DIRECT-CONTACT CONDENSATION IN THE PRESENCE OF NONCONDENSABLES OVER FREE-FALLING FILMS WITH INTERMITTENT LIQUID FEED

An intermittent (periodic) liquid flow rate is examined as a means to enhance local condensation rates in a vertical column where quasi-stagnant vapor-gas mixtures come in direct contact with falling wavy liquid layers. The gas mixture being effectively stagnant is responsible for the major resistance to heat transfer due to its relatively large concentration of noncondensables. Flow intermittency is found to improve heat transfer rates by as much as an order of magnitude. The local condensation heat transfer coefficients depend greatly on both liquid flow mode and liquid flow rate. For the range of frequencies encountered here, the flow cycling frequency seems to have only a minor effect on the transport process, whereas the asymmetry in the intermittency (flow/pause time periods) appears to be more significant. Statistical analysis of the measured local fluctuations of film thickness reveals that condensation is responsible only for slight modifications of the isothermal liquid surface morphology. There is also evidence that the interfacial transport process is aided by increased liquid wave velocities.