2-33 2-70 A compressed air pipe is subjected to uniform heat flux on the outer surface and convection on the inner surface The mathematical formulation, the variation of temperature in the pipe, and the surface temperatures are to be determined for steady one-dimensional heat transfer Assumptions Heat conduction is steady and one-dimensional since the pipe is long relative to its thickness, and there is thermal symmetry about the center line Thermal conductivity is constant There is no heat generation in the pipe Properties The thermal conductivity is given to be k = 14 W/m⋅°C Analysis (a) Noting that the 85% of the 300 W generated by the strip heater is transferred to the pipe, the heat flux through the outer surface is determined to be Q& Q& s 0.85 × 300 W = = 169.1 W/m q& s = s = A2 2πr2 L 2π (0.04 m)(6 m) Noting that heat transfer is one-dimensional in the radial r direction and heat flux is in the negative r direction, the mathematical formulation of this problem can be expressed as d ⎛ dT ⎞ ⎜r ⎟=0 dr ⎝ dr ⎠ and −k dT ( r1 ) = h[T∞ − T ( r1 )] dr r r2 Heater Air, -10°C r1 dT ( r2 ) k = q& s dr (b) Integrating the differential equation once with respect to r gives L=6 m dT r = C1 dr Dividing both sides of the equation above by r to bring it to a readily integrable form and then integrating, dT C1 = dr r T (r ) = C1 ln r + C where C1 and C2 are arbitrary constants Applying the boundary conditions give q& r C k = q& s → C1 = s r = r2: r2 k ⎛ ⎛ C1 k ⎞ k ⎞ q& s r2 ⎟⎟C1 = T∞ − ⎜⎜ ln r1 − ⎟ = h[T∞ − (C1 ln r1 + C )] → C = T∞ − ⎜⎜ ln r1 − r1 hr1 ⎠ hr1 ⎟⎠ k ⎝ ⎝ Substituting C1 and C2 into the general solution, the variation of temperature is determined to be r = r1: −k ⎛ r ⎛ ⎛ k ⎞ k ⎞ k ⎞ q& s r2 ⎟⎟ ⎟⎟C1 = T∞ + ⎜⎜ ln + ⎟⎟C1 = T∞ + ⎜⎜ ln r − ln r1 + T (r ) = C1 ln r + T∞ − ⎜⎜ ln r1 − hr1 ⎠ hr1 ⎠ ⎝ r1 hr1 ⎠ k ⎝ ⎝ ⎞ (169.1 W/m )(0.04 m) ⎛ r ⎞ ⎛ r 14 W/m ⋅ °C ⎟ = −10 + 0.483⎜⎜ ln + 12.61⎟⎟ = −10°C + ⎜ ln + ⎜ r (30 W/m ⋅ °C)(0.037 m) ⎟ 14 W/m ⋅ °C ⎠ ⎝ r1 ⎠ ⎝ (c) The inner and outer surface temperatures are determined by direct substitution to be ⎛ r ⎞ Inner surface (r = r1): T (r1 ) = −10 + 0.483⎜⎜ ln + 12.61⎟⎟ = −10 + 0.483(0 + 12.61) = −3.91°C ⎝ r1 ⎠ ⎛ r ⎞ ⎛ 0.04 ⎞ Outer surface (r = r2): T (r1 ) = −10 + 0.483⎜⎜ ln + 12.61⎟⎟ = −10 + 0.483⎜ ln + 12.61⎟ = −3.87°C ⎝ 0.037 ⎠ ⎝ r1 ⎠ Note that the pipe is essentially isothermal at a temperature of about -3.9°C PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-34 2-71 EES Prob 2-70 is reconsidered The temperature as a function of the radius is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" L=6 [m] r_1=0.037 [m] r_2=0.04 [m] k=14 [W/m-C] Q_dot=300 [W] T_infinity=-10 [C] h=30 [W/m^2-C] f_loss=0.15 "ANALYSIS" q_dot_s=((1-f_loss)*Q_dot)/A A=2*pi*r_2*L T=T_infinity+(ln(r/r_1)+k/(h*r_1))*(q_dot_s*r_2)/k "Variation of temperature" "r is the parameter to be varied" r [m] 0.037 0.03733 0.03767 0.038 0.03833 0.03867 0.039 0.03933 0.03967 0.04 T [C] 3.906 3.902 3.898 3.893 3.889 3.885 3.881 3.877 3.873 3.869 -3.87 T [C] -3.879 -3.888 -3.897 -3.906 0.037 0.0375 0.038 0.0385 0.039 0.0395 0.04 r [m ] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-35 2-72 A spherical container is subjected to uniform heat flux on the outer surface and specified temperature on the inner surface The mathematical formulation, the variation of temperature in the pipe, and the outer surface temperature, and the maximum rate of hot water supply are to be determined for steady onedimensional heat transfer Assumptions Heat conduction is steady and one-dimensional since there is no change with time and there is thermal symmetry about the mid point Thermal conductivity is constant There is no heat generation in the container Properties The thermal conductivity is given to be k = 1.5 W/m⋅°C The specific heat of water at the average temperature of (100+20)/2 = 60°C is 4.185 kJ/kg⋅°C (Table A-9) Analysis (a) Noting that the 90% of the 500 W generated by the strip heater is transferred to the container, the heat flux through the outer surface is determined to be Q& Q& s 0.90 × 500 W = = 213.0 W/m q& s = s = A2 4πr2 4π (0.41 m) Noting that heat transfer is one-dimensional in the radial r direction and heat flux is in the negative r direction, the mathematical formulation of this problem can be expressed as d ⎛ dT ⎞ ⎟=0 ⎜r dr ⎝ dr ⎠ Insulation T1 and T (r ) = T = 100°C k 1 dT (r2 ) Heater k = q& s dr (b) Integrating the differential equation once with respect to r gives dT r2 = C1 dr Dividing both sides of the equation above by r2 and then integrating, dT C1 = dr r C T (r ) = − + C r where C1 and C2 are arbitrary constants Applying the boundary conditions give q& r C r = r2: k 21 = q& s → C1 = s k r2 r1 r2 q& r C1 C + C → C = T1 + = T1 + s r1 r1 kr1 Substituting C1 and C2 into the general solution, the variation of temperature is determined to be ⎛ 1⎞ ⎛ 1 ⎞ q& r C C C T (r ) = − + C = − + T1 + = T1 + ⎜⎜ − ⎟⎟C1 = T1 + ⎜⎜ − ⎟⎟ s r r r1 ⎝ r1 r ⎠ ⎝ r1 r ⎠ k r = r1: T (r1 ) = T1 = − ⎞ (213 W/m )(0.41 m) 1⎞ ⎛ ⎛ = 100°C + ⎜ − ⎟ = 100 + 23.87⎜ 2.5 − ⎟ r⎠ 1.5 W/m ⋅ °C ⎝ 0.40 m r ⎠ ⎝ (c) The outer surface temperature is determined by direct substitution to be ⎛ 1⎞ ⎞ ⎛ Outer surface (r = r2): T (r2 ) = 100 + 23.87⎜⎜ 2.5 − ⎟⎟ = 100 + 23.87⎜ 2.5 − ⎟ = 101.5°C 0.41 ⎠ r2 ⎠ ⎝ ⎝ Noting that the maximum rate of heat supply to the water is 0.9 × 500 W = 450 W, water can be heated from 20 to 100°C at