Chapter 9 Natural Convection Review Problems 9-94E A small cylindrical resistor mounted on the lower part of a vertical circuit board. The approximate surface temperature of the resistor 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 Radiation effects are negligible. 5 Heat transfer through the connecting wires is negligible. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T ∞ )/2 = (220+120)/2 = 170 ° F are (Table A-15E) Q Resistor 0.1 W D = 0.2 in Air T ∞ = 120 ° F 1-2 3 R 001587 .0 R ) 460 170 ( 1 1 7161 .0 Pr /s ft 10 222 .0 F Btu/h.ft. 01692 .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 220
This is the end of the preview. Sign up
access the rest of the document.
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.