Flow patterns of a refrigerant, with and without small amounts of oil, in smooth and micro-fin tubes, were investigated. The refrigerant was R-12 and the oil was a 300 SUS naphthenic mineral oil. Foaming associated with refrigerant-oil mixtures in smooth tubes washes the side walls of the tube with roll waves, but foam was not observed in the core of the tube. For pure R-12 flows, the micro-fin surface promoted thin flowing films on the side and upper walls, which represents a significant improvement in surface wetting characteristics over the smooth tube. In both the smooth and micro-fin tubes, oil effects caused semi-annular flow to occur at lower mass fluxes than had been possible in either case for pure R-12. Foaming was substantially reduced in the micro-fin tube, however. The experiments have shown that both oil and micro-fin effects can improve surface wetting characteristics. Because surface wetting is an important factor in determining the heat transfer coefficient for two-phase flow, the results also contribute to our understanding of heat transfer effects due to refrigerant-oil mixtures and micro-fin surfaces.

KEYWORDS: refrigerants, oils, heat transfer fluids, R12, fluid flow, properties, heat flow, experiment, heat transfer coefficient, mixtures, smooth tube heat exchangers, extended surfaces heat exchangers.