Thermodynamics HW Solutions 676

Thermodynamics HW Solutions 676 - Chapter 8 Internal Forced...

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Chapter 8 Internal Forced Convection 8-57 Air 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. Assumptions 1 Steady flow conditions exist. 2 Heat generated is uniformly distributed over the two surfaces of the PCB. 3 Air is an ideal gas with constant properties. 4 The pressure of air is 1 atm. Properties We 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.s 10 08 . 2 kg/m.s 10 89 . 1 710 . 0 Pr C J/kg. 1006 /s m 10 67 . 1 C W/m. 0268 . 0 kg/m 143 . 1 5 K 350 @ , 5 2 5 - 3 × = × = = ° = × = ° = = s b p C k μ υ ρ Air 32 ° C 0.8 L/s T e Electronic components, 20 W L = 18 cm Air channel 0.25 cm × 12 cm Analysis (a) The mass flow rate of air and the exit temperature are determined from & & (. mV == × × ρ 1143 kg / m )(0.8 10 m / s) = 9.14 10 kg / s
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