a rate of Q& 0.450 kJ/s Q& = m& c p ΔT → m& = = = 0.00134 kg/s = 4.84 kg/h c p ΔT (4.185 kJ/kg ⋅ °C)(100 − 20)°C PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission r 2-36 2-73 EES Prob 2-72 is reconsidered The temperature as a function of the radius is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" r_1=0.40 [m] r_2=0.41 [m] k=1.5 [W/m-C] T_1=100 [C] Q_dot=500 [W] f_loss=0.10 "ANALYSIS" q_dot_s=((1-f_loss)*Q_dot)/A A=4*pi*r_2^2 T=T_1+(1/r_1-1/r)*(q_dot_s*r_2^2)/k "Variation of temperature" r [m] 0.4 0.4011 0.4022 0.4033 0.4044 0.4056 0.4067 0.4078 0.4089 0.41 T [C] 100 100.2 100.3 100.5 100.7 100.8 101 101.1 101.3 101.5 101.6 101.4 101.2 T [C] 101 100.8 100.6 100.4 100.2 100 0.4 0.402 0.404 0.406 0.408 0.41 r [m ] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-37 Heat Generation in Solids 2-74C No Heat generation in a solid is simply the conversion of some form of energy into sensible heat energy For example resistance heating in wires is conversion of electrical energy to heat 2-75C Heat generation in a solid is simply conversion of some form of energy into sensible heat energy Some examples of heat generations are resistance heating in wires, exothermic chemical reactions in a solid, and nuclear reactions in nuclear fuel rods 2-76C The rate of heat generation inside an iron becomes equal to the rate of heat loss from the iron when steady operating conditions are reached and the temperature of the iron stabilizes 2-77C No, it is not possible since the highest temperature in the plate will occur at its center, and heat cannot flow “uphill.” 2-78C The cylinder will have a higher center temperature since the cylinder has less surface area to lose heat from per unit volume than the sphere 2-79 A 2-kW resistance heater wire with a specified surface temperature is used to boil water The center temperature of the wire is to be determined Assumptions Heat transfer is steady since there is no change with time Heat transfer is one-dimensional since there is thermal symmetry about the center line and no change in the axial direction Thermal conductivity is constant Heat generation in the heater is uniform 110°C Properties The thermal conductivity is given to be k = 20 W/m⋅°C Analysis The resistance heater converts electric energy into heat at a rate of kW The rate of heat generation per unit volume of the wire is E& gen E& gen 2000 W e& gen = = = = 1.768 × 10 W/m V wire πro L π (0.002 m) (0.9 m) r D The center temperature of the wire is then determined from Eq 2-71 to be To = T s + e& gen ro2 4k = 110°C + (1.768 × 10 W/m )(0.002 m) = 118.8°C 4(20 W/m.°C) PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-38 2-80 Heat is generated in a long solid cylinder with a specified surface temperature The variation of temperature in the cylinder is given by T (r ) = e& gen ro2 ⎡ ⎛ r ⎢1 − ⎜ k ⎢ ⎜⎝ ro ⎣ ⎞ ⎟ ⎟ ⎠ 2⎤ ⎥ + Ts ⎥ ⎦ 80°C (a) Heat conduction is steady since there is no time t variable involved (b) Heat conduction is a one-dimensional k egen (c) Using Eq (1), the heat flux on the surface of the cylinder at r = ro is determined from its definition to be ⎡ e& gen ro2 dT (ro ) q& s = − k = −k ⎢ dr ⎢⎣ k ⎡ e& gen ro2 = −k ⎢ ⎢⎣ k ⎛ 2ro ⎜− ⎜ r2 ⎝ o r D ⎛ 2r ⎞ ⎤ ⎜− ⎟⎥ ⎜ r ⎟⎥ ⎝ o ⎠⎦ r = r0 ⎞⎤ ⎟⎥ = 2e& gen ro = 2(35 W/cm )(4 cm) = 280 W/cm ⎟⎥ ⎠⎦ 2-81 EES Prob 2-80 is reconsidered The temperature as a function of the radius is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" r_0=0.04 [m] k=25 [W/m-C] g_dot_0=35E+6 [W/m^3] T_s=80 [C] "ANALYSIS" T=(g_dot_0*r_0^2)/k*(1-(r/r_0)^2)+T_s "Variation of temperature" 2500 T [C] 2320 2292 2209 2071 1878 1629 1324 964.9 550.1 80 2000 T [C] r [m] 0.004444 0.008889 0.01333 0.01778 0.02222 0.02667 0.03111 0.03556 0.04 1500 1000 500 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 r [m ] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-39 2-82 Heat is generated in a large plane wall whose one side is insulated while the other side is subjected to convection The mathematical formulation, the variation of temperature in the wall, the relation for the surface temperature, and the relation for the maximum temperature rise in the plate are to be determined for steady one-dimensional heat transfer Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since the wall is large relative to its thickness Thermal conductivity is constant Heat generation is uniform Analysis (a) Noting that heat transfer is steady and one-dimensional in x direction, the mathematical formulation of this problem can be expressed as d T e&gen + =0 k dx and dT (0) = (insulated surface at x = 0) dx k egen dT ( L) = h[T ( L) − T∞ ] dx (b) Rearranging the differential equation and integrating, e& gen e& gen d 2T dT = − → = − x + C1 k dx k dx −k T∞ h Insulated L x Integrating one more time, T ( x) = − e&gen x + C1 x + C (1) 2k Applying the boundary conditions: −e& gen dT (0) B.C at x = 0: =0→ (0) + C1 = → C1 = dx k B C at x = L: ⎞ ⎛ − e& gen L2 ⎛ − e& gen ⎞ L ⎟⎟ = h⎜ − k ⎜⎜ + C − T∞ ⎟ ⎟ ⎜ 2k ⎝ k ⎠ ⎠ ⎝ e& gen L = Dividing by h: C2 = e&gen L h + − he&gen L2 2k e& gen L2 2k − hT∞ + C → C = e& gen L + he& gen L2 2k + hT∞ + T∞ Substituting the C1 and C2 relations into Eq (1) and rearranging give T ( x) = − e&gen x + e&gen L + e&gen L2 + T∞ = e&gen ( L2 − x ) + e&gen L + T∞ h h 2k 2k 2k which is the desired solution for the temperature distribution in the wall as a function of x (c) The