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Solution Manual for Fundamentals of Heat and Mass Transfer, 7th Edition

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    PROBLEM 1.1 KNOWN: Thermal conductivity, thickness and temperature difference across a sheet of rigid extruded insulation. FIND: (a) The heat flux through a 2 m × 2 m sheet of the insulation, and (b) The heat rate through the sheet. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state conditions, (3) Constant properties. ANALYSIS: From Equation 1.2 the heat flux is 1 2 x T - T dT q = -k = k dx L ′′ Solving, " x W 10 K q = 0.029 × m K 0.02 m ⋅ x 2 W q = 14.5 m ′′ < The heat rate is 2 x x 2 W q = q A = 14.5 × 4 m = 58 W m ′′ ⋅ < COMMENTS: (1) Be sure to keep in mind the important distinction between the heat flux (W/m 2 ) and the heat rate (W). (2) The direction of heat flow is from hot to cold. (3) Note that a temperature difference may be expressed in kelvins or degrees Celsius. q cond A = 4 m 2 T 2 T 1 k = 0.029 ⋅ W m K x L = 20 mm T 1 – T 2 = 10 ˚ C q cond A = 4 m 2 T 2 T 1 k = 0.029 ⋅ W m K x L = 20 mm T 1 – T 2 = 10 ˚ C

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    PROBLEM 1.2 KNOWN: Thickness and thermal conductivity of a wall. Heat flux applied to one face and temperatures of both surfaces. FIND: Whether steady-state conditions exist. SCHEMATIC : ASSUMPTIONS : (1) One-dimensional conduction, (2) Constant properties, (3) No internal energy generation. ANALYSIS: Under steady-state conditions an energy balance on the control volume shown is 2 in out cond 1 2 ( ) / 12 W/m K(50 C 30 C) / 0.01 m 24,000 W/m q q q k T T L ′′ ′′ ′′ = = = − = ⋅ ° − ° = Since the heat flux in at the left face is only 20 W/m 2 , the conditions are not steady state. < COMMENTS: If the same heat flux is maintained until steady-state conditions are reached, the steady-state temperature difference across the wall will be Δ T = 2 / 20 W/m 0.01 m /12 W/m K 0.0167 K q L k ′′ = × ⋅ = which is much smaller than the specified temperature difference of 20 ° C. q ” = 20 W/m 2 L = 10 mm k = 12 W/m·K T 1 = 50 ° C T 2 = 30 ° C q ″ cond

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    PROBLEM 1.3 KNOWN: Inner surface temperature and thermal conductivity of a concrete wall. FIND: Heat loss by conduction through the wall as a function of outer surface temperatures ranging from -15 to 38 ° C. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state conditions, (3) Constant properties. ANALYSIS: From Fourier’s law, if x q ′′ and k are each constant it is evident that the gradient, x dT dx q k ′′ = − , is a constant, and hence the temperature distribution is linear. The heat flux must be constant under one-dimensional, steady-state conditions; and k is approximately constant if it depends only weakly on temperature. The heat flux and heat rate when the outside wall temperature is T 2 = -15 ° C are ( ) 2 1 2 x 25 C 15 C dT T T q k k 1W m K 133.3 W m dx L 0.30 m − − − ′′ = − = = ⋅ = o o . (1) 2 2 x x q q A 133.3 W m 20 m 2667 W ′′ = × = × = . (2) < Combining Eqs. (1) and (2), the heat rate q x can be determined for the range of outer surface temperature, -15 ≤ T 2 ≤ 38 ° C, with different wall thermal conductivities, k. For the concrete wall, k = 1 W/m ⋅ K, the heat loss varies linearly from +2667 W to -867 W and is zero when the inside and outer surface temperatures are the same. The magnitude of the heat rate increases with increasing thermal conductivity. COMMENTS: Without steady-state conditions and constant k, the temperature distribution in a plane wall would not be linear. -20 -10 0 10 20 30 40 Ambient air temperature, T2 (C) -1500 -500 500 1500 2500 3500 Heat loss, qx (W) Wall thermal conductivity, k = 1.25 W/m.K k = 1 W/m.K, concrete wall k = 0.75 W/m.K Outside surface -20 -10 0 10 20 30 40 Ambient air temperature, T2 (C) -1500 -500 500 1500 2500 3500 Heat loss, qx (W) Wall thermal conductivity, k = 1.25 W/m.K k = 1 W/m.K, concrete wall k = 0.75 W/m.K Outside surface

