Thermodynamics HW Solutions 585

Thermodynamics HW Solutions 585 - . 3 /s m 10 774 . 1 m)...

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Chapter 7 External Forced Convection 7-55 The components of an electronic system located in a horizontal duct is cooled by air flowing over the duct. The total power rating of the electronic device 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 The properties of air at 1 atm and the film temperature of (T s + T )/2 = (65+30)/2 = 47.5 ° C are (Table A-15) Air 30 ° C 200 m/min 20 cm 65 ° C 7235 . 0 Pr /s m 10 774 . 1 C W/m. 02717 . 0 2 5 - = × = ° = υ k Analysis The Reynolds number is [ ] 4 2 5 10 758
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Unformatted text preview: . 3 /s m 10 774 . 1 m) (0.2 m/s (200/60) Re = = = D V Using the relation for a square duct from Table 7-1, the Nusselt number is determined to be 2 . 112 ) 7235 . ( ) 10 758 . 3 ( 102 . Pr Re 102 . 3 / 1 675 . 4 3 / 1 675 . = = = = k hD Nu The heat transfer coefficient is C . W/m 24 . 15 ) 2 . 112 ( m 2 . C W/m. 02717 . 2 = = = Nu D k h Then the rate of heat transfer from the duct becomes 2 m 2 . 1 m) 5 . 1 )( m 2 . 4 ( = = s A W 640.0 = C 30)-)(65 m C)(1.2 . W/m 24 . 15 ( ) ( 2 2 = = T T hA Q s s & 7-40...
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