On Condensation Heat Transfer for Water and Ethanol Vapor Mixture. Observation of Drop Formation and Departure Processes for Marangoni Dropwise Condensation.

Abstract

The pseudo-dropwise condensation driven by concentration difference, which is called Dropwise Condensation in this paper, appears in the positive system of the condensation of vapor mixture. The observation and measurement were performed for Marangoni dropwise condensation of water and ethanol vapor mixture. The processes of initial droplets formation from thin condensate film and drop departure were taken by using a high speed digital camera simultaneously with the heat transfer measurement. The variations of the initial drop distances exhibited V-or U-shape against the surface subcooling. Their minimum values were in the range of 0.03mm∼0.15mm depending upon the etanol concentrations. The surface subcooling in which the minimum initial drop distances appeared coincided with those for maximum heat transfer coefficient. Also the variation of departing drop diameters had similar trend. Since the augmentation of driving force of Marangoni dropwise condensation made the initial drop distance and the departing drop diameter smaller and the condensate film among droplets thinner, the heat transfer was enhanced.