Electrical characterization of bell-type electrostatic paintingsystems

A numerical model that determines the electrical characteristics of a bell-type electrostatic painting system is presented. The model is capable of handling the complex geometries of practical sprayers and accounts for the space charge effect of charged paint droplets. The influence of aerodynamic forces is also considered in tracing individual droplet trajectories. As a test of tile simulation, the restricted case of a sprayer emitting ions, rather than charged paint droplets, was modeled. The simulation results were compared with experimental data and showed good agreement. For the more general case, maps of equipotential contours and the paint droplet trajectories are generated for a bell-type sprayer. It was found that the space charge tends to increase the deposition field, although not as strongly as increased bell potential or decreased bell-to-target spacing. However, the space charge also caused the spray pattern to expand, possibly leading to increased overspray     

Characterization of tribocharging properties of powder paint

The charge-to-mass ratio (q/m) of individual particles plays a vital role in the powder coating process. Simultaneous measurements of particle size and electrostatic charge were performed on triboelectrically charged powder paint and other powders in the particle size range 1.0 to 65.0 μm in diameter. An electrical single particle aerodynamic relaxation time (ESPART) analyzer was used to measure electrostatic charge (q) and aerodynamic size (d_a) distributions on a single particle basis and in a noninvasive manner. The powders were triboelectrically charged against nonconducting surfaces. Experimental data show that (1) the magnitude of the charge increases linearly with the surface area of the particle and consequently, the charge-to-mass ratio of the particles decreases inversely with respect to d_a in the entire size range, and (2) twenty to sixty percent of the particles were charged with opposite polarity. The average charge attained by particles larger than 4.0 μm in diameter was less than the Gaussian limit of saturation charge. The charge-to-mass ratio of the bulk powder sample depends upon the range of particle size in the sample. The mass median aerodynamic diameter of the sample studied was 30.0 μm with the geometric standard deviation of 2.0     

Measurement of particle size and electrostatic charge distributionson toners using E-SPART analyzer

Performance characteristics of a laser-based instrument called the electrical-single particle aerodynamic relaxation time (E-SPART) analyzer for measuring aerodynamic size and electrostatic charge distribution of particles in real time and on a single-particle basis are described. Applications of this analyzer along with experimental data of the characterization of different toners for their size and charge distributions are presented, including operational principles and methods of data acquisition. Size distribution ranged from 2.0 to 20.0 μm in aerodynamic diameter, and the charge-to-mass distribution ranged from 0 to ±20 μC/g. The method is noncontact and provides both magnitude and polarity of electrostatic charge on an individual particle basis. The aerodynamic diameter and charge are measured simultaneously at an average count rate of approximately 100 particles/s     

Analysis of the behavior of ions produced by pulsed coronadischarge

The motion of an ion cloud which is produced by a corona discharge with the high-voltage pulse of a short duration and is introduced into parallel plate electrodes was calculated numerically. The ion cloud initially placed at the tip of a needle electrode was simulated by a number of ring charges and the trajectories of individual ring charges were calculated by using electric field strength obtained with a charge simulation method. Not only movement of the ion cloud but also the waveform of the induced current flowing through a ring electrode located at the center of the parallel plate electrode were simulated and analyzed. The ion cloud with an initial diameter of 0.5 mm expands to 5 mm by electrostatic repulsion within 10 μs after the beginning of drift. During drifting toward the counter electrode, the ion cloud extends wider to a diameter of around 20-25 mm. The size and velocity of ion clouds agree with those estimated by experiments and the waveform of induced current obtained by experiments was reproduced by this simulation     

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     

Numerical computation of conducting particle trajectories in plate-type electrostatic separators

Development of new applications of plate-type electrostatic separators need increased controllability of particle trajectories. The aim of this paper is to model the particle behavior after lift-off from the plate electrode. A superficial charge simulation program was employed for the computation of the electric field in the active zone of the separator. This enabled the evaluation of the charge acquired by a conducting particle in contact with the plate electrode and the electric force exerted on it before and after detachment. Based on a previously derived mathematical model, the particle trajectories were calculated for various electrode configurations. The data of such numerical simulations can guide the experimental studies needed for the assessment of new industry applications of electrostatic sorting or classifying techniques.     

Evaluation of continuum regime theories for bipolar charging ofparticles in the 0.3-13 μm diameter size range

Bipolar charging experiments were performed on particles in the 1-13 μm diameter size range, and the data were used to evaluate continuum regime models for predicting charge acquisition. Particles were exposed to countercurrents of positive and negative ions in the presence of an external electric field. Particle charge was determined from observations of particle trajectories in a uniform electric field. Data were obtained for dimensionless charging times from approximately 1 to 40, dimensionless electric fields from 1 to 20, and positive-to-negative ion conductivity ratios of 3, 10 and ∞. The data were compared to predictions of field-diffusion theory, classical field theory, classical diffusion theory, and an empiricism formed by adding the field and diffusion approximations. Comparisons were also performed using data obtained previously for particles in the 0.3-1.1 μm diameter size range. Predictions were found to be in excellent agreement with the data     

