Mechanism of the particle collision and adhesion in flow field vortex using CFD-DEM(XDLVO) simulation

Abstract

The vortex characteristics of the flow field in the coagulation equipment are one of the key factors affecting the agglomeration efficiency of fine particles such as coal slurry and water. To analyze the impact of vortex flow fields on the collision and adhesion of particles, in this study, the vortex street generated by the flow around a cylinder was chosen as a representative vortex flow field. Computational Fluid Dynamics (CFD) was used to simulate the influence of flow velocity and cylinder diameter on the strength and scale of the vortex in the flow field. Additionally, the Extended Derjaguin-Landau-Verwey-Overbeek (XDLVO) theory was introduced to describe the interaction forces between particles in the Discrete Element Method (DEM), and the CFD-DEM simulations of the collision and adhesion processes of particles with diameters ranging from 25−100 μm in the aforementioned flow field were conducted, followed by experimental validation. The results indicated that within the range of flow velocities of 0.06−0.12 m/s, cylinder diameters of 2−6 mm, and Reynolds numbers of 120−720, the relationship between the vortex radius (r) and the cylinder diameter (D) in the cylinder flow field was approximately r=0.133 3D+0.421 4, but with no significant correlation with the flow velocity. The maximum vorticity at the vortex center was directly proportional to the flow velocity, and the vortex intensity was positively correlated with the square of cylinder diameter. The characteristic parameters of the adhesive aggregates obtained from the CFD-DEM (XDLVO) simulation method were in good agreement with the experimental results, indicating its ability to accurately describe the collision and adhesion processes of particles in the cylinder flow field. Analysis of the vortex distribution characteristics in the flow field and the distribution of aggregates in the flow field zones revealed that the vortices in the flow field enriched the particles away from the vortex center towards the viscous shear zone through inertial centrifugal force. This altered the direction of particle motion and increased the local concentration of particles in the viscous zone, thereby enhancing the probability of particle collisions and effectively promoting particle adhesion and agglomeration. When the vortex scale was approximately 10 times the particle size, it was most conducive to particle collision and adhesion. Therefore, the design of the flow-around cylinder diameter can be optimized based on the particle size of the application scenario to achieve the best adhesion effect.

Keywords

• collision and adhesion
• xdlvo
• cfd-dem
• flow around a cylinder
• vortices