MEASUREMENT OF BUBBLE DYNAMICS ON AN ENHANCED BOILING SURFACE

This work is a continuation of a major study performed by the authors to understand the mechanism of boiling heat transfer in structured boi ling surfaces. The tests were performed on a tubular surface composed of an integral-fin tube [1575 fins/m (40 fins/in.) and 0.8 mm fin heig ht] having a copper foil wrapped over the fin tips. Pores of known dia meter and pitch are pierced in the foil cover. The resulting surface i s similar to the Hitachi Thermoexcel-E tube. The tests were conducted using R-123 at 26.7 degrees C on foil covers having 0.18 or 0.23 mm po re diameter and 0.75 or 1.5 mm pore pitch. The present study reports b ubble dynamics data (bubble diameter. bubble frequency and nucleation site density). The data were obtained using a high speed video camera at 30 x magnification, The bubble growth mechanism on the enhanced sur face is different from that on plain surfaces. A significant fraction of the vaporization occurs at menisci in the corners of the tunnels. T his evaporation process controls the bubble frequency and nucleation s ite density. Further, evaporation and bubble growth occur after the bu bbles emerge from the surface pores. Smaller bubbles are generated on the enhanced surface and at a greater frequency than on a plain surfac e for the same heat flux. The enhanced surface has a greater nucleatio n site density than on a plain surface. (C) 1998 Elsevier Science Inc. All rights reserved.