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    PROBLEM 1.4 KNOWN: Dimensions, thermal conductivity and surface temperatures of a concrete slab. Efficiency of gas furnace and cost of natural gas. FIND: Daily cost of heat loss. SCHEMATIC: ASSUMPTIONS: (1) Steady state, (2) One-dimensional conduction, (3) Constant properties. ANALYSIS: The rate of heat loss by conduction through the slab is ( ) ( ) 1 2 T T 7 C q k LW 1.4 W / m K 11m 8 m 4312 W t 0.20 m − ° = = ⋅ × = < The daily cost of natural gas that must be combusted to compensate for the heat loss is ( ) ( ) g d 6 f q C 4312 W $0.02 / MJ C t 24 h / d 3600s / h $8.28 / d 0.9 10 J / MJ η × = Δ = × = × < COMMENTS: The loss could be reduced by installing a floor covering with a layer of insulation between it and the concrete.

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    PROBLEM 1.5 KNOWN: Thermal conductivity and thickness of a wall. Heat flux through wall. Steady-state conditions. FIND: Value of temperature gradient in K/m and in ° C/m. SCHEMATIC : L = 20 mm k = 2.3 W/m·K q ” x = 10 W/m 2 x ASSUMPTIONS : (1) One-dimensional conduction, (2) Constant properties. ANALYSIS: Under steady-state conditions, " 2 10 W/m 4.35 K/m 4.35 C/m 2.3 W/m K x dT q dx k = − = − = − = − ° ⋅ < Since the K units here represent a temperature difference , and since the temperature difference is the same in K and ° C units, the temperature gradient value is the same in either units. COMMENTS: A negative value of temperature gradient means that temperature is decreasing with increasing x , corresponding to a positive heat flux in the x -direction.

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    PROBLEM 1.6 KNOWN: Heat flux and surface temperatures associated with a wood slab of prescribed thickness. FIND: Thermal conductivity, k, of the wood. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state conditions, (3) Constant properties. ANALYSIS: Subject to the foregoing assumptions, the thermal conductivity may be determined from Fourier’s law, Eq. 1.2. Rearranging, ( ) L W 0.05m k=q 40 T T m 40-20 C x 2 1 2 ′′ = − o k = 0.10 W / m K. ⋅ < COMMENTS: Note that the ° C or K temperature units may be used interchangeably when evaluating a temperature difference.

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    PROBLEM 1.7 KNOWN: Inner and outer surface temperatures of a glass window of prescribed dimensions. FIND: Heat loss through window. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction in the x-direction, (2) Steady-state conditions, (3) Constant properties. ANALYSIS: Subject to the foregoing conditions the heat flux may be computed from Fourier’s law, Eq. 1.2. ( ) T T q k L 15-5 C W q 1.4 m K 0.005m q 2800 W/m . 1 2 x x 2 x − ′′ = ′′ = ⋅ ′′ = o Since the heat flux is uniform over the surface, the heat loss (rate) is q = qx A q = 2800 W / m 2 3m 2 ′′ × × q = 8400 W. < COMMENTS: A linear temperature distribution exists in the glass for the prescribed conditions.

