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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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