LES analysis of the flow in a simplified PWR assembly with mixing grid

Abstract

The flow in fuel assemblies of PWRs with mixing grids has been analyzed with CFD calculations by numerous authors. The comparisons between calculation and experiment are focused on the flow in the near wake of the mixing grid, i.e. on the flow in the first 10 to 20 hydraulic diameters (dh) downstream of the grid. In the study presented here, the comparison between the measurements in the AGATE facility (5x5 tube bundle) and Trio_U calculations is done for the whole distance between two successive mixing grids that is up to 0.6m downstream of the grid. The AGATE experiments have originally not been designed for CFD validation but to characterize different types of mixing grids. Nevertheless, the quality of the experimental data allows the quantitative comparison between measurement and calculation. The conclusions of the comparison are summarized below: Linear turbulent viscosity models seem to work rather well as long as the cross flow velocity in the rod gaps is advection controlled, that is directly downstream of the mixing grid, Further downstream, when the cross flow velocity is reduced and isotropic turbulence becomes a more and more important mixing phenomena, linear viscosity models will fail, The mixing grid affects the cross flow velocity up to the successive grid at a distance of about 50dh. The flow in fuel assemblies is never similar to that in undisturbed rod bundles. The test section of the AGATE facility has been discretized on 300 million control volumes by using a staggered grid approach on tetrahedral meshes. 20 days of CPU on 4600 nodes of the HPC machine CURIE of the CCRT was necessary to calculate the statistics of the turbulent flow, in particular the mean velocity and the RMS of the turbulent fluctuations.