Thermodynamics HW Solutions 987

Thermodynamics HW Solutions 987 - Chapter 12 Radiation Heat...

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Unformatted text preview: Chapter 12 Radiation Heat Transfer 12-103 Combustion gases flow inside a tube in a boiler. The rates of heat transfer by convection and radiation and the rate of evaporation of water are to be determined. Assumptions 1 Steady operating conditions exist. 2 The inner surfaces of the duct are smooth. 3 Combustion gases are assumed to have the properties of air, which is an ideal gas with constant properties. Properties The properties of air at 1200 K = 927 ° C and 3 atm are (Table A-15) /s m 10 5287 . /s)/3 m 10 586 . 1 ( C W/m. 07574 . kg/m 2944 . 2 5- 2 5- 3 × = × = υ ° = = ρ k 7221 . Pr C J/kg. 1173 = ° = p C T s = 105 ° C D = 15 cm Analysis ( a ) The Reynolds number is Combustion gases, 3 atm T i = 1200 K 3 m/s 114 , 85 /s m 10 5287 . m) m/s)(0.15 (3 Re 2 5 = × = υ = − D m V which is greater than 10,000. Therefore, the flow is turbulent and the entry lengths in this case are roughly m 5 . 1 m) 15 . ( 10 10 = = ≈ ≈ D L L t h which is much shorter than the total length of the duct. Therefore, we can assume fully developed turbulent which is much shorter than the total length of the duct....
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