Thermodynamics HW Solutions 624

Thermodynamics HW Solutions 624 - Chapter 7 External Forced...

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Chapter 7 External Forced Convection 7-94 A fan is blowing air over the entire body of a person. The average temperature of the outer surface of the person is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The pressure of air is 1 atm. 4 The average human body can be treated as a 30-cm-diameter cylinder with an exposed surface area of 1.7 m 2 . Properties We assume the film temperature to be 35 ° C. The properties of air at 1 atm and this temperature are (Table A-15) D = 0.3 m Person, T s 90 W ε = 0.9 V = 5 m/s T = 32 ° C 7268 . 0 Pr /s m 10 655 . 1 C W/m. 02625 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is 4 2 5 10 063 . 9 /s m 10 655 . 1 m) m/s)(0.3 (5 Re × = × = υ = D V The proper relation for Nusselt number corresponding to this Reynolds number is () [] 6 . 203 000 , 282 10 063 . 9 1 7268 . 0 / 4 . 0 1 ) 7268 . 0 ( ) 10 063 . 9 ( 62 . 0 3 . 0 000 , 282 Re 1 Pr / 4 . 0 1 Pr Re 62 . 0 3 . 0 5 / 4 8 / 5 4 4 / 1 3 / 2 3 / 1 5 . 0 4 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . 0 = × + + × + =
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