Study on CFD Approach for Gas Entrainment Phenomenon; Evaluation of Applicability of Finite Element Flow Analysis

Eguchi, Yuzuru*

The report is concerned with the evaluation of applicability of numerical modelling methods for the prediction of gas entrainment in an upper plenum of a sodium-cooled fast breeder reactor (FBR). Special attention was paid to applicability of variational multiscale (VMS) modelling in the context of the Finite Element Method. Two flow problems, which were experimentally shown to induce gas entrainment, are solved by a VMS code (MISTRAL). First, computing a benchmark problem of a gas entrainment swirl flow in a cylindrical vessel has led to the following results;(1) the VMS solution is able to resolve the precise vortex core structure more accurately than the non-VMS solution. The circumferential velocity obtained from VMS computation rises almost double in comparison with the non-VMS solution, though it still underestimates the experimental values.(2) the half-value radius of the negative region of the second invariant of velocity gradient matches well between the VMS and the non-VMS solutions.(3) the negative/positive boundary of the second invariant obtained from the VMS solution is closer to the vortex core radius observed in the experiment than that of the non-VMS solution, though the vortex dip length computed from the VMS result is shorter than the experimental value. Second, computing a benchmark problem of open channel flow with a square pillar and downstream suction pipe has led to the following results;(4) 2Dx-type spatial oscillation was observed due to lack of mesh subdivisions.(5) the distributional profile of the second invariant is similar to that of the first problem, characterized by a strong negative region surrounded by a weak positive region. As a possible future plan, it may be necessary to more precisely analyze the features of unsteady vortices obtained in the second benchmark problem and to identify the difference from the steady vortex of the first benchmark problem.