The Importance of Drainage in Dropwise Condensation From Flowing Air-Steam Mixtures

In this study, the importance of drainage on dropwise condensation of a flowing air-steam mixture is investigated. The initial phase of drop growth, when diffusion is not limiting, is artificially made more important to separate the diffusion resistance to heat transfer. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and tested. The dropwise condensation process is frequently interrupted upon which nucleation restarts each time. Non-artificial drainage occurs at low frequencies of typically 4 large drops per second per dm². Condensate removal at such a frequency that does not allow formation of large drops artificially makes the initial phase of drop growth more important. The results are believed to be important for the explaining of differences between filmwise and dropwise condensation heat transfer. It is found that the total heat transfer resistance decreases with increasing droplet removal frequency, f. When f is increased, the ratio of condensate mass flow rate to gas mass flow rate increased as well, by 11% for a droplet removal frequency of 0.8 Hz. With increasing f, the relative importance of convective heat transfer decreases.