Dynamics Of Fine Particles During Fiber Filtration In Presence Of Electrostatic Field

The migration,agglomeration,clogging,deposition and some other processes of micro-particles under the effect of electrostatic field and fluid widely exist in both nature and industry.Taking the hybrid electrostatic precipitator used in the coal-fired power plants as an example,the micro-particles move toward and deposit on the collecting plates after being charged and polarized in the presence of an external electric field.After that some particles may re-fly into the gas,flow with the gas and finally get captured by the fibers together with the penetrating particles.Pursuing a further understanding of the underlying physical mechanism of particle behaviors and the correlation between the macroscopic particle motions and the microscopic particle-particle interactions,helps to improve the filtration performance of the related equipment.Previous studies on the filtration mechanism of microparticles in the presence of the electrostatic field still need to go further.The difficulty lies in the existence of complex long-range electrostatic/dipole interactions,fluid interactions and frequent contact interactions among microparticles.This difficulty results in a great challenge with the large span of calculation time scale in simulations and real-time observations in experiments.Based on the background above,this thesis focuses on the dynamics of adhesive microparticles under the effect of electrostatic/dipole interactions,including migration,agglomeration,clogging and deposition,using a multiscale discrete element method(DEM)coupled with computational fluid dynamics(CFD).Experiments are also conducted to observe the behavior of microparticles and the filtration performance of typical filters.Firstly,a two-fiber filtration system is employed so that the simple,underlying physical mechanism of clogging is revealed at a single pore level.By changing the work of adhesion,we study the effect of inter-particle adhesion on both macroscopic filtration characteristics and microscopic particle behaviors.A clogging phase diagram in the form of the Stokes number(St)and an adhesion parameter(Ad)is constructed.Decomposing the clogging time into chain growing time and the bridging time,we further reveal the underlying mechanism for particles to get clogged with different inter-particle adhesion.Secondly,a micro-meter particle motion observation platform is built up with a high-speed camera and micro-lens to continuously photograph the motion of microparticles.A kinetic model of single particle levitation is established.The mechanism of the formation of microparticle chains in the presence of a strong external electric field is then revealed.Based on that,the filtration performance of microparticles when polarized is simulated with the two-fiber filtration system.It is found that polarization helps to enhance the filtration performance.We further investigate the particle migration,agglomeration,clogging and formation of particle layers,and the effects of polarization on the whole filtration process are clearly presented.Furthermore,we consider the influence of both adhesion and polarization on the filtration of micoparticles,and the effect of dipole attraction under different adhesive forces on particle dynamics is elaborated.By comparing the filtration performance of charged and uncharged particles in the presence of polarization,we find that the dipole force has significant impact on the particle deposition structures.The mechanism of clogging of charged particles in the presence of dipole force is then summarized.Finally,using the self-built filtration experimental platform,we carry out a series of filtration experiments under different conditions with different types of filters.The particle filtration and deposition on filters are observed in complex external fields(e.g.particles are charged,polarized or in a humid environment).Combined with the measure of macroscopic filtration parameters and the observation of microscopic deposition morphology,we analyze the particle behaviors at each stage during filtration.The mechanisms of clogging revealed by DEM-CFD simulation are verified to some extent,and it will help to improve the design of fiber filters used in industry.