Discrete Element Methods Of Fine Particle Dynamics In Presence Of Van Der Waals And Electrostatic Forces

Fundamental of fine particles plays an important role in either basic naturalphenomena or various industrial processes. Both van der Waals (vdW) adhesion forceand electrostatic force are the main factors affecting those processes as well as thecountermeasures to control fine particles from becoming hazards. Soft-sphere discreteelement method (DEM), considering inter-particle interactions as its basis and beingable to reveal the details of multi-particle dynamics, has been recognized one of themost powerful methods for fine particulate system. However, in contrast to its mature infield of granular solids, the development and application of DEM into fine adhesiveparticles encounters a broad array of challenges. In this work, we firstly develop andbuild the mathematical models, computational methods and experimental validation ofsoft-sphere DEM in the presence of van der Waals and electrostatic forces, and thenapply it to solve problems of two prototypical particulate systems: the transport andremoval of fine particle in an oscillating electric field, and the deposition of fineparticles on a single fiber.The applicability of different adhesion models for fine particle contact is firstlydiscussed and the corresponding quantitative adhesion map is presented. Based on theJKR model that is valid for static/quasi-static contact of fine particles, the energydissipation mechanisms including the first-contact energy loss and dissipative energyloss corresponding to the attractive and repulsive force components in the contact areaare developed to establish a complete dynamic model for impact process. Compared toprevious work, the current model provides a clearer physical basis. Next, boundaryelement method (BEM) is incorporated into current DEM platform to solve for theelectric field of macroscopic bodies (like fiber or sphere collectors) in presence ofsuspension of fine particles. The additional challenges due to the small scale of fineparticles are found for the traditional BEM. By utilizing the image theory ofelectrostatics and local subdivision of boundary elements, the solution precision andcomputational speed for this fine particulate system have been greatly improved. Then amultipole expansion method is particularly incorporated and combined with BEM toenable high-efficiency calculation of the electrostatic interactions between a large number of particles. On basis of these achievements, a general DEM platform has beenfinally constructed to accurately and effectively predict the fine particle flow systemswith both vdW and electrostatic forces.Secondly, by adopting the above DEM, the particle transport on traveling-waveand standing-wave electric curtains is studied. The effects of various factors, includingthe adhesion force and electrostatic forces between particles, as well as the ACfrequency, have been investigated. The transport mechanisms of particles onstanding-wave electric curtain are also investigated. Three different modes of trappedmotion and two modes for effective transport are found, and the correspondingnondimensional criteria are proposed. From the aspect of detailed DEM simulations,this work has clearly revealed the mechanism of effective particle transportation withstanding-wave electric curtain, which was believed to be, at least theoretically,impossible. Experimental studies, on the other hand, are also conducted and differentcharacteristic modes of particle motion are observed. Comparative studies on the basisof particle dynamics are carried out and the mechanisms behind above phenomena arerevealed.Finally, we originally apply DEM to investigate the effects of adhesion force andelectrostatic interactions on the deposition of fine particles on a single fiber that is aclassic prototypical system for particle filtration. A microscopic view on the depositionof neutral, polarized or charged particles is built, in which some comprehensiveconclusions are drawn as:(1) the deposition of small and big micron particles with a bigsize ratio exhibits synergistic deposition;(2) the pre-polarization of fine particlesenhances the deposition by nearly an order of magnitude;(3) the charging of particlesslightly increases the deposition at the early period, but finally inhibits the depositionbecause of the repulsion between incident particles with deposited particles.