Chapter 9 Natural Convection9-26A printed circuit board (PCB) is placed in a room. The average temperature of the hot surface of the board is to be determined for different orientations. Assumptions1Steady operating conditions exist. 2Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 3The heat loss from the back surface of the board is negligible. PropertiesThe properties of air at 1 atm and the anticipated film temperature of (Ts+T∞)/2 = (45+20)/2 = 32.5°C are (Table A-15) 1-25K003273.0K)2735.32(117275.0Pr/sm10631.1CW/m.02607.0=+===×=°=−fTkβυAnalysisThe 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 L = 0.2 m Insulation Air T∞= 20°C PCB, Ts8 W (a) Vertical PCB . We start the solution process by “guessing” the surface temperature to be 45°C for the evaluation of the properties and h. We will check the accuracy of this guess later and repeat the calculations
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