Influence of a hydrophobic membrane on evaporation rate of water placed in a top-cooled circular cavity

Abstract

The present study investigates changes in the evaporation rate of an initially warm body of water covered by a hydrophobic membrane in a top-cooled circular cavity. Evaporation from pure water and saline solution of NaCl have been considered. The interfacial heat fluxes are measured in a Mach-Zehnder interferometer. Membrane-controlled evaporation is numerically modeled for coupled heat and moisture transport on air and water sides of their interface. The model predictions are in reasonably good agreement with measured heat fluxes. The two are also found to be in good agreement for evaporation without the membrane cover. Transport mechanisms in the membrane pores, namely, Knudsen diffusion, ordinary diffusion, and Poiseuille flow are compared. Interfacial heat fluxes in an open-surface evaporation arrangement are about twice the value realized in membrane-controlled evaporation but decrease with increasing salinity. A circular cavity shows higher heat and mass transfer rates compared to the rectangular, bringing in the effect of the confining walls. Instantaneous Nusselt and Sherwood numbers show good correlation, owing to the coupling between the heat and mass transfer processes. The presence of salt reduces the evaporation mass fluxes below that of pure water for both open-surface and membrane evaporation.