Circuit boards are mounted to interior vertical surfaces

Chapter 9, Problem 9.30

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Circuit boards are mounted to interior vertical surfaces ofa rectangular duct of height H?400 mm and lengthL?800 mm. Although the boards are cooled by forcedconvection heat transfer to air flowing through the duct,not all of the heat dissipated by the electronic componentsis transferred to the flow. Some of the heat is insteadtransferred by conduction to the vertical walls of the ductand then by natural convection and radiation to the ambi-ent (atmospheric) air and surroundings, which are atequivalent temperatures of T??Tsur?20?C. The wallsare metallic and, to a first approximation, may beassumed to be isothermal at a temperature Ts.(a) Consider conditions for which the electronic com-ponents dissipate 200 W and air enters the duct at aflow rate of m.?0.015 kg/s and a temperature ofTm,i?20?C. If the emissivity of the side wallsiss?0.15 and the outlet temperature of the air isTm,o?30?C, what is the surface temperature Ts?(b) To reduce the temperature of the electronic compo-nents, it is desirable to enhance heat transfer fromthe side walls. Assuming no change in the airflowconditions, what is the effect on Tsof applying ahigh emissivity coating (s?0.90) to the side walls?(c) If there is a loss of airflow while power continuesto be dissipated, what are the resulting values of Tsfor s?0.15 and s?0.90?