Evaporation and condensation of refrigerant-oil mixtures were studied inside a low-fin tube and compared with earlier results for smooth tube and micro-fin tube tests. The refrigerant was R-22 and the oil was a 150 SUS naphthenic mineral oil. The test section was a straight, horizontal tube with a 0.375 in (9.52 mm) outside diameter and a length of 12 ft (3.67 m). The test section was heated or cooled by water flowing in a surrounding annulus. The mass concentration of the oil was varied from 0% to 5%, and mass fluxes were varied from 92,000-294,000 lb/hft² (125-400 kg/m²•s). Nominal evaporation conditions were 37 F (3° C) with inlet and outlet qualities of 15% and 85%, respectively. Nominal conditions for condensation were 105 F (41° C) with inlet and outlet qualities of 85% and 15%, respectively.

For pure refrigerant, the low-fin tube enhanced evaporative heat transfer by a factor as high as 2.5 compared to the smooth tube, but the enhancement relative to the smooth tube decreased as oil was added. Condensation heat transfer was enhanced by a factor as high as 2.2 with pure refrigerant, and the enhancement relative to the smooth tube tended to increase as oil was added. Although the smooth tube showed enhancement of evaporative heat transfer with small quantities of oil, the low-fin tube showed virtually no enhancement. The performance of pure refrigerant evaporation in the low-fin tube was equivalent to that in the previously tested micro-fin tube, but the relative performance of the lowfin tube decreased as oil was introduced. During condensation, the heat transfer performance of the lowfin tube was approximately 10% to 15% below that of the micro-fin tube over the range of oil concentrations tested.

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