Transition Mechanism of Supercritical Carbon Dioxide Between Gas-Like and Liquid-Like States

Abstract In recent years, the supercritical carbon dioxide power cycle has made great breakthroughs and has been applied to new energy fields such as solar heat and waste heat recovery. In the field of nuclear energy, the supercritical power cycle has the advantages of high efficiency, compact structure, and good compatibility with active media, so it has broad application prospects in the new generation of nuclear energy systems combined with high-temperature gas cooled reactors, sodium-cooled fast reactors, and even fusion reactors. Supercritical fluids are typically regarded as homogeneous, continuous, indistinguishable, single-phase substances without undergoing phase transitions. However, recent research has demonstrated the existence of a transition mechanism between gas-like and liquid-like structures within supercritical fluids. Considering that this structural transition may lead to the mixing of nitrogen and supercritical carbon dioxide within the pressurizer, this paper devised a straightforward visual experiment. The experiment investigated the characteristics of the interface between supercritical carbon dioxide and nitrogen under different pressure and temperature conditions. The results revealed that, under the same pressure, as the temperature rises, the interface between supercritical carbon dioxide and nitrogen evolves from being clear to gradually becoming blurry and ultimately vanishing. Under a pressure of approximately 7.7 MPa, nitrogen and supercritical carbon dioxide will completely mix at temperatures above 31 °C. To address the significant interference caused by this issue in thermal-hydraulic experiments, this paper also introduces an effective pressurizer design.