LOCAL CONDENSATION RATES OF STEAM-AIR MIXTURES IN DIRECT-CONTACT WITHA FALLING LIQUID-FILM

Local condensation heat transfer rates are measured for steam-air mixt ures in direct contact with subcooled water layers inside a vertical t ube over a wide range of liquid flow rates. The gas mixture is maintai ned effectively stagnant during the tests and the major resistance to heat transfer is due to the large amount of noncondensables. A theoret ical model is developed to account for some additional thermal resista nce on the liquid side, by decomposing the liquid film into a wavy 'no nresistant' region and a substrate region where temperature gradients may prevail. Heat transfer coefficients are found to depend not only o n the steam concentration but also on the liquid flow rate. Interestin gly, wave characteristics of the falling liquid layer, such as the dom inant wave velocity and frequency, demonstrate that condensation may o nly be responsible for minor modifications of the isothermal liquid su rface morphology. Furthermore, the dependence of heat transfer coeffic ients on the liquid flow rate is attributed to the dynamic interaction between the interfacial waves and the gas layer. This notion is utili zed by correlating measured gas Sherwood numbers with the gas Grashof number and a dimensionless parameter which characterizes the interface .