Unformatted text preview: Chapter 8 Internal Forced Convection 853 Hot air enters a sheet metal duct located in a basement. The exit temperature of hot air and the rate of heat loss are to be determined. Assumptions 1 Steady flow conditions exist. 2 The inner surfaces of the duct are smooth. 3 The thermal resistance of the duct is negligible. 4 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. Properties We expect the air temperature to drop somewhat, and evaluate the air properties at 1 atm and the estimated bulk mean temperature of 50 ° C (Table A15), Air 60 ° C 4 m/s L = 12 m ε = 0.3 Air duct 20 cm × 20 cm T ∞ = 10 ° C 7228 . Pr C J/kg. 1007 /s; m 10 797 . 1 C W/m. 02735 . ; kg/m 092 . 1 2 5 3 = ° = × = υ ° = = ρ p C k Analysis The surface area and the Reynolds number are m 9.6 m) m)(12 2 . ( 4 4 = × = = aL A s D A p a a a h c = = = = 4 4 4 0 2 2 . m 509 , 44 /s m 10 797 . 1 m) m/s)(0.20 (4 Re 2 5 = × = = − υ h m D V which is greater than 10,000. Therefore, the flow is turbulent and the entry lengths in this case are roughly which is greater than 10,000....
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 Fall '10
 Dr.DanielArenas
 Fluid Dynamics, Thermodynamics, Convection, Force, Mass, Heat, Heat Transfer, hot air, Dh

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