Packed pebble beds occur in many industrial applications, including the very high temperature and molten salt nuclear reactor design concepts. These designs are currently being researched as possible fourth-generation nuclear power system designs. In order to ensure proper cooling of the reactor cores in these systems during normal operation, as well as under accident conditions, a detailed understanding of the coolant flow behavior is required. Direct numerical simulation (DNS) can be used to simulate specific pebble bed flow and geometry conditions in order to develop high-fidelity fluid flow data and hence improve scientists’ understanding and enhance lower-fidelity modeling.

We have used Nek5000, a spectral-element computational fluid dynamics code, to develop DNS fluid flow data for pebble bed flow, including budgets for the Reynolds stresses. The geometry is a structured pebble bed with a face-centered cubic packing arrangement. The flow domain features periodic boundaries in both streamwise and spanwise directions except for a single bounding wall parallel to the flow direction. In a fully periodic domain, the flow was found to be asymmetric. In this work we focus on turbulence properties in the near-wall region and their effect on the overall flow behavior. A set of preliminary Reynolds-averaged Navier-Stokes calculations was also performed to investigate the effect of geometric parameters such as the distance of the wall from the first row of pebbles.

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