Numerical investigation and modeling of two-phase flow sweeping in rod bundles with mixing vane grid spacers

Grid spacers with mixing vanes are used in nuclear fuel assemblies in order to increase the lateral cross-flows between sub-channels that homogenize the temperature distribution within the fuel rod bundle. For the thermal-hydraulic design of such fuel bundles, sub-channel analysis codes are used in order to predict the mass and temperature distribution. These analysis codes compute sub-channel averaged values and contain empirical correlations that account for the effects of microscopic flow phenomena, such as wall friction, turbulent mixing or heat transfer. Experimental investigations show a strong swirling flow in the vicinity of a mixing vane grid spacer. No model was found in literature that accounts for the swirling flow effect on the lateral cross-exchange of mass, momentum or energy. Systematic CFD investigations of the flow in a 5×5 rod bundle containing a single grid spacer with split-type mixing vanes were performed with single-phase and two-phase flow conditions with void fractions ranging between αg=0%and15%. A cubic non-linear k-ε model was used to simulate the typical rod bundle flow effects, such as secondary flow and non-isotropic turbulence. A detailed analysis of the forces and flows in the gap showed that mixing vane induced swirling flow in the sub-channels strongly affects the inter-sub-channel cross-flow. A new model was developed, based on the CFD simulation results, that predicts the flow sweeping between the sub-channels. This model was implemented into the sub-channel analysis code COBRA-FLX™ that was developed and is used by AREVA GmbH. The new model significantly improves the predictions for the cross-flow and mass flow distribution within the rod bundle in the vicinity of the mixing vane grid spacer.