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Thermodynamics HW Solutions 557

# Thermodynamics HW Solutions 557 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-27 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. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Heat exchange on the back surface of the absorber plate is negligible. 4 Air is an ideal gas with constant properties. 5 The local atmospheric pressure is 1 atm. Properties The properties of air at the film temperature of are (Table A-15) C 30 2 / ) 25 35 ( ° = + V = 30 km/h T = 25 ° C T sky = -40 ° C 700 W/m 2 7282 . 0 Pr /s m 10 608 . 1 C W/m. 02588 . 0 2 5 - = × = ° = υ k Solar collector T s = 35 ° C Analysis ( a ) Assuming wind flows across 2 m surface, the Reynolds number is determined from L = 2 m 6 2 5 10 036 . 1 /s m 10 608 . 1 m) m/s(2 ) 3600 / 1000 30 ( Re × = × × = = L L V which 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 ) × 10 5 C . W/m 83 . 17 ) 1378 ( m 2 C W/m. 02588 . 0 1378 ) 7282 . 0 ](
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