Chapter 7 External Forced Convection7-27 Solar radiation is incident on the glass cover of a solar collector. The total rate of heat loss from the collector, the collector efficiency, and the temperature rise of water as it flows through the collector are to be determined. Assumptions1Steady operating conditions exist. 2The critical Reynolds number is Recr= 5×105. 3 Heat exchange on the back surface of the absorber plate is negligible. 4Air is an ideal gas with constant properties. 5 The local atmospheric pressure is 1 atm. PropertiesThe properties of air at the film temperature of are (Table A-15) C302/)2535(°=+V∞= 30 km/h T∞= 25°C Tsky= -40°C 700 W/m27282.0Pr/sm10608.1CW/m.02588.025-=×=°=υkSolar collector Ts= 35°CAnalysis(a) Assuming wind flows across 2 m surface, the Reynolds number is determined from L= 2 m 62510036.1/sm10608.1m)m/s(2)3600/100030(Re×=××==−∞LLVwhich is greater than the critical Reynolds number (5. Using the Nusselt number relation for combined laminar and turbulent flow, the average heat transfer coefficient is determined to be )×105C.W/m83.17)1378(m2CW/m.02588.01378)7282.0](
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