The study of the performance of an electrostatic valve used forbulk transport of particulate materials

In many mass transfer processes, it is necessary to accurately control the flow of particulate materials. Commonly used mechanical valves have serious drawbacks which can be overcome by the use of electric field, which can locally originate interparticle compressive forces throughout the bulk material as a result of the greatly enhanced electric field and charge flux densities occurring at the contact points between the particles or between the particles and the boundary. Such interparticle electroclamping forces can be established by applying an electric potential gradient between a separated pair of conductive electrode grids placed perpendicularly across the flow within the duct where the material flows. The flow control of particulate materials is, thus, achieved using no moving parts. When an electric field is applied to a packed bed of particulate solids, several types of electrical force (electrostatic attractive force, dielectrophoretic force, and electroclamping force) may be generated, depending on the bulk and surface resistivities of the particle, the geometry of the electrodes, as well as the nature of the applied field. The influence of the electrode geometry on flow control was investigated using computer modeling of the potential based on finite element techniques. Furthermore, the effect of the applied field with respect to the magnitude, frequency, pulsewidth, and pulse shape on flow controllability was experimentally investigated. The influence of the moisture content of turnip seeds on flow controllability and specific charge was investigated, and the results obtained are discussed in this paper     

Slope Stability of Electropacked Beds

The mechanics of packed or fluidized beds of semi- insulating particles are substantially altered by strong applied electric fields. The angle of repose of a loosely packed bed of glass beads increases with the applied electric field; a sufficiently intense field will freeze the bed. An extensive series of angle of repose measurements are reported which suggest three distinct regimes of critical slope equilibria: infinite slope equilibrium for low electric field intensities; finite slope equilibrium for somewhat stronger fields, and the frozen bed limit for strong electric fields. The failure of an infinite slope equilibrium is characterized by individual particle motions confined to a thin layer near the surface. This regime is amenable to analysis, and the electrically induced cohesion can be inferred from the experimental data. The results are compared to Dietz's published expression for the cohesive electrical force acting between two contacting spherical particles. SEM photographs of the particles used experimentally provide evidence that surface asperities and very fine particles may be more important than previously suspected.     

Numerical modeling of inhaled charged aerosol deposition in human airways

A new numerical modeling of inhaled charge aerosol has been developed based on a modified Weibel's model. Both the velocity profiles (slug and parabolic flows) and the particle distributions (uniform and parabolic distributions) have been considered. Inhaled particles are modeled as a dilute dispersed phase flow in which the particle motion is controlled by fluid force and external forces acting on particles. This numerical study extends the previous numerical studies by considering both space- and image-charge forces. Because of the complex computation of interacting forces due to space-charge effect, the particle-mesh (PM) method is selected to calculate these forces. In the PM technique, the charges of all particles are assigned to the space-charge field mesh, for calculating charge density. The Poisson's equation of the electrostatic potential is then solved, and the electrostatic force acting on individual particle is interpolated. It is assumed that there is no effect of humidity on charged particles. The results show that many significant factors also affect the deposition, such as the volume of particle cloud, the velocity profile and the particle distribution. This study allows a better understanding of electrostatic mechanism of aerosol transport and deposition in human airways.     

循环流化床内稠密气固两相流动的数值模拟

颗粒团聚是稠密气固两相流动中的一个重要现象，该文定义了颗粒团聚合力的概念来表征稠密气固两相流动中颗粒所受到的团聚效应，对单个颗粒进行了全受力分析，得到了聚合力的线形模型表达式。采用聚合力的线形模型，将两相流场分为稀相区和浓相颗粒团，将颗粒团视为整体离散相，文中数值模拟了循环流化床内的稠密气固两相流动，得到了床内颗粒团分布、颗粒团大小、床内空隙率、气相速度、颗粒相速度的详细分布，揭示了循环流化床内稠密气固两相流场的规律，以及循环流化床内两相流场的核心—环形流动结构。计算结果与前人实验结果相符并表明，采用该模型及其算法模拟循环流化床内稠密气固两相流动是可行的。     

