Dynamic motion of a conductive particle in viscous fluid under DCelectric field

When plastic waste is liquefied for recycling, the impurities present could influence the quality of the reprocessed products. In order to explore the possible method for removing solid impurities using electrostatic forces, the motion of spherical conductive particles under a uniform electric field has been carefully investigated. In this paper, a solid impurity and liquefied plastics are simulated by a conductive spherical particle and viscous insulating liquid, silicone oil, respectively. Experimental results indicate that the particle undergoes repeated motion between the parallel electrodes. The motion of the particle can be divided into four modes: settling on the lower electrode, moving upward, settling on the upper electrode, and moving downward. The higher the applied voltage, the faster the average particle velocity and the shorter the resting time of the particle. The particle accelerates after leaving the electrode and decelerates before reaching the other electrode. This deceleration could be explained by the viscous effect of the liquid layer between the particle and the electrode. The settling mode could be explained by the fact that there is a liquid flow induced by the particle motion and it pushes the particle against the electrode until the flow decreases. It is, therefore, suggested that the hydrodynamic effect is dominant in our case, as well as the electrostatic force     

Unipolar charging of cylindrical insulating particles nearelectrode surfaces

Numerous papers have discussed the ionic charging of insulating spheres in uniform electric fields. However, in certain electrostatic technologies, such as separation and flocking, the particles are often cylindrical in shape, and they get charged on the surface of an electrode or in its proximity, so that existing formulas cannot be used. This paper addresses this problem from both a computational and an experimental point of view. The charge acquired by cylindrical particles of various dielectric constants was evaluated with an original computer program, based on the boundary-element method of field analysis. The computed results show that the position of the particle with respect to the electrodes changes the value of the saturation charge. The experimental setup simulated the charging conditions in a roll-type electrostatic separator. The unipolar space charge was generated by a needle-type electrode. An electrometer was used to measure the charge acquired by millimeter-size calibrated cylinders of polyethylene and polyvinyl chloride on a rotating roll electrode. The experimental results, which were in good agreement with the theoretical predictions, put forward a particle self-discharge effect, at field intensities beyond a well-defined threshold. This kind of information may guide the design of the electrostatic technologies based on the corona charging of granular matter     

Numerical Modeling of Conductive Particle Trajectories in Roll-Type Corona-Electrostatic Separators

Several attempts have already been made to simulate particle trajectories in roll-type electrostatic separators. However, the predictive value of the results is limited by an excessive number of simplifying assumptions regarding the electric field distribution, as well as particle charging and discharging mechanisms. The present work is aimed at improving the existing models by taking into account: (1) the non-uniformity of the electric field in the active zone of the separator and (2) the effect of spark discharges occurring between the electrodes. Based on previous observations, the conductive particles were assumed to lift-off when no longer exposed to corona discharge. The numerical simulations were performed for particles of various sizes. The electric field was computed in each point of the trajectory using a finite element program. It was found that: (1) some of the smaller particles impact the static electrode and are deviated to the middling compartment of the collector and (2) field annealing which accompanies spark discharges significantly affects the trajectories of conductive particles. The results of this study could guide the design of new electrostatic separation applications.     

Unipolar charging of insulating spheres in rectified AC electricfields

Nonfiltered rectifiers and pulsed power supplies are often used for the energization of the high-voltage electrodes in various electrostatic installations. The aim of this paper is to investigate the ionic charging of insulating particles in the pulsatory electric fields specific to such applications. In a first set of numerical simulations, the space charge was considered constant in time, which means that the ion generation is not related to the voltage drop between the electrodes. A second set of simulations was carried out using the assumption that the space charge is generated by the pulsed corona from one of the electrodes. The computed results, which were found in good agreement with the experimental data, show that the amount of charge acquired by a particle depends on the following factors: (1) particle transit time through the electric field zone; (2) space charge density; and (3) ratio between the corona onset voltage and the amplitude of the variable voltage applied to the electrodes. These factors are discussed in correlation with the operating parameters of roll-type corona-electrostatic separators, but the conclusions are valid for a wider group of industry applications     

