Numerical treatment of pebble contact in the flow and heat transfer analysis of a pebble bed reactor core

This paper studies the numerical treatment of the inter-pebble regions in the modeling of a packed bed geometry for the computational fluid dynamics (CFD) analysis of a pebble bed reactor (PBR) core, where the pebbles are physically in contact with each other. In some studies, the inter-pebble regions have been approximated with gaps, in consideration of the problems on mesh quality or economy of the CFD calculation. To examine such a methodology, a sensitivity analysis for the gap size was conducted with two spherical pebbles, where the inter-pebble region was modeled by means of two kinds of inter-pebble gap and two kinds of direct contact. The cases of direct contact showed numerous differences in the results of the flow regime around the pebbles as well as in the wake, compared to the cases of the inter-pebble gap. No large differences were found between the two cases of direct contact. Based on the result of the sensitivity analysis, the two cases of inter-pebble modeling, i.e., the 1-mm gap and area-contact, were applied to the PBR simulation. It was concluded that the flow regimes and their relevant flow-induced local heat transfer were significantly dependent on the modeling of the inter-pebble region.