Thermodynamics HW Solutions 587

Thermodynamics HW Solutions 587 - ) 7228 . / 4 . ( 1 ) 7228...

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Chapter 7 External Forced Convection 7-57 A cylindrical electronic component mounted on a circuit board is cooled by air flowing across it. The surface temperature of the component 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 local atmospheric pressure is 1 atm. Properties We assume the film temperature to be 50 ° C. The properties of air at 1 atm and at this temperature are (Table A-15) Air V = 150 m/min T = 40 ° C Q Resistor 0.4 W D = 0.3 cm 7228 . 0 Pr /s m 10 798 . 1 C W/m. 02735 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is 1 . 417 /s m 10 798 . 1 m) m/s)(0.003 (150/60 Re 2 5 = × = = D V The proper relation for Nusselt number corresponding to this Reynolds number is [] [] 43 . 10 000 , 282 1 . 417 1
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Unformatted text preview: ) 7228 . / 4 . ( 1 ) 7228 . ( ) 1 . 417 ( 62 . 3 . 000 , 282 Re 1 Pr) / 4 . ( 1 Pr Re 62 . 3 . 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . 5 / 4 8 / 5 4 / 1 3 / 2 3 / 1 5 . = + + + = + + + = = k hD Nu The heat transfer coefficient is C . W/m 09 . 95 ) 43 . 10 ( m 003 . C W/m. 02735 . 2 = = = Nu D k h Then the surface temperature of the component becomes 2 m 0001696 . m) 018 . )( m 003 . ( = = = DL A s C 64.8 = = + = = ) m 96 C)(0.00016 . W/m 09 . 95 ( W 0.4 + C 40 ) ( 2 2 hA Q T T T T hA Q s s s & & 7-42...
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