Thermodynamics HW Solutions 679

Thermodynamics HW Solutions 679 - Chapter 8 Internal Forced...

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Chapter 8 Internal Forced Convection 8-60 A computer is cooled by a fan blowing air through its case. The flow rate of the air, the fraction of the temperature rise of air that is due to heat generated by the fan, and the highest allowable inlet air temperature are to be determined. Assumptions 1 Steady flow conditions exist. 2 Heat flux is uniformly distributed. 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 300 K. The properties of air at 1 atm and this temperature are (Table A-15) C J/kg. 1005 /s m 10 57 . 1 C W/m. 0261 . 0 kg/m 177 . 1 2 5 - 3 ° = × = ° = = p C k υ ρ kg/m.s 10 08 . 2 kg/m.s 10 85 . 1 712 . 0 Pr 5 K 350 @ , 5 × = × = = s b μ Analysis ( a ) Noting that the electric energy consumed by the fan is converted to thermal energy, the mass flow rate of air is kg/s 0.01045 = ° ° + × = + = = C) C)(10 J/kg. (1005 W 25) 10 (8 ) ( ) ( fan elect, i e p i e
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