부산대학교 도서관

In-tube evaporation tests for R-407C and R-407C/oil are reported for a plain copper tube. The tests were run at a nominal inlet pressure of 645 kPa (93.5 psia) at mass velocities of 100, 200, and 300 kg/(m{sup 2}{center{underscore}dot}s)(20.5, 41, and 61 lb/s{center{underscore}dot}ft{sup 2}) over nearly the entire vapor quality range. Pure R-407C performed very similarly to pure R-134a run previously in similar tests, at all three mass velocities. The only difference was at high vapor qualities where the peaks in the refrigerant heat transfer coefficient versus vapor quality were shifted slightly. For local vapor qualities from 10--70%, the oil tended to have little effect on local R-407C/oil heat-transfer coefficients at the lowest mass velocity, while at the higher mass velocities the effect was to increase or decrease the coefficients within {+-}20% of the pure R-407C values. At vapor qualities higher than 70%, the effect of the oil was very dramatic, decreasing performance by as much as 80--90%, even with small amounts of oil. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. A new method for predicting local boiling coefficients of refrigerant-oil mixtures is presented. Using the refrigerant-oil mixture viscosity in place of the pure refrigerant viscosity in the recent Kattan-Thome-Favrat flow boiling model and flow pattern map without further modification predicted the R-134a/oil and R-407C/oil data quite accurately. In addition, the Friedel two-phase friction multiplier was found to work adequately for pure R-134a and pure R-407C. Finally, a new local refrigerant-oil viscosity ratio was developed that accurately predicted two-phase pressure drops of R-134a/oil and R-407C/oil mixtures at high vapor qualities.