Effect of PAG-type lubricating oil on heat transfer characteristics of supercritical carbon dioxide cooled inside a small internally grooved tube

Heat transfer characteristics of supercritical CO2 cooled inside a small internally grooved tube were investigated. The mean inner diameter (ID) and helix angle of this tube were 2 mm and 6.3°, respectively. Experiments were conducted in the pressure range of 8–10 MPa and mass flux range of 400–1200 kg m−2 s−1. The effect of PAG-type lubricating oil on the heat transfer characteristics was investigated by changing the oil concentration from 0% to 5%. The drop in the heat transfer coefficient was 30–50% and 50–70% at oil concentrations of 1% and 3%, respectively. The heat transfer coefficient of CO2 in the grooved tube was higher than that in the smooth tube at all oil concentrations tested. The experimental results indicated that the grooved tube has a large heat transfer area and facilitates the breaking up of the oil film.