Predicting reverse flow characteristics in equal-height-difference passive heat removal system for ocean nuclear power

Abstract

The equal-height-difference passive heat removal system (EHDPHRS) via natural circulation driving and utilizes ocean as infinite heat sink for heat dissipation, showing broad application prospect in ocean nuclear power field; but the heat removal pipe normally encounters the reverse flow of cool seawater into the pipe, leading to operation instability and even condensation-induced water hammer. In this study, the integration of volume of fluid model and condensation model based on surface renewal theory is used to simulate the reverse flow characteristics in the heat removal pipe under single-and two-phase natural circulation conditions. The interfacial heat and mass transfer are calculated by user-defined functions (UDF). The numerical results reveal that the buoyancy force (Fb) and condensation driving force (FΔP) both accelerate the reverse flow, while the inertial force (Fi) restrains the reverse flow; the reverse flow length presents monotonous decrease with increasing the flow velocity under single-phase conditions and parabolic-like trend for two-phase flow. A prediction correlation for the reverse flow length is developed using dimensionless Reynolds number, Grashof number and vapor volume fraction, covering single- and two-phase conditions. The predicted reverse flow length by the correlation agrees with 94% of the results within deviations of ±15%.