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Unformatted text preview: . )( m/s 81 . 9 ( Pr ) ( = = = D T T g Ra s Air T = 25 C Lamp 60 W = 0.9 D = 8 cm Light, 6 W ( ) [ ] ( ) [ ] 42 . 20 7116 . / 469 . 1 ) 10 694 . 2 ( 589 . 2 Pr / 469 . 1 589 . 2 9 / 4 16 / 9 4 / 1 6 9 / 4 16 / 9 4 / 1 = + + = + + = Ra Nu Then 2 2 2 2 m 02011 . m) 08 . ( C . W/m 854 . 7 ) 42 . 20 ( m 08 . C W/m. 03077 . = = = = = = D A Nu D k h s Considering both natural convection and radiation, the total rate of heat loss can be written as ] ) K 273 25 ( ) 273 )[( .K W/m 10 67 . 5 )( m 02011 . )( 9 . ( C ) 25 )( m 02011 . )( C . W/m 854 . 7 ( W ) 60 90 . ( ) ( ) ( 4 4 4 2 8 2 2 2 4 4 + + + = + = s s surr s s s s T T T T A T T hA Q & Its solution is T s = 169.4 C which is sufficiently close to the value assumed in the evaluation of properties and h . Therefore, there is no need to repeat calculations. 940...
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Mass, Light

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