Effect of lubricating oil on the flow and heat-transfer characteristics of supercritical carbon dioxide

Effects of lubricating oil on heat-transfer performance of supercritical CO2 were studied by applying three lubricants: PAG, PVE, and ECP. Heat-transfer coefficient measurements and flow-pattern visualization were conducted in a horizontal tube of 2 mm I.D. at CO2 pressures from 8 to 10 MPa and mass fluxes from 800 to 1200 kg m−2 s−1. The solubility of lubricants with CO2 was found having remarkable influence on both the flow pattern and heat-transfer coefficient. For PVE, which has the highest CO2 solubility, oil droplets can only be observed occasionally and the oil film can hardly be identified at temperatures lower than Tpc, and the heat-transfer coefficient does not greatly change with oil concentration. At higher temperatures, a decrease in the heat-transfer coefficient with increasing oil concentration was observed for all three lubricants due to the formation of oil film. The experiments show that while ECP is inferior to PVE, it provides better heat-transfer performance than PVG.