Experimental Study of Pumping of Liquid Film With Electric Conduction Phenomenon

In electrohydrodynamics (EHD) conduction pumping, Coulomb force is the main driving force for fluid motion. The process of dissociation and recombination of electrolytic species produces heterocharge layers in the vicinity of electrodes. Asymmetric electrodes are used to arrange heterocharge layers to induce net motion in fluid. In this paper, EHD conduction pumping is experimentally studied with stratified liquid film in a channel. Liquid flow rate and the generated pressure are measured for different applied voltages and liquid film heights. Two different designs of electrode pairs, including flushed electrodes having virtually no drag force, are considered in this paper. Liquid pumping using EHD conduction phenomenon is significant for heat transfer and mass transport in thermal equipment of macro- and microsizes, in the presence and absence of gravity. This is particularly true for nonmechanical pumping of liquid film in phase change equipment.     

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.     

Fluid circulation in an enclosure generated by electrohydrodynamic conduction phenomenon

Fluid circulation/mixing inside an enclosure, nonmechanically, is of significant importance in various applications. This study investigates, theoretically, the circulation of an isothermal dielectric liquid inside an enclosure based on the electrohydrodynamic (EHD) conduction phenomenon. The theoretical model and the numerical solutions are presented in dimensionless forms to illustrate the effects of the controlling parameters on the induced flow. The EHD induced flow, based on the conduction phenomenon, is associated with the process of dissociation of the neutral electrolytic species and recombination of the generated ions. The charges generated by dissociation are redistributed in the region by the applied electric field resulting in heterocharge layers in the immediate vicinity of the electrodes. The Coulombic attraction between a given electrode and the charges within the corresponding heterocharge layer induces a fluid motion near that electrode. With a proper electrode design, a significant flow circulation/mixing can be achieved within an enclosure based on the conduction phenomenon.     

Innovative electrode designs for electrohydrodynamic conduction pumping

In the absence of a direct charge injection, pumping of an isothermal liquid can be achieved through the charges associated with the heterocharge layers of finite thickness in the vicinity of the electrodes. These charges are based on the process of dissociation of a neutral electrolytic species and recombination of the generated ions. This type of pumping is referred to as electrohydrodynamic (EHD) conduction pumping. The EHD conduction pumping mechanism is experimentally investigated with four innovative electrode designs, with refrigerant R-123 as the working fluid in this paper. Specifically, two different types of perforated disks and two different types of porous disks are used as the high-voltage electrode while a ring is used as the common ground electrode for the four designs. Pressures up to approximately 1400 Pa (at 18.5-kV potential with 0.23 W of power consumption) are generated with one electrode pair. Such pressure generations make the EHD conduction pumping attractive to certain applications in the presence and absence of gravity, such as capillary pumped loops and heat pipes.     

Control of liquid flow distribution utilizing EHD conduction pumping mechanism

A maldistribution of liquid flow in parallel evaporators may cause local hot areas, leading to dry-out of branch lines and resulting in a nonoptimum performance. The electrohydrodynamic (EHD) conduction pumping shows its potential as an active control method of the flow distribution. The EHD conduction pumping is associated with the heterocharge layers of finite thickness in the vicinity of the electrodes, which are based on the process of dissociation of the neutral electrolytic species and recombination of the generated ions. This paper presents the successful control of liquid flow distribution between two branch lines utilizing an EHD conduction pump at total mass flux levels of 100 and 200 kg/m/sup 2//spl middot/s.     

The influence of electrodes and conditioning on space chargeaccumulation in XLPE

The accumulation of space charge in planar crosslinked polyethylene (XLPE) samples under dc electric fields at room temperature was investigated using the pulsed electroacoustic (PEA) method. Three different organic semiconductor (semicon) materials containing carbon black at concentrations ~30 %wt, and vacuum-evaporated gold, were used as electrodes. Three different conditioning procedures were investigated. Unconditioned samples developed heterocharge with density increasing from zero at the electrodes, to a maximum at roughly one-third of the thickness, and then falling to zero around the center. They also developed homocharge close to the electrodes. The choice of electrode material had little effect on the heterocharge profile in unconditioned samples. Conditioning by holding at 80°C for four days, at rotary pump pressure or at atmospheric pressure, suppressed the accumulation of heterocharge. Homocharge accumulation close to the electrodes in samples with semicon electrodes was affected little by this conditioning, but was reduced considerably in samples with gold electrodes. Conditioning by holding at room temperature for seven days at rotary pump pressure had little effect. The heterocharge originates in inhomogeneous polarization due to a spatially inhomogeneous distribution of permanent dipole molecules, probably acetophenone. The dipole concentration decreases in going from the electrodes towards the center of the samples, as a result of diffusion of the molecules in the opposite direction. The homocharge originates in electronic charge injection/extraction across the electrode/XLPE interfaces     