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    PROBLEM 1.8 KNOWN: Net power output, average compressor and turbine temperatures, shaft dimensions and thermal conductivity. FIND: (a) Comparison of the conduction rate through the shaft to the predicted net power output of the device, (b) Plot of the ratio of the shaft conduction heat rate to the anticipated net power output of the device over the range 0.005 m ≤ L ≤ 1 m and feasibility of a L = 0.005 m device. SCHEMATIC : ASSUMPTIONS : (1) Steady-state conditions, (2) Constant properties, (3) Net power output is proportional to the volume of the gas turbine. PROPERTIES: Shaft (given): k = 40 W/m ⋅ K. ANALYSIS: (a) The conduction through the shaft may be evaluated using Fourier’s law, yielding ( ) ( ) 2 3 2 ( ) 40W/m K(1000 400) C " / 4 (70 10 m) / 4 92.4W 1m h c c k T T q q A d L π π − − ⋅ − ° = = = × = The ratio of the conduction heat rate to the net power output is 6 6 92.4W 18.5 10 5 10 W q r P − = = = × × < (b) The volume of the turbine is proportional to L 3 . Designating L a = 1 m, d a = 70 mm and P a as the shaft length, shaft diameter, and net power output, respectively, in part (a), d = d a × ( L/L a ); P = P a × ( L/L a ) 3 and the ratio of the conduction heat rate to the net power output is ( ) ( ) ( ) 2 2 2 3 2 3 2 6 6 2 2 2 ( ) ( ) ( ) / 4 / / 4 / " 4 ( / ) 40W/m K(1000 400) C (70 10 m) 1m / 5 10 W 18.5 10 m 4 = h c h c h c a a a a a c a a k T T k T T k T T d d L L d L P q A L L r P P P L L L L L π π π π − − − − − = = = = ⋅ − ° × × × × = Continued… L = 1m d = 70 mm P = 5 MW Turbine Compressor Shaft T h = 1000 ° C T c = 400 ° C Combustion chamber L = 1m d = 70 mm P = 5 MW Turbine Compressor Shaft T h = 1000 ° C T c = 400 ° C Combustion chamber k = 40 W/m · K

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    PROBLEM 1.8 (Cont.) The ratio of the shaft conduction to net power is shown below. At L = 0.005 m = 5 mm, the shaft conduction to net power output ratio is 0.74. The concept of the very small turbine is not feasible since it will be unlikely that the large temperature difference between the compressor and turbine can be maintained. < COMMENTS: (1) The thermodynamics analysis does not account for heat transfer effects and is therefore meaningful only when heat transfer can be safely ignored, as is the case for the shaft in part (a). (2) Successful miniaturization of thermal devices is often hindered by heat transfer effects that must be overcome with innovative design. Ratio of shaft conduction to net power 0 0.2 0.4 0.6 0.8 1 L (m) 0.0001 0.001 0.01 0.1 1 r

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    PROBLEM 1.9 KNOWN: Width, height, thickness and thermal conductivity of a single pane window and the air space of a double pane window. Representative winter surface temperatures of single pane and air space. FIND: Heat loss through single and double pane windows. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction through glass or air, (2) Steady-state conditions, (3) Enclosed air of double pane window is stagnant (negligible buoyancy induced motion). ANALYSIS: From Fourier’s law, the heat losses are Single Pane : ( ) T T 35 C 2 1 2 q k A 1.4 W/m K 2m 19, 600 W g g L 0.005m − = = ⋅ = o < Double Pane : ( ) T T 25 C 2 1 2 q k A 0.024 2m 120 W a a L 0.010 m − = = = o < COMMENTS: Losses associated with a single pane are unacceptable and would remain excessive, even if the thickness of the glass were doubled to match that of the air space. The principal advantage of the double pane construction resides with the low thermal conductivity of air (~ 60 times smaller than that of glass). For a fixed ambient outside air temperature, use of the double pane construction would also increase the surface temperature of the glass exposed to the room (inside) air.

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    PROBLEM 1.10 KNOWN: Dimensions of freezer compartment. Inner and outer surface temperatures. FIND: Thickness of styrofoam insulation needed to maintain heat load below prescribed value. SCHEMATIC: ASSUMPTIONS: (1) Perfectly insulated bottom, (2) One-dimensional conduction through 5 walls of area A = 4m 2 , (3) Steady-state conditions, (4) Constant properties. ANALYSIS: Using Fourier’s law, Eq. 1.2, the heat rate is q = q A = k T L A total ′′ ⋅ Δ Solving for L and recognizing that A total = 5 × W 2 , find L = 5 k T W q 2 Δ ( ) ( ) 5 0.03 W/m K 35 - -10 C 4m L = 500 W 2 ⎡ ⎤ × ⋅ ⎣ ⎦ o L = 0.054m = 54mm. < COMMENTS: The corners will cause local departures from one-dimensional conduction and a slightly larger heat loss.

