Numerical Analysis of Heat Transfer Characteristics of Supercritical Nitrogen in Horizontal Circular Tubes Available to Purchase

Supercritical nitrogen (SCN₂) has an important role in thermal equipment such as liquid air energy storage (LAES) due to its low-temperature inertness, good compressibility, richer resources, and easier-to-reach critical state, and its critical parameters are relatively lower than those of other supercritical fluids. This study focuses on the heat transfer mechanism of supercritical nitrogen in a horizontal circular tube, reveals the law of nitrogen heat transfer under different operating parameter conditions, and creates two new heat transfer correlations based on the buoyancy effect classification, with the maximum errors of 10% and 20%, respectively. The results show that increasing the mass flux and decreasing the heat flux suppress the occurrence of heat transfer deterioration (HTD), which leads to the improvement of supercritical nitrogen heat transfer; the strongest buoyancy effect at the inlet is the main reason for heat transfer deterioration occurring at the inlet. Under the condition of low mass flux, buoyancy plays a dominant role in supercritical nitrogen heat transfer, and increasing the mass flux and decreasing the heat flux can effectively mitigate the heat transfer deterioration caused by buoyancy effect dominance. When the mass flux is larger than 1590 kg/(m² · s), the effect of the buoyancy effect on the fluid heat transfer is negligible, and the mass flux at the buoyancy-driven transition point in the range of the heat flux (q = 40–60 kW/m²) is predicted. A theoretical reference is provided for the design optimization of a liquid air energy storage system.