temperatures at two surfaces and the temperature difference between these surfaces are e& gen L2 e& gen L T ( 0) = + + T∞ 2k h e& gen L T ( L) = + T∞ h e& gen L2 ΔTmax = T (0) − T ( L) = 2k Discussion These relations are obtained without using differential equations in the text (see Eqs 2-67 and 2-73) PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-40 2-83E A long homogeneous resistance heater wire with specified convection conditions at the surface is used to boil water The mathematical formulation, the variation of temperature in the wire, and the temperature at the centerline of the wire are to be determined Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since there is thermal symmetry about the center line and no change in the axial direction Thermal conductivity is constant Heat generation in the wire is uniform Properties The thermal conductivity is given to be k = 8.6 Btu/h⋅ft⋅°F Analysis Noting that heat transfer is steady and r T∞ one-dimensional in the radial r direction, the Water h mathematical formulation of this problem can be ro expressed as d ⎛ dT ⎞ e&gen =0 ⎜r ⎟+ r dr ⎝ dr ⎠ k dT (ro ) and − k = h[T (ro ) − T∞ ] (convection at the outer surface) Heater dr dT (0) = (thermal symmetry about the centerline) dr Multiplying both sides of the differential equation by r and rearranging gives e&gen d ⎛ dT ⎞ r ⎜r ⎟=− dr ⎝ dr ⎠ k Integrating with respect to r gives e& gen r dT r =− + C1 (a) dr k It is convenient at this point to apply the second boundary condition since it is related to the first derivative of the temperature by replacing all occurrences of r and dT/dr in the equation above by zero It yields e& gen dT (0) 0× =− × + C1 → C1 = B.C at r = 0: dr 2k Dividing both sides of Eq (a) by r to bring it to a readily integrable form and integrating, e& gen dT =− r dr 2k e& gen T (r ) = − r + C2 and (b) 4k Applying the second boundary condition at r = ro , e& gen ro e& gen ⎛ e& gen ⎞ = h⎜⎜ − ro + C − T∞ ⎟⎟ → C = T∞ + + ro 2k 2h 4k ⎝ 4k ⎠ Substituting this C relation into Eq (b) and rearranging give e& gen e& gen ro T ( r ) = T∞ + (ro − r ) + 4k 2h which is the desired solution for the temperature distribution in the wire as a function of r Then the temperature at the center line (r = 0) is determined by substituting the known quantities to be e& gen e& gen ro T ( 0) = T ∞ + ro + 4k 2h B C at r = ro : k e& gen ro (1800 Btu/h.in )(0.25 in) ⎛ 12 in ⎞ (1800 Btu/h.in )(0.25 in ) ⎛ 12 in ⎞ ⎜ ⎟+ ⎜ ⎟ = 290.8°F × (8.6 Btu/h.ft.°F) ⎝ ft ⎠ × (820 Btu/h ⋅ ft ⋅ °F) ⎝ ft ⎠ Thus the centerline temperature will be about 80°F above the temperature of the surface of the wire = 212°F + PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-41 2-84E EES Prob 2-83E is reconsidered The temperature at the centerline of the wire as a function of the heat generation is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" r_0=0.25/12 [ft] k=8.6 [Btu/h-ft-F] e_dot=1800 [Btu/h-in^3] T_infinity=212 [F] h=820 [Btu/h-ft^2-F] "ANALYSIS" T_0=T_infinity+(e_dot/Convert(in^3, ft^3))/(4*k)*(r_0^2-r^2)+((e_dot/Convert(in^3, ft^3))*r_0)/(2*h) "Variation of temperature" r=0 "for centerline temperature" 320 T0 [F] 229.5 238.3 247 255.8 264.5 273.3 282 290.8 299.5 308.3 317 300 T0 [F] e [Btu/h.in3] 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 280 260 240 220 250 700 1150 1600 2050 2500 e [Btu/h-in ] 2-85 A nuclear fuel rod with a specified surface temperature is used as the fuel in a nuclear reactor The center temperature of the rod is to be determined Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since there is thermal symmetry about the center line and no change in the axial direction Thermal conductivity is constant Heat generation in the rod is uniform Properties The thermal conductivity is given to be k = 29.5 W/m⋅°C Analysis The center temperature of the rod is determined from To = T s + e& gen ro2 4k = 220°C + 220°C egen Uranium rod (4 × 10 W/m )(0.005 m) = 228°C 4(29.5 W/m.°C) PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-42 2-86 Both sides of a large stainless steel plate in which heat is generated uniformly are exposed to convection with the environment The location and values of the highest and the lowest temperatures in the plate are to be determined Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since the plate is large relative to its thickness, and there is thermal symmetry about the center plane Thermal conductivity is constant Heat generation is uniform Properties The thermal conductivity is given to be k =15.1 W/m⋅°C k egen Analysis The lowest temperature will occur at surfaces of plate while the highest temperature will occur at the midplane Their values are determined directly from T s = T∞ + To = T s + e& gen L h = 30°C + e& gen L2 2k (5 × 10 W/m )(0.015 m) = 155°C + 60 W/m ⋅ °C T∞ =30°C h=60 W/m2⋅°C 2L=3 cm T∞ =30°C h=60 W/m2.