Triboelectric charging of polymer powders in fluidization andtransport processes

In this paper, we investigated the role of tribocharging in fluidization, flowability, and q/m distribution as functions of particle size distribution (PSD), fluidization time, and transport tubes of different materials. A charge separator was used to determine mass fractions of powder that had positive, zero, and negative charges. For the two acrylic powders tested, one (Sample A) had volume median diameter d₅₀ 18.73 μm while the other (Sample B) had d₅₀ 24.17-μm diameter. During fluidization for 1 h at a relative humidity of 57%, powder B acquired positive charge with charge-to-mass ratio Q/M=0.3 μC/g, and 25% of the powder mass had negative charge with Q/M=-0.36 μC/g. About 35% of the powder mass had particles with nearly zero net charge. Tribocharging during fluidization and transport processes was attributed to particle-particle and particle-wall collisions. Of the total charged particles, about 60% were positively charged while 40% were negatively charged. Each particle may have had patches of charges on its surface, some positive and some negative. The sum total of all patches of charges may yield positive, negative, or essentially neutral polarities on a given particle. For a relatively narrow size distribution of powder, the PSD is often assumed to be conserved during fluidization. A shift in PSD toward larger diameter particles was anticipated because of the possible elutriation of the fine fraction of the powder, but the actual shift was only about 1 μm after 1 h of fluidization. Under identical operational conditions using a fluidized bed, powder pump, transport hose, and corona gun, powder B had nearly twice the mass flow rate of Powder A. Such differences in flowability of powders influence charging and deposition efficiencies in powder coating applications     

高压静电液体雾化技术

高压静电雾化具有雾滴粒径细小、粒径尺度单一、空间弥散程度广等优点,广泛应用于农牧林业病虫害防治、工业喷涂、燃烧、脱硫除尘及材料薄膜制备等领域。为有效提高燃油燃烧效率、烟气脱硫效率及药剂灭菌效率等,从破碎动力学、不稳定理论及雾化模式等出发对高压静电雾化理论进行了详细的阐述,测试了平口雾化喷嘴在针-环状组合电极下的喷雾特性。获得了雾化角、射程、沉积量分布及雾滴荷质比、索太尔(SMD)平均直径和PIV雾化流场图象等信息,并进行了分析。实验结果表明:高压静电减小了液体的表面张力和粘滞阻力,使液体容易破碎成更为细小的液滴,使雾滴尺寸分布更均匀。雾滴荷电后,带电雾滴在高压静电场的作用下容易发生二次雾化,进一步减小雾滴粒径;同时带电雾滴在电荷之间斥力作用下,弥散程度加大,且能在目标物感应出与本身电荷极性相反的电荷,从而在极化力、引力等作用下更容易被目标物所捕获。     

Modeling of electrostatic-based pesticide spray systems

A microscopic model for the electrostatic spraying system is presented to investigate the effect of the voltage applied to the induction nozzle on the droplet's charge, mobility, and charge-to-mass ratio. The variation of these parameters along the jet was also included. The model also lays particular emphasis on the effect of the applied voltage on the spray current and the charge density at the nozzle. A macroscopic model for the electrostatic spraying system is also presented. The objective of the model was to study the spatial distribution of the droplet charge density, transit time, and trajectory in the region between the nozzle and the target in terms of the flow velocity of the spray and the space-charge-produced electric field. On the macroscopic scale, both the droplet charge density and the spray current increase with the voltage applied to the charging electrode. With the decrease of the spray flow velocity, the space-charge-produced electric field becomes dominant and tends to: contract the droplet trajectories toward the axis of the spray system, and hence enhance the droplet deposition efficiency; and decrease the charge density at the target with the possibility of minimizing back-ionization. On the microscopic scale, induction charging eliminates the ion current from the current to the target and the associated back-ionization. The calculated droplet charge and charge-to-mass ratio which increase with the voltage applied to the charging electrode, agreed with the values reported previously     

Capture of fine particles on charged moving spheres: a newelectrostatic precipitator

It was verified experimentally that electrostatic cleanup of fly ash dust is possible using the scrubbing action of charged copper particles with diameters 10-100 times the size of the dust particles. Results show that target efficiencies in excess of unity (up to 1.8) are possible as a result of the induced charge on the target particles. Power consumption to the precipitator was confirmed to be very small (~5×10^-4 W). Values of the total collection efficiency of 0.65 (65%) were obtained for a precipitator zone length of 1.2 cm (in the direction of air flow) at air velocities of about 3 cm/s. This value decreased with increasing air flow rate. Collection efficiencies up to 0.99 (99%) are projected with a longer test section of at least 5.3 cm in length for low air velocities of about 3 cm/s, an electric field strength of 10.91 kV/cm, and 114.5 μm copper particles     

Modeling of Corona Characteristics in a Wire-Duct Precipitator Using the Charge Simulation Technique

The charge simulation technique has been adapted to model the electrostatic and the corona characteristics in clean air of a duct-type electrostatic precipitator. The study involves the evaluation of the electric potential, electric field, and charge density in the interelectrode space as a function of corona current. The results show good agreement with published experimental data. The method developed can be applied to other geometries in the presence of space charge. The commonly used assumption that the space charge affects the magnitude but not the direction of the electric field is shown to be inadequate for large values of corona current. Also, the effect of using different values for the mobility of negative ions is presented.     

