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-stag nant vapor-gas mixtures come in direct contact with falling wavy liqui d layers. The gas mixture being effectively stagnant is responsible fo r the major resistance to heat transfer due to its relatively large co ncentration of noncondensables. Flow intermittency is found to improve heat transfer rates by as much as an order of magnitude. The local co ndensation heat transfer coefficients depend greatly on both liquid fl ow 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 (fl ow/pause time periods) appears to be more significant. Statistical ana lysis of the measured local fluctuations of film thickness reveals tha t condensation is responsible only for slight modifications of the iso thermal liquid surface morphology. There is also evidence that the int erfacial transport process is aided by increased liquid wave velocitie s.