Numerical-simulation for gas entrainment from free surface (I); Calculation of free surface vortex in the cylinder

Ito, Kei; Sakai, Takaaki; Yamaguchi, Akira

Design study is made on a large-scale sodium-cooled fast breeder reactor in the feasibility studies of the next generation reactors. In this design, coolant velocity in the upper plenum of reactor vessel is higher than conventional designs. Therefore special attention should be paid to gas entrainment behavior at the free surface. Gas entrainment due to a free surface vortex is particularly important, because it has downward velocity near the vortex center. The velocity is large enough to carry a gas bubble to the primary circuit, which connects to the core region. In this research, a free surface vortex test, conducted in Central Research Institute of Electric Power Industry, was numerically simulated, for the purpose of establishing an evaluation method of gas entrainment. VOF (Volume of Fluid) model of PLIC (Piecewise Linear Interface Calculation) type and CSF (Continuum Surface Force) model for surface tension are used to consider the free surface dynamics. As a result, it is noted that calculation accuracy of the vortex greatly depends on the mesh size in the analysis model. Required accuracy was obtained when the mesh size is 1/10 of the vortex scale length (i.e., radius where the tangential velocity reaches the maximum). Effect of surface tension was also investigated. It was cleared that higher surface tension coefficient tend to decrease the gas core dent of the vortex. In addition, 1 upwind scheme and 2 upwind scheme was compared. It is concluded that the effect of numerical diffusivity is significant near the vortex center.