Three-dimensional ionic wind and electrohydrodynamics of tuft/point corona electrostatic precipitator

The two-dimensional flow interaction between the primary flow and the secondary flow (often called an electric wind or ionic wind) in the wire-duct electrostatic precipitator (ESP) has been investigated theoretically and experimentally in the past. However, the analysis was limited to the two-dimensional, which is acceptable only for the positive polarity and small tuft spacing. The negative corona, commonly used for the industrial ESPs, generates tufts along the corona wire and the point coronas on the discharge wire and requires three-dimensional analysis. Three-dimensional electric field and space-charge density distributions, and the flow interaction between the primary flow and secondary flow, i.e., electrohydrodynamics were investigated. The computational results show that the secondary flow distribution consists of a donut-shaped ring from each tuft or corona point, which was predicted by the first author 16 years ago. When the primary flow exists, a pair of spiral rings, like Goertler vortices, is formed in the direction of the primary flow. The flow interaction was described using dimensionless number N/sub EHD/, which is the ratio of the ionic wind velocity to the primary flow velocity. The effects of particle motion in the electrohydrodynamic field in the tuft/point corona ESPs are discussed.     

Numerical Simulation of the Electrohydrodynamic Flow in a Single Wire-Plate Electrostatic Precipitator

In this paper, the electrohydrodynamic (EHD) flow in a single wire-plate electrostatic precipitator has been analyzed numerically by employing a hybrid Finite-Element Method-Method of Characteristics numerical algorithm and the commercial computational fluid dynamic software FLUENT6.2. Parameters defining both the electric and the flow fields were predicted. The interaction between the electric and the flow fields has been discussed by adopting the proposed two-way coupling algorithm. With the help of the dimensionless analysis, an entire flow regime map has been developed for different Reynolds (Re) and EHD numbers. It was found that, at very low Re (15) and high EHD (2000) (based on the corona wire) numbers, the EHD flow dominates in the channel; as Re increases, the significance of the EHD wakes gradually decreases; at a higher Re (50) or a lower EHD (500) numbers, the EHD flow becomes the near-corona-wire phenomena, and the main flow dominates the channel. The possible influence of the EHD flow on the particle-collection efficiency was also discussed.     

Effects of EHD and External Airflows on Electric Corona Discharge in Point-Plane/Mesh Configurations

The effect of airflow, including the electrohydrodynamic (EHD) flow produced by the electric corona discharge, and the flow generated externally on the corona discharge has been investigated numerically in this paper. Contrary to the common assumption that the corona discharge and the EHD flow can be decoupled by neglecting the convection term in the current density equation, the equations for electric field, charge transport, and fluid flow have been solved simultaneously. The numerical algorithm is based on boundary and finite element methods, the method of characteristics, and the finite volume method. The simulation results confirm that the EHD flow has a very small effect on the corona discharge. However, in some configurations, the external airflow may have considerable effect on the I- V characteristics of the system and the current density distribution on the ground electrode.     

Effect of Discharge Electrode Parameters on the Flow Velocity Profile of the Wire?rod Type Electrohydrodynamic Gas Pump Exit

Experimental investigation has been conducted to study the effects of corona wire diameter, pipe length, and corona polarity on outlet flow velocity distribution profile of a wire-rod type electrohydrodynamic (EHD) gas pump. Upon applying negative or positive dc high voltage between a wire electrode (outer diameter (o.d.) 60 μm, 200 μm, or 300 μm) and a rod electrode (o.d. 3 mm) in atmospheric air, corona discharge occurs and EHD gas flow is generated in the direction from the wire electrode to the rod electrode through a cylindrical pipe (inner diameter (i.d.) 20 mm). For both polarities, the discharge current and average flow velocity increase monotonically on increasing the applied voltage before the onset of spark discharge. Using wire electrodes with a smaller diameter, stable corona discharge between corona onset and spark onset is generated in a wider voltage range, and the discharge current becomes larger, resulting in a higher flow velocity. The maximum average flow velocity of 2.0 m/s, corresponding to a flow rate of 38 l/min, was achieved with a wire of diameter 60 μm by applying a voltage of ?16 kV.     

