Thermodynamics HW Solutions 541

Thermodynamics HW Solutions 541 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-15 The top surface of a hot block is to be cooled by forced air. The rate of heat transfer is to be determined for two cases. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. 4 Air is an ideal gas with constant properties. Air V = 6 m/s T = 30 ° C T s = 120 ° C Properties The atmospheric pressure in atm is L atm 823 . 0 kPa 101.325 atm 1 kPa) 4 . 83 ( = = P For an ideal gas, the thermal conductivity and the Prandtl number are independent of pressure, but the kinematic viscosity is inversely proportional to the pressure. With these considerations, the properties of air at 0.823 atm and at the film temperature of (120+30)/2=75 ° C are (Table A-15) 7166 . 0 Pr /s m 10 2.486 = 823 . 0 / ) /s m 10 046 . 2 ( / C W/m. 02917 . 0 2 5 - 2 5 1 @ = × × = υ = υ ° = atm atm P k Analysis ( a ) If the air flows parallel to the 8 m side, the Reynolds number in this case becomes
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