Chapter 9 Natural Convection 9-99 An electric resistance space heater filled with oil is placed against a wall. The power rating of the heater and the time it will take for the heater to reach steady operation when it is first turned on are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 Heat transfer from the back, bottom, and top surfaces are disregarded. 4 The local atmospheric pressure is 1 atm. Properties The properties of air at 1 atm and the film temperature of ( T s + T ∞ )/2 = (45+25)/2 = 35 ° C are (Table A-15) 1-2 5 K 003247 .0 K ) 273 35 ( 1 1 7268 .0 Pr /s m 10 655 . 1 C W/m. 02625 .0 = + = = = × = ° = − f T k β υ Analysis Heat transfer from the top and bottom surfaces are said to be negligible, and thus the heat transfer area in this case consists of the three exposed side surfaces. The characteristic length is the height of the box, L c = L = 0.5 m. Then, Air T ∞ =25 ° C ε = 0.8 T s = 45 ° C
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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.