Agglomeration,Migration And Deposition Of Microparticles In Presence Of Electrostatic And Flow Fields

Agglomeration,migration,and deposition of microparticles ubiquitously exist in nature and industrial processes,including aerosol aggloemratation in turbulence,migration of charged particles toward the collecting plate in an electrostatic precipitator and deposition of particles on fibers during filtration.An in-depth understanding of the relationship between microscopic interparticle interactions and the collective behavior of a large number of particles would be helpful to understand and further design large-scale devices.However,the picture of the transport of microparticles is far from complete,the difficulty lies in the complicated interacting modes between particles,namely the electrostatic interaction,the hydrodynamic interaction,and the contacting interactions,across several orders of magnitude in time scales.This work proposes new approaches for modeling contacting interactions and electrostatic interactions between particles in the framework of discrete element methods and presents a systematic investigation on the agglomeration,migration and deposition of microparticles in presence of electrostatic and flow fields.Firstly,we propose a Fast DEM based on the reduced particle Young's modulus in the framework of Johnson-Kendall-Roberts(JKR)-based contact theory.It allows one to resolve the collision between particles with a much larger time step.A novel inversion method is then presented to help users to quickly determine the input parameters in Fast DEM.After validating this inversion method,we apply the Fast DEM to packing problems of microparticles.The computational time is significantly reduced and measures of packing structures are in good agreement with results simulated using the original value of particle properties.We then carry out direct numerical simulation together with Fast DEM to investigate the agglomeration of particles in homogeneous isotropic turbulence.We report an exponential-form scaling for the size distribution of early-stage agglomerates.An agglomeration kernel is then constructed containing the information of agglomerate structures and the sticking probability.An explicit relationship between the sticking probability and microscale particle properties is also proposed.Our results extend Smoluchowski's theory to the condition of non-coalescing solid adhesive particles and can reproduce DNS-DEM results with a simple one-dimensional PBE model.The evolution of spherical clouds of charged particles that migrate under the action of a uniform external electrostatic field is then investigated.Our results reveal that,with sufficiently strong repulsion,the cloud undergoes a universal self-similar expansion.Scaling laws of cloud radius and particle number density are obtained by solving a continuum convection equation.Finally,we present an elaborate investigation of the deposition of charged particles on a flat plane and on fibers.A dimensionless adhesion parameter is constructed to predict the structure of deposits.The temporal evolution of the deposit structure,particle capture efficiency,and the pressure drop are displayed with varying values of Coulomb repulsion and adhesion magnitudes.Our results,together with previous results on the filtration of single fiber and the packing of neutral particles,form a more complete picture of filtration and deposition of microparticles.