Numerical simulation of pebble bed flows using high-fidelity methods

Pebble bed based nuclear reactor concepts are currently being developed for deployment in small modular reactors in the coming decades. To aid in the verification of turbulence models and develop an understating of the detailed dynamics of flows through domains of close packed spheres, we present high-fidelity numerical results for a number of incompressible and isothermal direct numerical simulations carried out using the Nek5000 spectral element computational fluid dynamics code. The presented cases include flows over single and multiple spheres in free-stream conditions at low Reynolds numbers. Flow over single spheres represent a classical problem in fluid dynamics research, thus we used this as base calculation and increased the complexity of the flow by adding additional spheres in close proximity to investigate the effects of this increased complexity on the formation of persistent flow recirculation regions, and near wake effects. Results of calculations carried in domains composed of single and multiple regularly packed spherical unit cell geometries are also presented. These idealized domains conform to packing fractions encountered in practical reactor designs. Results include engineering parameters of interest such as first and second order flow statistics. Finally, results are presented for a wall-bounded, regularly packed domain, with a focus on the near wall region. The effects of the near-wall spheres on velocity and Reynolds stress distributions on the wall surface are investigated.