelon is at a uniform temperature of 25°C, and the heat transfer coefficient on the surfaces is 30 W/m2 · °C. Assuming the watermelon to have the properties of water, determine how long it will take for the center of the exposed cut surfaces of the watermelon to drop to 3°C. Freezer –12°C Watermelon, 25°C FIGURE P4–112 FIGURE P4–110 4–113 The thermal conductivity of a solid whose density and specific heat are known can be determined from the relation k / Cp after evaluating the thermal diffusivity . Consider a 2-cm-diameter cylindrical rod made of a sample material whose density and specific heat are 3700 kg/m3 and 920 J/kg · °C, respectively. The sample is initially at a uniform temperature of 25°C. In order to measure the temperatures of cen58933_ch04.qxd 9/10/2002 9:13 AM Page 263 263 CHAPTER 4 Tsurface Thermocouples Rod Tcenter Boiling water 100°C FIGURE P4–113 the sample at its surface and its center, a thermocouple is inserted to the center of the sample along the centerline, and another thermocouple is welded into a small hole drilled on the surface. The sample is dropped into boiling water at 100°C. After 3 min, the surface and the center temperatures are recorded to be 93°C and 75°C, respectively. Determine the thermal diffusivity and the thermal conductivity of the material. 4–114 In desert climates, rainfall is not a common occurrence since the rain droplets formed in the upper layer of the atmosphere often evaporate before they reach the ground. Consider a raindrop that is initially at a temperature of 5°C and has a diameter of 5 mm. Determine how long it will take for the diameter of the raindrop to reduce to 3 mm as it falls through ambient air at 18°C with a heat transfer coefficient of 400 W/m2 · °C. The water temperature can be assumed to remain constant and uniform at 5°C at all times. 4–115E Consider a plate of thickness 1 in., a long cylinder of diameter 1 in., and a sphere of diameter 1 in., all initially at 400°F and all made of bronze (k 15.0 Btu/h · ft · °F and 0.333 ft2/h). Now all three of these geometries are exposed to cool air at 75°F on all of their surfaces, with a heat transfer coefficient of 7 Btu/h · ft2 · °F. Determine the center temperature of each geometry after 5, 10, and 30 min. Explain why the center temperature of the sphere is always the lowest. Plate 1 in. Sphere Cylinder 1 in. 1 in. FIGURE P4–115 4–116E Repeat Problem 4–115E for cast iron geometries (k 29 Btu/h · ft · °F and 0.61 ft2/h). 4–117E Reconsider Problem 4–115E. Using EES (or other) software, plot the center temperature of each geometry as a function of the cooling time as the time varies fom 5 min to 60 min, and discuss the results. 4–118 Engine valves (k 48 W/m · °C, Cp 440 J/kg · °C, and 7840 kg/m3) are heated to 800°C in the heat treatment section of a valve manufacturing facility. The valves are then quenched in a large oil bath at an average temperature of 45°C. The heat transfer coefficient in the oil bath is 650 W/m2 · °C. The valves have a cylindrical stem with a diameter of 8 mm and a length of 10 cm. The valve head and the stem may be assumed to be of equal surface area, and the volume of the valve head can be taken to be 80 percent of the volume of steam. Determine how long will it take for the valve temperature to drop to (a) 400°C, (b) 200°C, and (c) 46°C and (d) the maximum heat transfer from a single valve. 4–119 A watermelon initially at 35°C is to be cooled by dropping it into a lake at 15°C. After 4 h and 40 min of cooling, the center temperature of watermelon is measured to be 20°C. Treating the watermelon as a 20-cm-diameter sphere and using the properties k 0.618 W/m · °C, 0.15 10 6 m2/s, 3 995 kg/m , and Cp 4.18 kJ/kg · °C, determine the average heat transfer coefficient and the surface temperature of watermelon at the end of the cooling period. 4–120 10-cm-thick large food slabs tightly wrapped by thin paper are to be cooled in a refrigeration room maintained at 0°C. The heat transfer coefficient on the box surfaces is 25 W/m2 · °C and the boxes are to be kept in the refrigeration room for a period of 6 h. If the initial temperature of the boxes is 30°C determine the center temperature of the boxes if the boxes contain (a) margarine (k 0.233 W/m · °C and 0.11 10 6 m2/s), (b) white cake (k 0.082 W/m · °C and 0.106 0.10 10 6 m2/s), and (c) chocolate cake (k W/m · °C and 0.12 10 6 m2/s). 4–121 A 30-cm-diameter, 3.5-m-high cylindrical column of a house made of concrete (k 0.79 W/m · °C, 5.94 1600 kg/m3, and Cp 0.84 kJ/kg · °C) cooled 10 7 m2/s, to 16°C during a cold night is heated again during the day by being exposed to ambient air at an average temperature of 28°C with an average heat transfer coefficient of 14 W/m2 · °C. Determine (a) how long it will take for the column surface temperature to rise to 27°C, (b) the amount of heat transfer until the center temperature reaches to 28°C, and (c) the amount of heat transfer until the surface temperature reaches to 27°C. 4–122 Long aluminum wires of diameter 3 mm ( 2702 0.896 kJ/kg · °C, k 236 W/m · °C, and kg/m3, Cp 9.75 10 5 m2/s) are extruded at a temperature of 350°C and exposed to atmospheric air at 30°C with a heat transfer coefficient of 35 W/m2 · °C. (a) Determine how long it will take for the wire temperature to drop to 50°C. (b) If the wire is extruded at a velocity of 10 