This study investigates the effect of the R134a/POE68 mixture on the condensation heat transfer coefficient (HTC) in horizontal smooth tubes with an outer diameter of 8 mm. Experiments were conducted at a condensation temperature of 40 °C, with mass fluxes ranging from 100 to 500 \({\text{kgm}}^{ - 2} {\text{s}}^{ - 1}\) and oil concentrations between 0 and 10%. Results confirm that the presence of oil reduces the HTC, with a more pronounced deterioration at higher oil concentrations. The enhancement factor (EF) decreased from 0.95 to 0.76 at 5% oil and from 0.89 to 0.69 at 10% oil. Based on the experimental data, two new correlations were developed for predicting the HTC of R134a/POE68 mixtures. The first correlation uses a two-phase multiplier approach, yielding an average prediction error of 11.29%. The second correlation is based on mixture properties and achieves an average prediction error of 9.38%. These correlations provide improved predictive accuracy and are suitable for engineering applications involving refrigerant–oil mixtures.
