Chapter 8 Internal Forced Convection8-57Air enters a hollow-core printed circuit board. The exit temperature of the air and the highest temperature on the inner surface are to be determined. √Assumptions1Steady flow conditions exist. 2Heat generated is uniformly distributed over the two surfaces of the PCB. 3Air is an ideal gas with constant properties. 4 The pressure of air is 1 atm. PropertiesWe assume the bulk mean temperature for air to be 310 K since the mean temperature of air at the inlet will rise somewhat as a result of heat gain through the hollow core whose surface is exposed to a constant heat flux. The properties of air at 1 atm and this temperature are (Table A-15) kg/m.s1008.2kg/m.s1089.1710.0PrCJ/kg.1006/sm1067.1CW/m.0268.0kg/m143.15K350@,525-3−−×=×==°=×=°==sbpCkμυρAir 32°C 0.8 L/s TeElectronic components, 20 W L= 18 cmAir channel 0.25 cm ×12 cm Analysis(a) The mass flow rate of air and the exit temperature are determined from &&(.mV==××ρ1143 kg / m )(0.8 10 m / s) = 9.14 10 kg / s
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