Direct Numerical Simulation of the Flow in a Pebble Bed



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The flow in a tightly packed array of spheres is important to various engineering fields. In nuclear engineering applications, for instance, researchers have proposed core geometries of the pebble bed reactor (PBR) type cooled by gas or molten salt. Proper core cooling, both at operation and during accident conditions, is a key issue that must be addressed in any reactor design; and the limited amount of data available for the complicated geometry of PBR cores makes this task even more complex. A detailed understanding of coolant flow patterns and properties must be developed in order to meet safety requirements and ensure core longevity.

We addressed this issue by using the spectral-element computational fluid dynamics code Nek5000, developed at Argonne National Laboratory, to conduct both large eddy simulation (LES) and direct numerical simulation (DNS) of fluid flow through a single face-centered cubic sphere lattice with periodic boundary conditions. Multiple LES were conducted with varying Reynolds numbers in an effort to determine how the Reynolds number affects the development of asymmetries within the flow patterns. The DNS focused on the development of turbulence and were used to compute the turbulent kinetic energy budgets. A set of statistical analyses were also conducted to support the validity of the results.