Wetting Mode Evolution of Steam Dropwise Condensation on Superhydrophobic Surface in the Presence of Noncondensable Gas

Abstract

It is well known that heat transfer in dropwise condensation (DWC) is superior to that in filmwise condensation (FWC) by at least one order of magnitude. Surfaces with larger contact angle (CA) can promote DWC heat transfer due to the formation of “bare” condensation surface caused by the rapid removal of large condensate droplets and high surface replenishment frequency. Superhydrophobic surfaces with high contact angle (> 150°) of water and low contact angle hysteresis (< 5°) seem to be an ideal condensing surface to promote DWC and enhance heat transfer, in particular, for the steam-air mixture vapor. In the present paper, steam DWC heat transfer characteristics in the presence of noncondensable gas (NCG) were investigated experimentally on superhydrophobic and hydrophobic surfaces including the wetting mode evolution on the roughness-induced superhydrophobic surface. It was found that with increasing NCG concentration, the droplet conducts a transition from the Wenzel to Cassie-Baxter mode. And a new condensate wetting mode—a condensate sinkage mode—was observed, which can help to explain the effect of NCG on the condensation heat transfer performance of steam-air mixture on a roughness-induced superhydrophobic SAM-1 surface.