【作者】 陈晟；

【导师】 李水清；

【作者基本信息】 清华大学 ， 动力工程及工程热物理， 2019， 博士

【摘要】 微米颗粒在静电场与流场作用下的团聚、迁移与沉积过程广泛存在于自然界与工业生产中,例如细颗粒物的湍流团聚、静电除尘器内细颗粒向收尘极板的迁移、布袋过滤中细颗粒在纤维表面的沉积等。深入理解上述过程中颗粒间微观相互作用与宏观集群行为之间的关联,有助于高效除尘设备的开发、污染物控制与处理以及功能材料的合成等。而当前对于微米颗粒输运规律的理解仍有局限,其挑战在于颗粒间存在极其复杂的长程静电、流体相互作用以及短程接触相互作用,同时流场计算、颗粒运动与颗粒碰撞过程的时间尺度跨度大。基于上述背景,本文在粘附性颗粒离散动力学理论框架内,提出碰撞过程模拟与静电力模化的新方法,并对细颗粒在流场、静电场作用下的团聚、迁移与沉积过程开展了系统性的研究。本文首先针对现有粘附性颗粒离散动力学算法计算量大、参数选取规则不明晰的问题,提出了基于JKR接触理论的粘附性颗粒离散动力学加速算法(Fast DEM),利用约化弹性模量的方法增大计算颗粒碰撞过程的时间步长,大幅度降低计算量,并结合实验数据制定了参数选取规则。进一步利用标准的微米颗粒沉积过程进行算法检验,分析了弹性模量约化比例系数对于沉积结构、力分布、重叠量以及计算机时的影响。其次,将Fast DEM与直接数值计算(DNS)相结合,研究了微米颗粒在均匀各向同性湍流场内的团聚过程,建立了颗粒团聚体粒径分布演化与流场参数、颗粒性质之间的关联。提出了包含颗粒团聚体结构信息与颗粒碰撞后粘附概率的团聚核函数,利用该核函数可将群平衡方法拓展至粘附性微米颗粒系统。最后通过颗粒碰撞方程的无量纲分析,建立粘附概率模化公式。进而,探究了荷电微米颗粒群在不同电场强度下迁移行为及形状演化规律。重点关注多体效应(即颗粒间库伦及流体相互作用)对颗粒群迁移速度与形状演化的影响机制。针对强荷电条件下稳定演化的颗粒群,提出描述颗粒群浓度场演化的连续介质简化模型,求解出荷电颗粒群电迁移速度的解析表达式。最后,针对荷电颗粒在极板平面以及滤料纤维表面的沉积过程,提出了决定细颗粒物沉积结构的控制参数。建立了纤维滤料堵塞过程中沉积结构、颗粒过滤效率、压降的动态变化与微米颗粒荷电量、粘附性强弱的之间关联规律。该部分工作结合已有的单纤维过滤及中性颗粒沉积机理,共同形成微米颗粒过滤与沉积动力学理论基础。更多还原

【Abstract】 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.更多还原