hw2 - h = 4 W/m 2 ºC h = 6 W/m 2 ºC A i r a t º C Air at...

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ME 322 HW #2 14.02.2008 1. Write down the general form of the heat conduction equation in both cylindrical and spherical coordinates. (You can find it from the textbook.) 2. A composite wall is composed of two layers as shown below. One layer whose thermal conductivity is 1.0 W/m ºC has a thickness of 30 cm. Another layer, 5 cm in thickness, which provides insulation to the wall, has the thermal conductivity of 0.05 W/m K. One side of the composite wall is exposed to air at 20ºC with convection heat transfer coefficient of 4.0 W/m 2 ºC. The other side is exposed to air at 0ºC with h = 6.0 W/m 2 ºC. (1) Calculate the steady-state heat transfer rate per unit area through the composite wall. (Answer: 11.7 W/m 2 ) (2) If there were no insulating layer, what would be the steady-state heat transfer rate through the wall? (Answer: 27.9 W/m 2
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Unformatted text preview: ) h = 4 W/m 2 ºC h = 6 W/m 2 ºC A i r a t º C Air at 20ºC 3 c m 5 c m 3. Two separate experiments are performed on a small refrigerator. The refrigerator is placed in ambient air at 20ºC. In one experiment, an electrical heater is suspended inside the refrigerator cavity with the door closed, while the refrigerator is unplugged. With the heater dissipating 20 W, a steady-state temperature of 90ºC is recorded within the cavity. With the heater removed and the refrigerator now in operation, the second experiment involves maintaining a steady-state cavity temperature of 5ºC for a fixed time interval and recording the electrical energy consumption. In such an experiment, the input electrical energy is 125 kJ over 12-hour period. Determine the refrigerator’s coefficient of performance (COP). (Answer: 1.48)...
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