Thermodynamics HW Solutions 768

# Thermodynamics HW Solutions 768 - Chapter 9 Natural...

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Unformatted text preview: Chapter 9 Natural Convection 9-100 A horizontal skylight made of a single layer of glass on the roof of a house is considered. The rate of heat loss through the skylight is to be determined for two cases. 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 = (-4-10)/2 = -7 ° C are (Table A-15) T in = 20 ° C Outdoors T ∞ = -10 ° C T sky = -30 ° C Skylight 2.5 m × 1 m ε = 0.9 t = 0.5 cm 1- 2 5 K 003759 . K ) 273 7 ( 1 1 738 . Pr /s m 10 278 . 1 C W/m. 0231 . = + − = = = × = ° = − f T k β υ Analysis We assume radiation heat transfer inside the house to be negligible. We start the calculations by “guessing” the glass temperature to be 4 ° C for the evaluation of the properties and h . We will check the accuracy of this guess later and repeat the calculations if necessary. The characteristic length in this case is accuracy of this guess later and repeat the calculations if necessary....
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