Thermodynamics HW Solutions 556

Thermodynamics HW Solutions 556 - F . Btu/h.ft 427 . 9 ) 10...

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Chapter 7 External Forced Convection 7-26E A refrigeration truck is traveling at 55 mph. The average temperature of the outer surface of the refrigeration compartment of the truck is to be determined. Assumptions 1 Steady operating conditions exist. 2 The critical Reynolds number is Re cr = 5 × 10 5 . 3 Radiation effects are negligible. 4 Air is an ideal gas with constant properties. 5 The local atmospheric pressure is 1 atm. Properties Assuming the film temperature to be approximately 80 ° F, the properties of air at this temperature and 1 atm are (Table A-15E) Air V = 55 mph T = 80 ° F L = 20 ft Refrigeration truck 7290 . 0 Pr /s ft 10 1697 . 0 F Btu/h.ft. 01481 . 0 2 3 - = × = ° = υ k Analysis The Reynolds number is 6 2 3 10 506 . 9 /s ft 10 1697 . 0 ft) ft/s](20 /3600) 5280 55 [ Re × = × × = υ = L L V We assume the air flow over the entire outer surface to be turbulent. Therefore using the proper relation in turbulent flow for Nusselt number, the average heat transfer coefficient is determined to be
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Unformatted text preview: F . Btu/h.ft 427 . 9 ) 10 273 . 1 ( ft 20 F Btu/h.ft. 01481 . 10 273 . 1 ) 7290 . ( ) 10 506 . 9 ( 037 . Pr Re 037 . 2 4 4 3 / 1 8 . 6 3 / 1 8 . = = = = = = = Nu L k h k hL Nu L Since the refrigeration system is operated at half the capacity, we will take half of the heat removal rate & ( ) , Q = = 600 60 2 18 000 Btu / h Btu / h The total heat transfer surface area and the average surface temperature of the refrigeration compartment of the truck are determined from A = 2 20 ( ft)(9 ft) + (20 ft)(8 ft) + (9 ft)(8 ft) = 824 ft 2 F 77.7 = = = = ) ft F)(824 . Btu/h.ft 427 . 9 ( Btu/h 18,000 F 80 ) ( 2 2 s conv s s s hA Q T T T T hA Q & & 7-18...
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This note was uploaded on 01/22/2012 for the course PHY 4803 taught by Professor Dr.danielarenas during the Fall '10 term at UNF.

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