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.     

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.     

高压脉冲负电晕荷电喷雾试验研究

为减少化学农药带来的污染,采用了荷电喷雾新技术。首先对负高压脉冲电晕荷电机理进行了理论分析,再用网状目标法通过调节电压、极距、电极直径、电极数目等参数进行电晕荷电试验,比较各因素对电晕放电伏-安特性影响及荷电后雾化效果;测量不同电压下雾滴的索太尔直径;并用高速摄影拍摄雾滴带电射流破碎时的状态,得到射流破碎时的雾滴形状。理论分析和试验结果表明,电压越高,雾滴谱较窄,雾滴粒径变细,均匀度越高;揭示了不同影响因素下的雾滴荷电特性,为负高压脉冲电晕荷电喷雾技术的应用和设计提供了理论基础和试验数据。     

阵列式静电传感器灵敏场分布特性

粉体的气体输送过程中,颗粒的荷电包含了粉体速度、浓度及颗粒尺寸等大量信息,阵列式静电传感器是获取这些信息的重要手段,但由于静电传感器灵敏场的非均匀性,流型变化会给测量造成较大的误差。该文基于点电荷思想建立了阵列式静电传感器的三维静电场数学模型,并应用有限元法系统地分析了阵列式静电传感器的结构参数(包括电极的长宽尺寸、屏蔽罩的长度和半径,绝缘管道的厚度等)对传感器灵敏场分布特性的影响。最后结合理论分析和实验,证实了模拟结果的准确性,从而为实现阵列式静电传感器的优化设计提供了理论依据。     

Design and optimization of electrostatic fluid accelerators

Electrostatic air propulsion is a promising technology with such potential applications as energy-efficient ventilation, air sterilization, cooling of electronics, and dehumidification. The challenges of existing designs include the need to increase air speed, backpressure, energy efficiency, and heat exchange capability. The ultimate goal of this direction of research is to create multi-channel energy efficient ionic pumps. In the described project, a single cell analysis is conducted in this study as a building block of future designs. This paper presents the numerical simulation and experimental results of electrostatic fluid accelerators. This study was conducted for the purpose of optimizing device characteristics through the control of the electric field distribution. Simulations were performed for multiple collector electrode voltage distributions. A method to quantify the change in pump performance between different voltage distributions is presented. The influence of space charge on pump performance is also discussed. A significant improvement of air velocity generated by optimized electrostatic fluid accelerators has been achieved using the proposed approach.     

Characterization of chargeability of biological particulates bytriboelectrification

Triboelectric chargeability of pecan pollen and lycopodium spores, using commercial Teflon and nylon chargers, has been investigated as reported in this paper. The charge-to-mass ratio of lycopodium, measured at different powder feed rates, i.e., 50-500 mg/s, indicated that both Teflon and nylon chargers increased particle charge by approximately elevenfold over background with Teflon charging particles positively to +11.06 mC/kg and nylon negatively to -11.52 mC/kg. Pecan pollen charge increased by approximately 11 times for Teflon charging positively to +7.59 mC/kg, and five times for nylon charging negatively to a value of -3.72 mC/kg. Pollen charging was not affected by carrier-air humidity in the range of 18.1%-47.6% relative humidity (RH) and at a temperature of 22.5°C. Pollen chargeability by Teflon measured as a function of the duration of air flow over the charger surface indicated that triboelectric charging was not degraded, even by 3.5 h continuous flow of humid air (44.2% RH at 19.2°C). Results indicate that tribocharging is a feasible method for imparting charge to pollen in electrostatic pollination technology currently under development     

Parameterization of Continuum Theories for Single Wall Carbon Nanotube Switches by Molecular Dynamics Simulations

We propose continuum models for simulation of carbon nanotube based electromechanical switches. Mechanical behavior of a single wall nanotube has been modeled by a continuum model parameterized by using molecular dynamics simulations. By coupling the continuum mechanical and electrostatic models, electromechanical analysis of a single wall nanotube is performed to compute the pull-in voltage.     

直流电压下盆式绝缘子表面电荷积聚效应的仿真

为了研究直流电压下盆式绝缘子表面电荷的积聚效应,建立了GIL线路中所应用的盆式绝缘子的仿真模型,采用有限元分析法,应用Maxwell 2D软件进行仿真分析:分析了处于直流高压作用下的盆式绝缘子在表面不同位置产生不同性质、不同量的电荷时电场发生的改变。从仿真结果可知,盆式绝缘子内表面不同位置积聚电荷时,对原电场的影响不同;积聚电荷的极性不同时,对原电场的影响也不同,且电荷积聚到一定程度时,可能导致绝缘性能的下降。通过在仿真中逐渐减小所加电荷量的方法,可以找到不影响原电场分布的电荷极限值。