Circuit boards are mounted to interior vertical surfaces of a rectangular duct of height H = 400 mm and length L = 800 mm. Although the boards are cooled by forced convection heat transfer to air flowing through the duct, not all of the heat dissipated by the electronic components is transferred to the flow. Some of the heat is instead transferred by conduction to the vertical walls of the duct and then by natural convection and radiation to the ambient (atmospheric) air and surroundings, which are at equivalent temperatures of T∞ = Tsur = 20°C. The wal1s are metallic and. to a first approximation, may be assumed to be isothermal at a temperature Ts.

(a) Consider conditions for which the electronic components dissipate 200 Wand air enters the duct at a flow rate of m = 0.015 kg/s and a temperature of Tm.i = 20°C. If the emissivity of the side walls is εs = 0.15 and the outlet temperature of the air is Tm.o = 30°C, what is the surface temperature Ts?

(b) To reduce the temperature of the electronic components. it is desirable to enhance heat transfer from the side walls. Assuming no change in the air flow conditions, what is the effect on Ts of applying a high emissivity coating (∞s = 0.90) to the side walls?

(c) If there is a loss of air flow while power continues to be dissipated, what are the resulting values of Ts for εs = 0.15 and Ss = 0.90

Tme Quiescent air H. Sidewalls Electronic components, q Air m. Tmi