Thermodynamics HW Solutions 628

Thermodynamics HW Solutions 628 - 1 3 / 2 3 / 1 5 . = + + +...

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Chapter 7 External Forced Convection 7-98E A cylindrical transistor mounted on a circuit board is cooled by air flowing over it. The maximum power rating of the transistor is to be determined. Assumptions 1 Steady operating conditions exist. 2 Radiation effects are negligible. 3 Air is an ideal gas with constant properties. 4 The pressure of air is 1 atm. Properties The properties of air at 1 atm and the film temperature of are (Table A-15) F 150 2 / ) 120 180 ( ° = + = f T 7188 . 0 Pr /s ft 10 210 . 0 F Btu/h.ft. 01646 . 0 2 3 - = × = υ ° = k Analysis The Reynolds number is 5 . 727 /s ft 10 210 . 0 ft) /12 ft/s)(0.22 (500/60 Re 2 3 = × = υ = D V The Nusselt number corresponding to this Reynolds number is () [] () [] 72 . 13 000 , 282 5 . 727 1 7188 . 0 / 4 . 0 1 ) 7188 . 0 ( ) 5 . 727 ( 62 . 0 3 . 0 000 , 282 Re 1 Pr / 4 . 0 1 Pr Re 62 . 0 3 . 0 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . 0 5 / 4 8 / 5 4 /
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Unformatted text preview: 1 3 / 2 3 / 1 5 . = + + + = + + + = = k hD Nu Air 500 ft/min 120 F Power transistor D = 0.22 in and F . Btu/h.ft 32 . 12 ) 72 . 13 ( ft) 12 / 22 . ( F Btu/h.ft. 01646 . 2 = = = Nu D k h Then the amount of power this transistor can dissipate safely becomes ( ) [ ] Btu/h) 3.412 = W (1 C ) 120 180 ( ft) 2 ft)(0.25/1 (0.22/12 F) . Btu/h.ft 32 . 12 ( ) ( ) ( 2 W 0.26 = Btu/h 0.887 = = = = T T DL h T T hA Q s s s & 7-83...
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