Theoretical/numerical study of electrohydrodynamic pumping through conduction phenomenon

In an isothermal liquid, only the Coulomb force which is the force acting on the free charges, can contribute to the net electrohydrodynamic (EHD) motion. In the absence of a direct charge injection or induction, the charges can be generated through the dissociation process of the fluid. The generated charges by dissociation are redistributed by the applied electric field, resulting in the heterocharge layers around the electrodes. The pumping of an isothermal liquid without ion injection is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes. This type of pumping is referred to as the conduction pumping. This paper investigates the pressure head generated by the conduction pumping mechanism theoretically through the numerical solutions. For this purpose, a theoretical model for the static case (i.e., without a fluid motion) is established and a numerical code using finite volume method is developed. Electric potential, electric field, charge density, and electric body force distributions for the selected electrode configuration are presented. The generated pressure as a function of the applied voltage is also presented. The numerical results confirm the EHD conduction pumping concept theoretically.     

Electrical conduction of electrolyte solutions in nonpolar liquids

Certain nonpolar liquids, when doped with specific salts, are able to produce sustained and reproducible ion injections from one of the electrodes. Electronic transfer to the ionic pairs attached to the electrodes and the subsequent extraction of the free ions by the Schottky effect are the mechanisms responsible for this injection. The electrical conduction of these liquids is studied, using a recently proposed electric-field dependent law for the injection of ions and Onsager theory for the dissociation of ionic pairs in the bulk. Recombination is allowed between electrolytic ions and injected ions. The current-voltage characteristics predicted with this model have been presented in term of dimensionless parameters, so that an easy comparison with experiments can be done. Special attention has been paid to distinguish the contribution of the dissociation and the injection in the total current. The electric field distortion and the charge density at the injector has also been determined     

The onset of electrothermal convection in nonpolar liquids on thebasis of a dissociation-injection conductivity model

The onset of convection for a layer of nonpolar liquid subjected to an adverse thermal gradient and intense electric field is examined. The electrical conductivity is determined via a recent physiochemical model based on dissociation and injection charge generation. Convective instabilities and overstabilities are predicted as a function of strength of injection C and residual conduction C₀. Physical and mathematical arguments show that for values of C bigger (smaller) than 2√2C₀ , the regime is approximately injection dominated (conduction dominated). It is also shown that dissociation always plays a stabilizing role, which is contrary to the injection which can have either a stabilizing or a destabilizing effect due to its indirect coupling with thermal perturbations     

Unipolar injection induced instabilities in plane parallel flows

The behavior of small disturbances in parallel flows of insulating liquids subjected to charge injection, with imposed electric field orthogonal to the primary flow, is studied. The resulting convective patterns and the instability thresholds, for plane Couette and Poiseuille flows, have been determined. Particular attention has also been given to the energy transfer between the primary flow and the secondary flow in order to gain physical insight into the way the inertial and the electric physical mechanisms act on the liquid     

采用光学方法研究陶瓷绝缘材料的沿面闪络特性

对比研究了真空中交流电压下采用和未采用真空溅射金属电极的氧化铝陶瓷材料,在沿面闪络发生前不同的表面发光特性。基于固体的能带理论,提出了２类试品在金属电极－介质的界面处不同的能量状态分布模型;并在此基础上同时考虑电荷注入复合和场致电子发射对发光的影响,解释了２类试品发光的差异性,指出了采用和未采用真空溅射金属电极的氧化铝陶瓷存在不同的沿面闪络起始机理及发展过程。     

Measurement of space charge distributions in PLZT using Kerr effect

Kerr effect technique is an optical measurement technique used to determine space charge distributions of a dielectric liquid under applied electric stress. In other words, such measurement can be used to study the conduction mechanisms of a dielectric liquid, such as charge injection threshold, bulk electric field stress, and aging. In this study, an instrument was set up to measure the absolute value of the electric field distributions in lead–lanthanum–zirconium–titanium (PLZT). PLZT was selected for the study of the instrumentation because the material has a high Kerr constant. In the experiment, charge injection threshold and bulk field strength of PLZT were measured under low applied voltages. © 2012 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

Comparative study of theoretical Kerr electro-optic fringe patternsin 2-D and axisymmetric electrode geometries

Kerr electro-optic fringe patterns have long been used to study space charge injection and transport phenomena in highly birefringent materials such as nitrobenzene, Most past experimental work has been limited to 1 or 2-dimensional geometries where the electric field magnitude and direction have been constant along the light path such as two concentric or parallel cylinders or parallel plate electrodes. For these geometries the extrema in the fringe patterns can be used directly to find the electric field magnitude and direction. In this work we extend the fringe based Kerr electro-optic measurement technique to a point/plane electrode geometry which often is used in HV research to create large electric fields for charge injection at known location and at reasonable voltages. We calculate theoretical Kerr electro-optic fringe patterns for this point/plane electrode geometry with and without space charge distributions, for which the electric field magnitude and direction vary along the light path. We particularly compare the calculated space charge free optical patterns for the point/plane electrodes to the optical patterns of the 2-dimensional analog blade/plane geometry. We underline the differences and study how these fringe patterns can be used to reconstruct the axisymmetric electric field components in practice     

