Thermodynamics HW Solutions 677

Thermodynamics HW Solutions 677 - Chapter 8 Internal Forced...

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Chapter 8 Internal Forced Convection 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) ρ υ μ μ = = 1143 0 0268 167 1006 0710 189 10 208 10 5 350 5 . . . Pr . . . ,@ kg / m W/m. C 10 m / s J/kg . C kg / m.s kg / m.s 3 -5 2 K k C p b s Air 32 ° C 0.8 L/s T e Electronic components, 35 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 3- 3 3 - 4 & & () & & Qm CT T T T Q mC pe i e i p =− = + = ° ×° 32 914 10 4 C+ 35 W kg / s)(1006 J / kg. C) 70.1 C (b) The mean fluid velocity and hydraulic diameter are V V A D A P m c h c ×
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