Thermodynamics HW Solutions 733

Thermodynamics HW Solutions 733 - Chapter 9 Natural...

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Chapter 9 Natural Convection 9-39 A circuit board containing square chips is mounted on a vertical wall in a room. The surface temperature of the chips is to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 4 The heat transfer from the back side of the circuit board is negligible. Air T = 25 ° C T surr = 25 ° C PCB, T s ε = 0.7 121 × 0.05 W L = 30 cm Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T )/2 = (35+25)/2 = 30 ° C are (Table A-15) 1 - 2 5 K 0033 . 0 K ) 273 30 ( 1 1 7282 . 0 Pr /s m 10 608 . 1 C W/m. 02588 . 0 = + = = = × = ° = f T k β υ 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 35 °
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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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