Recent Improvements to the Multi-Dimensional Semi-Implicit Two-Phase Flow Solver, CUPID

Abstract

A two-phase analysis code, CUPID, has been developed for a realistic simulation of transient two-phase flows in light water nuclear reactor components. In the CUPID code, a two-fluid three-field model is adopted and the governing equations are solved on unstructured grids for flow analyses in complicated geometries. For the numerical solution scheme, the semi-implicit method of the RELAP5 code, which has been proved to be stable and accurate for most practical applications, was used with some modifications for an application to unstructured non-staggered grids. In the present study, constitutive relations of the two-fluid model were employed including models for an interfacial drag force, interfacial heat transfer, interfacial area, wall heat partition and interfacial non-drag forces. The inter-phase surface topology concept which was proposed for CFD-BWR was applied in order for the present numerical solver to be applicable not only to dispersed flows but also to a flow with a sharp interface. The current paper presents some preliminary calculation results which have been performed for the verification of the inter-phase surface topology. This paper is also concerned with the effects of interpolation schemes on the simulation of two-phase flows. So as to assure a high numerical accuracy, the second-order upwind scheme is implemented into the CUPID code. The calculation results with the second-order upwind scheme were compared with those with the first-order upwind difference. For the comparison, a single-phase laminar flow and a phase separation were simulated. The comparison results among the two interpolation schemes and the exact solutions apparently showed a reduced numerical diffusion with the second order scheme.