Thermodynamics HW Solutions 699

Thermodynamics HW Solutions 699 - rate of 80 W can be...

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Chapter 8 Internal Forced Convection 8-79 A computer is cooled by a fan blowing air through the case of the computer. The flow rate of the fan and the diameter of the casing of the fan are to be specified. Assumptions 1 Steady flow conditions exist. 2 Heat flux is uniformly distributed. 3 Air is an ideal gas with constant properties. Properties The relevant properties of air are (Tables A-1 and A-15) /kg.K kPa.m 287 . 0 C J/kg. 1007 3 = ° = R C p Analysis We need to determine the flow rate of air for the worst case scenario. Therefore, we assume the inlet temperature of air to be 50 ° C, the atmospheric pressure to be 70.12 kPa, and disregard any heat transfer from the outer surfaces of the computer case. The mass flow rate of air required to absorb heat at a
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Unformatted text preview: rate of 80 W can be determined from kg/s 007944 . C ) 50 60 )( C J/kg. 1007 ( J/s 80 ) ( ) ( = ° − ° = − = ⎯→ ⎯ − = in out p in out p T T C Q m T T C m Q & & & & In the worst case the exhaust fan will handle air at 60 ° C. Then the density of air entering the fan and the volume flow rate becomes /min m 0.6497 3 = = = = = = = /s m 01083 . kg/m 0.7337 kg/s 007944 . kg/m 7337 . 273)K + /kg.K)(60 kPa.m 287 . ( kPa 70.12 3 3 3 3 ρ m V RT P & & For an average velocity of 120 m/min, the diameter of the duct in which the fan is installed can be determined from cm 8.3 = = = = ⎯→ ⎯ = = m 083 . ) m/min 120 ( ) /min m 6497 . ( 4 4 4 3 2 ππ π V V D V D V A V c & & Cooling air 8-61...
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