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.     

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。     

Numerical modeling of near corona wire electrohydrodynamic flow in a wire-plate electrostatic precipitator

Numerical modelling of the flow velocity fields for the near corona wire electrohydrodynamic (EHD) flow was conducted. Solutions of the steady, two-dimensional momentum equations have been computed for a wire-plate type electrostatic precipitator (ESP). The equations were solved in the conservative finite-difference form on a fine uniform rectilinear grid of sufficient resolution to accurately capture the momentum boundary layers. The numerical procedure for the differential equations was used by SIMPLEST algorithm. The CFD code coupled with Poisson's electric field, ion transport equations and the momentum equation with electric body force, was used for the numerical simulation with the Chen-Kim k-epsiv turbulent model. The numerical results show that EHD secondary flow was clearly visible in the downstream regions of the corona wire despite the low Reynolds number for the electrode (Re _cw=12.4). Secondary flow vortices caused by the EHD increases with increasing discharge current or EHD number, hence pressure drop of ESP increases     

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.     

Numerical Simulation of Three-Dimensional Tuft Corona and Electrohydrodynamics

The importance of high-voltage and low-current operation in the wire-duct precipitator has focused attention on collecting high-resistivity dust. The local current density of individual tufts is considerably higher even at a low average current level and therefore could contribute to both the formation of back corona in the collected-dust layer and the generation of the secondary flow. Numerical simulation for three-dimensional tuft corona is, successfully solved. The electrical characteristics of the tuft corona is investigated and the structure and role of the three-dimensional secondary flow and electrohydrodynamics in relation to transport of the fine particles is described.     

Corona discharge in the hyperbolic point-plane configuration: direct ionization criterion versus an approximate formulations

The paper presents an algorithm to simulate the electric corona discharge in the two-dimensional hyperbolic needle-ground plate configuration. The boundary conditions for the space charge density are derived from the direct ionisation criterion. This technique doesn't rely on the Kaptzov hypothesis and can be used for any shape of the discharge electrode. The proposed algorithm has been compared with two approximate ones. One uses the Kaptzov hypothesis that the electric field on the corona electrode surface remains constant, with the value derived from the ionisation criterion, assuming that the electric field is Laplacian. The other one mechanically applies the Peek formula for the cylindrical wire, after replacing the wire radius with the surface curvature radius at given point of the electrode. The differences between all three approaches are negligible for small corona currents; they are noticeable, but still small, for higher currents.     

Experimental studies of free conducting wire particle behavior between nonparallel plane electrodes with AC voltages in air

When a spherical conducting particle exists in an electrode system where electrical gradient force acts along the electrode surface, the particle tends to move towards the high field region. The possibility of appearance of this phenomenon is experimentally investigated with wire particles, which can easily produce the corona discharge on their ends resulting the action of a reactive force on the particle by a corona wind. The results show that a hovering particle travels deeper into the high electric field regions regardless of the corona discharge. Moreover, it is found that wire particle with corona discharge hovers near the negative electrode under DC voltage in atmospheric air due to different characteristics of corona discharge by the polarity. But under the AC voltage, the particle hovers near the bottom electrode only and invades in the high field region deeper than under the DC voltage.     

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     

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。     

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.     

AC Corona Charging of Particles

Measurements of the mobility statistics of dioctyl-phthalate (DOP) particles leaving an ac corona charger with various electrode configurations are reported. Mobilities are determined using a laser-Doppler velocimeter (LDV) to measure particle migration velocities in response to a transverse electric field imposed downstream of the charger. Charging models, which do not include effects of finite ion transit times, nonuniform fields, or differences between the distributions of positive and negative wire coronas, predict some of the mobility statistics, especially below 100 Hz, but cannot explain the high-frequency charging. In the turbulent flow with significant corona wind effects the disparate corona current distributions are shown to have a significant effect on the ac corona charging processes.     

注入功率及耗能密度对离子输运率的影响

大气压非平衡等离子体的输运特性直接制约着工业电收尘和飞行器等离子体隐身的进一步发展,影响该输运特性的因素较多,包括放电区的压力、气体流速、电场强度、注入功率密度及其耗能密度等。为提高大气压非平衡等离子体输运效率,以线板形式电晕放电为手段进行电离流经放电区气体的实验,运用DLY-3型大气离子测量仪测量脱离电场约束的正负离子浓度,研究了注入功率密度及耗能密度对离子浓度输运率的影响。研究结果表明,在一定程度上随着注入功率密度、耗能密度的增加,离子浓度输运率越高.但当注入功率密度和耗能密度达到一定值时,离子输运率变化趋于平缓;同时,高风速比低风速的等离子体输运率要高出几个数量级。     

多回HVDC输电线路地面离子流场的计算

HVDC输电线路离子流场的计算对于输电线路的设计和电磁分析具有重要意义,为此,笔者在Deutsch假设的基础上对地面离子流场进行解析求解。根据输电线路电力线分布特点,建立了适用于多回输电线路的离子流场算法并探讨了输电线路极性排列方式对于离子流场的影响。该算法可考虑正、负极性导线的起晕差异,实例计算表明:该方法的计算结果与实测值有较高的吻合度,并对各种线路结构具有良好的适应性;双回线路极性排列方式对于合成电场最大值影响不大,而离子流密度则受到较大影响;在不考虑输电线其他结构参数的情况下,双回输电线路以--/++的极性排列方式最优;地线的存在对于双回线路地面离子流场并无明显影响。     

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.     

