URANS simulations of the single-phase mixing in a 4*4 LWR fuel bundle with mixing vanes, validated against a sub-channel experiment

Abstract

A relatively uniform coolant temperature profile at the outlet of reactor core, resulted from good mixing behavior inside fuel assemblies, is desirable for better safety and economic performance. In order to enhance mixing between adjacent sub-channels, various kinds of mixing vanes have been designed and attached to the edge of spacer grids. Both experimental and computational investigations have been ongoing on this topic for decades. However due to the complexity of the geometrical configuration of the spacer grids and mixing vanes, the flow field in the rod bundle involves complicated mechanism, and is not yet fully understood by researchers.A sub-channel mixing test facility SUBFLOW was designed by PSI jointly with ETH Zurich. The test section is an adiabatic 4*4 rod bundle at atmospheric pressure. Both single-phase and two-phase mixing experiments were carried out and valuable data has been obtained. This paper presents the numerical prediction of the single-phase mixing at a Reynolds number of 28000. Unsteady Reynolds Averaged Navier-Stokes (URANS) simulations were performed on a combination of hexahedral and trimmed meshes. A passive scalar source was introduced as an indicator of the mixing behavior. Various turbulence models have been studied. Lateral mixing of passive scalar, velocity profiles and swirl behavior along the channel have been investigated. Fluctuations induced by the mixing vanes were analyzed. The results showed that among the tested methods only low-Reynolds number model v2f with unsteady solver was capable of obtaining quantitatively correct lateral mixing rate and swirl decay rate along the channel, and capturing the fluctuations of the flow field with qualitative agreement with experimental data.