循环流化床锅炉炉内颗粒分布平衡模型

在对循环流化床锅炉炉内颗粒特性分析的基础上,提出同时以颗径和密度两参数来描述炉内颗粒特性。结合循环流化床锅炉特殊的炉内流体动力特性,建立了包括炉膛密相区和稀相区在内的循环流化床锅炉炉内宽筛分的颗粒分布模型,其中密相区假设为浓度分布均匀的湍流床,而稀相区则为和核心-边壁区的流动结构。模型同时耦合炉内颗粒所经历的爆裂、燃烧、磨损及气固分离等物理和化学过程。应用以所建的颗粒分布平衡模型为子模型的循环流化床唤炉总体数学模型模拟了一台12MW循环流化床锅炉燃用烟煤时满负荷运行的工况。计算时把炉内颗粒分为70档,其中颗粒粒径在0～8mm之间分为10档,密度在1100～2400kg/m∧3之间分为7档,模拟计算所得的炉内颗粒分布合理正确,与试验研究研究结果吻合良好,表明所建立的颗粒分布模型可以用来描述循环流化床锅炉炉内颗粒分布特性。     

The effect of electric field strength on the induction charge of freely levitating particles

Many industrial processes such as electrostatic separation, fluidization, and coating rely upon induction charging of fine particles. This paper considers the effects of electric field strength on the magnitude of the induction charge on freely levitating particles. The charging time and charge on a freely levitating particle depend on a number of properties, mainly the electric field strength, particle size, density, and resistivity. A charging model showing the dependence upon the electric field strength is presented and analyzed, along with a model of the levitation process. A high-speed digital imaging system was used to measure individual particle motion during levitation. Using these data along with the developed models, it was possible to determine the charge on the particle. Semiconductive particles with a mass mean diameter (MMD) of 156 /spl mu/m were used in these experiments and tested at electric fields of 6.8, 8.5, 15, and 21 kV/cm, respectively. In addition, some experiments using particles 97-/spl mu/m and 412-/spl mu/m MMD at an electric field of 15 kV/cm were carried out to confirm the results obtained for the 156-/spl mu/m particles. It was found that the particle charge was dependent upon both the charging time and electric field strength. In particular, for high electric fields the particle did not achieve its saturation charge before liftoff occurred. This shows that higher electric field strength is not necessarily the optimum condition for levitation of semiconductive particles.     

Viscous flow model for charged particle trajectories around asingle square fiber in an electric field

The trajectories and deposition efficiency of aerosol particles on a single rectangular fiber are studied in this paper. It is assumed that the particles are electrically charged and an external electric field is applied. A conductive, infinitely long fiber can be rotated by some angle with respect to the air flow. A single aerosol particle moves under the influence of the inertial, air drag, and electrical forces. The air-flow distribution is determined by solving the Navier-Stokes equation assuming laminar flow. The vorticity-stream function approximation is used and solved by means of the finite-element method. Results of numerical simulation for the particle trajectories and deposition efficiency are shown for different Reynolds, Stokes, and Coulomb numbers     

On the dielectrophoretic separation of solid particles

An analysis of an idealized high-gradient dielectrophoretic separator that is simply a metal circular filament placed in a uniform electric field between two parallel plates is presented. The analysis is aimed at determining the spatial distribution of the electric field inside the volume of the separator, so that the dielectrophoretic force on a dielectric particle can be evaluated. The conditions necessary for producing dielectrophoretic forces comparable to those in electrostatic separators are determined     

Modeling of conductive particle behavior in insulating fluidsaffected by DC electric fields

Several electrostatic technologies, such as separation of granular mixtures, flocking, printing, or biological cell manipulation, are based on the accurate control of conductive particle motion in insulating gases or liquids by means of relatively high DC electric fields. This paper is aimed at characterizing the behavior of such particles by numerical modeling of two aspects: (1) particle motion under the action of electric field forces and (2) insulation breakdown triggered by mobile particles. The equations of particle motion were written by taking into account both gravitational and drag forces, as well as the rebound at particle impact with the electrodes. If the particles move in ionized air, their charge varies in time. In that case, the equation of particle charge should be added to the mathematical model. The output data of the programs for numerical simulation of particle behavior are in good agreement with the available experimental results. Particle movements were shown to be influenced by the intensity of the electric field, by the density of the space charge, by size and mass density of the particles, as well as by their coefficient of restitution at impact with the electrodes. The conclusions regarding the behavior of conductive particles in insulating fluids are useful for the development of improved electrostatic separation technologies; they are of particular interest to all manufacturers of high-voltage equipment     

