Modeling of spacer grid mixing effects through mixing vane crossflow model in subchannel analysis

Abstract

Spacer grids, especially those with mixing vanes or other mixing devices in the rod bundle, have great influence on local flow fields in the reactor core. This mixing effect has been demonstrated by both experimental and numerical simulations, however, the spacer grids’ effect on flow field is not well reflected in most subchannel codes that are currently used as main instruments in calculating local conditions and safety analysis for the nuclear industry. This paper focuses on improvement of the mixing vane crossflow model in subchannel analysis for simulating the mixing effects of spacer grids. The distributed resistance method (DRM) is applied, and the source terms associated with the spacer grids are added to momentum equations to account for the mixing vane grids’ influence on local flow fields in rod bundles (U.S. patent, application number: 61841961, confirmation number: 8224; U.S. patent, application number: 62375023, confirmation number: 2306; Chinese patent, application number: 201611179395.1). The improved DRM model is used to analyze the thermal hydraulic performance in the 5×5 rod bundle. The calculation results are compared with experimental data and CFD simulation results respectively. The subchannel calculation results are also compared with those from the conventional code without the DRM model. The flow characteristics, such as the crossflow direction, the crossflow values at grid zone, and the decay trend of the crossflow downstream of the mixing vane grid are analyzed. The improved mixing vane crossflow model is assessed by comparison analysis, and some conclusions are obtained.