静电场与流场作用下细颗粒团聚、迁移与沉积动力学（英文版）

本书入选“清华大学优秀博士学位论文丛书”系列,结合国家能源与环境重大需求，从多相流离散相动力学出发，进行基础理论研究。研究内容涵盖表面物理、静电学与流体力学等多学科知识，涉及国际流体力学界 particle-laden flow领域前沿与难点问题；研究成果不仅对细颗粒生成与控制具有重要指导意义，同时对大气云滴生成动力学、行星形成初期理论以及地表风沙运动科学具有重要借鉴意义。

本书以静电场、流场等复杂多场作用下细颗粒团聚、迁移与沉积行为为研究对象，发展了粘附性微米颗粒接触相互作用及长程相互作用的快速算法(Fast DEM)，并将该算法与直接数值模拟结合，揭示了微米颗粒在湍流场内的碰撞与团聚机理，构建了湍流团聚核函数；进一步结合Oseen 动力学算法，给出了荷电颗粒群电迁移率及形状演化与荷电强度、流体惯性之间的关联规律；最后，针对细颗粒在极板及纤维表面的沉积过程，明晰了沉积体结构及过滤效率与颗粒微观特性之间的关系。

陈晟，华中科技大学能源与动力学院副研究员。2019年毕业于清华大学，获动力工程及工程热物理博士学位，获2019年度清华大学优秀博士学位论文。研究方向包括燃烧污染物控制、多场耦合作用下颗粒动力学理论、太阳能光热利用与储能技术开发。在J. Fluid Mech.、Phys. Rev. E、Phys. Rev. Fluids、Chem. Eng. Sci. 等国际知名期刊上发表SCI论文13篇，授权发明专利1项。参与国家重点研发计划项目1项、973项目1项、国家自然科学基金1项。

1 Introduction 1
1.1 Adhesive Particle Flow 1
1.2 Example Systems 2
1.3 Collision and Agglomeration of Particles in Turbulence 4
1.4 Migration of Microparticles in an Electrostatic Field 6
1.5 Deposition of Microparticles and Clogging Phenomenon 9
1.6 Discrete Element Methods for Adhesive Particles 12
1.7 A Road Map to Chaps. 2–6 13
References 14

2 A Fast Discrete Element Method for Adhesive Particles 17
2.1 Introduction 17
2.2 Discrete Element Method for Adhesive Particles 18
2.3 Critical Sticking Velocity for Two Colliding Particles 20
2.3.1 Temporal Evolution of the Collision Process 22
2.3.2 Prediction of the Critical Sticking Velocity 25
2.3.3 Effect of Particle Size 29
2.4 A Fast Adhesive DEM 31
2.4.1 Accelerating Adhesive DEM Using Reduced Stiffness 31
2.4.2 Modi?ed Models for Rolling and Sliding Resistances 34
2.5 Determination of Parameters in Adhesive DEM 36
2.5.1 An Inversion Procedure to Set Parameters in Adhesive DEM 36
2.5.2 Comparison Between Experimental and DEM Results 39
2.6 Test on Packing Problem 40
2.6.1 Packing Fraction and Coordination Number 43
2.6.2 Local Structure of Packings 45
2.6.3 Interparticle Overlaps and Normal Forces 46
2.7 Summary 48
References 49

3 Agglomeration of Microparticles in Homogenous Isotropic Turbulence 51
3.1 Introduction 51
3.2 Methods 52
3.2.1 Fluid Phase Calculation 52
3.2.2 Equation of Motion for Solid Particles 53
3.2.3 Multiple-time Step Framework 54
3.2.4 Simulation Conditions 55
3.2.5 Identi?cation of Collision, Rebound and Breakage Events 57
3.2.6 Smoluchowski’s Theory 59
3.3 Collision Rate, Agglomerate Size and Structure 60
3.4 Effect of Stokes Number 62
3.5 Exponential Scaling of Early-Stage Agglomerate Size 62
3.6 Agglomeration Kernel and Population Balance Modelling 64
3.7 Effect of Adhesion on Agglomeration 65
3.8 Effect of Adhesion on Breakage of Agglomerates 68
3.9 Formulation of the Breakage Rate 68
3.10 Agglomerate Size Dependence of the Breakage Rate 76
3.11 Role of Flow Structure 76
3.12 Conclusions 78
References 79

