A DEM-CFD study of the effects of size distributions and packing fractions of pebbles on purge gas flow through pebble beds

In a solid-type ceramic breeding blanket, the functional materials, such as the breeder, multiplier, and reflector, are equipped in the form of pebble beds, while helium purge gas flows through pebble beds. The flow characteristics of purge gas need to be investigated in order to design and evaluate the performance of the blanket system, which depends on the configuration of pebble beds. In the current study, various pebble bed models were constructed according to the discrete uniform distribution and normal distribution of pebble sizes, and the packing fractions of the pebble beds were controlled using the discrete element method. Computational fluid dynamics simulations for the laminar flow of helium purge gas through pebble beds were then conducted. In particular, the effects of size distributions and packing fractions of pebble beds on the flow resistance were investigated in terms of pressure drop. Through a series of numerical analyses, it was found that the pressure drop increases not only, obviously, in proportion to the packing fraction of pebbles, but also in inverse proportion to the difference in pebble size. Since the pressure drop is related to the surface areas of pebbles, a smaller difference in the pebble size leads to a higher pressure drop due to a larger surface effect at the same volume of pebbles. The numerical results were also validated by comparing them to the semi-empirical formulae derived for the prediction of pressure drop in packed beds, such as the Ergun equation and the Kozeny-Carman equation. The current study will provide a basis for resolving the practical issues on blanket system design by extending it further to the purge gas flow and conjugate heat transfer that involve fragmentation and resettlement of pebbles.