Understanding the Flow Anisotropic Turbulent Flow in the Subchannel of Fuel Assembly Under Effect of the Bare Rod and Mixing Vane Grid by LES

Abstract The grid has the functions of supporting fuel rod bundles and mixing the coolant. The flow of the coolant through the mixing vanes increases its perturbation, which directly affects the Critical Heat Flux Density (CHF) of the reactor core, enhances the wall heat transfer performance, and improves the safety and economy of the reactor. The geometric asymmetry of the sub-channels in the Pressurized Water Reactor (PWR) fuel assemblies leads to anisotropy in the flow, and accurate prediction of the flow mixing characteristics in the rod bundle channels plays a significant role in the prediction of the onset of nucleate boiling and CHF. Traditional RANS turbulence models struggle to accurately predict the anisotropic turbulent flow in rod bundle channels. This paper, based on LES simulation calculations, investigates the anisotropic flow characteristics in 2 × 1 and 3 × 3 bare rod channels, comparing the effects of different sizes of rod bundle geometries on flow anisotropy. On this basis, the study examines the anisotropic turbulent flow in rod bundle channels under the influence of a 3 × 3 grid. By conducting Large Eddy Simulations (LES) on the flow field of a simple 3 × 3 rod bundle sub-channel, this paper analyzes the impact of grid mixing vane angle variations on the flow anisotropy between nuclear fuel rods. The results show that changes in the grid mixing vane angles can produce varying degrees of flow field perturbations. The study of the evolution of turbulent anisotropy between rods under the coupled effect of rod bundle channels and grids, and flow field characteristics, lays the foundation for the construction of URANS models that can accurately reflect anisotropic effects, and also establishes a basis for the design of high-performance fuel assembly grids.