IN-TUBE FLOW BOILING OF R-407C AND R-407C OIL MIXTURES PART II - PLAIN TUBE RESULTS AND PREDICTIONS

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(2).s) (20.5, 41, and 61 lb/s.ft(2)) over nearly the entire vapor quality ran ge. Pure R-407C performed very similarly to pure R-134a run previously in similar tests, at all three mass velocities. The only difference w as at high vapor qualities where the peaks in the refrigerant heat tra nsfer coefficient versus vapor quality were shifted slightly. For loca l vapor qualities from 10-70%, the oil tended to have little effect on local R-407C/oil heat-transfer coefficients at the lowest mass veloci ty, 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 dramat ic, decreasing performance by as much as 80-90%, even with small amoun ts of oil. Two-phase pressure drops were increased by the presence of oil, especially at high vapor qualities. A new method for predicting l ocal boiling coefficients of refrigerant-oil mixtures is presented. Us ing the refrigerant-oil mixture viscosity in place of the pure refrige rant viscosity in the recent Kattan-Thome-Favrat flow boiling model an d flow pattern map without further modification predicted the R-134a/o il 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.