Dropwise condensation from flowing air-steam mixtures: Diffusion resistance assessed by controlled drainage

In this study, the initial phase of drop growth, when diffusion is not limiting, is artificially made more important. An apparatus with controlled removal of condensate droplets from the condenser plates is designed and applied. The dropwise condensation process is frequently interrupted upon which nucleation restarts upon each sweep. Condensate growth and surface temperatures are assessed by simultaneous video and infrared recordings. Cold wakes downstream of big drops on the condenser plate were observed. A single controlled droplet removal action enables a 'reset' of the condenser surface. This allowed measurement of droplet growth histories. It is found that droplet growth follows a power law of D ∝ tβ, with β increasing with increasing ωvap,in. Direct contact condensation on drops at condenser plate dominates drop growth. The main finding is that the total heat transfer resistance decreases with increasing droplet removal frequency, while two measures for mass transfer simultaneously increase. Increasing diffusion limitation is one explanation for the observed decreasing mass transfer rate with time. After initial fast growth of drops, the slight increase in interfacial temperature observed offers another explanation.