Experimental study of the coalescence phenomenon during nucleate pool boiling

This work deals with the study of the coalescence phenomenon (merging of two or more bubbles into a single larger one) during pool boiling on a duraluminium (AU4G) vertical heated wall. Various boiling curves characterising boiling (with or without coalescence) from three artificial nucleation sites a variable distance apart are presented. The heat flux ranges from 100 to 900 W/cm(2) and the wall superheat from 5 to 35 K. It is pointed out that the coalescence of bubbles growing on three sites results in higher heat transfer coefficients than single-site boiling. However, the highest heat transfer coefficients are obtained for an optimal distance between the sites for which coalescence does not occur. It is also shown that coalescence results in a decrease in the bubble frequency. Lastly, a map is proposed to determine the conditions leading to coalescence or to site activation. (C) 2000 Elsevier Science Inc. All rights reserved.