°C = 155 °C (5 × 10 W/m )(0.015 m) = 158.7 °C 2(15.1 W/m ⋅ °C) 2-87 Heat is generated uniformly in a large brass plate One side of the plate is insulated while the other side is subjected to convection The location and values of the highest and the lowest temperatures in the plate are to be determined Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since the plate is large relative to its thickness, and there is thermal symmetry about the center plane Thermal conductivity is constant Heat generation is uniform Properties The thermal conductivity is given to be k =111 W/m⋅°C Analysis This insulated plate whose thickness is L is equivalent to one-half of an uninsulated plate whose thickness is 2L since the midplane of the uninsulated plate can be treated as insulated surface The highest temperature will occur at the insulated surface while the lowest temperature will occur at the surface which is exposed to the environment Note that L in the following relations is the full thickness of the given plate since the insulated side represents the center surface of a plate whose thickness is doubled The desired values are determined directly from T s = T∞ + To = T s + e& gen L h e& gen L2 2k = 25°C + (2 × 10 W/m )(0.05 m) = 252.3°C + 44 W/m ⋅ °C k egen Insulated L=5 cm T∞ =25°C h=44 W/m2.°C = 252.3 °C (2 × 10 W/m )(0.05 m) = 254.6 °C 2(111 W/m ⋅ °C) PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-48 297.4 317.9 338.4 358.9 379.5 400 115.9 113.6 111.5 109.7 108.1 106.7 800 700 600 T [C] 500 400 300 200 100 0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 r [m ] 1200 1000 T [C] 800 600 400 200 0 50 100 150 200 250 300 350 400 k [W /m -C] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-49 2-94 A long homogeneous resistance heater wire with specified surface temperature is used to heat the air The temperature of the wire 3.5 mm from the center is to be determined in steady operation Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since there is thermal symmetry about the center line and no change in the axial direction Thermal conductivity is constant Heat generation in the wire is uniform Properties The thermal conductivity is given to be k = W/m⋅°C Analysis Noting that heat transfer is steady and onedimensional in the radial r direction, the mathematical formulation of this problem can be expressed as d ⎛ dT ⎞ e&gen =0 ⎜r ⎟+ r dr ⎝ dr ⎠ k and r 180°C ro e&gen T (ro ) = Ts = 180°C (specified surface temperature) Resistance wire dT (0) = (thermal symmetry about the centerline) dr Multiplying both sides of the differential equation by r and rearranging gives e&gen d ⎛ dT ⎞ r ⎜r ⎟=− dr ⎝ dr ⎠ k Integrating with respect to r gives r e& gen r dT =− + C1 dr k (a) It is convenient at this point to apply the boundary condition at the center since it is related to the first derivative of the temperature It yields B.C at r = 0: e& gen dT (0) =− × + C1 dr 2k 0× → C1 = Dividing both sides of Eq (a) by r to bring it to a readily integrable form and integrating, e& gen dT =− r dr 2k and T (r ) = − e& gen 4k r + C2 (b) Applying the other boundary condition at r = ro , B C at r = ro : Ts = − e& gen 4k ro2 + C → C = Ts + e& gen 4k ro2 Substituting this C2 relation into Eq (b) and rearranging give T (r ) = Ts + e& gen 4k (ro2 − r ) which is the desired solution for the temperature distribution in the wire as a function of r The temperature 3.5 mm from the center line (r = 0.0035 m) is determined by substituting the known quantities to be T (0.0035 m) = Ts + e&gen 4k (ro2 − r ) = 180°C + ×10 W/m [(0.005 m) − (0.0035 m) ] = 200°C × (8 W/ m ⋅ °C) Thus the temperature at that location will be about 20°C above the temperature of the outer surface of the wire PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-50 2-95 Heat is generated in a large plane wall whose one side is insulated while the other side is maintained at a specified temperature The mathematical formulation, the variation of temperature in the wall, and the temperature of the insulated surface are to be determined for steady one-dimensional heat transfer Assumptions Heat transfer is steady since there is no indication of any change with time Heat transfer is one-dimensional since the wall is large relative to its thickness, and there is thermal symmetry about the center plane Thermal conductivity is constant Heat generation varies with location in the x direction Properties The thermal conductivity is given to be k = 30 W/m⋅°C Analysis (a) Noting that heat transfer is steady and one-dimensional in x direction, the mathematical formulation of this problem can be expressed as d 2T dx + e& gen ( x) k =0 k e&gen Insulated where e& gen = e& e −0.5 x / L and dT (0) = (insulated surface at x = 0) dx T2 =30°C and e& = 8×106 W/m3 L x T ( L) = T2 = 30°C (specified surface temperature) (b) Rearranging the differential equation and integrating, d 2T dx =− e& − 0.5 x / L e → k e& e −0.5 x / L dT =− + C1 → dx k − 0.5 / L dT 2e& L − 0.5 x / L = e + C1 dx k Integrating one more time, T ( x) = 2e& L e −0.5 x / L 4e& L2 + C1 x + C → T ( x) = − e −0.5 x / L + C1 x + C k − 0.5 / L k (1) Applying the boundary conditions: 2e& L 2e& L dT (0) 2e& L −0.5×0 / L e = + C → = + C1 → C1 = − dx k k k B.C at x = 0: B C at x = L: T ( L) = T2 = − 4e& L2 − 0.5 L / L 4e& L2 2e& L2 + C1 L + C → C = T2 + e − 0.5 + e k k k Substituting the C1 and C2 relations into Eq (1) and rearranging give T ( x ) = T2 + e& L2 [4(e −0.5 − e −0.5 x / L ) + 2(1 − x / L)] k which is the desired solution for the temperature distribution in the wall as a function of x (c) The temperature at the insulate surface (x = 0) is determined by substituting the known quantities to be e& L2 [4(e − 0.5 − e ) + (2 − / L)] k (8 × 10 W/m )(0.05 m) = 30°C + [4(e −0.5 − 1) + (2 − 0)] = 314°C (30 W/m ⋅ °C) T ( 0) = T + Therefore, there is a temperature difference of almost 300°C between the two sides of the plate PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-51 2-96 EES Prob 2-95 is reconsidered The heat generation as a function of the distance is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" L=0.05 [m] T_s=30 [C] k=30 [W/m-C] e_dot_0=8E6 [W/m^3] "ANALYSIS" e_dot=e_dot_0*exp((-0.5*x)/L) "Heat generation as a function of x" "x is the parameter to be varied" e [W/m3] 8.000E+06 7.610E+06 7.239E+06 6.886E+06 6.550E+06 6.230E+06 5.927E+06 5.638E+06 5.363E+06 5.101E+06 4.852E+06 x [m] 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.04 0.045 0.05 8.000x106 7.500x106 e [W/m ] 7.000x106 6.500x106 6.000x106 5.500x106 5.000x106 4.500x106 0.01 0.02 0.03 0.04 0.05 x [m] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-52 Variable Thermal Conductivity 2-97C During steady one-dimensional heat conduction in a plane wall, long cylinder, and sphere with constant thermal conductivity and no heat generation, the temperature in only the plane wall will vary linearly 2-98C The thermal conductivity of a medium, in general, varies with temperature 2-99C During steady one-dimensional heat conduction in a plane wall in which the thermal conductivity varies linearly, the error involved in heat transfer calculation by assuming constant thermal conductivity at the average temperature is (a) none 2-100C No, the temperature variation in a plain wall will not be linear when the thermal conductivity varies with temperature 2-101C Yes, when the thermal conductivity of a medium varies linearly with temperature, the average thermal conductivity is always equivalent to the conductivity value at the average temperature 2-102 A plate with variable conductivity is subjected to specified temperatures on both sides The rate of heat transfer through the plate is to be determined Assumptions Heat transfer is given to be steady and onedimensional Thermal conductivity varies quadratically There is no heat generation k(T) T2 T1 Properties The thermal conductivity is given to be k (T ) = k (1 + βT ) Analysis When the variation of thermal conductivity with temperature k(T) is known, the average value of the thermal conductivity in the temperature range between T1 and T2 can be determined from T k avg ∫ = T2 k (T )dT T1 T2 − T1 ∫ = T2 T1 ( k (1 + β T )dT T2 − T1 ) = β ⎛ ⎞ k ⎜T + T ⎟ ⎝ ⎠ T1 T2 − T1 L ( x ) β ⎡ ⎤ k ⎢(T2 − T1 ) + T23 − T13 ⎥ ⎣ ⎦ = T2 − T1 ⎡ β ⎤ = k ⎢1 + T22 + T1T2 + T12 ⎥ ⎣ ⎦ This relation is based on the requirement that the rate of heat transfer through a medium with constant average thermal conductivity kavg equals the rate of heat transfer through the same medium with variable conductivity k(T) Then the rate of heat conduction through the plate can be determined to be ( T − T2 ⎡ β Q& = k avg A = k ⎢1 + T22 + T1T2 + T12 L ⎣ )⎤⎥ A T ⎦ − T2 L Discussion We would obtain the same result if we substituted the given k(T) relation into the second part of Eq 2-76, and performed the indicated integration PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-53 2-103 A cylindrical shell with variable conductivity is subjected to specified temperatures on both sides The variation of temperature and the rate of heat transfer through the shell are to be determined Assumptions Heat transfer is given to be steady and one-dimensional Thermal conductivity varies linearly There is no heat generation Properties The thermal conductivity is given to be k (T ) = k (1 + βT ) Solution (a) The rate of heat transfer through the shell is expressed as k(T) Q& cylinder where L is the length of the cylinder, r1 is the inner radius, and r2 is the outer radius, and kavg T1 T − T2 = 2πk avg L ln(r2 / r1 ) T2 r1 r2 T +T ⎞ ⎛ = k (Tavg ) = k0 ⎜1 + β ⎟ ⎠ ⎝ r is the average thermal conductivity (b) To determine the temperature distribution in the shell, we begin with the Fourier’s law of heat conduction expressed as dT Q& = − k (T ) A dr where the rate of conduction heat transfer Q& is constant and the heat conduction area A = 2πrL is variable Separating the variables in the above equation and integrating from r = r1 where T (r1 ) = T1 to any r where T (r ) = T , we get Q& ∫ r r1 dr = −2πL r ∫ T k (T )dT T1 Substituting k (T ) = k (1 + βT ) and performing the integrations gives r Q& ln = −2πLk [(T − T1 ) + β (T − T12 ) / 2] r1 Substituting the Q& expression from part (a) and rearranging give T2 + β T+ 2k avg ln(r / r1 ) (T1 − T2 ) − T12 − T1 = βk ln(r2 / r1 ) β which is a quadratic equation in the unknown temperature T Using the quadratic formula, the temperature distribution T(r) in the cylindrical shell is determined to be T (r ) = − β ± β − 2k avg ln(r / r1 ) (T1 − T2 ) + T12 + T1 β k ln(r2 / r1 ) β Discussion The proper sign of the square root term (+ or -) is determined from the requirement that the temperature at any point within the medium must remain between T1 and T2 PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-54 2-104 A spherical shell with variable conductivity is subjected to specified temperatures on both sides The variation of temperature and the rate of heat transfer through the shell are to be determined Assumptions Heat transfer is given to be steady and one-dimensional Thermal conductivity varies linearly There is no heat generation Properties The thermal conductivity is given to be k (T ) = k (1 + βT ) T2 Solution (a) The rate of heat transfer through the shell is expressed as T − T2 Q& sphere = 4πk avg r1 r2 r2 − r1 k(T) r1 T1 r2 where