Streamer current in a three-electrode system

A study has been conducted on positive streamer discharges in air at atmospheric conditions for a three-electrode system, The electrode system consisted of two parallel planes (one grounded and one supplied with a negative dc voltage) and a small, insulated needle, sticking out from the center of the grounded plane. A triggering positive square impulse voltage of 5 μs duration was applied to the insulated needle and the currents associated with the streamer discharge were measured simultaneously on all three electrodes. During the streamer propagation, the current measured at the needle was the conduction current while the other two were the displacement (or capacitive) currents generated by the movement of charge in the electrode gap. The objective of this study is to identify the three currents and to investigate if simple representations of the streamer can reproduce the displacement currents measured at the plane electrodes. Two models for the streamer were applied: (1) a charged sphere moving in the background field and (2) a channel with a constant voltage gradient extending in the gap. In both models it was assumed that the streamer propagated with a constant velocity, which was estimated from the measurements. The motion of the streamer was simulated by a series of electrostatic calculations, using a field calculation program. Comparison of the measurements with the simulations indicates that the charge of the streamer is confined to a spherical region (i.e. streamer head) and it is increasing continuously during its advancement in the electrode gap. A discussion on advantages and disadvantages with the two investigated models (sphere vs. channel with potential gradient) is conducted, and a possible hybrid model is suggested. In the proposed model, features from both considered streamer representations are included     

温度梯度下双层油纸绝缘系统的空间电荷分布特性

换流变压器为直流高压输电系统中的重要设备,但因其运行工况的特殊性,其内部油纸等绝缘材料往往承受较大的内外温度差异作用。针对换流变压器中双层油纸绝缘系统存在的内外温度不同的温度梯度效应对油纸绝缘中空间电荷分布的影响,利用电声脉冲法测量了不同温度梯度下（Δθ=0、20、40°C）,电压分别为-3.4、-10.2、-20.4kV（即平均电场强度分别为10、30、60MV/m）时双层油纸中空间电荷在20min内随时间变化规律及电场分布。实验结果表明：在低场强下,上下电极均有明显的同极性电荷注入;温度梯度使低温侧出现异极性电荷,并且随着温度梯度的增大、电压的升高,低温侧异极性电荷越来越多,从而使低温侧的场强畸变越来越大;加压后,2层油纸界面处开始积累与上电极相同极性的电荷,并且界面处电荷量随温度梯度及电压的增大而增大,但电荷密度出现饱和现象。经分析可知,油纸界面起到阻挡正负电荷通过的作用,而温度对电极注入特性及试样电导特性的影响为温度梯度影响空间电荷分布的主要原因。     

接地网垂直接地极地表传导电流分析

垂直接地极作为一种有效降低接地网接地电阻的方法,在接地网图纸缺失或图纸与实际不符情况下,面临准确探测位置等问题。根据安培环路定律,提出通过测量地表传导电流来探测接地网垂直接地极位置的方法。通过理论公式推导,推算垂直接地极及其连接地网支路在地表产生的传导电流分布情况;再构建有/无垂直接地极的接地网模型,仿真垂直接地极对地表传导电流的影响。理论分析和模型仿真验证表明,测量地表传导电流能够有效地探测垂直接地极位置,方法简单,可进一步用于接地网3D结构成像。     

On Electroconvection Between Nonparallel Electrodes and Its Possible Use to Model General Atmospheric Circulation

From the knowledge of ionic injection from metal electrodes in an insulating liquid, the modeling of geophysical flows is proposed. More particularly, it would be desirable to the general atmospheric circulation with a laboratory experiment where an insulating liquid, lying between two metal electrodes (a spherical one and an ellipsoid one) is subjected to both an electric force and the effect of rotation around an axis. The first step in solving this problem is to study the hydrodynamic stability of an insulating incompressible liquid layer lying between two concentric conducting spheres subjected to a unipolar injection by one or the other of the two spheres. The linear critical conditions are determined for various values of the different parameters. Secondly, we take into account the ellipticity of one electrode in a circulation model by studying the flow of a liquid between two planes lying at a small angle to each other. The experiments exhibit such a flow in conditions where volumic conduction is still dominant. An explanation is proposed which accounts for the variation law and the order of magnitude of the liquid velocity.     