Induction of Waves on a Horizontal Water Film by an Impinging Corona Wind

The electric wind produced by corona discharge of a high-voltage electrode in air is employed for destabilizing a horizontal water film. In wire-to-plane geometry, the phenomenon is characterised by current versus voltage curves and visual observations of the onset of free-surface oscillations. The effect of the following parameters is examined for both positive and negative coronas: distance between the wire and the film (S), film thickness (h), wire diameter (phi) and composition, applied voltage (HV), and relative humidity (RH). The free-surface destabilisation is retarded by increasing d and phi and is insensitive to h in the tested range. The onset of corona discharge is predicted by Peek's law and compared with the experimentally observed threshold. In negative corona discharge, the current values are higher and the film is destabilised at lower HV than in positive polarity. Humidity tends to decrease the corona current at a given HV. Correlations are proposed for the current-voltage curves, in terms of the mean electric field in the inter-electrode gap and of RH, satisfactorily agreeing with the experimental data. Both positive and negative corona currents turn out to be stable for days of operation. The power loss by corona discharge is in any case lower than 12 W. Wave induction on the liquid-gas interface can effectively enhance heat and mass exchange between the two phases.     

Flow visualization inside a wire-plate electrostatic precipitator

A flow visualization study was performed in a wire-plate precipitator with a plate-plate spacing of 20.32 cm and wire-wire spacing of 20.32 cm. Smoke was used as the flow tracer, where injection and illumination were accomplished by two different techniques. In one method smoke was released from a single heated probe and illuminated by electronic flash. The other method employed a uniform smoke flow made visible by a laser sheet. The latter technique proved to be superior, providing well-defined views of the flow in both streamwise and spanwise planes over a wide range of velocities (0.2-2 m/s) and current densities (0-0.5 mA/m). The illuminated flow patterns were recorded by still 35 mm photography and color video taping. Of particular interest was the interaction between the precipitator gas flow and the corona-generated electric wind for both polarities of discharge. Results showed that positive corona discharge produces a stable two-dimensional smoke flow with negligible turbulent dispersion for precipitator velocities greater than 0.7 m/s. Lower velocities allowed the electric wind to dominate producing unstable, recirculating flow with widespread turbulence. Negative discharges were inherently unsteady and three-dimensional at all operating conditions but displayed extreme instability and recirculation (similar to positive polarity) for precipitator velocities less than 0.7 m/s     

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.     

Distribution of electric field strength around a large-scalecharged particle cloud

To investigate an electrical discharge occurring from or in a space-charge cloud, a large-scale charged particle cloud was formed by using a cloud generator consisting of a blower and corona charger. The distribution of the electric field strength around a charged particle cloud has been investigated to determine the behavior of charged particles. The soil-conditioning particles were charged by corona charging and blown by high-speed air flow in a test room, 5 m wide, 10 m long, and 3 m high. The average charge-to-mass ratio of the particles blown by this method was 170 μC/kg. The space-charge density of the cloud was calculated at the order of 10 μC/m³ from the electric field strength outside of the cloud. While the electric field strength at the outside of the cloud increased up to 52 kV/m within 2 m downstream from the cloud generator, it decreased below 25 kV/m farther than 2 m away from the cloud generator due to dispersion of charged particles. The change in the electric field strength due to dispersion of charged particles can be qualitatively explained by a simple cloud model. The velocity of charged particles transported by air flow and mobility of charged particles are found to be effective factors increasing the electric field strength around the large-scale charged particle cloud     

The electric corona discharge in the triode system

In this paper, the charge density, electric field and corona current distributions in the corona triode are calculated using a hybrid technique; the finite-element method and the boundary-element method are combined with the method of characteristics. The characteristic lines are traced backward from points of the analyzed domain to the corona wire. The current density, electric field and space-charge density distributions can be controlled by changing the configuration of the system. The voltage applied to the grid plays an important role in controlling the corona current. Results of calculations in a number of different cases show the influence of different parameters on the characteristics of the corona triode     

Parallel streamer discharges between wire and plane electrodes in water

Streamer discharges in tap water and distilled water have been generated by applying a voltage pulse from 120 to 175 kV and 500 ns duration to a wire-to-electrode configuration. Electrical and optical diagnostics were used to explore the temporal development of the streamers in tap and distilled water, at various applied voltages and both polarities. With the wire serving as anode, multiple, parallel streamer discharges were generated. The number density of these streamers along the wire decreases with decreasing electric field on the surface of the wire. The dependence of the streamer density on electric field indicates the role of field enhancement at inhomogeneous microstructures along the wire as streamer initiation mechanism. The appearance of the discharge was different for tap and distilled water. However, the measured average streamer propagation velocity from the positive wire to the grounded plane electrode, of 32 mm//spl mu/s, was independent of the water conductivity and the applied voltage. This suggests the existence of a self-sustained electric field at the streamer head. With the wire serving as cathode, only a weak light emission from the area close to the wire was observed, and streamers did not appear for the same voltage amplitude as with the positive polarity. This suggests that an ionic current flowing in the water is not dominant in the streamer propagation process.