Measurement of bubble dynamics on an enhanced boiling surface

Abstract

This work is a continuation of a major study performed by the authors to understand the mechanism of boiling heat transfer in structured boiling 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 height] having a copper foil wrapped over the fin tips. Pores of known diameter and pitch are pierced in the foil cover. The resulting surface is similar to the Hitachi Thermoexcel-E tube. The tests were conducted using R-123 at 26.7°C on foil covers having 0.18 or 0.23 mm pore diameter and 0.75 or 1.5 mm pore pitch. The present study reports bubble dynamics data (bubble diameter, bubble frequency and nucleation site density). The data were obtained using a high speed video camera at 30 × magnification. The bubble growth mechanism on the enhanced surface is different from that on plain surfaces. A significant fraction of the vaporization occurs at menisci in the corners of the tunnels. This evaporation process controls the bubble frequency and nucleation site density. Further, evaporation and bubble growth occur after the bubbles emerge from the surface pores. Smaller bubbles are generated on the enhanced surface and at a greater frequency than on a plain surface for the same heat flux. The enhanced surface has a greater nucleation site density than on a plain surface.