CFD modelling of turbulent flows through 5 × 5 fuel rod bundles with spacer-grids

• CFD post-test analysis of MATiS-H 5-by-5 rod bundles benchmark with split spacers.
• Sensitivity to domain length, mesh density and turbulence models were investigated.
• Simulations were assessed against MATiS-H experiments performed at KAERI.
• Calculated turbulence intensity was dependent on the turbulence model used.
• Good agreement with axial velocity was achieved.

In this investigation, post benchmark evaluation of the OECD/NEA-KAERI MATiS-H benchmark was performed to predict the turbulence intensities as well as velocity variation downstream of the split-vane spacer grid. A three-dimensional Computational Fluid Dynamics (CFD) tool (STAR-CCM+) was used to model the single phase flow through a cold 5-by-5 rod bundle with a split-vane spacer grid. The effects of the domain length upstream of the split-vane spacer grid and the mesh resolution at the split-vane spacer grid regions were assessed for their impact on the predicted attenuation of turbulence intensity downstream of the spacers. Polyhedrons were used to discretize the computational domain, along with prismatic cells near the walls, with an overall mesh count that spanned from 25 M to 64 M cell volumes. The Realizable k–ε, k–ω SST and Reynolds stress models were tested to assess their capability in predicting the turbulence intensities as well as the velocities measured in the experiment. The calculated turbulence intensities were found to be dependent on the choice of the turbulence model used, the mesh density on the spacers as well as on the domain length upstream of the spacers. The CFD simulations underpredicted the measured turbulence intensity at the downstream region of the split-vane spacer grid. The predicted velocities, however, were in good agreement with the measurements downstream of spacers.