Dem Study Of Packing And Connectivity Of Binary-Sized Pebbles According To Their Size And Mixing Ratios Under Vibration Conditions

In the breeding zone of solid-type blankets, the packing configurations of pebble beds significantly affect the efficiency of thermal energy transfer to the coolant and tritium recovery by purge gas. Binary-sized pebbles with significant size difference can be used to increase the number of contacts between them, thereby increasing the efficiency of heat transfer in the breeding zone. In this study, we simulate a packing process of mono- and binarysized pebble beds using the discrete element method. Vibrations at a constant frequency are imposed to enhance the connectivity of pebbles, and periodic boundary conditions are applied to mimic the dynamic behavior of a large number of pebbles while reducing computational burden. Subsequently, the final packing fractions of the pebble beds are analyzed according to the size and volume ratios of binary-sized pebbles as well as the vibration frequency and direction of pebble beds. The results reveal that a more significant difference in pebble size generally leads to a higher packing fraction; however, this trend is governed by the volume mixing ratio of binary-sized pebbles and the vibration conditions. For binary-sized pebble beds, the vibration frequency yields no significant change in the packing fractions but causes pebble segregation because of the significant difference in pebble size. Furthermore, we discover that the vertical vibration of a bottom wall can reduce the segregation more effectively than the horizontal vibration of lateral walls. By contrast, the packing of mono-sized pebbles is extremely sensitive to the vibration frequency. Moreover, the pebble connectivity is investigated in terms of the effective thermal conductivity calculated considering heat conduction through the contact area between the pebbles. The effective thermal conductivity of binary-sized pebble beds decreases as the pebble size difference decreases and the vibration frequency increases. For mono-sized pebble beds, the effective thermal conductivity can be extremely high or low depending on the vibration.