LOCAL HEAT-TRANSFER COEFFICIENTS FOR FORCED-CONVECTION CONDENSATION OF STEAM IN A VERTICAL TUBE IN THE PRESENCE OF A NONCONDENSABLE GAS

An experimental investigation has been conducted to determine the loca l condensation heat transfer coefficient (HTC) of steam in the presenc e of air or helium flowing downward inside a 46-mm-i.d. vertical tube. The gas-steam mixture flow rate was measured with a calibrated vortex flowmeter before it entered the 2.54-m-long test condenser. Cooling w ater flow rate in an annulus around the tube was measured with a calib rated rotameter. Temperatures of the cooling water, the gas-steam mixt ure, and the tube inside and outside surfaces were measured at 0.3-m i ntervals in the test condenser. Inlet and exit pressures and temperatu res of the gas-steam mixture and of the cooling water were also measur ed. The local heat flux was obtained from the slope of the coolant axi al temperature profile and the coolant mass flow rate. For the air-str eam experiments, the ranges of the test variables were as follows: mix ture inlet temperatures of 100, 120, and 140-degrees-C; inlet air mass fraction of 10 to 35%; and mixture inlet Reynolds number of approxima tely 5000 to 22 700. For the helium-steam experiments, the ranges of t he test variables were as follows: mixture inlet temperatures of 100, 120, and 140-degrees-C; inlet helium mass fraction of 2 to 10%; and mi xture inlet Reynolds number of approximately 5000 to 11 400. The local HTC varied from 100 to approximately 25 000 W/m2.degrees-C The local Nusselt number calculated from the obtained data was correlated in ter ms of the local mixture Reynolds number, Jakob number, Schmidt number, and gas mass fractions. It was found that for the same mass fraction of the noncondensable gas, compared with air, helium has a more inhibi ting effect on the heat transfer, but for the same molar ratio, air wa s found to be more inhibiting.