Effect of lubricating oil on cooling heat transfer of supercritical carbon dioxide
Abstract In this research, the cooling heat transfer coefficient and pressure drop of supercritical CO 2 with PAG-type lubricating oil entrained were experimentally investigated. The inner diameter of the test tubes ranged from 1 to 6 mm. The experiments were conducted at lubricating oil concentrations from 0 to 5%, pressures from 8 to 10 MPa, mass fluxes from 200 to 1200 kg m −2  s −1 , and heat fluxes from 12 to 24 kW m −2 . In comparison to the oil-free condition, when lubricating oil entrainment occurred, the heat transfer coefficient decreased and the pressure drop increased. The maximum reduction in the heat transfer coefficients—about 75%—occurred in the vicinity of the pseudocritical temperature. The influence of oil was significant for a small tube diameter and a large oil concentration. From visual observation, it was confirmed that this degradation in the heat transfer was due to the formation of an oil-rich layer along the inner wall of the test tube. However, when the oil concentration exceeded 3%, no further degradation in the heat transfer coefficient could be confirmed, which implies that the oil flowing along with CO 2 in the bulk region does not influence the heat transfer coefficient and the pressure drops significantly. For a large tube at a lower mass flux, no significant degradation in the heat transfer coefficient was observed until the oil concentration reached 1%. This is due to the transition of the flow pattern from an annular-dispersed flow to a wavy flow for a large tube, with CO 2 flowing on the upper side and the oil-rich layer on the lower side of the test section.