Production of Nano-silica Particles in Liquid-liquid System by Pulsed Voltage Application

Electrostatic atomization in liquid-in-liquid media has been performed to produce fine droplets with narrow size distribution. The droplet diameter was controlled from mm to mum with applying dc voltage. When ac superimposed on dc voltage was applied, uniformly-sized droplets were produced. The high frequency pulsed voltage was effective to disperse liquid into nm-sized droplets. Water glass aqueous solution was dispersed in alcohol solution, and then they were solidified into solid particles by dehydration. The diameter of the dispersed particles was measured with a laser scattering particle analyzer. Particle diameter was depended on the applied pulse frequency and voltage, flow rate of the water glass, and water glass concentration. With increasing applied pulse frequency, the particle diameter and size distribution were decreased. Particles with 200 nm average diameter were produced at 1 kHz and 5.3 kV of applied pulse.     

Production of oil/water type uniformly sized droplets using aconvergent AC electric field

The authors proposed a new method to produce uniformly sized insulating liquid droplets (such as kerosene or plastic monomer) in immiscible liquid media (distilled water) by means of an applied convergent electric field generated using AC or pulsed voltage. The disintegration mechanism of the liquid column was observed precisely using video images and still photographs. Kerosene droplets with an essentially uniform diameter ranging from 100 to 250 μm were produced synchronously with the applied AC frequency using a nozzle diameter of 100 μm. When the flow rate of the co-flowing liquid surrounding the oil-phase liquid jet was increased, the synchronous frequency became higher and the size of the resulting droplets was decreased due to the elongation of the liquid jet. The disintegration mechanism is most likely the forced oscillation of the liquid jet stimulated by each cycle change of the applied voltage. The droplet size can be widely controlled by varying the AC frequency, nozzle diameter, liquid flow rate and velocity ratio between the oil-phase and co-flowing water     

Emulsification and Demulsification Processes in Liquid–Liquid System by Electrostatic Atomization Technique

A new electrical emulsification and demulsification apparatus using an electrostatic atomization technique was developed, and the investigation of both processes in a liquid–liquid system was conducted. First, fine water droplets were generated in silicone oil using a nozzle electrode with ac high voltage, and then, a stable water-in-oil (W/O) emulsion was produced without surfactants. The mode of electrostatic atomization and the trajectory of the fine droplets' flow were observed. The diameter of fine droplets and their distributions were measured as a function of the applied voltage and its frequency. Experiments indicated that electrohydrodynamic (EHD) induced liquid flow with water droplets played an important role in the formation of the emulsion. Next, the demulsification of W/O emulsion was carried out by two injections of oppositely charged water droplets, using the method of electrostatic atomization. The test liquid, which was prepared by the electrostatic atomization technique, was maintained in a state of emulsion for a longer time if no electric field was applied. When the positive- and negative-charged droplets were injected into the test liquid, the emulsion, with a dull color, gradually became transparent as time elapsed. The aggregated droplets fell down due to gravity and the EHD flow effect, resulting in the separation of water and oil. It was found that two injections of oppositely charged water droplets were effective for resolving the emulsion.      

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。     

高压静电雾化雾滴粒径双峰分布概率密度模型

为预测毛细管—环电极配置的高压静电雾化系统下水静电雾化雾滴粒径的分布规律,采用颗粒动态分析仪测试了该系统的雾化雾滴粒径。当环电极电压在15~25 kV之间时,静电雾化模式为泰勒锥射流模式,雾滴粒径呈显著双峰分布规律,对此利用一种可双峰分布的概率密度函数建立了预测该模式下粒径分布规律的统计学模型,并与试验数据对比进行了误差分析,讨论了误差原因。结果表明:随着电压的增加,电场力耦合因素的影响将导致误差增大;在每个电压工况下最大误差均出现在双峰之间的谷值附近,但均<15%,总体上该模型能较好地与试验数据相符。     

