The highly turbulent flow circulating between the rods inside a Pressurised Water Reactor (PWR) core creates turbulence induced vibrations. To get a better understanding of the flow characteristics, the French Alternative and Atomic Energy Commission (CEA) had built and operated CALIFS 5x5-test section. This mock up composed of a 5-by-5 rod bundle allowed velocity measurements using laser imaging techniques downstream an analytical spacer grid with and without mixing vanes. The CALIFS experiments and other experiments, revealed turbulence anisotropy downstream of spacer grids. The current study aims to assess the impact of spacer grid geometry on the turbulence anisotropy in a rod bundle using fine scale CFD simulation. The study focuses on the characterisation of the anisotropic behaviour of the flow in a rod bundle downstream simplified mixing grids inspired from the CALIFS 5x5 design. Wall-Resolved Large Eddy Simulations (WRLES) are used at a reduced Reynolds number of 14 000. The computation domain is limited to four subchannels around a central rod. Two simulations are performed, using different spacer grids designs (with and without mixing vanes). The results confirm mixing grid design strongly influences the overall turbulence anisotropy. In the case without mixing vanes, the impact of the grid on turbulence anisotropy seems to vanish 10 Hydraulic diameter (Dh) downstream of the grid. In the case with mixing vanes, the anisotropy remains in the subchannels up to 15 Dh downstream of the grid. In both cases, the turbulence anisotropy in the rod vicinity is strongly impacted close to the grid.
Charton et al. (Fri,) studied this question.
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