Particle Charging in Combined Corona-Electrostatic Fields

The association of several ionizing and nonionizing electrodes generates combined corona-electrostatic fields, characterized by space charge zones of well-defined extensions. In a previous paper, the authors presented an effective numerical method for the computation of such fields. The aim of this present work is to show how these results can be employed for estimating the charge acquired by insulating and conducting particles when passing through the space charge zones generated by various corona-electrostatic electrode geometries. The study is done under several assumptions that authorize the use of Pauthenier's formula. Diffusion charging can be neglected, the applied electric field is quasi-uniform in the vicinity of particles, and particle speed is low compared with that of air ions. The charging model takes into account the computed spatial distribution of the electric field and charge density. The computations were performed for various values of the geometrical parameters of the electrode system and of the particle transit time through the corona discharge zone. The results can be used for the design of the electrode system of any electrostatic process employing corona discharge fields.      

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.     

Electrical characteristics of an electrostatic valve used for bulktransport of agricultural seeds

In an electrostatic valve used for bulk transport of agricultural seeds, the interparticulate electroclamping forces can be established by applying electric potential gradient between a separated pair of conducting electrode grids placed perpendicularly across the flow. The flow control of particulate material is thus achieved using no moving parts. When an electric field is applied, several types of electric field forces are generated, depending on the bulk and surface resistivities of the particles, the geometry of the electrodes, the applied field, and the geometry and the conductivity of the materials used for the transport channel. In this study the current-voltage characteristics of the valve were experimentally investigated for different flow control parameters. The triboelectrification of turnip seeds caused by the frictional contact on the channel walls was investigated and compared with the valve current. A range of wall liner materials with different electrical properties conductive to insulating were tested. The materials used for the wall liner did not significantly influence the current characteristics of the electrostatic valve. The effect of pulse duration of the applied potential on charge-to-mass ratio of the discharged material was studied. The results obtained show that the level of net charge acquired by the particles could be controlled by controlling the duty cycle of the electroclamping field     

Charges and forces on conductive particles in roll-typecorona-electrostatic separators

Mathematical modelling is a powerful tool in the optimization of electroseparation technologies and equipment. This paper proposes the use of a numerical method of field analysis to evaluate more accurately the charges and forces on millimeter-size conductive particles of virtually any shape in contact with an electrode. A program based on the boundary-element method was tested for this purpose, and provided the input data to an algorithm for the computation of particle trajectories. The results were in good agreement with the observations made on a laboratory electroseparator. The effect of space charge on the behavior of conductive particles was studied with a three-electrode system (a grounded grid, between a corona electrode and a metallic plate, energized by two high-voltage supplies of opposite polarities). The accuracy of numerical simulations of electroseparation processes is expected to increase when a mathematical model of particle motion in DC corona fields will be incorporated in the existing computer programs     

High-voltage electrode position: a key factor of electrostatic separation efficiency

Industry application of electrostatic separation technologies still faces a major difficulty: good results can be obtained only by adequate control of a multitude of operating parameters. The aim of the present paper is to analyze a key factor of electrostatic separation efficiency: the position of the high-voltage electrodes. Experiments were performed with two types of granular materials: chopped electric wire wastes and foundry sands. The electrostatic separator employed for the tests was provided with a wire-type corona electrode, associated - in some experiments - with a tubular-type electrostatic electrode, at various angular and radial positions, with respect to a rotating roll electrode connected to the ground. The experimental data are discussed in relation to the results of the numerical analysis of the electric field, carried out with a charge simulation program. They show that the outcome of the separation process (i.e., the weight percentage of the conductor and nonconductor fractions, as well as the purity of the recovered materials) depends on the configuration of the electrode system. The position of the electrodes affects both the particles charging conditions on the surface of the roll electrode, and the trajectories of the charged particles in the high-intensity electric field of the separator. Some recommendations could be formulated for the industrial application of the electrostatic separation technology.     

