Modeling particle settling behavior using the discrete element method

The discrete element model DMX was used to simulate the settling behavior of a unique particle in an assembly of other particles in a hopper filling process. No fluid effects or cohesive forces were modeled so the results are applicable for non-cohesive particles with diameters greater than about 1 mm where air effects can be neglected. No vibration or other such external load was included. The settling behavior of the unique particle was investigated in terms of the density ratio and the diameter ratio, and of varying conditions of chronological entry position, initial assembly velocity and system friction. The model showed satisfactory agreement with several observations reported regarding this behavior. The model demonstrated that at a high density ratio ‘sinking’ is dominant irrespective of the size of the particle, initial position or initial velocity, whereas a balance between particle size and density determine whether a particle sinks or rises at other densities. Based on this, a preliminary phase diagram was derived showing when a particle will sink or rise or do neither (termed ‘neutral’) as a function of particle size and diameter ratios. The model also showed that the initial position has only a qualifying effect on the behavior of the unique particle and the initial velocity is only significant at low-density ratios where with higher velocities the lighter particle will rise more. At lower friction these effects are enhanced. Repeating some simulation runs on larger binary systems showed that these effects could cause some segregation during settling.