Numerical study of supercritical CO₂ heat transfer characteristics in twisted oval tubes

Twisted oval tubes are commonly used in heat exchange systems, but the heat transfer characteristics of S-CO₂ in the twisted oval tube are less studied in the field. The combination of twisted oval tubes with S-CO₂ was not common in previous studies. This study aims to improve the heat transfer efficiency of S-CO₂ in twisted oval tubes using the field synergy principle. In this paper, the SST k-ω model is used to compare the twisted oval tubes with the conventional straight tubes, and the effects of flow direction, heat flux, and twist pitch on the performance of S-CO₂ are studied in these tubes. The study finds that better mixing of temperature and speed in twisted oval tubes improves the flow and heat transfer of CO₂, and the heat transfer coefficient of the twisted oval tube is about 1.1 ~ 1.18 times that of the straight tube. In the vertical flow direction, the heat transfer performance of twisted oval tube is poor, and the heat transfer coefficient of the horizontal twisted oval tube is about 1.26 ~ 1.68 times that of the vertical twisted oval tube. At high Re, the overall performance of the twisted oval tube with smaller twist pitch is better, and the highest overall performance is 1.05. Compared with the straight tube, the overall performance of the twisted oval tube is better.