Chapter 7 External Forced Convection7-99Wind is blowing over the roof of a house. The rate of heat transfer through the roof and the cost of this heat loss for 14-h period are to be determined. Assumptions1Steady operating conditions exist. 2The critical Reynolds number is Recr= 5×105. 3 Air is an ideal gas with constant properties. 4 The pressure of air is 1 atm. PropertiesAssuming a film temperature of 10°C, the properties of air are (Table A-15) Tsky= 100 K Q&7336.0Pr/sm10426.1CW/m.02439.025-=×=υ°=kAnalysisThe Reynolds number is 72510338.2/sm10426.1m)(20m/s)3600/100060(Re×=××=υ=−∞LVLwhich is greater than the critical Reynolds number. Thus we have combined laminar and turbulent flow. Then the Nusselt number and the heat transfer coefficient are determined to be Air V∞= 60 km/h T∞= 10°C Tin= 20°C C.W/m0.31)10542.2(m20CW/m.02439.010542.2)7336.0](871)10338.2(037.0[Pr)871Re037.0(2443/18.073/18.0°=×°==×=−×=−==NuLkhkhLNuL
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