Numerical simulation of three-dimensional electrohydrodynamics of spiked-electrode electrostatic precipitators

The three-dimensional flow interaction between the primary flow and the secondary flow (often called an electric wind or ionic wind) for tuft/point corona in the wire-duct type electrostatic precipitators (ESPs) has been investigated in the past. This study was further extended to incorporate the alternately oriented point corona on the wire-plate type electrodes, which are commonly used for the industrial ESPs. The secondary flow distribution without gas flow consists of a pair of long-elliptic and circulatory cells between spikes points along the wire. However, the flow rotation between spiked points is the same direction, which is opposite to that of the point corona electrode. The flow interaction is described using dimension-less number, N/sub EHD/, which is the ratio of the ionic wind velocity to the primary flow velocity. When the primary flow exists, a pair of long-elliptic spiral flows is formed in the direction of the gas flow between ground plates. When the relative distance between the spike point spacing and the wire-to-plate spacing (or the ratio of S/sub z//d) is greater than 0.17, the organized long-elliptic spiral flow is formed. When S/sub z//d is less than 0.1, the organized spiral flow is diminished, resulting in turbulence.     

Flow visualization and image analysis of gas-phase AC corona discharge induced electrohydrodynamic liquid flow in a stratified fluid

A phenomenon of AC gas-phase needle-plate corona discharge induced electrohydrodynamic (EHD) liquid flow in a stratified fluid has been investigated experimentally. The flow visualization of flow pattern is carried out by a fluorescent dielectric liquid tracer, where the steady EHD liquid motion induced by the corona discharge is presented in visual impressions. The results show that the EHD liquid flow direction on an electrode axis is not from needle to plate electrodes axis when the needle position was above the liquid layer. For the case of the needle electrode tip immersed into the dielectric liquid, it is well known that the dielectric liquid flows in anticlockwise rotation by contraries. The flow velocity distribution is experimentally obtained by a particle image velocimetry with image processing, where the measurement system takes sequential digital images of flow field illuminated by a laser light sheet for computed tomography. The mechanistic model based on interfacial momentum transfer effects on liquid-phase fluid motion is discussed in detail.     

Numerical Simulation of the 2-D Gas Flow Modified by the Action of Charged Fine Particles in a Single-Wire ESP

A numerical model for simulating precipitation of submicrometer particles in a singlewire electrostatic precipitator is discussed in this paper. It includes all important phenomena affecting the process: electric field, space charge density, gas flow, including the secondary electrohydrodynamic flow caused by the corona discharge and charged particles, and particle transport. A simplified corona model assumes just one ionic species and neglects the ionization zone. The fully coupled model for the secondary EHD flow, considering the ion convection, has been implemented. The dust particles are charged by ionic bombardment and diffusion. The gas flow pattern is significantly modified by the secondary EHD flow, which depends on the particle concentration. As for fine particles the drift velocity is small and particles practically follow the gas streamlines, the particle concentration has a very strong effect on the precipitation efficiency.     

An EHD Gas Pump Utilizing a Wet Porous Point Electrode

A point-to-mesh type electrohydrodynamic (EHD) gas pump, utilizing a wet porous point electrode, has been proposed, and the effect of the wet porous point electrode on the gas flow velocity and flow generation yield of the EHD gas pump was investigated. The wet porous point is used as a corona discharge electrode, and the mesh is used as an ion collecting electrode. It was observed that the EHD gas pump with the wet porous point electrode can generate a higher gas flow velocity and flow generation yield than the same design EHD gas pump with a non-porous metal point electrode. As a result with the wet porous electrode, a peak gas flow velocity of V_W=2.54, 2.74, and 1.93 m/s has been obtained for positive, negative, and ac corona discharges. This is 1.95, 1.31, and 1.30 times higher than the velocity V_W =1.30, 2.09, and 1.49 m/s measured with the same design EHD gas pump and a non-porous) metal point electrode. At 0.1 W of input corona power for dc and ac powers, the flow generation yields of Y=13.88, 12.80, and 11.70 m/s/W were obtained with the wet porous point electrode for the positive, negative, and ac corona discharges, which are 1.23, 1.24, and 1.15 times higher as compared with those of the metal point electrode with Y=11.32, 10.29, and 10.20 m/s/W, respectively. These enhancements may be due to the elevated input corona powers and the water particles generated from the wet porous point electrode.     

Corona inception in typical electrode configurations forelectrostatic processes applications

The corona inception electric field at the surface of a wire electrode is usually evaluated by an empirical formula established by Peek. That formula was found to be valid in several simple electrode configurations (wire cylinder, wire plate), which are typical to electrostatic precipitators. The aim of this paper is to extend the study to other situations encountered in electrostatic applications. The experimental setup specifically modeled the several electrode arrangements which are commonly used with roll-type electrostatic separators. It consisted of wire-type corona electrodes, connected to a regulated DC high-voltage supply, and a rotating roll electrode, connected to the ground. The effect of associating the corona wire to one or several tubular electrodes of various shapes and sizes was investigated. The experimentally determined corona inception voltage was used as input data of a boundary-element-method program for the electric field computation. The computed values of the electric field were compared with those given by Peek's formula for wire electrodes of the same radius. The derived conclusions can be of help in the custom design of the corona electrode arrangements for various electrostatic applications     

Electrohydrodynamically Enhanced Capillary Evaporator

A two-phase capillary evaporator has been enhanced using electrohydrodynamic (EHD) forces. The evaporator was experimentally investigated under DC applied electric field operating modes as well as without an applied electric field. The results without the applied EHD forces show that the induced flow increases with increasing applied heat loads until a dry out condition is reached. The applied DC voltage electric field provides flow enhancements as high as 200%. A slight polarity effect was observed.     

