A detailed CFD analysis was performed to investigate the effect of mixing vanes on single-phase flow distribution in a 5×5 rod bundle simulating PWR operating conditions. The simulation model was 1 m long and contains one split-type mixing vane grid. For the inlet, a fully developed velocity profile from a previous rod-bundle simulation was used. The simulation case was isothermal to avoid a superposition with wall boiling effects. For the simulation of turbulence effects a non-linear k-ε model was used. This model is able to reproduce the swirling flow in the wake of the grid spacer as well as secondary flow patterns that typically occur in sub-channel flows and needs less computational time compared to Reynolds Stress models. The results show that two large vortices are formed at the mixing vane tips in the center of an interior sub-channel. These vortices are co-rotating within the sub-channel and counter-rotating compared to the adjacent sub-channels. The presence of these vortices influence the radial pressure distribution and the cross-flows significantly. A new model approach was developed, which accounts for the effects of mixing vanes and the vortex structures. The new model requires additional input parameters that can be derived from the CFD results. The proposed model was implemented into the sub-channel analysis tool COBRA-FLX™. A comparison between the old and the new approaches for modeling the forced cross-flow shows a significant improvement of prediction of the mass flux distribution.

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