Effects of density difference and loading ratio on pebble flow in a three-dimensional two-region-designed pebble bed

Highlights: • Pebble flow in two-region reactor is studied on effects of density & loading ratio. • Pebble has invariable central boundary & stable discharge ratio (=loading ratio). • L-H-L makes central region reduced, stagnant zone larger & total retention time longer. • H-L-H slows middle-pebble flow, enlarges central area & increases side-pebble density. • H-L-H leads to shorter total retention time and smaller stagnant zone. - Abstract: The pebble flow of a two-region-designed dynamic reactor core is simulated by discrete element method. The aims are to verify the feasibility of the two-region-designed reactor and explore the influence of loading ratio and pebble density on flow pattern. Results show that after a period of recirculation flow, the pebble bed can reach equilibrium states with invariable central boundary and stable discharging number ratio of pebbles in the middle to the side regions, which is consistent with the loading ratio of them. The mixing region at different heights and the dispersion of pebbles are analyzed. The mixing zone between the two regions is constrained within reasonable and acceptable ranges. The loading ratio has no influence on the retention rate. But it could significantly affect the two-region configuration, i.e. a larger loading ratio corresponds to a larger central region. Besides, both the shape of the central region and the stagnant zone could be affected by pebble density. Compared to the single-density condition, increasing the middle pebble density (the L-H-L condition) can accelerate the pebble flow and reduce the size of central region. Meanwhile the stagnant zone may be larger and the total retention time may be longer, which are not beneficial to the core safety. On the other hand, although the flow of middle pebbles may be much slower and the central area size may be larger by increasing the side pebble density (the H-L-H condition), all pebbles will flow out of the bed in shorter time, leading to smaller stagnant zone and shorter total retention time. Finally, the vertical flow can be greatly affected by pebble density distribution, and the axial velocity profiles show different patterns between the bottom and the upper part of the packed bed.