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.     

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     

Separation of Small Particles Suspended in Liquid by Nonuniform Traveling Field

A study of size and charge-dependent separation of small particles in liquid using a traveling-field-type electric curtain device is made. The principle of the separation is to make use of the spatial harmonic components of the rotating traveling field produced by such a device, the first harmonic propagating in one direction, which plays a dominant role in the region distant from the electrodes, and the second harmonic propagating in the opposite direction, which becomes dominant near the electrodes. Small particles brought into this field undergo circular motion and, as a result of field nonuniformity, are repelled from the electrodes and drift in the direction of the dominant harmonics. The lighter or more charged particles are strongly repelled from the electrodes and swept by the first harmonic, while the heavier or less charged particles can approach the electrodes and are transported by the second harmonic in the opposite direction, thus enabling separation by mass and charge. First a theoretical investigation of this method is made to clarify the operation conditions for the separation, then the experimental observations of particle motion are made and scaling laws of transport velocity with the applied voltage and frequency are confirmed. Finally, an example of a cell separator design using this method is presented.     

Charging and behavior of a spherically conducting particle on a dielectrically coated electrode in the presence of electrical gradient force in atmospheric air

This paper discusses the charging and behavior of a spherically conducting particle on a dielectrically coated electrode in non-uniform dc and ac electric fields between non-parallel plane electrodes in atmospheric air. The charging mechanism of a particle resting on a dielectrically coated electrode is investigated by observing light emissions from partial discharges and by utilizing the dust figure technique. The charge acquired by a particle is estimated from the results of particle motion onset voltage measurement and direct measurement with a Faraday cage. The basic particle movement as well as particle-triggered breakdown characteristics under non-uniform electric field distribution along the electrode surface is also investigated. The results suggest that the effect of the electrical gradient force/dielectro-phoretic force on particle motion on a dielectrically coated electrode should be adequately considered in the design of gas insulated systems (GIS). This is because a particle can move laterally on the coated electrode toward increasing electric field regions by the action of the electrical gradient force which is independent of the charge on the particle. Also the possibility of the particle charging and lifting at high field regions or reaching and adhering onto an insulating spacer further increases the risk of breakdown.     

Charging of insulating spheres on the surface of an electrodeaffected by monopolar ions

The charge acquired by an insulating sphere in a uniform monoionized electric field has been accurately evaluated by Pauthenier. In certain electrostatic applications, such as the electroseparation of mixed granular solids, the particles to be charged are on the surface of an electrode. Under these circumstances, Pauthenier's formula is no longer valid, because the field is nonuniform. This paper addresses this problem from both a computational and an experimental point of view. A numerical method of field analysis was employed for the evaluation of the charge acquired by spheres of various dielectric constants, on the surface of a plate electrode. The numerically computed values of the saturation charge in this situation were always greater than those given by Pauthenier's formula. The experiments were carried out on laboratory equipment provided with various types of corona electrodes. An electrometer was used to measure the charge acquired by calibrated spheres of polyamide (3 mm diameter) when subjected to the positive or negative corona discharge generated between these electrodes and a metallic rotating roll electrode (150 mm diameter) connected to the ground. The experimental data were in good agreement with the theoretical predictions; the saturation charge increases linearly with the applied voltage, up to a threshold at which the self discharge of the particle occurs. The efficiency of ionic charging was shown to depend on the type of corona electrode that is employed     

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     

同轴电极下非线性绝缘介质阶跃响应仿真分析

为了验证非线性绝缘材料在非均匀电极结构下均化电场分布的内在机理,以典型非均匀电极结构(同轴电极)下填充电导率为非线性的绝缘介质所构成的绝缘结构为研究对象,采用有限元软件ElecNet仿真分析了不同幅值阶跃电压作用下非线性绝缘介质中瞬态响应的空间分布,得到的非线性绝缘介质内部空间电荷的分布揭露了非线性绝缘介质能自行均化电场的根本原因是空间电荷效应。稳态电场的计算结果表明:阶跃电压幅值越高,绝缘介质非线性程度越高,电场改善的程度越好。     

