Chapter 9 Natural Convection 9-17 A power transistor mounted on the wall dissipates 0.18 W. The surface temperature of the transistor is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 Any heat transfer from the base surface is disregarded. 4 The local atmospheric pressure is 1 atm. 5 Air properties are evaluated at 100 ° C. Properties The properties of air at 1 atm and the given film temperature of 100 ° C are (Table A-15) 1-2 5 K 00268 .0 K ) 273 100 ( 1 1 7111 .0 Pr /s m 10 306 . 2 C W/m. 03095 .0 = + = = β = × = υ ° = − f T k Analysis The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown. We start the solution process by “guessing” the surface temperature to be 165 ° C for the evaluation of h . This is the surface temperature that will give a film temperature of 100 ° C. We will check the accuracy of this
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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.