$2027$ A $50-$cm $50-$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 vertical surface in a room at $25$C$.$ Each chip dissipates $0\mathrm{.}18$ W of power, and the emissivity of the chip surfaces is $0\mathrm{.}7.$ 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? Answer: $36\mathrm{.}2$C

$2028$ Repeat Prob. $2027$ assuming the circuit board to be positioned horizontally with $($a$)$ chips facing up and $($b$)$ chips facing down.