A finite-element analysis of the electrostatic force on a uniformlycharged dielectric sphere resting on a dielectric-coated electrode in adetaching electric field

In the electrophotographic process, charged toner particles are transferred from one surface to another with an electric field. To enable electric field transfer of toner, the externally applied field strength must be greater than a threshold value, so that the Coulomb force can overcome the toner adhesion force at the supporting surface. In this paper, the threshold field strength to detach a charged dielectric particle is determined efficiently by using the Galerkin finite-element method to simultaneously solve the Laplace equation for the field distribution and an overall constraint equation for the force balance. This computational method also enables calculation of the electrostatic adhesion force and is applicable to various particle-electrode configurations. For illustrative purposes, however, we consider the axisymmetric problem of electric field detachment of a dielectric sphere with uniform surface charge resting on a planar dielectric-coated electrode. The analysis is particularly focused on the dependence of the electrostatic force upon the dielectric overcoating thickness and spacing between parallel plate electrodes. The electrostatic force on a uniformly charged particle in contact with a surface is found to be influenced significantly by the thickness of dielectric overcoating and the spacing between electrodes when either becomes less than five times the particle radius     

Proposal for new particle deactivation methods in GIS

This paper deals with consideration of particle motion control and its trapping in the deactivation of foreign contaminating moving particle in simulated GIS. The particle motion behavior in nonuniform field gap without/with different shaped spacers was investigated experimentally under DC and 60 Hz AC voltages. The obtained results were confirmed experimentally as well as by calculating the electrostatic force acting on the particle in the nonuniform field gap and are reported elsewhere. Whereas, in this paper, the observed particle motion behavior around simple shaped spacer is simulated by solving the particle motion equations analytically in the presence of electrical gradient and image forces. The simulation agreed fairly well with the experimental results. Furthermore, in the nonuniform electric field, the effect of dielectric coating on the grounded electrode on the particle motion and particle deactivation is also briefly discussed. On the basis of the obtained results of particle motion behavior without/with different shaped spacers and the effect of dielectric coating on the ground electrode, different methods of particle deactivation are suggested as well as confirmed experimentally     

Electric Field Effects on the Equilibrium and Small Signal Stabilization of Electrofluidized Beds

A continuum analysis is developed of electric field coupling to a charge-free fluidized bed of polarizable particles, with the electric field at an arbitrary angle to the gas flow. The electric force density and stress tensor is derived from the principle of virtual work and includes polarization and electrostriction forces but neglects Coulombic free charge forces. Effective medium theory for various particle structures derives the effective bed permittivity as a function of particle voidage and is used with the energy method to derive the electric. force density acting on the particles. Necessary force-free boundary conditions at the top free surface of the bed are developed. A linear stability analysis is used to find the conditions for electric field stabilization. Experiments with Rochelle salt show elimination of bubbling with collinear electric field and flow but never complete stabilization with crossed field and flow as predicted by the analysis. There is qualitative agreement between the stability analysis and measurements. Numerical differences are attributed to particle viscosity, which was not included in the fluid model.     

Electrostatic devices for particle microhandling

Devices for handling microparticles using an AC electric field are proposed and experimentally validated. With the application of balanced multiphase high voltages in the 0.2 kV_p-p-2 kV_p-p range to a series of encased parallel electrodes with a typical center-to-center pitch width of 400 μm, particles on a thin protecting and insulating film become charged and are conveyed by the created traveling field. Devices with various novel transportation and manipulation features, utilizing multiphase AC electric fields that instantly generate particle driving forces, have been designed and produced. Their features include newly developed methods for particle conveyance, smoothing, grouping and sorting, as well as two-dimensional handling. Traced particle conveyance characteristics have been studied under application of three and six-phase voltages with three different voltage wave profiles and also in diverse environments. Various particle materials with a 5-400 μm diameter range have been experimentally examined; Fe, Al, glass and plastic spheres showed the best performances