Numerical analysis of transit conduction current in an insulating liquid with added ionic electrolyte

We simulated the phenomena of transit conduction in an insulating liquid with added ionic molecules such as surfactant, by using the charge‐injection model with dissociation‐recombination. When the parameters of C_i (density of injected charge) and C₀ (density of created charge in the liquid) are C_i>C₀ in this simulation, the waveform of current density showed an injection type with a peak, and when C_i<C₀, it showed a dissociation‐recombination type. A decreasing characteristic appeared in the low electric field, maximum one in the middle field, and saturating one in the high field. © 2007 Wiley Periodicals, Inc. Electr Eng Jpn, 161(1): 10– 16, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/eej.20327     

Computation methods for one-dimensional bipolar charge injection

Kerr electrooptic field-mapping measurements have shown strong space charge effects in high-voltage stressed dielectrics, where the magnitude and sign of injected charge depends on electrode material. With appropriate choice of electrode material combinations and voltage polarity is possible to have uncharged, unipolar positively or negatively charged, or bipolar charged dielectrics. The bipolar homocharge case lowers the electric field at both electrodes due to space-charge shielding, and thus allows a higher voltage without breakdown. Using charge-transport analysis the authors consider the general case of charge injection from both electrodes and solve time and space dependences of the electric field and charge distributions and the time dependences of the terminal voltage and current. This analysis extends earlier work by including nonequilibrium charge dissociation and recombination so that the effective ohmic conductivity is not constant but varies with time and position due to local net charge density. Specific cases discussed are the open-circuit voltage decay of an initially charged capacitor, the charging and discharging by a Marx generator, and alternating voltage excitations     

Electrohydrodynamically induced dielectric liquid flow through pure conduction in point/plane geometry

Mildly polar liquids generally exhibit an ohmic behavior when subjected to electric fields of limited values. The resulting conduction is then associated with heterocharge layers of finite thickness in the vicinity of the electrodes. In the absence of charge injection or induction, a simple conduction model based on the processes of dissociation of a neutral electrolytic species and recombination of the generated ions is presented. This model is first applied to parallel plane electrode geometry to describe the build-up of the heterocharge. Then, the case of point/plane configuration is considered where the effect of Coulomb force is different in the two layers next to the electrodes. A net motion toward the point electrode is predicted to occur. With a rough approximation for the harmonic field, an analysis is presented which leads to an expression for the axial component of the net force exerted on the liquid. A simple static pump is designed and built to experimentally investigate the pressure head generated through pure conduction. Two working fluids (refrigerant R-123 and n-hexane) and two different electrode designs are considered in this study. The experimental results are qualitatively compared to the theoretical predictions.     

High repetition rate PEA system for in-situ space charge measurement during breakdown tests

In order to investigate transient space charge phenomena, it is essential that the space charge profile be observed at a high repetition rate. We have developed a new space charge measurement system using the pulsed electroacoustic (PEA) method, which can measure the space charge profiles every 10 /spl mu/s. It employs the most recent digitising oscilloscope model and a semiconductor switch. The effect of prestressing on impulse breakdown voltage of a low-density polyethylene sheet was investigated by using the new system. Experimental results suggest that positive charge injection was dominant immediately before the breakdown, and charge injection during the prestressing causes distortion of the electric field near the electrode, and enhances the subsequent charge injection due to the impulse voltage.     

Electrohydrodynamic instability and motion induced by injectedspace charge in insulating liquids

A survey is given of different aspects of fluid motion induced by the Coulomb force exerted by the electric field on an injected space charge. The instability problem in highly symmetrical electrode configurations is considered with positive coupling between velocity and charge perturbations, linear and nonlinear criteria and the phenomena during the transient regime of unipolar injection. Then the features of electroconvection are described for both strong and weak injection and also in the case of coaxial cylinders. The last part deals with the liquid motion induced by injection from very restricted areas, as occurs in configurations of low symmetry with particular attention devoted to charge plumes     

SEM-mirror methods and application to insulator characterization

The SEM mirror method (SEMM) is a nondestructive technique developed for insulators characterization. It provides measurements on the charge trapping ability under high electric field with a spatial resolution down to 1 μm. SEMM consists of experimental measurements, ground current and minor plot, and a simulation part using Monte Carlo calculations. The mirror plot gives information on the trapped charge distribution at an equilibrium state. Many experimental results on different kind of materials, e.g. polymers, ceramics, single crystals, have shown that the mirror plot is useful to study insulators properties. But, the charge distribution deduced from the experimental minor plot is not unique and more information about charge buildup is needed to reduce the set of solutions. More recently, the ground current measurement during the injection phase gave complementary results on the charge trapping rate. Under specific conditions, peaks of electron emission occur during injection which have been associated with a local flashover. This result clearly shows that SEMM is able to study insulators under high field stress close to the breakdown strength. To understand the mirror experiments, a more complete simulation of the injection phase is needed. This can be achieved through Monte Carlo simulation of the electron-matter interactions by taking into account charge trapping and detrapping processes under high field. Using these three complementary approaches, it is possible to get a better understanding of charge trapping and detrapping processes in insulators under high electric field with a fine spatial resolution     

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.     

