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Unformatted text preview: Chapter 9 Natural Convection 998 A doublepane window consisting of two layers of glass separated by an air space is considered. The rate of heat transfer through the window and the temperature of its inner surface are to be determined. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 Radiation effects are negligible. 4 The pressure of air inside the enclosure is 1 atm. Properties We expect the average temperature of the air gap to be roughly the average of the indoor and outdoor temperatures, and evaluate The properties of air at 1 atm and the average temperature of ( T 1 + T 2 )/2 = (20 +0)/2 = 10 C are (Table A15) 1 2 5 K 003534 . K ) 273 10 ( 1 1 7336 . Pr /s m 10 426 . 1 C W/m. 02439 . = + = = = = = f T k Analysis We guess the temperature difference across the air gap to be 15 C = 15 K for use in the Ra relation. The characteristic length in this case is the air gap thickness, L c = L = 0.03 m. Then, = 0....
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 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Mass, Heat

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