Harmonic spraying of conducting liquids employing AC-DC electricfields

This paper reports the results of the work carried out to investigate the breakup process of charged conducting liquid jet under the influence of an AC field superimposed onto a DC field. The main aim of this paper is to optimize the spraying parameters and to generate uniform droplets of highly conducting liquid in a controlled fashion by varying the strength and the frequency of the applied AC field. The electric field at the tip of a blunt needle facing a ground electrode was examined using a finite-element modeling technique. Three types of ground electrode configurations were investigated. A mathematical model for the electrostatic force on the leading edge of the droplet as a function of the liquid flowrate, the applied voltage, and distance between the capillary tip and the ground electrode has been developed. The breakup and droplet formation processes were investigated using a telemicroscopic lens and charge-coupled device (CCD) camera. The video data was processed using frame grabber software installed on a PC. The digital images were then analyzed utilizing image analysis software to give information on the breakup process and the size of droplets. A frequency range was identified where the formation of the droplets was synchronized with the applied AC field frequency. The effects of liquid flowrate on the synchronous bands were examined. The experimental results showed the dependency of the droplets size and the emission frequency on the applied AC frequency     

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     

双流体雾化荷电特性试验

为了有效改善石灰浆喷雾烟气脱硫中的浆液雾化,提高烟气脱硫效率,在引入双流体静电喷雾技术中,建立了环形电极的感应荷电的数学模型,计算了电极诱导的电位;采用目标网状法设计了雾滴荷质比Aq的测定装置,测量了不同浆液质量分数w(slurry)、气体流量Qg、液体流量Ql、荷电电压Uq下雾滴的Aq。结果表明,Aq随着Uq的增大而增加;在同一Uq下,Ql增大则Aq变小,Qg增加则Aq增大。随着w(slurry)的增大,Aq呈现下降趋势,在w(slurry)>1.0%时,Aq变化不明显。     

水滴高速撞击下汽轮机叶片的动力响应

为获得汽轮机叶片在高速水滴撞击下的应力状况,从而为进一步研究汽轮机叶片水蚀提供撞击应力等基础参数,该文建立了高速水滴与叶片钢撞击模型。模型中采用光滑粒子动力学无网格方法描述水滴部分、常规有限元有网格方法描述叶片钢材部分,充分考虑了水滴与钢材间的流固耦合效应,研究了多种速度和粒径的水滴对叶片的撞击情况,得到了水滴内部的瞬态压力分布以及水滴撞击以后的压缩、扩展、飞溅的情况,叶片钢内部应力影响区域以及应力随时间的变化,撞击应力与水滴尺寸和撞击速度的关系等。与文献果的对比表明：文中的分析结果是合理的,试验结相关参数可以应用于汽轮机叶片的水蚀疲劳研究。     

等离子体射流雾滴的荷电特性研究

为了从理论上探讨等离子体荷电过程出现的瞬时不稳定和突变现象,在高压电场下用网状目标法测量雾滴群体电流,用荷质比评价雾滴的荷电效果,试验研究了电晕产生的等离子体射流喷雾荷电过程。电极间气体电离过程的伏安特性分析、荷电过程的电学模型理论分析和试验结果表明:等离子体荷电喷雾产生的突变现象是极间气体电离,阻抗变化,从而引起极间电压降低,电流(荷质比)增大,是等离子荷电过程的固有特性;等离子体荷电喷雾雾滴的荷质比与荷电电压间存在线性关系,其荷电效果受电极的结构形式和尺寸、极间气体介质的厚度、雾化液体的介电性能及喷头和喷雾参数等因素的影响;安全高效的荷电电压要高于起晕电压,宜为15-20 kV。     

碱回收锅炉脱硫喷枪的模拟研究

以碱回收锅炉的脱硫喷枪为对象,运用数值模拟的方法研究雾化粒径大小、喷射速度和雾化角度对脱硫效果的影响。结果表明:液滴初始雾化直径对蒸发完全时间和水蒸气分布均有较大影响,粒径为400μm的液滴蒸发完全所需时间是粒径100μm的3.96倍;喷射速度和雾化角度对蒸发完全时间和水蒸气分布的影响程度各有不同,但均远小于雾化粒径的影响。     

Experimental investigation of droplet formation mechanisms byelectrostatic dispersion in a liquid-liquid system