r1 is the inner radius, r2 is the outer radius, and kavg r T +T ⎞ ⎛ = k (Tavg ) = k0 ⎜1 + β ⎟ ⎠ ⎝ is the average thermal conductivity (b) To determine the temperature distribution in the shell, we begin with the Fourier’s law of heat conduction expressed as dT Q& = − k (T ) A dr where the rate of conduction heat transfer Q& is constant and the heat conduction area A = 4πr2 is variable Separating the variables in the above equation and integrating from r = r1 where T (r1 ) = T1 to any r where T (r ) = T , we get Q& r dr r1 ∫ r = −4π ∫ T k (T )dT T1 Substituting k (T ) = k (1 + βT ) and performing the integrations gives ⎛ 1⎞ Q& ⎜⎜ − ⎟⎟ = −4πk [(T − T1 ) + β (T − T12 ) / 2] ⎝ r1 r ⎠ Substituting the Q& expression from part (a) and rearranging give T2 + β T+ 2k avg r2 (r − r1 ) (T1 − T2 ) − T12 − T1 = βk r (r2 − r1 ) β which is a quadratic equation in the unknown temperature T Using the quadratic formula, the temperature distribution T(r) in the cylindrical shell is determined to be T (r ) = − β ± β − 2k avg r2 ( r − r1 ) (T1 − T2 ) + T12 + T1 βk r (r2 − r1 ) β Discussion The proper sign of the square root term (+ or -) is determined from the requirement that the temperature at any point within the medium must remain between T1 and T2 PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-55 2-105 A plate with variable conductivity is subjected to specified temperatures on both sides The rate of heat transfer through the plate is to be determined Assumptions Heat transfer is given to be steady and one-dimensional Thermal conductivity varies linearly There is no heat generation Properties The thermal conductivity is given to be k (T ) = k (1 + βT ) Analysis The average thermal conductivity of the medium in this case is simply the conductivity value at the average temperature since the thermal conductivity varies linearly with temperature, and is determined to be k ave k(T) T1 T2 T +T ⎞ ⎛ = k (Tavg ) = k ⎜⎜1 + β ⎟⎟ ⎠ ⎝ (500 + 350) K ⎞ ⎛ = (25 W/m ⋅ K)⎜1 + (8.7 × 10 - K -1 ) ⎟ ⎝ ⎠ = 34.24 W/m ⋅ K L Then the rate of heat conduction through the plate becomes T − T2 (500 − 350)K Q& = k avg A = (34.24 W/m ⋅ K)(1.5 m × 0.6 m) = 30,820 W = 30.8 kW 0.15 m L Discussion We would obtain the same result if we substituted the given k(T) relation into the second part of Eq, 2-76, and performed the indicated integration PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-56 2-106 EES Prob 2-105 is reconsidered The rate of heat conduction through the plate as a function of the temperature of the hot side of the plate is to be plotted Analysis The problem is solved using EES, and the solution is given below "GIVEN" A=1.5*0.6 [m^2] L=0.15 [m] T_1=500 [K] T_2=350 [K] k_0=25 [W/m-K] beta=8.7E-4 [1/K] "ANALYSIS" k=k_0*(1+beta*T) T=1/2*(T_1+T_2) Q_dot=k*A*(T_1-T_2)/L T1 [W] 400 425 450 475 500 525 550 575 600 625 650 675 700 Q [W] 9947 15043 20220 25479 30819 36241 41745 47330 52997 58745 64575 70486 76479 80000 70000 60000 Q [W ] 50000 40000 30000 20000 10000 400 450 500 550 600 650 700 T [K] PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-57 Special Topic: Review of Differential equations 2-107C We utilize appropriate simplifying assumptions when deriving differential equations to obtain an equation that we can deal with and solve 2-108C A variable is a quantity which may assume various values during a study A variable whose value can be changed arbitrarily is called an independent variable (or argument) A variable whose value depends on the value of other variables and thus cannot be varied independently is called a dependent variable (or a function) 2-109C A differential equation may involve more than one dependent or independent variable For ∂ T ( x, t ) e& gen ∂T ( x, t ) example, the equation has one dependent (T) and independent variables (x + = k α ∂t ∂x and t) the equation ∂ T ( x, t ) + ∂x independent variables (x and t) ∂W ( x, t ) ∂T ( x, t ) ∂W ( x, t ) has dependent (T and W) and = + ∂x α ∂t α ∂t 2-110C Geometrically, the derivative of a function y(x) at a point represents the slope of the tangent line to the graph of the function at that point The derivative of a function that depends on two or more independent variables with respect to one variable while holding the other variables constant is called the partial derivative Ordinary and partial derivatives are equivalent for functions that depend on a single independent variable 2-111C The order of a derivative represents the number of times a function is differentiated, whereas the degree of a derivative represents how many times a derivative is multiplied by itself For example, y ′′′ is the third order derivative of y, whereas ( y ′) is the third degree of the first derivative of y 2-112C For a function f ( x, y ) , the partial derivative ∂f / ∂x will be equal to the ordinary derivative df / dx when f does not depend on y or this dependence is negligible 2-113C For a function f (x) , the derivative df / dx does not have to be a function of x The derivative will be a constant when the f is a linear function of x 2-114C Integration is the inverse of derivation Derivation increases the order of a derivative by one, integration reduces it by one 2-115C A differential equation involves derivatives, an algebraic equation does not 2-116C A differential equation that involves only ordinary derivatives is called an ordinary differential equation, and