Adhesion of charged powders to a metal surface in the powdercoating process

Substrate-particle adhesion of electrostatically charged, nonconducting particles deposited on electrically grounded substrates is discussed. Glass microspheres of diameters ranging from 25.5-74.1 μm, charged by corona and tribe-charging, were deposited in a monolayer on conducting stannic-oxide coated surfaces of glass plates (NESA). The total force of adhesion due to electrostatic, van der Waals, and gravitational forces was measured by observing the removal of particles by applying a known electric field between the particle coated surface and a clean surface of a second NESA glass, placed parallel to it at a distance of 0.013 m. The adhesive force was measured as a function of particle size and charge. The net average charge on the particles was measured using a Faraday cup. The experimental values agree well with the calculated force of adhesion for a single layer deposition. The charge decay of the particles was studied using a noncontact electrostatic voltmeter. The charge relaxation time of the deposited powder was found to increase with time. A physical model of the adhesion of charged powder paints deposited on a grounded metal substrate is presented. The role of the forces acting on a spherical polymer particle deposited on the surface of a uniform coating of powder paint is investigated as a function of particle diameter and charge. The particles are assumed to be unipolarly charged and deposited uniformly on the substrate. The relative magnitudes of the electrostatic attractive and repulsive forces are analyzed as functions of powder film thickness and particle size     

GIL内金属微粒在直流电压下的运动特性分析

气体绝缘金属封闭输电线路(gas-insulated metal-enclosed transmission line,GIL)以其输送容量大、占地面积小、传输损耗小等优点,在许多场合是替代电缆和架空线路的首选方案。但GIL内部的金属微粒会在电场的影响下发生起跳和运动,严重威胁了GIL的绝缘性能。为更好地捕获金属微粒,掌握GIL内金属微粒的受力和运动特性是十分有必要的。本文首先忽略了盆式绝缘子对GIL轴向场强的影响,分析了金属微粒在同轴圆柱间的受力和运动特性,使用金属微粒谐振频率表征金属微粒在直流电压下的活跃度,并分析了电压和微粒半径对金属微粒谐振频率的影响规律。然后考虑了盆式绝缘子对GIL轴向场强的影响,得到了金属微粒的几种典型的运动轨迹。最后给出了对于金属微粒陷阱布置的建议,认为在盆式绝缘子凸面侧下方布置金属微粒陷阱是必要的。     

Induction charging of granular materials in an electric field

It is well known that particles can be freely levitated in an electric field due to the charge induced on the particles by the external field. The charge depends upon the electric field strength and particle properties. This paper studies some of these factors to investigate the fundamentals of induction charging for granular materials. An experimental apparatus was set up to collect the levitated particles in a filter contained in a Faraday pail and the charge-to-mass (Q/M) ratio was obtained based on the charge and mass measurements for the samples in the filter. Furthermore, the particle size distribution was measured and analyzed by laser diffraction and microscopy and the surface mean diameter (D/sub s/) and volume mean diameter (D/sub v/) were obtained. In these experiments irregular shaped Al/sub 2/O/sub 3/ particles and spherical glass beads with a size range of 42-390 /spl mu/m were used and tested at different electric field strengths. By combining the results of Q/M, D/sub s/ and D/sub v/, the charge per particle was calculated and the results compared with theoretical values. It was confirmed that the particle charge is dependent upon the electric field strength and the particle properties of size, shape, density, resistivity, and adhesive force.     

Mathematical modeling of powder paint particle trajectories inelectrostatic painting

The trajectories of charged powder particles in an electrostatic powder coating system were modeled considering electrical and fluid forces. The mathematical model employed an iterative technique wherein the charge simulation method was used to compute the electric field strength and the method of characteristics was used to compute the charge density in the gun-to-target region. The fluid flow between the electrostatic gun and the target was modeled using interpolated experimental data assuming stagnation point flow. Particle trajectories were simulated for size range 10-40 μm and charge-to-mass ratios of -0.1 to -1 μC/g. The simulation results showed good agreement with experimental data (charge and mass measurements) at several collection points on the painting target and provided valuable information concerning particle deposition     

Numerical simulation of the electric field distribution intribo-powder coating of conducting cylindrical objects

The successful implementation of powder coating technology requires detailed investigation of different factors, which can improve, or disturb, the process. The two most important are the air entrainment force and the electric deposition force acting on the powder particles. The authors present a numerical model for predicting the electric field distribution between a tribe gun and a cylindrical object. It is assumed, that the particle distribution is governed primarily by the air flow and that the only source of the electric field is the space charge of the tribocharged particles. Simplified models for the field lines and design configuration have been assumed and the electric field is determined by means of the charge simulation method     

Suppression of Metallic Particle Lift‐Off in GIS by a Nonlinear Resistivity Coating

To suppress metallic particle lift‐off in compact gas‐insulated switchgear (GIS), a coating with nonlinear resistivity characteristics was developed for the inner surface of the GIS tank. An electrical field analysis that measured the current density and electric field (J–E) characteristic indicates that a nonlinear resistivity coating can reduce field intensification at triple junctions between metallic particles and the coating, and can effectively suppress particle lift‐off. The results of experiments using a GIS mock‐up model indicate that a nonlinear resistivity coating increases the lift‐off electric field of metallic particles with less statistical dispersion than a dielectric coating.     

