HEAT-TRANSFER OF OIL-CONTAMINATED HFC134A IN A HORIZONTAL EVAPORATOR

The introduction of chlorine-free refrigerants to the market requires experimental investigations of their behaviour in heat pumps and refrigerators. One particular area of interest is the effect of the new oils on the heat transfer in evaporators and condensers. Oil can either increase or decrease the heat transfer coefficient. This paper presents the results from an experimental investigation of the effect of three different ester-based oils on the heat transfer of HFC134a in a horizontal evaporator. The tests were carried out at heat fluxes between 2 and 8 kW m(-2) (corresponding to mass fluxes between approximately 40 and 170 kg s(-1) m(-2)). The evaporation temperature was varied from -10 to +10 degrees C. The global oil concentration ranged from 0 to 4.5 mass percentage based on the total liquid flow. The heat transfer coefficient decreased in most of the cases. The results indicate that the decrease seems to depend on the viscosity of the oil. The decrease can fairly well be estimated with the correlation for pure refrigerants by Shah if the viscosity of the mixture is used in the calculations. The data for the oil-contaminated refrigerant also agree well with data for pure refrigerants in a plot of alpha(tp)/alpha(lo)* versus the inverse Martinelli-Lockhart parameter when alpha(lo)* is calculated with a modified Dittus-Boelter correlation and the mixture viscosity is used in the calculations. The heat transfer is found to increase when introducing oil in the special cases where the flow rate is low and the viscosity is low (oil A, 2 and 4 kW m(-2), oil B, 6 kW m(-2) at +10 degrees C). This is most likely due to surface tension effects. It has been suggested that the increased surface tension leads to a better tube wetting and thus an increased heat transfer.