Homework 4 - ME 114 Homework #4 Due February 21, 2008 3—1...

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Unformatted text preview: ME 114 Homework #4 Due February 21, 2008 3—1 16 A 25-W power transistor is to be cooled by attaching it to one of the commercially available heat sinks shown in Table 3~6. Select a heat sink that will allow the case temperature of the tran- sistor not to exceed 55°C in the ambient air at 18°C. Steel rods of 2-mm diameter and 50-m length, are attached to a tube (axis of the rods perpendicular to the Surface of the tube) whose surface temperature is 100 °C. The convective heat transfer coefficient between the rods and ambient air (20 °C) is 20 W/lm2 K. Determine I ‘ -‘ ~“""1‘.v'..EE-'r‘-§':i=ii--= : » n » e - _ --;==;: .: :--- -.- C — I n . _ . (c) The heat transfer rate and the tip temperature assuming the tips of the rods to be perfectly insulated. h = 20 W/m2 l’C Tm = 20 “c 100 "C 3—1123 A hot surface at 100°C is to be cooled by attach- ing 3-cm-long, 0.25-cm-diameter aluminum pin fins (k = 237 W/m - °C) to it, with a center-to-center distance of 0.6 cm. The temperature of the sulrounding medium is 30°C, and the heat transfer coefficient on the surfaces is 35 W/m2 - “C. Determine the rate of heat transfer from the sur- face fora 1-111 X l-m section of the plate. Also determine the overall effectiveness of the fins. FHGURE 533—123 Liquid nitrogen is stored in a S-m diameter sphere buried below the .surface of the earth, with its center 7 m below the surface. The surface of the sphere is maintained at —180 “C. Nitrogen vaporizes as a result of heat transfer to the tank. Constant ‘ pressure (and temperature) is maintained in the tank by venting the vapor generated to the atmosphere. If the surface of the earth is at 10 °C and its thermal conductivity is 0.17 W/m K, estimate the heat transfer rate to the sphere and the percentage of nitrogen vaporized per hour when the tank is full of the liquid. The specific enthalpy and density of liquid nitrogen are 198.6 kJ/kg and 808.6 kg/ma, respectively. Vent pipe A copper cylinder, l-cm diameter and 3-cm long, is initially at 20 °C. It is placed in a 100 °C air stream, which gives a convective heat transfer coefficient of 20 W/m2 K over the entire cylinder including the end surfaces. How long will it take for the cylinder temperature to reach 95 °C? Stainless steel balls of 4-mm diameter, initially at 30 °C are heated for l min by blowing air at 95 °C (convective heat transfer coefficient of 40 W/m1 °C) and then cooled in 25 °C air for l min (convective heat transfer coefficient of '20 W/m2 °C). What is the temperature of the balls When they are removed from the cool air? ...
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Homework 4 - ME 114 Homework #4 Due February 21, 2008 3—1...

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