Particle scale modelling of the multiphase flow in a dense medium cyclone: Effect of near gravity material

Dense medium cyclone (DMC) is widely used to upgrade the run-of-mine coal in the coal industry. It is known that the amount of near gravity material (NGM) fed into a DMC is an important parameter since it may cause problems such as vortex finder/spigot overloading, surging phenomenon, and system instability. Until now, the underlying mechanism of this phenomenon is not well understood. Here, this phenomenon is studied numerically using a previously developed method of combined computational fluid dynamics and discrete element method (CFD-DEM), facilitated by a parcel-particle model to account for fine particles. The simulated results are analyzed for fundamental understanding, in terms of medium and coal flow patterns, particle-fluid, particle-particle and particle-wall interaction forces. It is found that the amount of NGM has a significant effect on the stability inside a DMC. When there are excessive NGM fed into a DMC, the solid concentration below the vortex finder increases drastically, resulting in high local tangential particle-fluid and particleparticle interaction forces. Correspondingly, the pressure drop is high there, and so is the pressure gradient force. This unstable flow structure has been identified as a cause of the vortex finder overloading phenomenon in the DMC operation.