Thermodynamics HW Solutions 775

Thermodynamics HW Solutions 775 - Chapter 9 Natural...

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Unformatted text preview: Chapter 9 Natural Convection 9-104E The components of an electronic system located in a horizontal duct of circular cross section is cooled by forced air. The heat transfer from the outer surfaces of the duct by natural convection and the average temperature of the duct are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 4 Radiation effects are negligible. 5 The thermal resistance of the duct is negligible. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T ∞ )/2 = (150+80)/2 = 115 ° F are (Table A-15E) 180 W L = 4 ft Air duct D = 4 in Air 85 ° F 22 cfm 100 ° F Air 80 ° F 1- 2 3 R 001739 . R ) 460 115 ( 1 1 7268 . Pr /s ft 10 1894 . F Btu/h.ft. 01564 . = + = = = × = ° = − f T k β υ Analysis (a) Using air properties at the average temperature of (85+100))/2 = 92.5 ° F and 1 atm for the forced air, the mass flow rate of air and the heat transfer rate by forced convection are determined to be...
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