Prediction of gas cooling heat transfer coefficients for CO2–oil mixtures

The transport properties and the gas cooling heat transfer coefficients of CO2–oil mixtures were investigated under gas cooling conditions for a CO2 refrigeration system. The viscosity and thermal conductivity of the CO2–oil mixture were predicted using the TRAPP model under the high pressure condition of around 10 MPa. Depending on the oil concentration, two different models: the homogenous model and separate model, were utilized to estimate the gas cooling heat transfer coefficient. Significant variations of the thermophysical properties of pure CO2 with oil concentration heavily influenced the gas cooling characteristics of the CO2–oil mixture. The homogenous model for low oil concentrations (i.e. 1.0 wt.%) and the separate model for relatively high oil concentrations (i.e. 5.0 wt.%) predictions matched the previous experimental data with mean deviations of ±13.2% and ±33.4%, respectively.