Analysis of the Disruption of Evaporating Charged Droplets

Charged droplets undergo periodic convulsive disruption during evaporation. This makes their behavior fundamentally different from that of uncharged droplets. During the disruption, a number of small "sibling" droplets (about 15?m diameter) are released and carry 5 percent of the mass and about 25 percent of the charge. An analytical model of this process which is based on a "macroscopic" approach is presented. The model predicts characteristics of the final, postdisruption state (e. g., charge and mass ratios, number of droplets produced) which are in good agreement with experimental data for droplets having initial sizes less than about 100-pm diameter. A most interesting result of this analysis is the prediction that a limited number of sibling droplets (about seven) can be produced.     

Influence of charged droplet cloud on flashover of air gaps for lightning impulse voltage

Transmission lines passing through a mountainous region often are struck by winter lightning. As a result, numerous double-circuit faults occur. Space charge might be responsible for this phenomena. Several investigations on flashover characteristics have been performed focusing on reduction of flashover voltage due to ionic space charge formed by corona discharge. In this paper, flashover characteristics of an air gap within a charged droplet cloud ejected from an airless nozzle using an induction charging method are discussed. The droplets are charged in the range -200 to 200 μC/kg, forming the space-charge density of approximately 13 μC/m³. The flashover voltage and the time lag to flashover were measured when 1.2 × 50 μs lightning impulse voltage applied to a rod-sphere gap is placed within the charged cloud. Flashover voltage for a positive rod increased markedly within a positive charged cloud by increasing the charge-to-mass ratio. The increase of flashover voltage was up to 80 percent in comparison with uncharged droplets. Flashover voltage for the negative rod decreased about 20 percent in the cloud of either polarity. The results show that the presence of charged droplets in the air gaps affects the discharge process significantly.     

Dielectric material properties investigated through space charge measurements

This paper investigates means of exploitation of space charge measurement results in order to achieve information about chemical, physical and microstructural properties of dielectric materials. The derivation of quantities as mean volume density of space charge, threshold for DC space charge accumulation, apparent-trap controlled mobility is discussed and various applications to dielectric materials are shown. In particular, the differences of trapping behavior of polyethylene-based materials, as low-density polyethylene, cross-linked polyethylene and high-density polyethylene, the effect of additives containing polar groups, the evaluation of cellular polymeric electrets are discussed on the basis of the results of space charge measurements and the relevant extracted quantities.     

Theory of GaN Quantum Dots for Optical Applications

The optical properties of III-N wurtzite heterostructures are dominated by the built-in polarization potential. We first review the dependence of III-N bulk valence band structure on strain and the key factors determining the polarization vector in polar and nonpolar quantum wells, including electromechanical effects. We then present a surface integral technique to determine the built-in potential in quantum dots (QDs) of arbitrary shape. We show for polar QDs how the polarization potential spatially separates electrons and holes vertically but confines them laterally, causing the radiative recombination rate to decrease rapidly with increasing dot height and a strong blueshift with increasing carrier density. Finally, we show that although the polarization potential can be much reduced in nonpolar GaN/AlN QDs, it is likely to remain significant in nonpolar InN/GaN QD structures.      

Electric field and energy computation on wet insulating surfaces

The paper presents the results of electric field and energy computation on insulating surfaces covered by water in the form ranging from small discrete droplets to large wet patches. The goal of this study was to understand the differences in the flash-over performance of outdoor insulators, in particular nonceramic insulators, under contaminated conditions as a function of surface wettability. The computational algorithm is based on the charge simulation method. A decrease in the maximum electric field at the water droplet tip is obtained as the droplet size increases, however, the electrostatic potential energy increases with the size of the water droplet, which might increase the surface discharge intensity. The role of the coalescence of small water droplets into larger droplets and filaments on the electric field and energy has been investigated. The location of such larger water bodies with respect to the electrodes has been examined. These computations shed some light not only on the role of hydrophobic surfaces but also on the location of hydrophobic surfaces along the insulator on the flashover performance of outdoor insulators. It has been shown that these computations provide quantitative data that compliments information obtained from simpler techniques such as visual observation and surface resistance measurements, for the purpose of assessing the performance of insulators in service     

