Thermodynamics HW Solutions 748

Thermodynamics HW Solutions 748 - Properties The properties...

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Chapter 9 Natural Convection Combined Natural and Forced Convection 9-72C In combined natural and forced convection, the natural convection is negligible when Gr /Re . 2 01 < . Otherwise it is not. 9-73C In assisting or transverse flows, natural convection enhances forced convection heat transfer while in opposing flow it hurts forced convection. 9-74C When neither natural nor forced convection is negligible, it is not correct to calculate each separately and to add them to determine the total convection heat transfer. Instead, the correlation () n n n / 1 natural forced combined Nu Nu Nu + = based on the experimental studies should be used. 9-75 A vertical plate in air is considered. The forced motion velocity above which natural convection heat transfer from the plate is negligible is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The atmospheric pressure at that location is 1 atm.
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Unformatted text preview: Properties The properties of air at 1 atm and 1 atm and the film temperature of ( T s + T )/2 = (85+30)/2 = 57.5 C are (Table A-15) 1-2 5 K 003026 . K ) 273 5 . 57 ( 1 1 /s m 10 871 . 1 = + = = = f T Analysis The characteristic length is the height of the plate, L c = L = 5 m. The Grashof and Reynolds numbers are = = = = = = V V L V L T T g Gr s 5 2 5 11 2 2 5 3-1 2 2 3 10 67 . 2 /s m 10 871 . 1 ) m 5 ( Re 10 829 . 5 ) /s m 10 871 . 1 ( ) m 5 )( K 30 85 )( K 003026 . )( m/s 81 . 9 ( ) ( Air T = 30 C V Plate, T s = 85 C L = 5 m and the forced motion velocity above which natural convection heat transfer from this plate is negligible is m/s 9.04 = = = V V Gr 1 . ) 10 67 . 2 ( 10 829 . 5 1 . Re 2 5 11 2 9-71...
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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