The effect of oil on heat transfer and pressure drop during evaporation and condensation of refrigerant inside augmented tubes.

摘要

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.