Abstract Condensation from air-steam mixtures on falling water layers is investigated experimentally and theoretically. The thin film flows on the inner surface of a 5cmi.d. vertical pipe. This film is wavy turbulent while the gas phase is kept saturated with steam. Experiments are conducted with the gas mixture effectively stagnant, compared with the fast moving liquid film. Measurements are also made under a mild vacuum applied on the gas phase. Heat transfer coefficients averaged over the entire length of the condensing surface, tend to increase by decreasing the liquid flowrate, by increasing the steam fraction, and by applying a mild vacuum on the gas phase. However, for the cases examined, there is a liquid flowrate above which the heat transfer coefficient becomes almost constant. Numerical predictions are made for a fully developed turbulent film using an eddy diffusivity model. The results indicate that for a system with a large amount of noncondensable gases-as in this study-the temperature profile in the liquid film is nearly uniform and that the major resistance to condensation resides in the gas phase. The analysis also shows that the relative contribution of sensible heat transferred through the gas phase is small relative to the latent heat released upon condensation. Comparison of predictions with experimental data suggests that a significant parameter in these analyses is the gas diffusion boundary layer thickness which seems to be comparable in size with the liquid film thickness. Finally, the possibility is discussed of correlating condensation heat transfer coefficients with already available statistical characteristics of the falling wavy layer. Theoretical predictions based on this idea are in good agreement with data.
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