离子风对静电除尘器中颗粒物脱除的影响机制研究

静电除尘器中电晕放电产生的离子风是影响颗粒迁移和沉积的重要因素,通过建立多过程耦合的静电除尘器数值模型,研究了不同极配型式静电除尘器的电场分布特性及离子风对空间流场和颗粒物脱除的影响规律。结果表明,针刺电极电场强度和离子电荷密度的最高值均略高于芒刺电极,但是芒刺电极的静电场分布更加均匀。在外加电压为60k V时,针刺电极和芒刺电极的离子风最高流速分别可达7.91m/s和4.62m/s,离子风会促进荷电颗粒向收尘极板运动,同时也会形成近壁高速区造成堆积颗粒的不均匀分布并导致二次扬尘。由此,针对不同形式静电除尘器提出了合理调整放电电极结构方法,进行离子风调控以强化颗粒物脱除。     

特高压直流输电线下的直流离子流电场

针对特高压直流输电导线发生电晕的情况,采取测量试验来探究直流导线产生电晕以后导线下方的离子流对地面电场的影响。通过试验,得出直流导线产生电晕以后导线下方的合成场强、标称场强、空间电荷场强分别与导线电位成线性关系,正、负极导线放电电晕情况有差异以及标称场强在合成场强中的比重随高度变化而改变等。     

Three-dimensional (3-D) model of electric field and space charge inthe barbed plate-to-plate precipitator

A finite-element/method-of-characteristics model of three-dimensional (3-D) electrode geometries with corona discharge is used to predict space charge density, current density, electric potential and electric field in point-to-plane, single-barb plate-to-plane, and hexagonal multiple-barbed plate-to-plate electrostatic precipitator (ESP) geometries. Although a modification of Peek's formula for the hyperboid-to-plane was initially used to establish a boundary condition at the edge of the corona, predicted total current did not agree with measured values. As a result, it was necessary to use measured current-voltage characteristics to establish the space charge density at the outer surface of the corona sheath. An additional problem in modeling point discharges is specification of shape and size of the corona sheath. Both the authors' results and much earlier work by Trichel suggest that the thickness of the corona sheath cannot be automatically neglected     

Bi-Ionized Space Charges Generated by Means of Corona

Due to the extensive usage of unipolar corona in electrostatic precipitators, electrostatic painting, mineral ore separation, and other applications, most of the past studies of particle charging and electric fields in space-charge regions have focused on the unipolar corona. As such, while the basic equations for particle charging in bi-ionized fields are known, relatively little experimental work may be found in the literature. A test chamber and a wire probe assembly have been constructed allowing the investigation of electric fields in the bi-ionized space between parallel wire coronas for various gas mixtures, pressures, and spacings. Results are presented for experiments in air, CO2, and CO2-nitrogen mixtures, and they seem to indicate that a substantial amount of recombination of positive and negative ions occurs. The resulting average electric field intensity over a large section of the bi-ionized space between the corona wires was found to be uniform.     

不同海拔下电晕笼分裂导线起晕电压的计算分析

为了研究海拔高度对电晕笼分裂导线起始电晕电压特性的影响,建立电晕笼钢芯铝绞线起始电晕电压的计算模型,并开展相应试验研究。采用模拟电荷法计算钢芯铝绞线的空间电场强度。依据极不均匀电场下自持放电判据,建立不同海拔高度电晕笼分裂导线电晕起始电压的计算模型。在超/特高压人工环境气候试验室内,以500 m海拔高度为间隔,系统开展19~4-000-m海拔高度范围内六分裂导线起始电晕电压的试验研究。试验获得超高压电晕笼不同海拔高度下6-LGJ—400/50、6-LGJ—500/45分裂导线的起晕电压。计算获得不同海拔高度、分裂间距、导线分裂数及绞线表面粗糙系数下的导线起晕电压曲线族,以及不同绞线半径及最外层铝绞线股数的表面粗糙系数计算结果。分析结果表明:计算模型能够较好地计算电晕笼内绞线的起晕电压;在350~500-mm分裂间距范围内,分裂导线起晕电压随着分裂间距的增大而降低,随着导线分裂数的增加而升高;绞线表面粗糙系数与绞线最外层铝线半径与绞线半径之比相关。     

