| Although refrigerant in vapor compression refrigeration systems contains lubricating oil, and the refrigeration industry shows a growing interest in in-tube heat transfer augmentation, there has been no research into oil effects on two-phase heat transfer and pressure drop inside augmented tubes.;A literature review emphasized oil effects on evaporation and condensation inside smooth tubes and pure refrigerant evaporation and condensation inside augmented tubes. Also reviewed were papers dealing with mixture properties; pool boiling, vapor space condensation, and adiabatic transport of refrigerant-oil mixtures; and past correlations for heat transfer and pressure drop.;Experiments, using R-22 as the working fluid, measured the heat transfer coefficient and pressure drop during evaporation and condensation of refrigerant and refrigerant-oil mixtures inside smooth and augmented tubes. Naphthenic mineral oil (150 or 300 SUS) was added in concentrations ranging from 0 to 5 weight percent. The test section was a straight tube having a 9.52-mm outside diameter, heated or cooled by water in a surrounding annulus. Augmented tubes tested were: a spiral micro-fin tube having 60 fins with a height of 0.2 mm, and a low-fin tube having 21 fins with a height of 0.38 mm. Test conditions were: 125 to 400 kg/m;To compare results from the different tubes, enhancement factors and performance ratios were defined to quantify the effects of oil and/or augmentation. Augmented tubes have a heat transfer performance advantage relative to the smooth tube with refrigerant and refrigerant-oil mixtures, although the advantage tends to diminish with mixtures. Considering heat transfer and pressure drop together, the advantage of a particular tube is less clear and depends on test conditions and the application.;Predictive equations for heat transfer and pressure drop were developed. In some cases, existing correlations were modified; in other cases, only statistical expressions were obtained. |
