Repeat Prob. 20–28 assuming the circuit board to be positioned horizontally with (a) chips facing up and (b) chips facing down.

Repeat Prob. 20–28

A 50-cm × 50-cm circuit board that contains 121 square chips on one side is to be cooled by combined naturalconvection and radiation by mounting it on a vertical surface in a room at 25°C. Each chip dissipates 0.18 W of power, and the emissivity of the chip surfaces is 0.7. Assuming the heat transfer from the back side of the circuit board to be negligible, and the temperature of the surrounding surfaces to be the same as the air temperature of the room, determine the surface temperature of the chips. Evaluate air properties at a film temperature of 30°C and 1 atm pressure. Is this a good assumption?