Droplets having high uniformity and a widely controllable size range (from millimeters to micrometers) were obtained by means of electrostatic dispersion in an oil-in-water system, as reported previously by the authors. In liquid in-gas systems, many studies reported that the electrostatic force acting on the liquid due to the surface charge seemed to be the main factor affecting the atomization. However, using distilled water as a continuous-phase liquid causes the theoretical analysis to be very difficult, because distilled water has high conductivity and permittivity. Therefore, in the present study, experimental work is carried out on dispersion mechanisms in an oil-in-water system. The main factor affecting the atomization is considered to be electrohydrodynamic flow of continuous-phase liquid (distilled water) around the capillary nozzle tip. The amount of electric charge on the dispersed-phase liquid (kerosene) is considered to be negligible because of the very short relaxation time of the surrounding continuous-phase liquid. It is suggested that the droplet size and frequency distribution can be controlled by adjusting the operating conditions (applied voltage) and design parameters (nozzle shape)     

低浓度石灰浆雾滴荷电特性的试验研究

为深入探索石灰浆液荷电雾化脱硫的机理,对石灰浆雾滴荷电特性进行了研究。通过石灰浆雾滴荷质比、喷雾场的测试和机理分析表明:石灰浆雾滴的荷电效果与浆液流量、雾化角、充电装置结构参数、充电电压等诸多因素有关;荷质比随浆液流量、雾化角的增大而增大,但雾化角进入稳定值后继续增大流量对提高荷质比的作用不明显;电极放电电压随喷嘴与电极距离、名义雾化角和流量的增大而减小;在本试验条件下,流量<45 L/h时存在最佳电压值,约为11 kV,流量>45 L/h时荷电电压值宜控制在约12 kV;石灰浆雾滴荷电能有效地使雾滴细化而且分布趋向均匀。     

Understanding discharges initiated by water droplet on epoxy nanocomposites under DC voltages adopting UHF technique

Corona inception voltage due to a water droplet on epoxy resin in an electrode gap is high under DC voltage compared to AC voltage. It is observed that, as the contact angle of the epoxy nanocomposite material becomes higher, the corona discharge inception voltage increases. The droplet movement is observed, using a high‐speed camera, on application of the voltage. It is seen that a droplet moves toward the ground electrode under an AC or a negative DC voltage, whereas it moves toward the high‐voltage electrode under a positive DC voltage. It is also observed that carbonization occurs near the ground electrode under AC and negative DC voltages, and near the high‐voltage electrode under a positive DC voltage. During the evaporation of the water droplet (during arcing) on the surface of the insulating material under AC and DC voltages, carbonization of material occurs and is high both in pure epoxy resin and in nanocomposites with 5 wt% epoxy clay. The magnitude of the arcing current is nearly the same irrespective of the percentage of clay in the epoxy nanocomposites. The magnitude of discharge current flow is high under negative DC voltage compared to positive DC/AC voltages. The rise time of injected current pulses, at the time of corona inception and during arcing, under AC/DC voltages, is a few nanoseconds. Ultrahigh‐frequency signals were emitted as a result of the corona discharge from the water droplet on epoxy nanocomposites and at the time of arcing between the droplets and the electrodes, both under AC and DC voltages, with its dominant frequency in the range 1–2 GHz. © 2013 Institute of Electrical Engineers of Japan. Published by John Wiley & Sons, Inc.     

憎水性表面水珠的电场计算及放电研究

用ANSYS有限元分析软件对不均匀电场(针-板电极)中憎水性表面分离水珠的电场进行了计算,比较了无水珠、单个水珠以及多个水珠存在时憎水性表面沿面电场分布的变化情况,计算出水珠存在引起固、液、气三重介质连接点处的畸变,且水珠的存在对强场区电场的畸变作用更显著。同时在实验室中进行了模拟试验,用10000帧/s的高速数字摄影仪对整个放电过程进行了全程捕捉。结果表明,人工布液方式下憎水性表面分离水珠的放电是存在明显熄灭重燃现象并由两端向中间发展并沿水珠顶部贯通的细小电弧连接所致。     

雨天交流输电线路电晕放电判据

电磁环境是制约特高压输电线路结构设计与导线选型的关键因素,通过高速摄像机和放电脉冲的同步观测,研究了水滴下落、附着和离开导线时的放电特征;利用有限元软件分析了水滴和导线之间的电场分布;基于有效电离积分分析了水滴位置、大小、导线电压对水滴滴落放电的影响,提出了水滴滴落放电的起始判据。研究发现,雨滴与导线之间的间隙放电产生的放电电流脉冲远高于雨滴附着在导线表面时产生的电晕放电,是造成雨天交流线路电磁环境问题的主要原因;伴随水滴的滴落过程,有效电离积分经历了一个先升高后降低的过程;有效电离积分的极值随电压的升高而显著上升,随水滴半径的增加而小幅增加。     