Prospects for chiral nonlinear optical media

This paper describes the development and optimization of chiral, nonpolar media with large second-order nonlinear optical responses. We employ molecular engineering, quantum-mechanical sum-over-states theory, and measurements of molecular hyperpolarizability by means of Kleinman-disallowed hyper-Rayleigh scattering in order to understand molecular properties. Then we analyze the appropriate arrangement of the chromophores that produce an optimum axial nonlinear optical medium. Chromophores with large Kleinman disallowed traceless symmetric second-rank tensor hyperpolarizabilities β can be aligned so as to result in large susceptibilities χ⁽²⁾ in structures that lack polar order. We found that Λ-shaped chromophores with C_2v or similar symmetry are good candidates for these materials, as they can exhibit large second-rank components of the hyperpolarizability tensor. A wide variety of techniques can be used to fabricate bulk materials belonging to the chiral nonpolar symmetry groups D_∞ and D₂. The microscopic chromophore alignment schemes that optimize the nonlinear optical response in such materials are deduced from general symmetry consideration for both molecules and bulk. We also speculate on the possible application of such materials as high-bandwidth spatial light modulators     

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     

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     

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     

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     

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。     

Breakup of large water droplets by electric fields

The results are presented of an experimental study of the processes that lead to the breakup of a large water droplet doped with alcohol to approach the density of oil and immersed in an oil dielectric medium to which a strong electric field is applied. The study, which was carried out by means of photography, showed that the deformation of the droplets prior to breakup involves distinct characteristic stages of importance to the induction charging and to the ultimate charge separation phenomena. The experimental results show the conditions and the phenomena that lead to the breakup of an initially uncharged water droplet when subjected to an electric field that is not perfectly uniform and in a gravity field     

A dissociation-injection model for non-polar liquid conduction andwire-cylinder geometry

The conduction of a nonpolar liquid filling a cylindrical capacitor has been investigated with a model that includes both the dissociation of ionic pairs in the bulk of the liquid and charge injection at the inner electrode. Three different cases, of increasing complexity, have been analyzed. Firstly, injection has been neglected and the dissociation rate has been taken as constant. Then, the effect of the electric field on the dissociation rate has been considered. And, finally, a field dependent injection of charge has been also included in the model. The current voltage characteristics corresponding to these three models have been evaluated for different ratios of the electrodes radii. Approximate analytical expressions are provided for the charge density and electric field distributions over a wide range of parameters of physical interest     

Charge and mass flux in the radial electric field of an evaporatingcharged water droplet: an experimental analysis

In the electrostatic application of pesticide sprays, charge retention by evaporating droplets in transit to deposit surfaces is a limiting problem. The possible charge loss via evaporative mass transfer and the possible alteration of evaporation rate caused by the presence of the droplet surface charge were experimentally investigated. For studies of evaporation from a charged 3 mm diameter water droplet conducted as a function of droplet potential (-1, -2, and -3 kV) and atmospheric relative humidity (22, 64, and 100%), the current flowing in the radial electric field of the charged evaporating was compared with that flowing in the radial field of a nonevaporating metal sphere to determine if the departing water vapor was electrified. It is concluded that electric charge on evaporating liquid droplets comprising agricultural chemical sprays neither alters the mass transfer rate nor is dissipated by the evaporation     

Evaluation methods on polymer materials for surface of outdoor insulators

Performance of nonceramic insulators is strongly related to hydrophobicity of the surface. Therefore, it is useful to evaluate the characteristics of the surface of organic insulating materials. To characterize the methods for the evaluation, some measurements were conducted on silicon rubber (SR), EPDM, PMMA and polyethylene (PE). The influence of irradiation with a mercury lamp was detected through measurements of contact angle, formation time of water film, and mass analysis of released gases under irradiation of electron beam. Measurement of the contact angle by using polar and nonpolar liquids revealed that silicon rubber (SR) was the most resistant against UV radiation. SR also showed recovery of hydrophobicity after UV irradiation. Measurement of the current flowing across the surface of materials exposed to artificial fog showed good agreement to the measurement of contact angle. Infrared absorption spectroscopic analysis enabled the intensity of oxidation of highly UV-deteriorated organic insulating materials to be detected. Radicals on the surface of specimens were specified from the result of mass analysis of released gases. From these results, the nature of deterioration of organic insulating materials irradiated by UV is discussed.     

