Thermodynamics HW Solutions 785

Thermodynamics HW Solutions 785 - Chapter 9 Natural...

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Chapter 9 Natural Convection 9-111E A hot water pipe passes through a basement. The temperature drop of water in the basement due to heat loss from the pipe 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. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T )/2 = (150+60)/2 = 105 ° F are (Table A-15E) Water 4 ft/s 150 ° F L = 50 ft D i =1.0 in D o =1.2 in T s ε = 0.5 T sky = 60 ° F T = 60 ° F 1 - 2 3 R 00177 . 0 R ) 460 105 ( 1 1 7253 . 0 Pr /s ft 10 1837 . 0 F Btu/h.ft. 01541 . 0 = + = = = × = ° = f T k β υ Analysis We expect the pipe temperature to be very close to the water temperature, and start the calculations by “guessing” the average outer surface temperature of the pipe to be 150 ° F 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 the outer diameter of the pipe, L c = D o = 1.2 in. Then, 5 2 2
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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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