Predictions of in-tube cooling pressure drops for CO2 mixed with lubricating oil at supercritical pressures

• Viscosity and density are key affecting factors for the CO2/POE oil mixture flow.
• Solubility of CO2 into oil greatly affects the CO2/PAG oil mixture flow.
• Correlations for pressure drop are developed for CO2/POE and CO2/PAG mixtures.

The in-tube cooling pressure drop characteristics of supercritical pressure CO2 mixed with small amounts of lubricating oil are different with those for pure CO2, and are closely related to oil type. The effects of two typical oils, the POE oil, which has good miscibility with supercritical pressure CO2, and the PAG oil with poor miscibility, on the in-tube cooling pressure drops of supercritical pressure CO2 are compared and analyzed in the present paper. An empirical correlation is developed for POE oil with factors accounting for the oil concentration, the density and viscosity ratios of the oil to the CO2 which predicts most of the experimental data within 10%. For cases with entrained PAG oil, the solubility which heavily influences the flow patterns of the CO2–oil mixture, is included in addition to the above mentioned factors when correlating the experimental data. The correlation predicts the measured data within 20%.