Thermodynamics HW Solutions 723

Thermodynamics HW Solutions 723 - T A Q h T T hA Q s s s s...

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Chapter 9 Natural Convection Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. Analysis The heat transfer surface area of the cylinder is 2 m 05027 . 0 ) m 8 . 0 )( m 02 . 0 ( = = = ππ DL A Noting that in steady operation the heat dissipated from the outer surface must equal to the electric power consumed, and radiation is negligible, the convection heat transfer is determined to be Air T = 20 ° C Cylinder T s = 120 ° C ε = 0.1 L = 0.8 m D = 2 cm Resistance heater, 40 W C . W/m 7.96 2 ° = ° = = = C ) 20 120 )( m 05027 . 0 ( W 40 ) ( ) ( 2 T
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Unformatted text preview: T A Q h T T hA Q s s s s & & The radiation heat loss from the cylinder is W 7 . 4 ] ) K 273 20 ( ) K 273 120 )[( .K W/m 10 67 . 5 )( m 05027 . )( 1 . ( ) ( 4 4 4 2 8 2 4 4 = + − + × = − = − surr s s rad T T A Q σε & Therefore, the fraction of heat loss by radiation is % 8 . 11 1175 . W 40 W 7 . 4 fraction Radiation = = = = total radiation Q Q & & which is greater than 5%. Therefore, the radiation effect is still more than acceptable, and corrections must be made for the radiation effect. 9-24...
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This note was uploaded on 01/24/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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