cen58933_ch04

# The engine is then exposed to atmospheric air at 17c

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Unformatted text preview: ke 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 fo...
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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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