Flow and convection heat transfer characteristics of CO2 mixed with lubricating oil at super-critical pressures in small tube during cooling

This paper describes the heat transfer and the pressure drop characteristics of CO2 mixed with small amounts of compatible lubricating oil at super-critical pressures inside horizontal tubes with inner diameters of 1.98 mm and 4.14 mm during cooling. The heat transfer coefficients and pressure drops were measured. The results show that for the oil-free cases, the correlation proposed by Dang and Hihara accurately predicted the heat transfer coefficients and Petukhov’s correlation was found to predict the frictional pressure drops reasonably. Entrainment of the lubricating oil reduced the heat transfer coefficients and increased the pressure drops. Analysis showed that the heat transfer coefficients of the CO2/oil mixture are strongly related to the density and viscosity ratios of the oil to the CO2. An empirical correlation was developed based on the measured data, which predicts most of the experimental data within a deviation of 20%.