Space charge injected via interfaces and tree initiation inpolymers

Space charge is a recent hot topic in the field of electrical insulation, since it can be measured nondestructively as standard practice. It has been long suspected that space charge might be critically involved in local dielectric breakdown or tree initiation. It is widely accepted that space charge is formed under dc stress in dielectrics. The author demonstrated a hypothetical space charge-dominant model for tree initiation in 1979 in Japanese and in 1991 in English, and proposed that charge can be accumulated inside polymer dielectrics near the divergent high field region even under ac stress. The model consists of charge injection and extraction, charge trapping and space-charge forming, polymer scission due to high energy injected and extracted electrons, oxidation, electric field enhancement, and finally tree initiation. The author will explain his old hypothetical space charge dominant model for tree initiation, various relevant phenomena and data that could prove the validity of the model, try to interpret several recently discovered phenomena with his model, and re-evaluate it as compared with modern concepts for tree initiation     

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     

正极性雷电冲击电压下空间电荷电场测量研究

很多绝缘问题本质上都与电场相关,电场测量是高电压领域的重要研究手段。空间电荷导致电场畸变是流注理论的基础,因此定量测量空间电荷的泊松电场对于研究空气间隙的放电机制有重要意义。利用光电集成电场传感器测量了1m棒–板间隙未发生放电条件下各点的轴向电场,并与数值仿真结果进行对比,验证了电场传感器进行定量测量的可靠性;对3种电极条件下1m棒–板间隙空间电荷区域外部轴线上各点的电场时域波形进行测量,将空间电荷形成的泊松电场与外加电压形成的拉普拉斯电场分开,定量测量了不同电压幅值下、不同空间位置的泊松电场幅值与变化规律。研究结果为建立并验证空间电荷模型奠定了基础。     

温度对直流电压下油纸绝缘空间电荷特性的影响

油纸绝缘是直流输电设备的主要绝缘材料,其空间电荷特性变化会引起局部电场发生畸变,从而影响材料的介电强度,降低直流输电设备的运行可靠性和稳定性.使用电声脉冲法( PEA)测量装置,分析油纸绝缘中空间电荷的注入、迁移、消散规律,搭建试验平台,通过升压试验获得试品空间电荷注入的参考电压,然后对试品加3 kV的直流电压,分别在15℃、30℃、50℃温度下对油纸介质的空间电荷的特性进行分析.试验结果证明:在不同条件下发生空间电荷的注入,温度会对去压时空间电荷的消散产生很大的影响;温度会影响油纸绝缘介质内电场的大小和分布,使空间电荷严重畸变电场,引起绝缘的进一步破坏.     

基于Kerr电光效应的冲击电压下液体电介质空间电荷高速CCD测量

空间电荷的存在直接影响液体电介质的绝缘击穿性能,为了更好地研究空间电荷的动态变化过程,建立了基于Kerr电光效应的冲击电压下液体电介质空间电荷的高速电荷耦合器件(CCD)测量系统。运用高精度光电传感器的测量方法,得到了室温下液体碳酸丙烯酯的Kerr常数。进而通过高速CCD的动态光电测量,并结合图像处理技术,求取了冲击电压下平行板铜电极间电场和空间电荷的动态分布曲线。实验研究表明:在平行板铜电极间,当液体介质中电场达到一定程度时会出现空间电荷的注入与输运,使得极间电场呈现出中间高两边低的分布情况。另外还分析了空间电荷的分布和输运对液体绝缘特性的影响,该研究为分析冲击电压作用下液体介质中空间电荷的产生机制及空间电荷对流注放电的影响奠定了基础。     

Droplet charge-to-mass ratio measurement in an EHD liquid-liquidextraction system

The application of a high voltage electric field to enhance the rate of mass transfer in liquid-liquid extraction has been an active subject of investigation in recent years. The electrohydrodynamic (EHD) forces generated by electric fields at a liquid-liquid interface have a potential application in chemical processing industry solvent extraction processes. In this work, the charge-to-mass ratio of droplets formed in an EHD liquid-liquid extraction system has been investigated experimentally and theoretically. The results reported from the present investigation, extending from the single discrete droplet regime (at low applied voltages) to the dispersed multi-droplet regime (at high applied voltages), indicate that the modified Rayleigh instability model and Vonnegut and Neubauer model can explain maximum droplet charge acquired in liquid-liquid systems, especially the modified Vonnegut and Neubauer model which can predict most of the experimental results when the applied electric field is high enough and EHD forces become dominant