a differential equation that involves partial derivatives is called a partial differential equation 2-117C The order of a differential equation is the order of the highest order derivative in the equation PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-58 2-118C A differential equation is said to be linear if the dependent variable and all of its derivatives are of the first degree, and their coefficients depend on the independent variable only In other words, a differential equation is linear if it can be written in a form which does not involve (1) any powers of the dependent variable or its derivatives such as y or ( y ′) , (2) any products of the dependent variable or its derivatives such as yy ′ or y ′y ′′′ , and (3) any other nonlinear functions of the dependent variable such as sin y or e y Otherwise, it is nonlinear 2-119C A linear homogeneous differential equation of order n is expressed in the most general form as y ( n ) + f ( x) y ( n −1) + L + f n −1 ( x) y ′ + f n ( x) y = Each term in a linear homogeneous equation contains the dependent variable or one of its derivatives after the equation is cleared of any common factors The equation y ′′ − x y = is linear and homogeneous since each term is linear in y, and contains the dependent variable or one of its derivatives 2-120C A differential equation is said to have constant coefficients if the coefficients of all the terms which involve the dependent variable or its derivatives are constants If, after cleared of any common factors, any of the terms with the dependent variable or its derivatives involve the independent variable as a coefficient, that equation is said to have variable coefficients The equation y ′′ − x y = has variable coefficients whereas the equation y ′′ − y = has constant coefficients 2-121C A linear differential equation that involves a single term with the derivatives can be solved by direct integration 2-122C The general solution of a 3rd order linear and homogeneous differential equation will involve arbitrary constants PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-59 Review Problems 2-123 A small hot metal object is allowed to cool in an environment by convection The differential equation that describes the variation of temperature of the ball with time is to be derived Assumptions The temperature of the metal object changes uniformly with time during cooling so that T = T(t) The density, specific heat, and thermal conductivity of the body are constant There is no heat generation Analysis Consider a body of arbitrary shape of mass m, volume V, surface area A, density ρ, and specific heat cp initially at a uniform temperature Ti 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 transfer coefficient h During a differential time interval dt, the temperature of the body rises by a differential amount dT Noting that the temperature changes with time only, an energy balance of the solid for the time interval dt can be expressed as A ⎛ Heat transfer from the body ⎞ ⎛ The decrease in the energy ⎞ ⎟⎟ ⎟⎟ = ⎜⎜ ⎜⎜ during dt ⎠ ⎝ of the body during dt ⎠ ⎝ or h T∞ m, c, Ti T=T(t) hAs (T − T∞ )dt = mc p (−dT ) Noting that m = ρV and dT = d (T − T∞ ) since T∞ = constant, the equation above can be rearranged as hAs d (T − T∞ ) =− dt ρVc p T − T∞ which is the desired differential equation 2-124 A long rectangular bar is initially at a uniform temperature of Ti The surfaces of the bar at x = and y = are insulated while heat is lost from the other two surfaces by convection The mathematical formulation of this heat conduction problem is to be expressed for transient two-dimensional heat transfer with no heat generation Assumptions Heat transfer is transient and two-dimensional Thermal conductivity is constant There is no heat generation Analysis The differential equation and the boundary conditions for this heat conduction problem can be expressed as ∂ 2T ∂x + ∂ 2T ∂y = ∂T α ∂t ∂T ( x,0, t ) =0 ∂x ∂T (0, y, t ) =0 ∂x ∂T (a, y, t ) = h[T (a, y , t ) − T∞ ] ∂x ∂T ( x, b, t ) −k = h[T ( x, b, t ) − T∞ ] ∂x h, T∞ b h, T∞ −k a Insulated T ( x, y,0) = Ti PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-60 2-125 Heat is generated at a constant rate in a short cylinder Heat is lost from the cylindrical surface at r = ro by convection to the surrounding medium at temperature T∞ with a heat transfer coefficient of h The bottom surface of the cylinder at r = is insulated, the top surface at z = H is subjected to uniform heat flux q& h , and the cylindrical surface at r = ro is subjected to convection The mathematical formulation of this problem is to be expressed for steady two-dimensional heat transfer Assumptions Heat transfer is given to be steady and two-dimensional Thermal conductivity is constant Heat is generated uniformly Analysis The differential equation and the boundary conditions for this heat conduction problem can be expressed as ∂ ⎛ ∂T ⎞ ∂ T e&gen qH + =0 ⎜r ⎟+ r ∂r ⎝ ∂r ⎠ ∂z k ∂T (r ,0) =0 ∂z ∂T ( r , H ) k = q& H ∂z egen h T∞ z ∂T (0, z ) =0 ∂r ∂T (ro , z ) −k = h[T (ro , z ) − T∞ ] ∂r ro 2-126E A large plane wall is subjected to a specified temperature on the left (inner) surface and solar radiation and heat loss by radiation to space on the right (outer) surface The temperature of the right surface of the wall and the rate of heat transfer are to be determined when steady operating