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    PROBLEM 1.11 KNOWN: Heat flux at one face and air temperature and convection coefficient at other face of plane wall. Temperature of surface exposed to convection. FIND: If steady-state conditions exist. If not, whether the temperature is increasing or decreasing. SCHEMATIC : ASSUMPTIONS : (1) One-dimensional conduction, (2) No internal energy generation. ANALYSIS: Conservation of energy for a control volume around the wall gives st in out g dE E E E dt = − + & & & [ ] st in in 2 2 2 ( ) ( ) 20 W/m 20 W/m K(50 C 30 C) 380 W/m s s dE q A hA T T q h T T A dt A A ∞ ∞ ′′ ′′ = − − = − − = − ⋅ ° − ° = − ⎡ ⎤ ⎣ ⎦ Since dE st / dt ≠ 0, the system is not at steady-state. < Since dE st / dt < 0, the stored energy is decreasing, therefore the wall temperature is decreasing. < COMMENTS: When the surface temperature of the face exposed to convection cools to 31 ° C, q in = q out and dE st / dt = 0 and the wall will have reached steady-state conditions. q ” = 20 W/m 2 T s = 50 ° C h = 20 W/m 2 ·K T ∞ = 30 ° C Air q ” conv

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    PROBLEM 1.12 KNOWN: Dimensions and thermal conductivity of food/beverage container. Inner and outer surface temperatures. FIND: Heat flux through container wall and total heat load. SCHEMATIC: ASSUMPTIONS: (1) Steady-state conditions, (2) Negligible heat transfer through bottom wall, (3) Uniform surface temperatures and one-dimensional conduction through remaining walls. ANALYSIS: From Fourier’s law, Eq. 1.2, the heat flux is ( ) 0.023 W/m K 20 2 C T T 2 2 1 q k 16.6 W/m L 0.025 m ⋅ − − ′′ = = = o < Since the flux is uniform over each of the five walls through which heat is transferred, the heat load is ( ) q q A q H 2W 2W W W total 1 2 1 2 ′′ ′′ ⎡ ⎤ = × = + + × ⎣ ⎦ ( ) ( ) 2 q 16.6 W/m 0.6m 1.6m 1.2m 0.8m 0.6m 35.9 W ⎡ ⎤ = + + × = ⎣ ⎦ < COMMENTS: The corners and edges of the container create local departures from one- dimensional conduction, which increase the heat load. However, for H, W 1 , W 2 >> L, the effect is negligible.

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    PROBLEM 1.13 KNOWN: Masonry wall of known thermal conductivity has a heat rate which is 80% of that through a composite wall of prescribed thermal conductivity and thickness. FIND: Thickness of masonry wall. SCHEMATIC: ASSUMPTIONS: (1) Both walls subjected to same surface temperatures, (2) One- dimensional conduction, (3) Steady-state conditions, (4) Constant properties. ANALYSIS: For steady-state conditions, the conduction heat flux through a one- dimensional wall follows from Fourier’s law, Eq. 1.2, ′′ q = k T L Δ where Δ T represents the difference in surface temperatures. Since Δ T is the same for both walls, it follows that L = L k k q q 1 2 1 2 2 1 ⋅ ′′ ′′ . With the heat fluxes related as ′′ = ′′ q 0.8 q 1 2 L = 100mm 0.75 W / m K 0.25 W / m K 1 0.8 = 375mm. 1 ⋅ ⋅ × < COMMENTS: Not knowing the temperature difference across the walls, we cannot find the value of the heat rate.

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    PROBLEM 1.14 KNOWN: Expression for variable thermal conductivity of a wall. Constant heat flux. Temperature at x = 0. FIND: Expression for temperature gradient and temperature distribution. SCHEMATIC: ASSUMPTIONS: (1) One-dimensional conduction. ANALYSIS: The heat flux is given by Fourier’s law, and is known to be constant, therefore x dT q k constant dx ′′ = − = Solving for the temperature gradient and substituting the expression for k yields x x q q dT dx k ax b ′ ′ ′′ = − = − + < This expression can be integrated to find the temperature distribution, as follows: x q dT dx dx dx ax b ′ ′ = − + ∫ ∫ Since x q constant ′′ = , we can integrate the right hand side to find ( ) x q T ln ax b c a ′ ′ = − + + where c is a constant of integration. Applying the known condition that T = T 1 at x = 0, we can solve for c. Continued… q ” x k = a x + b T 1

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