Thermodynamics HW Solutions 759

Thermodynamics HW Solutions 759 - 2 2 km/h 12 window, km/h...

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Chapter 9 Natural Convection 9-92 The overall U-factor of a window is given to be U = 2.76 W/m 2 . ° C for 12 km/h winds outside. The new U-factor when the wind velocity outside is doubled is to be determined. Assumptions Thermal properties of the windows and the heat transfer coefficients are constant. Properties The heat transfer coefficients at the outer surface of the window are h o = 22.7 W/m 2 . ° C for 12 km/h winds, and h o = 34.0 W/m 2 . ° C for 24 km/h winds (from the text). Analysis The corresponding convection resistances for the outer surfaces of the window are C/W m 029 . 0 C W/m 0 . 34 1 1 C/W m 044 . 0 C W/m 7 . 22 1 1 2 2 km/h 24 o, km/h 24 o, 2 2 km/h 12 o, km/h 12 o, ° = ° = = ° = ° = = h R h R Inside Outside 12 km/h or 24 km/h Also, the R -value of the window at 12 km/h winds is C/W m 362 . 0 C W/m 76 . 2 1 1
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Unformatted text preview: 2 2 km/h 12 window, km/h 12 window, ° ⋅ = ° ⋅ = = U R Noting that all thermal resistances are in series, the thermal resistance of the window for 24 km/h winds is determined by replacing the convection resistance for 12 km/h winds by the one for 24 km/h: C/W m 347 . 029 . 044 . 362 . 2 km/h 24 o, km/h 12 o, km/h 12 window, km/h 24 window, ° ⋅ = + − = + − = R R R R Then the U-factor for the case of 24 km/h winds becomes C W/m 2.88 2 ° ⋅ = ° ⋅ = = C/W m 347 . 1 1 2 km/h 24 window, km/h 24 window, R U Discussion Note that doubling of the wind velocity increases the U-factor only slightly ( about 4%) from 2.76 to 2.88 W/m 2 . ° C. 9-82...
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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.

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