Intube Flow Boiling of R-407C and R-407C/Oil Mixtures. Part 2: Plain Tube Results and Predictions

Intube 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 6.45 bar (93.5 psia) at mass velocities of 100, 200 and 300 kg/m2s (73581, 147162 and 220743 lb/h ft2) over nearly the entire vapor quality range. Pure R-407C performed very similar to pure R-134a run previously in similar tests at all three mass velocities, differing only at high vapor qualities where the peaks in href vs. x were shifted slightly. For local vapor qualities from 10-70%, 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. Two methods for predicting local boiling coefficients of refrigerant-oil mixtures were 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. 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.