Steam flow condensation on superhydrophobic surfaces in a high aspect ratio microchannel

Steam flow condensation has a wide range of applications in the industry such as in air conditioning, refrigeration, and thermal power plants. Condensation of steam on highly hydrophobic surfaces has resulted in notable heat transfer improvement compared to conventional hydrophilic surfaces. Dropwise condensation and increased droplet mobility are the main reason for thermal performance enhancement of superhydrophobic surfaces. Although there are considerable reports of enhanced thermal transport behavior of highly hydrophobic surfaces on steam condensation, the literature lacks sufficient investigation on flow condensation of steam, such as the effect of average vapor quality change on heat transfer rate. Unlike gravity-driven droplet departure in quiescent dropwise condensation, droplet departure sizes in flow condensation are governed by flow-droplet shear forces and droplet-surface adhesive forces. This work experimentally investigates steam flow condensation on nanotextured highly hydrophobic and slightly hydrophobic surfaces. The experimental setup consists of a reservoir, boiler, superheater, condensation chamber (test section), pre-condenser (to adjust the inlet quality), a post condenser, and a pump. A high aspect ratio microchannel was used as the test section. Different mass fluxes and inlet vapor qualities were used for the experimentations. Visualization studies were performed to analyze droplet dynamics such as droplet departure and coalescence in flow condensation. It is shown that for both surfaces increase condensation heat transfer coefficient were a function of both average quality and mass flux. Increase in mass flux from G=8 kg/m2s to G=14 kg/m2s, resulted in 65% and 60% enhancement in condensation heat transfer coefficient of slightly hydrophobic and highly hydrophobic surfaces respectively.