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RESEARCH PAPERS: Boiling and Condensation

Theoretical and Experimental Studies on the Pseudo-Dropwise Condensation of a Binary Vapor Mixture

[+] Author and Article Information
K. Hijikata, Y. Fukasaku, O. Nakabeppu

Department of Mechano-Aerospace Engineering, Tokyo Institute of Technology, Meguro-ku, Tokyo 152 Japan

J. Heat Transfer 118(1), 140-147 (Feb 01, 1996) (8 pages) doi:10.1115/1.2824027 History: Received May 01, 1995; Revised October 01, 1995; Online December 05, 2007

Abstract

When a water–ethanol binary mixture condenses on a flat plate, one observes that the liquid film condensate rises locally and eventually forms many droplets on the film. Usually, filmwise condensation is expected because both substances are completely soluble in each other and they wet a copper plate well. This paper presents the droplet growth mechanism during so-called pseudo-dropwise condensation. Instability analysis is used to determine the transition from filmwise condensation to pseudo-dropwise condensation theoretically. In a stress balance at the vapor–liquid interface, the analysis considers not only the surface tension itself, but also the surface tension variation due to changes in temperature and concentration, assuming saturation conditions at the interface. Numerical results indicate that the Marangoni effect plays a more important role than the absolute value of the surface tension in pseudo-dropwise condensation. The change in surface tension with temperature is not always negative; it becomes positive for certain mixtures due to the dependence on concentration. Pseudo-dropwise condensation is only realized when surface tension increases with temperature. This analysis qualitatively predicts the critical Marangoni number experimentally observed during water–ethanol mixture condensation.

Copyright © 1996 by The American Society of Mechanical Engineers
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