The heat transfer techniques of supercritical CO₂ (S-CO₂) in small channels have been extensively used in nuclear reactor cooling, aerospace engine cooling and so on. To ensure the safety and high operation efficiency of the thermodynamic devices involved in the S-CO₂ Brayton cycle system, it is necessary to systematically study the flow and heat transfer characteristics of S-CO₂, and thoroughly analyze the abnormal heat transfer mechanism.

Through CFD numerical simulation, the heat transfer characteristics of S-CO₂ in a vertically upward tube with an internal diameter of 2 mm at different mass flow rates are studied. At the same time, under conditions of heat transfer enhancement and deterioration, the change of radial physical properties at different sections is studied, and the mechanism of abnormal heat transfer is analyzed in depth.

It is found that under conditions with mass flow rates of 500–1 000 kg/(m²·s), the heat transfer of S-CO₂ moves from heat transfer enhancement to heat transfer deterioration. In the heat transfer deterioration cases, the peak value of the heat transfer coefficient in the recovery region gradually decreases with the increase in heat flux, then finally disappears. As the operating pressure is far from the critical pressure, heat transfer enhancement and deterioration are both weakened. As shown by mechanism investigation, in heat transfer enhancement cases, the heat transfer of the fluid is stronger near the pseudo-critical region than in other regions, and the increased heat transfer coefficient comes from the increased portion of the cross-section area occupied by the fluid with large specific heat. In heat transfer deterioration cases, the velocity profile of the fluid at the cross-section where the wall temperature has its peak value presents an M-shaped curve, the velocity gradient at the inflection point of the velocity profile decreases to 0, the turbulent kinetic energy reaches its lowest level and heat transfer deterioration occurs.

The results of this study are significant for improving the heat transfer characteristics of S-CO₂ in small channels.