m/min, determine how far the wire travels after extrusion by the time its temperature drops to 50°C. What change in the cooling process would you propose to shorten this distance? (c) Assuming the aluminum wire leaves the extrusion room at 50°C, determine the rate of heat transfer from the wire to the extrusion room. Answers: (a) 144 s, (b) 24 m, (c) 856 W cen58933_ch04.qxd 9/10/2002 9:13 AM Page 264 264 HEAT TRANSFER 350°C body as 28-cm diameter, 1.80-m-long cylinder, estimate how long it has been since he died. Take the properties of the body to be k 0.62 W/m · °C and 0.15 10 6 m2/s, and assume the initial temperature of the body to be 36°C. Tair = 30°C 10 m/min Aluminum wire FIGURE P4–122 4–123 Repeat Problem 4–122 for a copper wire ( 8950 kg/m3, Cp 0.383 kJ/kg · °C, k 386 W/m · °C, and 1.13 10 4 m2/s). 4–124 Consider a brick house (k 0.72 W/m · °C and 0.45 10 6 m2/s) whose walls are 10 m long, 3 m high, and 0.3 m thick. The heater of the house broke down one night, and the entire house, including its walls, was observed to be 5°C throughout in the morning. The outdoors warmed up as the day progressed, but no change was felt in the house, which was tightly sealed. Assuming the outer surface temperature of the house to remain constant at 15°C, determine how long it would take for the temperature of the inner surfaces of the walls to rise to 5.1°C. 15°C 5°C FIGURE P4–124 4–125 A 40-cm-thick brick wall (k 0.72 W/m · °C, and 1.6 10 7 m2/s) is heated to an average temperature of 18°C by the heating system and the solar radiation incident on it during the day. During the night, the outer surface of the wall is exposed to cold air at 2°C with an average heat transfer coefficient of 20 W/m2 · °C, determine the wall temperatures at distances 15, 30, and 40 cm from the outer surface for a period of 2 hours. 4–126 Consider the engine block of a car made of cast iron (k 52 W/m · °C and 1.7 10 5 m2/s). The engine can be considered to be a rectangular block whose sides are 80 cm, 40 cm, and 40 cm. The engine is at a temperature of 150°C when it is turned off. The engine is then exposed to atmospheric air at 17°C with a heat transfer coefficient of 6 W/m2 · °C. Determine (a) the center temperature of the top surface whose sides are 80 cm and 40 cm and (b) the corner temperature after 45 min of cooling. 4–127 A man is found dead in a room at 16°C. The surface temperature on his waist is measured to be 23°C and the heat transfer coefficient is estimated to be 9 W/m2 · °C. Modeling the Computer, Design, and Essay Problems 4–128 Conduct the following experiment at home to determine the combined convection and radiation heat transfer coefficient at the surface of an apple exposed to the room air. You will need two thermometers and a clock. First, weigh the apple and measure its diameter. You may measure its volume by placing it in a large measuring cup halfway filled with water, and measuring the change in volume when it is completely immersed in the water. Refrigerate the apple overnight so that it is at a uniform temperature in the morning and measure the air temperature in the kitchen. Then take the apple out and stick one of the thermometers to its middle and the other just under the skin. Record both temperatures every 5 min for an hour. Using these two temperatures, calculate the heat transfer coefficient for each interval and take their average. The result is the combined convection and radiation heat transfer coefficient for this heat transfer process. Using your experimental data, also calculate the thermal conductivity and thermal diffusivity of the apple and compare them to the values given above. 4–129 Repeat Problem 4–128 using a banana instead of an apple. The thermal properties of bananas are practically the same as those of apples. 4–130 Conduct the following experiment to determine the time constant for a can of soda and then predict the temperature of the soda at different times. Leave the soda in the refrigerator overnight. Measure the air temperature in the kitchen and the temperature of the soda while it is still in the refrigerator by taping the sensor of the thermometer to the outer surface of the can. Then take the soda out and measure its temperature again in 5 min. Using these values, calculate the exponent b. Using this b-value, predict the temperatures of the soda in 10, 15, 20, 30, and 60 min and compare the results with the actual temperature measurements. Do you think the lumped system analysis is valid in this case? 4–131 Citrus trees are very susceptible to cold weather, and extended exposure to subfreezing temperatures can destroy the crop. In order to protect the trees from occasional cold fronts with subfreezing temperatures, tree growers in Florida usually install water sprinklers on the trees. When the temperature drops below a certain level, the sprinklers spray water on the trees and their fruits to protect them against the damage the subfreezing temperatures can cause. Explain the basic mechanism behind this protection measure and write an essay on how the system works in practice. ...
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This note was uploaded on 01/28/2010 for the course HEAT ENG taught by Professor Ghaz during the Spring '10 term at University of Guelph.

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