Problem 4 (1-57, page 51, 1 An aluminum pan whose thermal conductivity is 237 W/

Question

Problem 4 (1-57, page 51, 1 An aluminum pan whose thermal conductivity is 237 W/m,°C has a flat bottom with diameter 20 cm. and thickness 0.4 cm. Heat is transferred steadily to boiling water in the pan through its bottom at a rate of 1400 W. If the inner surface of the bottom of the pan is at 105°C, determine the temperature of the outer surface of the bottom of the pan. Problem 5 (1-74, closely related to page 52 SI units, [1) The inner and outer glasses of a 6 ft x 6 ft double pane window are at 60°F and 42°F, respectively. If the 0.25 in space between the two glasses is filled with still air, determine the rate of heat transfer through the window Answer: 439 Btu/h Problem 6 (1-50, page 50, 11]) Two surfaces of a 2 cm thick plate are maintained at O°C and 80°C, respectively. If it is determined that heat is transferred through the plate at a rate of 500 W/m2, determine its thermal conductivity. Problem 7 (1,9, p. 41, 14) What is the thickness required of masonry wall having thermal condcutivity 0.75 W/m K if the heat rate is to be 80% of the heat rate through a composite structure wall having a thermal conductivity of0.25 W/m-K and thickness of 100 mm? Both walls are subjected to the same surface temperature difference. Problem 8 (1.2, p. 41, 14

Explanation / Answer

Ans 4

Thermal conductivity k = 237 W/m-°C

Heat transfer area A = (3.14/4) x d2

= (3.14/4) x (20cm x 1m/100cm)2

= 0.0314 m2

At steady state conditions

From Fourier law of heat conduction

Q = kA(T1 - T2) /L

1400 W = 237 W/m-°C x 0.0314 m2 x (T1 - 105)°C / (4cm x 1m/100cm)

1400 x 0.04 = (7.4418 T1 - 781.389)

56 = 7.4418 T1 - 781.389

7.4418 T1 = 837.389

T1 = 112.53 °C

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