Experimental investigations on cooling heat transfer of CO₂-lubricant mixtures in horizontal tubes at supercritical pressure: A review

The presence of lubricating oil has been shown to have a significant effect on the heat transfer process of the CO₂ gas cooler. Previous review work has not comprehensively summarized and discussed the effect of lubricating oil on supercritical carbon dioxide (sCO₂) cooling heat transfer (CHT) in gas coolers. This study provides a comprehensive review of the effect of lubricants on the CHT of sCO₂. This study aims to provide a good insight for evaluating the influence of lubricating oil on the heat transfer performance of gas coolers and provide a reference to guide the actual design and optimization of gas coolers for oil-entrained cases. In the present work, the effects of temperature, the type and concentration of oil, mass flux, operating pressure, tube diameter, and tube configuration on the CHT of sCO₂-oil mixtures are explored in detail. The results show that the presence of lubricant contributes to the formation of oil films and droplets, thereby impairing heat transfer. Compared to oil droplets, the oil film has a more profound effect on weakening the heat transfer. Moreover, this paper also summarizes and evaluates the sCO₂-oil mixture heat transfer correlations and presents the recommended correlations. Finally, given the limitations in existing studies, two future research directions are put forward: research on CHT of sCO₂-oil mixtures under different heat fluxes and development of heat transfer correlations.