Enhanced condensation via surface modification: flow visualization.

The effectiveness of condensation heat transfer to a surface is affected by the type of condensation that occurs. Condensation heat transfer is shown to improve in dropwise condensation (DWC) compared to filmwise condensation (FWC). This is mainly because condensate more readily departs the condensing surface as droplets in DWC than as a film in FWC, making room for further condensation. Steam-based DWC can also be promoted by making the surface more hydrophobic or other surface modifications.

Two external condensation experiments have been developed to explore heat transfer and visualize condensation on hydrophobic surfaces. One is a tube condensing heat exchanger (TC-HEX) with carefully monitored coolant conditions inside the tube sample and ambient conditions in the surrounding chamber. The second is a plate condensing heat exchanger (PC-HEX) with a large viewport to the condensing surface of the plate sample.

The TC-HEX is used to obtain the condensation heat transfer coefficient enhancement for a treated tube compared to an untreated tube. A high-speed camera is used in the PC-HEX to visualize the condensation phenomenon on the surface of a plate sample. Image analysis of the high-speed footage allows for further characterization of hydrophobic treated surfaces, including droplet radii, droplet population densities, and condensate sweep rate, all of which are relevant in heat flux models of dropwise condensation.