conditions are reached Assumptions Steady operating conditions are reached Heat transfer is one-dimensional since the wall is large relative to its thickness, and the thermal conditions on both sides of the wall are T2 uniform Thermal properties are constant There is no heat generation in the wall Properties The properties of the plate are given to be k = 1.2 520 R qsolar Btu/h⋅ft⋅°F and ε = 0.80, and α s = 0.60 Analysis In steady operation, heat conduction through the wall must be equal to net heat transfer from the outer surface Therefore, taking the outer surface temperature of the plate to be T2 (absolute, in R), L T1 − T2 = εσAsT24 − α s As q&solar L Canceling the area A and substituting the known quantities, kAs (1.2 Btu/h ⋅ ft ⋅ °F) (520 R) − T2 = 0.8(0.1714 × 10 −8 Btu/h ⋅ ft ⋅ R )T24 − 0.60(300 Btu/h ⋅ ft ) 0.8 ft Solving for T2 gives the outer surface temperature to be Then the rate of heat transfer through the wall becomes q& = k T2 = 553.9 R T1 − T2 (520 − 553.9) R = (1.2 Btu/h ⋅ ft ⋅ °F) = −50.9 Btu/h ⋅ ft (per unit area) L 0.8 ft Discussion The negative sign indicates that the direction of heat transfer is from the outside to the inside Therefore, the structure is gaining heat PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission x 2-61 2-127E A large plane wall is subjected to a specified temperature on the left (inner) surface and heat loss by radiation to space on the right (outer) surface The temperature of the right surface of the wall and the rate of heat transfer are to be determined when steady operating conditions are reached Assumptions Steady operating conditions are reached Heat transfer is one-dimensional since the wall is large relative to its thickness, and the thermal conditions on both sides of the wall are uniform Thermal properties are constant There is no heat generation in the wall Properties The properties of the plate are given to be k = 1.2 Btu/h⋅ft⋅°F and ε = 0.80 Analysis In steady operation, heat conduction through the wall must be equal to net heat transfer from the outer surface Therefore, taking the outer surface temperature of the plate to be T2 (absolute, in R), kAs T1 − T2 = εσAs T24 L T2 520 R Canceling the area A and substituting the known quantities, (1.2 Btu/h ⋅ ft ⋅ °F) (520 R) − T2 = 0.8(0.1714 × 10 −8 Btu/h ⋅ ft ⋅ R )T24 0.5 ft Solving for T2 gives the outer surface temperature to be L x T2 = 487.7 R Then the rate of heat transfer through the wall becomes q& = k T1 − T2 (520 − 487.7) R = (1.2 Btu/h ⋅ ft ⋅ °F) = 77.5 Btu/h ⋅ ft (per unit area) L 0.5 ft Discussion The positive sign indicates that the direction of heat transfer is from the inside to the outside Therefore, the structure is losing heat as expected PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission 2-62 2-128 A steam pipe is subjected to convection on both the inner and outer surfaces The mathematical formulation of the problem and expressions for the variation of temperature in the pipe and on the outer surface temperature are to be obtained for steady one-dimensional heat transfer Assumptions Heat conduction is steady and one-dimensional since the pipe is long relative to its thickness, and there is thermal symmetry about the center line Thermal conductivity is constant There is no heat generation in the pipe Analysis (a) Noting that heat transfer is steady and onedimensional in the radial r direction, the mathematical formulation of this problem can be expressed as d dr −k and ⎛ dT ⎞ ⎜r ⎟=0 ⎝ dr ⎠ Ti hi dT (r1 ) = hi [Ti − T (r1 )] dr −k r1 dT (r2 ) = ho [T (r2 ) − To ] dr r2 r To ho (b) Integrating the differential equation once with respect to r gives r dT = C1 dr Dividing both sides of the equation above by r to bring it to a readily integrable form and then integrating, dT C1 = dr r T (r ) = C1 ln r + C where C1 and C2 are arbitrary constants Applying the boundary conditions give r = r1: −k C1 = hi [Ti − (C1 ln r1 + C2 )] r1 r = r2: −k C1 = ho [(C1 ln r2 + C2 ) − To ] r2 Solving for C1 and C2 simultaneously gives C1 = T0 − Ti r2 k k + ln + r1 hi r1 ho r2 and ⎛ ⎛ k ⎞ T0 − Ti k ⎞ ⎟ = Ti − ⎜ ln r1 − ⎟ C2 = Ti − C1⎜⎜ ln r1 − ⎟ ⎜ ⎟ r k k h r h r i1⎠ i ⎝ ⎝ ⎠ + ln + r1 hi r1 ho r2 Substituting C1 and C into the general solution and simplifying, we get the variation of temperature to be r k + k r1 hi r1 ) = Ti + T (r ) = C1 ln r + Ti − C1 (ln r1 − r2 k k hi r1 ln + + r1 hi r1 ho r2 ln (c) The outer surface temperature is determined by simply replacing r in the relation above by r2 We get r2 k + r1 hi r1 T (r2 ) = Ti + r2 k k ln + + r1 hi r1 ho r2 ln PROPRIETARY MATERIAL © 2007 The McGraw-Hill Companies, Inc Limited distribution permitted only to teachers and educators for course preparation If you are a student using this Manual, you are using it without permission ... 2-108C A variable is a quantity which may assume various values during a study A variable whose value can be changed arbitrarily is called an independent variable (or argument) A variable whose value... conduction heat transfer Q& is constant and the heat conduction area A = 2πrL is variable Separating the variables in the above equation and integrating from r = r1 where T (r1 ) = T1 to any r where... differential equation that involves only ordinary derivatives is called an ordinary differential equation, and a differential equation that involves partial derivatives is called a partial differential