Numerical study on heat transfer of SCW near the pseudo-critical temperature in a hexagon sub-channel

A numerical study was carried out to study the local heat transfer performance of supercritical water (SCW) within a hexagon bare inner sub-channel by using the commercial CFD code CFX. The main objective was to focus on the special thermal-hydraulic characteristics near the pseudo-critical temperature (PCT) at different working conditions. The results show that the secondary flow induced in the inner sub-channel is weak in comparison with the axial main flow, and becomes weaker near the PCT. A stagnation point of the secondary flow is created right at the narrow gap, which can partially explain the weak heat transfer near the narrow gap region. The nonuniform geometry of the sub-channel dominates the creation of a secondary flow in the subchannel near the PCT. The fluid temperature very close to the cladding surface dominates the heat transfer enhancement of SCW within the sub-channel. The maximum values of the local heat transfer coefficient appear when the PCT is within the buffer layer of that angular position, indicating that the buffer layer is directly related to the heat transfer enhancement. The system parameters evidently affect the heat transfer performance, and the corresponding local heat transfer performance varies differently in the circumferential direction.