Study on two-phase flow pattern of supercritical carbon dioxide with entrained PAG-type lubricating oil in a gas cooler

Abstract

In order to clarify the heat transfer mechanism of supercritical carbon dioxide flowing with a small amount of lubricating oil, visualization experiments were conducted using two sight glass tubes with inner diameters of 2 and 6 mm. The visualization images were recorded using a high-speed CCD camera with shutter speed changing from 500 to 10,000 pps (pictures per second). PAG-type oil, which is partially miscible with supercritical carbon dioxide, was used. The experiments were conducted with lubricating oil concentrations of 1 and 5 wt%, pressures between 8 and 10 MPa, and mass fluxes between 200 and 1200 kg m −2 s −1. The visualization images revealed that the two-phase flow pattern inside the gas cooler was determined by many factors, including the tube diameter, oil concentration, temperature, pressure, and mass flux. For a small size tube of 2 mm ID, the formation of both oil droplets in the bulk region and an oil film along the inner wall of the tube was confirmed. At low temperatures, a large number of oil droplets were observed flowing with CO 2 with a slip ratio of approximately 0.7. With an increase in the bulk temperature, both the dimension and number of oil droplets entrained with CO 2 decreased, and the flow of the oil film became clearly visible. For a large tube of 6 mm ID, the flow pattern at a low mass flow rate was a separated wavy flow; with an increase in mass flow, the flow pattern changed to annular flow, which corresponds to a distinct decrease in the heat transfer coefficient due to the heat resistance of oil layer. The transition of the flow pattern is considered due to the shear stress between the oil layer and bulk CO 2. In addition, the dissolution of CO 2 into PAG oil as well as the change in the solubility and thermodynamic properties of CO 2 with the temperature and pressure makes the prediction of the flow patterns a challenging task. The relationship between the flow pattern and heat transfer characteristics was also discussed.