Natural Periodic Electrostatic Spraying of Liquids

Liquid issuing from the bore of a metallic capillary tube maintained at a high potential may, under certain conditions, be dispersed in the form of regular current pulses of charged droplets. This phenomenon is well known but not fully understood. The pulsation process is considered in some detail, and experimental data obtained during the electrostatic dispersion of pure and impure glycerol, in air and under vacuum, are presented. The variations of pulse characteristics with capillary voltage are studied together with cinephotography of the pulsating liquid meniscus at the tip of the capillary. Dripping and periodic spraying modes are identified. In air, corona discharges severely perturb and limit the spraying mode. In the case of impure glycerol (doped with an ionizable salt) spraying in vacuum, highly charged particles, probably cluster ions, are emitted at the beginning of each pulse.     

Nanosecond, electrical triggering of water switches

Spark gaps, which use water as a switching medium, allow fast closing and high repetition rate operation for high power and high voltage switching. They are usually operated in the pulse-charging mode and have a large jitter. To reduce the jitter, a trigger electrode with positive polarity is used to initiate breakdown between a negative high voltage electrode and a ground electrode. The trigger electrode consists of a tungsten wire with a diameter of 50 μm, enclosed in glass tubing. At the triple point where the three dielectrics (glass, water and metal) meet, the electric field is greatly enhanced. From the triple point, a trigger voltage of +16 kV at a pulse duration of 100 ns produces two streamer branches traveling towards the high voltage and ground electrodes. These two streamer branches bridge the main gap and serve as preionized channels for the initiation of the breakdown between the main electrodes. Arc transition develops along the two channels and completes the switching. It allows for the gap to be fired at various voltages independent of the gap?s self-breakdown voltage. The delay between application of the trigger pulse and the electrical breakdown is on the order of hundred nanoseconds. The jitter in breakdown is on the order of ten nanoseconds. It is less by more than an order of magnitude than that obtained with pulse charging.     

活化器中钙基吸着剂的增湿活化过程模拟

建立了一维活化器中吸着剂颗粒的活化效率预测模型。模型全面考察了水雾蒸发、减速、粒径分布以及液－液凝并等的影响。计算结果表明,液滴初始直径和初速度对颗粒活化效率有最大的影响,使用大粒径、窄分布和高初速度的雾化方式可强化增湿活化效果。通过对颗粒活化效率与实测脱硫效率的相关性分析,证实了活化颗粒的溶液反应是脱硫过程的主要机理     

Pulse response measurement for locating water tree degradation in XLPE cables

Water treeing is a degradation mode of power cable with polymeric insulation. A water tree is composed of small droplets filled with water. As the conductivity in a water tree is very high, it leads to dielectric breakdown when it grows. Because the inside of the water tree is filled with trap sites, it is polarized with a certain distribution of relaxation time when a DC poling voltage is applied. Although its depolarization process after removing the poling voltage depends on the ambient temperature, applying a “depolarizing voltage” with the opposite polarity can accelerate the process. If a short pulse propagating through the cable is employed as a depolarization voltage, we may locate the water tree by looking at the time‐resolved pulse response. This would lead to a diagnosis method with spatial resolution. In order to retain 100‐m spatial resolution, the response should be as sharp as 1 μs. As a preliminary study, a coaxial communication cable was aged to form water trees. A DC poling voltage was applied followed by a pulse voltage with opposite polarity. The rise time of the pulse was several hundreds of microseconds. A sharp pulse current response 50 μs wide was observed, suggesting that rapid depolarization took place. No such response was seen when the cable specimen was not aged. We concluded that the technique is quite feasible. Because the response was found to be as quick as several microseconds, an experiment using a cable 405 m long with 5 m of degraded length in the middle was performed. It was shown that the degraded point was successfully located. © 2012 Wiley Periodicals, Inc. Electr Eng Jpn, 180(3): 18–24, 2012; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/eej.21294