CFD modelling of mixing vane spacer grids for ALLEGRO relevant gas cooled reactor fuel geometry

Abstract

The main objective of this paper is to present a newly developed CFD modelling method, suitable for the simulation of Mixing Vane spacer Grids (MVGs) that are designed for Gas Cooled Fast Reactors (GFRs). In order to prove the reliability of the model, a thorough validation is necessary. However, the available experimental data concerning the fluid dynamics of the ALLEGRO reactor, is limited to bare subchannel models.CFD models have been developed with a domain, that contains one spacer grid with mixing vanes. The mesh sensitivity study of ALLEGRO MVG CFD models was extended to transient calculations. Five different mesh densities were used/applied to get mesh independent results. The key parameters were crossflow factor, pressure drop, and heat transfer coefficient enhanced by the MVGs.We investigated the performance of different mixing vane types in gas cooled fuel rod bundles. The chosen vanes were of type split, advanced split, swirl and twisted mixing vanes. The different types were ranked by performance.According to our research, the twisted type mixing vane is the most efficient vane type for the ALLEGRO assembly. Calculations were performed in search of the optimal geometry in which the vane angle was modified between 15° and 40°.With the optimised twisted vanes, a full active length ALLEGRO fuel assembly CFD model was created, and the performance of different MVG arrangements was compared in three different CFD models. All the calculations were carried out using ANSYS CFX 17 code.