Unformatted text preview: Chapter 12 Radiation Heat Transfer 12103 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 A15) /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....
View
Full Document
 Fall '10
 Dr.DanielArenas
 Thermodynamics, Convection, Mass, Heat, Radiation, Heat Transfer, ρ, 15 cm, υ, 105°C

Click to edit the document details