An improved semi-implicit numerical scheme for transient three-dimensional two-phase flow analysis

KAERI has developed the CUPID-I code for the analysis of transient two-phase flows in light water nuclear reactor components. In the CUPID-I code, a two-fluid three-field model is used and the governing equations are solved on unstructured grids. For the numerical solution scheme, the existing semi-implicit method of the RELAP5 code was used with some modifications for an application to non-staggered unstructured grids. This paper presents the recent improvements of the CUPID-I numerical scheme. In the previous scheme, the mass and energy equations were coupled with the momentum equations to obtain the pressure equation. The resulting pressure equation was an asymmetric matrix, which requires a long computation time. In the modified scheme, only the mass equations are implicitly coupled with the momentum equations and the energy equations are explicitly coupled. These modifications lead to a symmetric pressure equation, resulting in a fast running feature. In addition, a second-order upwind scheme was implemented for convection terms. The Green-Gauss reconstruction method suggested by Frink was modified for unstructured grids and the slope limiter by Barth and Jesperson were adapted in the CUPID-I code. To verify the numerical improvements, conceptual problems with a variety of two-phase flow conditions are solved. (author)