A basic study on the effect of voltage stress on a water droplet on a silicone rubber surface

Observations and analyses have been conducted on the behavior of water droplets deposited on the surface of polymer insulators. As an index to characterize the repellency of water droplets on the polymer surface, the contact angle was measured for different situations. Experiments were conducted to see the behavior of water droplets of different volumes and their size distribution on silicone rubber. Electric field strength calculation was performed to analyze the effect on water droplets. Finally, effects of the Maxwell stress and the Coulomb?s force were compared, to conclude that, even if there is some true charge on the water droplet, the Maxwell stress is predominant.     

Charge Separation During Splashing of Liquid Droplets in the Presence of an Electric Field

When conductive liquid droplets splash on solid or liquid surfaces, fragments are ejected from the surface that in the presence of an electric field can be inductively charged to the opposite polarity. This phenomenon can reduce the deposition efficiency of electrostatically applied conductive coatings such as water base paints. The charge separation process was investigated by collecting and measuring the size and charge of splashed fragments ejected from surfaces in the presence of an electric field.     

空间电荷对脉冲电压下局部放电影响

研究了空间电荷对双极性脉冲电压下局部放电行为的影响。通过等温松弛法测量了纳米和非纳米聚酰亚胺膜的陷阱能级和密度,表明纳米材料中大量界面区的存在导致其陷阱密度和电导率比非纳米材料大;此外,通过测量不同频率和上升时间下的局部放电参量(平均放电量和放电次数等)表明,频率的增加和上升时间的缩短导致PD放电活动增强,纳米材料的放电活动低于非纳米材料。通过不同材料的陷阱参数测量结果,分析了纳米和非纳米材料在脉冲电压下PD特征的不同,其PD特征受到材料陷阱参数的影响。     

Electrostatic painting of nonconductive surfaces with water-basepaints

Electrostatic corona painting on nonconductive surfaces normally requires a ground-connected conductive precoating of the surface in order to dissipate the continuously incoming ionic charge from the corona gun. If the surface charge does not flow to ground, the resulting surface electric field will repel a substantial amount of the incoming paint. The authors present a new painting method consisting of generating ultrasonically atomized electrically charged water particles behind the nonconductive surface to be painted. The paint spraying is carried out with a mechanical spray gun maintained at ground potential. The water base paint becomes charged by induction under the influence of the space charge behind the nonconductive surface to be painted. As the resulting charge on the water base paint particles is of opposite polarity to that of the ultrasonically atomized electrically charged water particles cloud, all propelling electric fields converge toward the surface to be painted. The results show excellent transfer efficiencies     

高压静电雾化喷浆脱硫实验

湿法烟气脱硫是目前应用最广的一种脱硫方式。为了提高脱硫效率,采用高压静电雾化技术,对石灰石浆液进行雾化,以增强喷浆液滴与烟气的反应过程。初步喷浆实验及观测得出:液滴由于带电极性相同,其雾化弥散程度、分布均匀性得到较大提高,使浆液与SO2的反应面增加,同时静电也提高了液滴表面活性,促使其化学反应加速,SO2吸收加剧。为验证此脱硫技术的效果,模拟实际湿法烟气脱硫建立了高压静电喷浆脱硫实验装置,通过对清水及石灰浆液充电前后烟气脱硫效率的对比分析得出:在石灰水的量浓度为0.02 mol/L、液气比<1.0的情况下,静电比非静电脱硫效率提高约6%,证明此方法能够提高脱硫效率。