7-34 An array of power transistors, dissipating 5 W of power each, are to be cooled by mounting them on a 25-cm x 25-cm-square aluminum plate and blowing air at 35°C over the plate with a fan at avelocity of 4 m/s. The average temperature of the plate is not to exceed 65°C. Assuming the heat transfer from the back side of the plate to be negligible and disregarding radiation, determine the numberof transistors that can be placed on this plate.FIGURE P7-34AluminumPowerplatetransistor, 5 W35°C25 cmAir →4 m/s-65°C25 cm

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Asked Dec 11, 2019
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7-34 An array of power transistors, dissipating 5 W of power each, are to be cooled by mounting them on a 25-cm x 25-cm-square aluminum plate and blowing air at 35°C over the plate with a fan at a
velocity of 4 m/s. The average temperature of the plate is not to exceed 65°C. Assuming the heat transfer from the back side of the plate to be negligible and disregarding radiation, determine the number
of transistors that can be placed on this plate.
FIGURE P7-34
Aluminum
Power
plate
transistor, 5 W
35°C
25 cm
Air →
4 m/s
-65°C
25 cm
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7-34 An array of power transistors, dissipating 5 W of power each, are to be cooled by mounting them on a 25-cm x 25-cm-square aluminum plate and blowing air at 35°C over the plate with a fan at a velocity of 4 m/s. The average temperature of the plate is not to exceed 65°C. Assuming the heat transfer from the back side of the plate to be negligible and disregarding radiation, determine the number of transistors that can be placed on this plate. FIGURE P7-34 Aluminum Power plate transistor, 5 W 35°C 25 cm Air → 4 m/s -65°C 25 cm

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Expert Answer

Step 1

Determine the average film temperature by using the information given in the question. Here, Ts is the average temperature of the plate and T is the temperature of the blowing air.

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T+T, avg _(65 °C)+(35 °C) = 50 °C

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Step 2

Assuming the local atmospheric condition that means pressure should be 1 atm.

Refer to the property table of air and obtain the value of thermal conductivity of air (h), the kinematic viscosity of the air and the Prandtl number (Pr) of the air at the temperature 50 0C and pressure 1 atm.

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At, (50 °C, 1 atm); k = 0.02735 W/kg K m²/s n=1.798×10- Pr = 0.7228

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Step 3

Evaluate the Reynolds number of the ...

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vl Re, 10 m (4 m/s)| 25 cm 1 cm 1.798 x10- m²/s = 55617.35 ||

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