Numerical-simulation for gas entrainment from free surface (2). Fine mesh simulation of free surface vortex with gas core

The large-scale sodium-cooled fast breeder reactor, examined in the feasibility studies of the next generation reactors, must be prevented from the gas entrainment in the upper plenum of reactor vessel. However, the gas entrainment due to a free surface vortex has not been enough investigated to establish a prediction method for the onset of the gas entrainment, because the gas entrainment phenomena highly depend on the system configuration. From the viewpoints of the costs and the difficulties of measurements, full-scale experiments may not be the best way to evaluate the gas entrainment phenomena. Recently, numerical simulations can be considered to be promising ways as substitutions of the full-scale experiments owing to the progress of computer systems and numerical methods. However, there are few simulation results that reproduce the gas entrainment phenomena accurately. Therefore, it is necessary to validate the applicability of simulation to the gas entrainment phenomena. In the past few years, we have calculated the free surface vortex in a cylindrical tank as an example of steady vortices. In this research, the applicability of simulation is discussed through parameters surveys on the simulation accuracy. The simulations were conducted on both steady and unsteady vortices. The results showed that the numerical simulation had enough capability to calculate the free surface vortex in the steady state with high accuracy, when the fine mesh system was applied to the region of the vortex center. In addition, the numerical simulations for the vortex development, the unstable gas core oscillation and the breaking process of the gas core made it clear that the numerical simulation method could reproduce unsteady behaviors. The gas core oscillation was also considered theoretically and it was deduced that the gas core oscillation was caused by the Kelvin-Helmholtz instability on the gas core surface. (author)