Thermodynamics HW Solutions 718

Thermodynamics HW Solutions 718 - Chapter 9 Natural...

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Chapter 9 Natural Convection Heat loss by both natural convection and radiation heat can be expressed as ] ) K 273 20 ( ) 273 )[( 10 67 . 5 )( m 03 . 0 )( 8 . 0 ( C ) 20 )( m 03 . 0 )( C . W/m 696 . 6 ( W 8 ) ( ) ( 4 4 8 2 2 2 4 4 + + × + ° ° = + = s s surr s s s s T T T T A T T hA Q σε & Its solution is T s 42.6 C which is sufficiently close to the assumed value of 45 ° C in the evaluation of the properties and h . (c) Horizontal, hot surface facing down This time we expect the surface temperature to be higher, and assume the surface temperature to be 50 . We will check this assumption after obtaining result and repeat calculations with a better assumption, if necessary. The properties of air at the film temperature of ° C C 35 2 20 50 2 ° = + = + = T T T s f are (Table A-15) 1 - 2 5 K 003247 . 0 K ) 273 35 ( 1 1 7268 . 0 Pr /s m 10 655 . 1 C W/m. 02625 . 0 = + = = = × = ° = f T k β υ The characteristic length in this case is, from part (b), L c = 0.0429 m. Then, 379 , 166 ) 7268
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