A method for suppressing unstable whip of jet in electrospinning

Abstract

Jet whip is the main mechanical factor of fiber becoming nano-scale fiber, and its stability largely determines the stability and quality of spinning. This article analyzes the complex relationship of various forces on the jet, analyzes the motion process of the steady-state jet based on the conservation of momentum, establishes the controls equations, and analyzes the effect of the electrostatic field on the steady-state jet motion. Combining the finite element analysis software COMSOL Multiphysics 5.0 to optimize the electric field. A method space insulator method that is enhanced spatial electric field strength to suppress jet whip is proposed. By introducing a polar polymer as an electrode to enhance the field strength in the space region, the electric field force of the moving jet can be enhanced to eliminate the coupling of other forces other than the electric field force, and the effect of suppressing the unstable whip of the jet is enhanced and improved. The stability and quality of electrospinning is improved.     

Conducting particle motion and particle-initiated breakdown in dcelectric field between diverging conducting plates in atmospheric air

This paper deals with the analysis of spherical conducting particle motion as well as particle initiated breakdown in electric fields between diverging conducting plates with dc voltage in atmospheric air. Motion of spherical particle was estimated by solving the motion equation numerically, and the results agreed well with the experimental ones. It was found that when the particle is placed on the horizontal electrode surface where the electric field is nearly equal to the lifting field, the particle progresses towards a higher electric field region by the effect of Coulomb force and electrical gradient force. This is true for a nonspherical conducting particle. The obtained results for particle motion suggest that much attention should be paid in the design of GIS particle traps to forces acting on the particle directly towards the higher electric field region. Moreover, the particle-initiated breakdown is discussed on the basis of the experimental and theoretical results. It was found that one of the reasons for reduction of the breakdown voltage is the effect of microdischarge between the particle and an oppositely charged electrode     

Numerical Simulation of the 2-D Gas Flow Modified by the Action of Charged Fine Particles in a Single-Wire ESP

A numerical model for simulating precipitation of submicrometer particles in a singlewire electrostatic precipitator is discussed in this paper. It includes all important phenomena affecting the process: electric field, space charge density, gas flow, including the secondary electrohydrodynamic flow caused by the corona discharge and charged particles, and particle transport. A simplified corona model assumes just one ionic species and neglects the ionization zone. The fully coupled model for the secondary EHD flow, considering the ion convection, has been implemented. The dust particles are charged by ionic bombardment and diffusion. The gas flow pattern is significantly modified by the secondary EHD flow, which depends on the particle concentration. As for fine particles the drift velocity is small and particles practically follow the gas streamlines, the particle concentration has a very strong effect on the precipitation efficiency.     

采用多元颗粒模型对圆柱形颗粒在移动床中流动的离散单元法直接数值模拟

为了研究活性焦颗粒在移动床内的流动特性,通过离散单元法直接数值模拟方法对活性焦即圆柱形颗粒在移动床内的流动进行了数值模拟。采用多元颗粒模型来描述圆柱形颗粒,即圆柱形颗粒通过几个叠加在一起的球元构成。建立了圆柱形颗粒的碰撞机制,分析了圆柱形颗粒在流动时的受力情况,并通过实验验证多元颗粒模型的可靠性。从宏观和微观2个方面,比较圆柱形颗粒2种构建方式(2球元和3球元)下的模拟结果。结果表明:2球元和3球元构建的圆柱形颗粒在动能、转动动能、变形量、碰撞力、碰撞次数等微观方面的差别较大。且通过比较颗粒位置图和下料率可以看出,3球元圆柱形颗粒的模拟结果与实际结果更加相符,因此,选择3球元作为圆柱形颗粒的构建方式。     

静电场对移动颗粒层过滤高温除尘效率促进作用的探讨

通过研究尘粒在带电场颗粒中的振汇运动，提出采用尘粒荷电和对颗粒层施加外电场的方法来提高地细微尘粒除尘效率的方法，并从理论上证明了在一定条件下是可行的。这为细微法粒除尘效率提供了一种可选的方案。