4 Migration of Cloud of Low-Reynolds-Number Particles with Coulombic and Hydrodynamic Interactions 81
4.1 Introduction 81
4.2 Formulation of Problem 81
4.3 Effect of Coulomb Repulsion on Cloud Shape 84
4.3.1 Cloud Shape 84
4.3.2 Effect of Fluid Inertia 87
4.3.3 Stability of the Cloud 88
4.4 Evolution of Particle Cloud Under Strong Repulsion 91
4.4.1 Scaling Analysis and Continuum Description 91
4.4.2 Prediction of Cloud Size and Migrating Velocity 93
4.4.3 Discussion 97
4.5 Summary 98
References 99

5 Deposition of Microparticles with Coulomb Repulsion 101
5.1 Introduction 101
5.2 Models and Methods 102
5.2.1 Simulation Conditions 102
5.2.2 Forces on Particles 103
5.2.3 Average-Field Calculation for Coulomb Interactions
in 2D Periodic System 103
5.3 Effects of Coulomb Interaction on Packing Structure 106
5.4 Scaling Analysis of the Interparticle Force 109
5.5 Governing Parameters for the Packing Structure 112
5.6 Phase Diagram 115
5.7 Summary 117
References 118

6 Deposition of Charged Micro-Particles on Fibers: Clogging Problem 119
6.1 Introduction 119
6.2 Models and Method 120
6.2.1 Simulation Conditions: Two Fiber System 120
6.2.2 Gas Phase Simulation 121
6.2.3 Solid-Phase: Discrete-Element Method (DEM) 122
6.2.4 Governing Parameters 123
6.3 Clogging/Non-clogging Transition 124
6.4 Measurement of Particle Capture Ef?ciency 127
6.4.1 Repulsion Effect: The Critical State 128
6.4.2 Structure Effect 130
6.5 Summary 133
References 134

7 Conclusions and Perspective 135
7.1 Conclusions 135
7.2 Future Work 137
References 138

Adhesive particle ?ow arises in many applications in industry, nature, and life sciences and has driven great research interests in areas of aerosol ?ltration, dust mitigation, nanoparticle deposition, ceramics manufacturing, fouling of MEMS devices, sediment transport, and production of fuel cells. 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, linking the microscopic properties of discrete particles to the macroscopic behaviors of particle ?ow systems is never a simple task. The dif?culty lies in the complicated interacting modes between particles, namely the electrostatic interaction, the hydrodynamic interaction, and the contact interactions, across several orders of magnitude in time and length scales.
　Within the past few decades, the discrete element method (DEM), in which the motion, collision, and adhesion of individual particles are resolved in time and space, has been developed to model particle collective dynamics from single- particle level. DEM coupled with computational ?uid dynamics (i.e., CFD-DEM) has shown powerful capabilities in investigating particle-laden ?ows. Moreover, there has recently been rapid progress on understanding the physics related to the intermolec- ular and surface forces, which enable us to develop more rational adhesive contact models. Scalable and ef?cient computational frameworks have also been proposed for handling long-range many-body interactions and for collision resolution. It is recognized that merging the expertise across various disciplines of ?uid and solid mechanics, condensed matter physics, materials science, and applied mathematics will signi?cantly improve our understanding of particle dynamics in electrostatic and ?ow ?elds.
　The objective of this thesis is to propose new approaches for modeling contacting interactions and electrostatic interactions between microparticles in the framework of discrete element methods and to present an insightful view on the agglomeration, migration, and deposition of microparticles in electrostatic and ?ow ?elds. The ?rst chapter discusses various applications of adhesive particle ?ows. Chapter 2 starts with a simple case of binary collisions of adhesive particles to show how the discrete element method gives the information on the force, the displacement, and the energy conversion. A novel fast DEM based on the reduced particle Young’s modulus is then proposed to accelerate the computation. In Chap. 3, the fast DEM is coupled with direct numerical simulation to investigate the agglomeration of particles in homoge- neous isotropic turbulence. The structure and the size distribution of agglomerates are obtained. The agglomeration and collision-induced breakage rates are formu- lated based on the classic theory for particle collisions in turbulence. In Chap. 4, the evolution of spherical clouds of charged particles that migrate in a uniform external electrostatic ?eld is then investigated by Oseen dynamics and a continuum approach, and the scaling laws for evolution of cloud radius and particle number density are derived. Finally, in Chaps. 5 and 6, an elaborate investigation of the deposition of charged particles on a ?at plane and ?bers is presented. The ?ndings, together with previous results for neutral particles, form a more complete picture of ?ltration and deposition of microparticles.
　I believe that the results in this book will substantially impact the ?eld relevant to adhesive particle ?ows. Beyond that, the ?ndings here may also have broader implications for granular ?uidization, liquid–solid suspensions, and colloidal gels, where complicated particle–particle interactions exist.

Beijing, China January 2021

Prof. Shuiqing Li