Thermodynamics HW Solutions 762

Thermodynamics HW Solutions 762 - Chapter 9 Natural...

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Chapter 9 Natural Convection 9-95 An ice chest filled with ice at 0 ° C is exposed to ambient air. The time it will take for the ice in the chest to melt completely is to be determined for natural and forced convection cases. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 Heat transfer from the base of the ice chest is disregarded. 4 Radiation effects are negligible. 5 Heat transfer coefficient is the same for all surfaces considered. 6 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 = (15+20)/2 = 17.5 ° C are (Table A-15) Ice chest, 0 ° C 3 cm Q Air T = 20 ° C 30 cm 1 - 2 5 K 003442 . 0 K ) 273 5 . 17 ( 1 1 7316 . 0 Pr /s m 10 493 . 1 C W/m. 02495 . 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
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