Effect of geometry and fluid property parameters on performance of tunnel and pore enhanced boiling surfaces

This manuscript is an extension of the previous study of boiling on enhanced surfaces. High pressure refrigerants, R-134a and R-22, were tested in this extended work. The authors have previously published experimental work, which identifies the effect of geometric dimensions on the boiling performance of "tunneled" enhanced boiling surfaces for R-11 and R-123. In the present work, tests were performed on 18.5-to-19.1 mm diameter, horizontal tubes using R-134a and R-22 at 26.7°C for heat fluxes between 2-to-80kW/m². The surface was formed on a horizontal integral-fin tube having a copper foil wrapped over the fin tips. Pores of known diameter and pitch were pierced in the foil cover. Boiling maps for R-134a and R-22 were generated from the experimental data. These boiling maps allow one to select the optimum combination of pore diameter and pore pitch for specific tunnel dimensions. Effects of tunnel height, tunnel pitch, and tunnel base radius were also tested. The data show that greater tunnel height and smaller tunnel pitch are preferred. Sharp tunnel corners at the base of the fins in the tunnel provide greater enhancement.