Chapter 9 Natural Convection 9-106 A cold aluminum canned drink is exposed to ambient air. The time it will take for the average temperature to rise to a specified value 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 Any heat transfer from the bottom surface of the can is disregarded. 5 The thermal resistance of the can is negligible. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T ∞ )/2 = (6+25)/2 = 15.5 ° C are (Table A-15) Air 25 ° C COLA 5 ° C ε = 0.6 D = 6 cm 12.5 cm 1-2 5 K 003466 .0 K) 273 5 . 15 /( 1 / 1 7321 .0 Pr /s m 10 475 . 1 C W/m. 0248 .0 = + = = = × = ° = − f T k β υ Analysis We assume the surface temperature of aluminum can to be equal to the temperature of the drink in the can since the can is made of a very thin layer of aluminum. Noting that the temperature of the drink rises from 5 ° C to 7 ° C, we take the average surface temperature to be 6
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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.