A collective dynamics-based method for initial Pebble packing in pebble flow simulation

In the simulation of pebble flow in Pebble-Bed Reactors (PBR), high-fidelity methods, such as Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) methods, are usually employed to simulate the dynamic process of the pebble recirculation. Such simulation normally takes extraordinarily long time to reach the dynamic equilibrium state, i.e. a state that the pebble distribution is statistically steady. However, if an initial dense packing of pebbles can be provided, which is close to the realistic packing at the equilibrium state and can be easily implemented without much computational effort, then the time consuming high-fidelity simulation can be considerably more efficient and take much less time to reach dynamic equilibrium state. In this paper, a collective dynamics-based method is developed to generate an initial pebble packing for further high-fidelity flow simulations. In the new method, pebbles are packed by a fast sequential generation process allowing overlaps, and an efficient overlap elimination process based on a simplified normal contact force model. The latter process provides an adaptive and efficient mechanism to eliminate the overlaps accounting for different overlapping depth and pebble size, thus speeding up the initialization without loss of reliability. Comparisons with other existing random packing methods have shown that the developed method not only exhibits unique significance and good computation efficiency in speeding up the pebble flow simulation, but also presents desirable potential in other applications as a general packing algorithm for packing uniform- or variable-size spheres in a large container. (author)