Thermodynamics HW Solutions 728

Thermodynamics HW - Chapter 9 Natural Convection 9-35 An insulated electric wire is exposed to calm air The temperature at the interface of the

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Chapter 9 Natural Convection 9-35 An insulated electric wire is exposed to calm air. The temperature at the interface of the wire and the plastic insulation 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. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T )/2 = (50+30)/2 = 40 ° C are (Table A-15) Air T = 30 ° C T s ε = 0.9 L = 12 m D = 6 mm Resistance heater 1 - 2 5 K 003195 . 0 K ) 273 40 ( 1 1 7255 . 0 Pr /s m 10 702 . 1 C W/m. 02662 . 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 50 ° C for the evaluation of the properties and h . We will check the accuracy of this guess later and repeat the calculations if necessary.
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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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