A $60-cm60-cm$ circuit board that contains $121$ square chips on one side is to be cooled by combined natural convection and radiation by mounting it on a horizontal surface in a room at $25C.$ Each chip dissipates $0\mathrm{.}18$ W of power, and the emissivity of the chip surfaces is $0\mathrm{.}7.$ Assume 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. Evaluate air properties at a film temperature of $30C$ and $1$ atm pressure.

Given: The properties of air at $1$ atm and the anticipated film temperature of $30C.$

$k=0\mathrm{.}02588\frac{W}{m}C$

$v=1\mathrm{.}608{10}^{-5}\frac{m^2}{s}$

$Pr=0\mathrm{.}7282$

$=0\mathrm{.}0033K^{-1}$

Part $A$:

Assuming the circuit board to be positioned horizontally with chips facing UP$,$ determine the surface temperature of the chips.

The surface temperature of the chips is $C$

Part B:

Assuming the circuit board to be positioned horizontally with chips facing DOWN, determine the surface tenperature of the chips.

The surface temperature of the chips is $\_\_\_\_\_$ C