Study on Convective Heat Transfer Characteristics of Supercritical Liquid Hydrogen in a U-Type Tube inside a Moderator

The flow and heat transfer characteristics of supercritical fluid in a U-tube have an important influence on the safe operation of a moderator, and the variation of gravity direction is suitable for special working conditions of the moderator. In this study, the three-dimensional turbulence flow and heat transfers of supercritical liquid hydrogen in a U-tube were investigated at an Re number ranging from 16,425 to 54,750 under constant heat flux (q = 80 kW/m2). The total length of the U-tube was 1725 mm, which had an entrance length L/D of 23, with the inner diameter and wall thickness of D × δ = 10 × 2 mm. The finite volume method was adopted, and the grid independence was verified by the grid convergence index (GCI). The calculation results of three turbulence models (SST k-w, RNG k-ε, Standard k-ε) were compared with the corresponding experimental data to obtain the turbulence model with the smallest error. The convective heat transfer characteristics with different values of heat flux (q = 30 kW/m2~100 kW/m2), mass flow (G = 3 g/s~10 g/s), and gravity (gx, gy, gz) were compared. Meanwhile, the heat transfer characteristics of supercritical liquid and conventional liquid hydrogen were compared. The results show that Nu increased from 5 g/s to 10 g/s by 56.6%, and mass flow rate had a greater impact on the variation of Nu; when gravity direction was consistent with the flow direction of liquid hydrogen (gx direction), the Nu number inside the channel was 4.21% and 5.56% higher than that in gy and gz direction, respectively. Supercritical liquid hydrogen has a stronger heat transfer ability than conventional liquid hydrogen, of which the Nu number is 16.7% higher. This study can provide useful guidance for the design of flow and heat transfer of supercritical liquid hydrogen in a U-tube and its application in moderators. Furthermore, it provides reference technical values for thermal safety and thermal management of the target station to ensure its safe and stable operation.