Investigation of heat transfer characteristics of carbon dioxide transcritical flow in a reactor channel

Abstract

The fourth generation of supercritical carbon dioxide (S-CO₂) cooled advanced reactor has the characteristics of compact volume and high efficiency, and is widely used in space reactors and small reactors. For the purpose of the heat transfer characteristics of S-CO₂ under transcritical conditions, the effects of changing inlet velocity, inlet CO₂ fluid temperature, CO₂ fluid operating pressure and wall heat flux on h and Nu in a single cylinder without gravity were analyzed by numerical simulation. The influence of different inlet flow rates on transcritical conditions under horizontal tube gravity is analyzed, and the temperature and velocity distributions of normal transcritical conditions and heat transfer deterioration conditions are compared. The consequence show that the rise of wall heat flux and the decrease of pressure will make the critical position move towards the inlet end. Reducing the inlet rate and increasing the inlet temperature will also shift the critical position towards the inlet end and result the heat transfer to worsen at the inlet end. In the horizontal pipe, when the heat transfer deteriorates, the CO₂ fluid temperature differences between the top and lower parts exceeds 20 K, and the coefficient of heat transfer at underside is 2.5 times that at the top. When the deterioration degree of heat transfer deepens, the difference of heat transfer coefficients between the upper and lower parts of the channel will also increase due to ascent the temperature difference. Finally, the heat transfer deterioration can be attributed to the dual effects of the near-wall CO₂ fluid criticality and the increase in the CO₂ fluid mainstream thermal conductivity.