Thermodynamics HW Solutions 542

Thermodynamics HW Solutions 542 - kW 9.08 = = = = = = W...

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Chapter 7 External Forced Convection 7-16 Wind is blowing parallel to the wall of a house. The rate of heat loss from that wall is to be determined for two cases. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. 4 Air is an ideal gas with constant properties. Properties The properties of air at 1 atm and the film temperature of (T s + T )/2 = (12+5)/2 = 8.5 ° C are (Table A-15) 7340 . 0 Pr /s m 10 413 . 1 C W/m. 02428 . 0 2 5 - = × = υ ° = k Analysis Air flows parallel to the 10 m side : The Reynolds number in this case is 7 2 5 10 081 . 1 /s m 10 413 . 1 m) m/s](10 ) 3600 / 1000 55 [( Re × = × × = υ = L L V T s = 12 ° C Air V = 55 km/h T = 5 ° C L which is greater than the critical Reynolds number. Thus we have combined laminar and turbulent flow. Using the proper relation for Nusselt number, heat transfer coefficient and then heat transfer rate are determined to be C . W/m 43 . 32 ) 10 336 . 1 ( m 10 C W/m. 02428 . 0 10 336 . 1 ) 7340 . 0 ]( 871 ) 10 081 . 1 ( 037 . 0 [ Pr ) 871 Re 037 . 0 ( 2 4 4 3 / 1 8 . 0 7 3 / 1 8 . 0 ° = × ° = = × = × = = = Nu L k h k hL Nu L
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Unformatted text preview: kW 9.08 = = = = = = W 9081 C 5)-)(12 m C)(40 . W/m 43 . 32 ( ) ( m 40 = m) m)(10 4 ( 2 2 2 s s s T T hA Q wL A & If the wind velocity is doubled : 7 2 5 10 163 . 2 /s m 10 413 . 1 m) m/s](10 ) 3600 / 1000 110 [( Re = = = L L V which is greater than the critical Reynolds number. Thus we have combined laminar and turbulent flow. Using the proper relation for Nusselt number, the average heat transfer coefficient and the heat transfer rate are determined to be C . W/m 88 . 57 ) 10 384 . 2 ( m 10 C W/m. 02428 . 10 384 . 2 ) 7340 . ]( 871 ) 10 163 . 2 ( 037 . [ Pr ) 871 Re 037 . ( 2 4 4 3 / 1 8 . 7 3 / 1 8 . = = = = = = = Nu L k h k hL Nu L kW 16.21 = = = = = = W 206 , 16 C 5)-)(12 m C)(40 . W/m 88 . 57 ( ) ( m 40 = m) m)(4 10 ( 2 2 2 s s s T T hA Q wL A & 7-4...
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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