Low resolution modelling of mixing phenomena in PWR fuel assemblies

Abstract

Simulation of single-phase turbulent flow in the entire reactor core is a challenging task due to a complex geometry and a huge computational domain, which requires large number of computational cells. In order to reduce the size of a computational mesh, simplifications in the geometry are practically inevitable. Among the most challenging parts with many small details that affect the flow through a fuel assembly are mixing grids. There are several different designs of the mixing grids. One of the most frequently used designs consists of a strap, springs and dimples, which are used as a structural support for the fuel rods, and mixing vanes, which deflect the fluid and increase the mixing. Neglecting the effects of the mixing grid would result in a less accurate reproduction of the flow and temperature field, which may lead to too small inter sub-channel mixing in the fuel assembly. Hence, application of a porosity model is proposed for the mixing grid, which largely reduces the computational mesh. Additional momentum sources mimic the effect of the mixing vanes. This approach aims to fill the gap between three-dimensional well-resolved Computational Fluid Dynamics (CFD) and one-dimensional sub-channel or system codes. As such, they are able to provide valuable insights to the flow behaviour in the reactor core.The present study examines the possibility to model the effect of the mixing grid by applying momentum sources in the governing equations. A split-type mixing grid is considered, which is one of the most frequently used designs in a Pressurized Water Reactor (PWR) fuel assembly. The applied split-type mixing grid generates a particular pattern of swirl motion within each sub-channel as well as it increases the cross flow between neighbour sub-channels of the fuel bundle. Non-homogeneous momentum sources have been applied, which mimic the deflection of the flow and blockage by the mixing vanes. The obtained pattern of the secondary flow closely resembles the secondary flow, which has been obtained with a reference CFD simulation that included all detailed geometrical aspects of the grid spacer with mixing vanes. The magnitudes of the momentum sources have been tuned to generate similar magnitudes of the secondary flow and stream-wise vorticity as observed in the results of the reference CFD simulation. In the next step, this approach has been applied on successively coarser meshes to investigate its effectivity on low resolution (very coarse) meshes. It turned out that the proposed model is able to mimic the mixing effect between the sub-channels. However, discrepancies are observed in the development of the secondary flow in the stream-wise direction. In the last step, the coolant flow in a flow domain consisting of 9 fuel assemblies is reproduced for normal conditions inside a PWR core as well as for two off-normal scenarios, which consider two different blockages at the entrance of the central fuel assembly. This approach has a promising potential to perform a CFD simulation of a realistic fluid mixing inside and between adjacent fuel assemblies in an entire PWR reactor core.