Investigation on heat transfer of supercritical CO2/Xe mixture crossing pseudo-critical temperature cooled in a horizontal circular tube

Supercritical CO₂/Xe mixture has a potential application prospect in the Brayton cycle system from the perspective of thermodynamic. Supercritical cooler is one of the key components there. However, the cooling heat transfer characteristics and mechanism of supercritical CO₂/Xe mixture is still unclear, which restricts the development and design of supercritical cooler. The heat transfer of supercritical CO₂/Xe mixture crossing T_pc cooled in a horizontal tube with inner diameter 8 mm is numerically explored in this study. The influences of operating parameters on supercritical mixture heat transfer are thoroughly examined. Buoyancy effect is evaluated through the buoyancy criterion Gr/Re². Further, the mechanism of supercritical CO₂/Xe mixture cooling heat transfer is revealed. It is found that heat transfer enhancement may occur during supercritical CO₂/Xe cooled process, and heat transfer is weakened when mass fraction of Xe increases. Considering the bulk specific heat and buoyancy dominating jointly the behavior of supercritical cooling heat transfer, a modified Dittus-Boelter correlation is newly developed to predict heat transfer coefficient of supercritical CO₂ and its mixture with Xe. The correlation matches well with the experimental and numerical data, and the average relative deviations of the new correlation are 18.27 % and 14.14 %, respectively. The investigation provides insight into the characteristics and mechanism of supercritical CO₂/Xe mixture cooling heat transfer. The correlation can provide significant theoretical guidance for accurate design and optimization of supercritical cooler.