Thermodynamics HW Solutions 622

Thermodynamics HW Solutions 622 - F . Btu/h.ft 39 . 11 )...

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Chapter 7 External Forced Convection 7-92E A minivan is traveling at 60 mph. The rate of heat transfer to the van is to be determined. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. 4 Air flow is turbulent because of the intense vibrations involved. 5 Air is an ideal gas with constant properties. 5 The pressure of air is 1 atm. Properties Assuming a film temperature of T f = 80 ° F, the properties of air are evaluated to be (Table A-15E) 7290 . 0 Pr /s ft 10 1697 . 0 F Btu/h.ft. 01481 . 0 2 3 - = × = ° = υ k Analysis Air flows along 11 ft long side. The Reynolds number in this case is Air V = 60 mph T = 50 ° F L = 11 ft Minivan 6 2 3 10 704 . 5 /s ft 10 1697 . 0 ft) (11 ] ft/s ) 3600 / 5280 60 [( Re × = × × = υ = L V L which is greater than the critical Reynolds number. The air flow is assumed to be entirely turbulent because of the intense vibrations involved. Then the Nusselt number and the heat transfer coefficient are determined to be
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Unformatted text preview: F . Btu/h.ft 39 . 11 ) 8461 ( ft 11 F Btu/h.ft. 01481 . 8461 ) 7290 . ( ) 10 704 . 5 ( 037 . Pr Re 037 . 2 3 / 1 8 . 6 3 / 1 8 . = = = = = = = Nu L k h k L h Nu o L o R i R insulation R o T 1 T 2 The thermal resistances are [ ] 2 ft 8 . 240 ft) ft)(11 6 ( + ft) ft)(11 2 . 3 ( + ft) ft)(6 2 . 3 ( 2 = = s A F/Btu h. 0004 . ) ft F)(240.8 . Btu/h.ft 39 . 11 ( 1 1 F/Btu h. 0125 . ) ft (240.8 F/Btu . h.ft 3 ) 3 ( F/Btu h. 0035 . ) ft F)(240.8 . Btu/h.ft 2 . 1 ( 1 1 2 2 2 2 2 2 = = = = = = = = = s o o s value insulation s i i A h R A R R A h R Then the total thermal resistance and the heat transfer rate into the minivan are determined to be Btu/h 1220 = = = = + + = + + = F/Btu h. 0164 . F ) 70 90 ( F/Btu h. 0164 . 0004 . 0125 . 0035 . 2 1 total o insulation i total R T T Q R R R R & 7-77...
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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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