Kaiwei Chu

Discrete particle simulation of particle–fluid flow: model formulations and their applicability

The approach of combining computational fluid dynamics (CFD) for continuum fluid and the discrete element method (DEM) for discrete particles has been increasingly used to study the fundamentals of coupled particle–fluid flows. Different CFD–DEM models have been used. However, the origin and the applicability of these models are not clearly understood. In this paper, the origin of different model formulations is discussed first. It shows that, in connection with the continuum approach, three sets of formulations exist in the CFD–DEM approach: an original format set I, and subsequent derivations of set II and set III, respectively, corresponding to the so-called model A and model B in the literature. A comparison and the applicability of the three models are assessed theoretically and then verified from the study of three representative particle–fluid flow systems: fluidization, pneumatic conveying and hydrocyclones. It is demonstrated that sets I and II are essentially the same, with small differences resulting from different mathematical or numerical treatments of a few terms in the original equation. Set III is however a simplified version of set I. The testing cases show that all the three models are applicable to gas fluidization and, to a large extent, pneumatic conveying. However, the application of set III is conditional, as demonstrated in the case of hydrocyclones. Strictly speaking, set III is only valid when fluid flow is steady and uniform. Set II and, in particular, set I, which is somehow forgotten in the literature, are recommended for the future CFD–DEM modelling of complex particle–fluid flow.

Modelling the Multiphase Flow in Dense Medium Cyclones

Dense medium cyclone (DMC) is widely used in mineral industry to separate solids by density. It is simple in design but the flow pattern within it is complex due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. Recently, the so-called combined computational fluid dynamics (CFD) and discrete element method (DEM) (CFD-DEM) was extended from two-phase flow to model the flow in DMCs at the University of New South Wales (UNSW). In the CFD-DEM model, the flow of coal particles is modelled by DEM and that of medium flow by CFD, allowing consideration of medium-coal mutual interaction and particleparticle collisions. In the DEM model, Newtons laws of motion are applied to individual particles, and in the CFD model the local-averaged Navier-Stokes equations combined with the volume of fluid (VOF) and mixture multiphase flow models are solved. The application to the DMC studies requires intensive computational effort. Therefore, various simplified versio...

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.

Effect of cohesive force on the formation of a sandpile

This paper presents a numerical study on the piling processes of mono-sized wet particles by the discrete element method (DEM). The capillary force between particles due to liquid bridge is implemented in an existing DEM model. The effects of moisture content on the repose angle and structure of a pile are studied by a series of controlled numerical experiments. It is confirmed that the structure of a pile is similar to that of a packing for cohesive particles. Moreover, the averaged local porosity and repose angle have similar changes with the moisture content and can be linearly correlated. Therefore, the relationship between the repose angle and the cohesive force can be established based on the previous correlation between the porosity and the force ratio of the cohesive force to gravity developed in the packing of cohesive particles.

Particle scale modelling of the multiphase flow in a dense medium cyclone: Effect of medium-to-coal ratio

The effect of solids loading ratio or the medium-to-coal (M:C) ratio is the most important operational parameter of the Dense Medium Cyclones (DMC) that are widely used in the coal industry to upgrade the run-of-mine coal by separating gangue from product coal. However, its effect is still not well understood so far, since the flow pattern within a DMC is complicated due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. Recently, it is shown that the particle-laden flow in a DMC can be modelled by the so-called combined Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM) in which the flow of coal particles is modelled by DEM which applies Newton’s laws of motion to individual particles and that of medium flow by the conventional CFD which solves the local-averaged Navier-Stokes equations, allowing consideration of particle-fluid mutual interaction and particle-particle collisions. In this work, the effect of medium-to-coal (M:C) ratio is studied by a two-way coupling CFD-DEM approach for a large diameter DMC. The flow structure, and particle-particle and particle-fluid forces are analysed to understand the fundamentals governing this effect. The results suggest that the solids volume fraction of 20% (or M:C ratio of 4 by volume) is a critical point for the DMC performance under the conditions considered.The effect of solids loading ratio or the medium-to-coal (M:C) ratio is the most important operational parameter of the Dense Medium Cyclones (DMC) that are widely used in the coal industry to upgrade the run-of-mine coal by separating gangue from product coal. However, its effect is still not well understood so far, since the flow pattern within a DMC is complicated due to the size and density distributions of the feed and process medium solids, and the turbulent vortex formed. Recently, it is shown that the particle-laden flow in a DMC can be modelled by the so-called combined Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM) (CFD-DEM) in which the flow of coal particles is modelled by DEM which applies Newton’s laws of motion to individual particles and that of medium flow by the conventional CFD which solves the local-averaged Navier-Stokes equations, allowing consideration of particle-fluid mutual interaction and particle-particle collisions. In this work, the effect of medium-to-c...