Study on Cooling Heat Transfer of Supercritical Carbon Dioxide Applied to Transcritical Carbon Dioxide Heat Pump

The heat transfer characteristics of supercritical fluids can be regarded as an extension of the conventional single-phase heat transfer with drastic change in thermophysical properties with temperature and pressure. It is of fundamental importance in many industrial processes such as transcritical heat pump system, supercritical water-cooled reactor, supercritical separation and supercritical extraction processes. This paper addresses to recent experimental, theoretical and numerical studies on cooling heat transfer of supercritical CO2. A systematic study on heat transfer coefficient and pressure drop of supercritical CO2 was carried out at wide ranges of tube diameter, mass flux, heat flux, temperature and pressure. The experimental results were explained with the help of numerical simulation. In addition, the effect of lubricating oil was also discussed including the change in flow pattern and heat transfer performance of oil and supercritical CO2.The experimental study were conducted in horizontal smooth tubes with tube diameter varied among 1 ~ 6 mm. Effects of parameters such as mass flux, pressure, heat flux, and cooling-tube diameter on the heat transfer coefficient and pressure drop were experimentally analyzed. Based on the understanding of temperature and velocity distributions at cross-sectional direction provided by the numerical simulation, a new prediction correlation was proposed which agreed well with the experimental results. In addition, the applicability of several conventional turbulence models to supercritical condition was clarified by employing the turbulence models for constant property and supercritical pressure fluids in both heating and cooling processes.Effect of lubricating oil on supercritical CO2 was discussed through heat transfer measurement and flow pattern visualization by applying three different types of lubricants, Polyalkylene Glycol(PAG), Polyvinyl Ether(PVE) and PAG-PVE Copolymer (ECP). It was found that the varied flow pattern of two-phase fluid caused by the difference in miscibility of oil with supercritical CO2 resulted to the different behavior of heat transfer. In addition, experimental studies on heat transfer characteristics of supercritical carbon dioxide cooled inside a small-sized internally grooved tube were also conducted and the results were presented in this paper.