单区静电除尘器捕集烟尘过程的数值模拟

通过得到烟尘的颗粒尺度谱(PSD)演变过程的详细信息来定量描述静电除尘器(ESP)对烟尘的捕集过程,此时需要同时考虑静电力、流体对颗粒的水平输运力和颗粒横向湍流和布朗扩散作用等烟尘捕集机制。发展事件驱动常体积法来求解考虑烟尘捕集的通用动力学方程以得到颗粒尺度谱的演变过程。对于典型的火电厂单区ESP进行的数值模拟结果表明：ESP对直径大于10mm的烟尘具有良好的捕集效果,也能有效脱除直径小于0.02mm 的烟尘,但是对粒径范围在0.02~10mm的烟尘的除尘效率相对较低;这种"U"型分级除尘效率主要是由于颗粒自身惯性力与静电力之间的相互竞争的结果;布朗扩散机制对任何尺度颗粒的捕集效果影响很小;减小气流输运速度和增加电场强度有利于任何粒径的烟尘颗粒的脱除。     

Agglomeration of particles by ac corona discharge

Improving the collection efficiency for particles smaller than 1 μm on every precipitator is important. We sought to improve the collection of these particles on an ESP due to particle agglomeration. Particles are charged by ac corona discharge in a precharger and agglomerated by a dc electric field in an agglomerator downstream of the precharger. Diesel exhaust particles were used as particulate matter for the experiments. The distribution of particle size was measured using a particle counter and a scanning electron microscope. By these methods, particles as small as 0.01 μm could be counted. Results showed the agglomeration between particles at ac corona discharge operating mode. The concentration of particles smaller than approximately 0.35 μm decreases, and that of particles larger than approximately 0.35 μm increases in the agglomerator. The agglomeration rate increases with increasing applied voltage, then saturates. These results may be due to the size distribution and to decrease of concentration by agglomeration. © 1999 Scripta Technica, Electr Eng Jpn, 130(1): 30–37, 2000     

An EHD Gas Pump Utilizing a Ring/Needle Electrode

An electrohydrodynamic (EHD) gas pump, utilizing a ring/needle electrode, has been proposed and the effect of the ring electrode to the flow velocity and yield of flow generation of the EHD gas pump was investigated. The needle/ring is used as a corona discharge electrode and the mesh is used as the ion collecting electrode. It was observed that the proposed type of EHD gas pump can generate a higher flow velocity and yield of flow generation as compared with that from the same EHD gas pump without the ring electrode. As a result, a maximum flow velocity of V_W=4.54 m/s from the proposed EHD gas pump can be obtained at V=15.0 kV, which is 1.2 times higher than that of V_W =3.82 m/s of the same needle-to-mesh type EHD gas pump without the ring electrode. With the ring electrode, the yield of flow generation of Y_W=46.15 m/Ws can be obtained at V=11 kV, which is 2.5 times higher than that of the design without the ring electrode of Y_W=18.41 m/Ws. These enhancements, however, may be due to the effect of the ring electrode installed near the needle electrode.     

The effect of high-voltage waveforms on ESP current densitydistributions

The efficiency of an electrostatic precipitator (ESP) collecting high resistivity dust is degraded by maldistribution of corona current in the interelectrode space. The current distribution can be improved by applying novel high-voltage waveforms to the discharge electrodes. Online measurements are presented of collecting plate current density distributions in a precipitator collecting fly ash from a coal-fired electric generating plant. The precipitator was operated with both round-wire and barbed-strip discharge electrodes. The current density distributions are compared for three high-voltage waveforms: conventional, pulse, and intermittent energization. A direct relationship is established between the ability of the waveforms to distribute useful values of corona current and the collection efficiency of the ESP     

纳米TiO2填料对变频电机耐电晕电磁线绝缘性能的影响

测试和分析了某型耐电晕电磁线绝缘漆中无机填料的化学组成和微观形貌,并测试该电磁线绝缘漆的紫外-可见光吸收谱.从无机填料效用的角度研究了纳米TiO2在绝缘破坏过程中的电、光及热效应.纳米TiO2填充改性绝缘的电磁线遭受电晕破坏时,析出的纳米TiO2微粉层可以改善电场分布,提高热传导能力,并在绝缘表面形成电子和紫外线屏障,捕获电晕放电产生的电荷,吸收紫外线.这些研究可为开发变频电机用耐电晕电磁线提供理论参考依据.