Experimental study of the coalescence phenomenon during nucleate pool boiling

This work deals with the study of the coalescence phenomenon (merging of tw o or more bubbles into a single larger one) during pool boiling on a duralu minium (AU4G) vertical heated wall. Various boiling curves characterising b oiling (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 t he coalescence of bubbles growing on three sites results in higher heat tra nsfer coefficients than single-site boiling. However, the highest heat tran sfer coefficients are obtained for an optimal distance between the sites fo r which coalescence does not occur. It is also shown that coalescence resul ts in a decrease in the bubble frequency. Lastly, a map is proposed to dete rmine the conditions leading to coalescence or to site activation. (C) 2000 Elsevier Science Inc. All rights reserved.