Effects of constituents of XLPE on the formation of space charge

The effects of such constituents as the crosslinked part, non-crosslinked LDPE part, antioxidant, and residual byproducts on the formation of space charge in XLPE have been investigated. It has been found that homocharge is developed in the pure crosslinked part of XLPE. The non-crosslinked LDPE part encourages the formation of heterocharge, although its influence is not strong. Residual byproducts generated during the crosslinking reaction encourage the formation of heterocharge, whereas antioxidant was found to prohibit the formation of heterocharge. Of these, residual byproducts have been found to impose the most pronounced influence on the formation of heterocharge in XLPE, so that only heterocharge is found in an as-pressed XLPE with semiconductive electrodes     

多高压电极系统电晕放电的研究

研究了多高压电极的放电特性现象。结果表明，由于各高压电极间受电场相互屏蔽作用的影响，使电晕放电通道的光学图形发生了向电极外侧偏移，同时各高压电极间距处在一定范围内可提高电晕放电起始电压。还利用分割电极的方法测量了平面电极上的电流密度，其极大值点可相应向外侧偏移，在负极性放电时在一定条件下能产生正流注放电。     

触发真空开关的导通特性

介绍触发真空开关导通特性的实验研究结果,研究了不同的电极极性,电极距离,电极形状,不同的触发方式和触发能量以及主电路负载性质对开关导通特性,特别是最小导通电压的影响,实验结果表明,触发真空开关的最小导通电压比火花隙开关小两个数量级,因而具有工作电压范围很宽的优点。     

Multiple-needle corona electrodes for electrostatic processesapplication

Corona from high-voltage electrodes is employed in various electrostatic installations, such as ozonizers, air filters, powder sprayers and separators. Multiple-needle electrode designs are preferred whenever low corona onset voltage and good resistance to mechanical shocks are required. This paper aims at identifying a simple solution to overcome the main drawback of this type of electrode, the nonuniformity of the generated space charge. Experiments were carried out with various models of electrodes, having one or several rows of stainless steel needles. A current probe, consisting of an enameled copper wire (0.4 mm diameter), was embedded in the center of a plane collecting electrode. For a given interelectrode distance (5-50 mm) and a fixed position of the multiple-needle electrode, the collecting plate was translated along two orthogonal directions, so that the current probe could scan a 75 mm×75 mm square. The results depended on the interelectrode distance and on the high-voltage level. An explanation is given to some observations made on a roll-type electrostatic separator provided with two models of multiple-needle corona electrodes. An improved electrode design was proposed for the industrial installations     

Influence of co-field motion on the conduction current in oil undernonuniform dc fields

The effect of enforced co-field motion on conduction current in transformer oil was studied using a point-to-plane electrode geometry. Tests were carried out on degassed oil and gas-saturated oil for negative and positive point polarities using both a solid and a mesh-plane electrode, and gaps ranging from 200 to 900 μm. For stationary oil the presence of oxygen decreased the conduction current for both polarities of the point electrode. For degassed and O₂ saturated oil the axial flow produced a significant reduction in the conduction current; the faster the flow the larger the reduction in current. With oxygen in solution, current fluctuations were much larger than for degassed oil. An unexpected result was the consistent finding that for gas-saturated oil an increase in pressure produced an increase in the conduction current, but only for a solid plane electrode. There is an excellent correlation between the effect of flow on conduction current and on the breakdown voltage     

Electrical conduction and dielectric relaxation in polyethylene terephthalate succinate

Electrical conduction and complex permittivity are examined in polyethylene terephthalate succinate, focusing on their relations to dielectric relaxation processes. Both the real and imaginary parts of the complex permittivity, namely, the dielectric constant ε′_r and the dielectric loss factor ε′′_r, increase with a decrease in frequency, especially at high temperatures. They are both ascribed to the transport of ionic mobile carriers. Namely, the carrier transport forms a conduction current that should contribute to ε′′_r. On this occasion, if charge exchange does not occur at the two electrodes, heterocharge layers should be formed before the electrodes. This should increase the charge density on the electrodes, thus contributing to ε′_r. In addition to the increase in ε′_r and ε′′_r due to mobile ions, two relaxation processes, one due to micro‐Brownian motion of dipoles and the other due to orientation and magnitude change of the dipole moment induced by two end groups in the polymer main chain, are observed. Corresponding to these two relaxation processes, two thermally stimulated discharge current (TSDC) peaks appear. The two TSDC peaks as well as the increments in ε′_r and ε′′_r become larger when the crystallinity of the sample decreases. © 2009 Wiley Periodicals, Inc. Electr Eng Jpn, 170(4): 1–8, 2010; Published online in Wiley InterScience ( www.interscience. wiley.com ). DOI 10.1002/eej.20956     

The effect of low-molecular-weight species on space charge andconduction in LDPE

We have investigated the effect of low-molecular mass species on the space charge behavior such as space charge distribution and electrical conduction of LDPE (low density polyethylene). We also attempted to explain the relationship between the space charge distribution and electrical conduction. The heterocharge, conduction current and effective charge mobility decreased when the low-molecular-mass species in LDPE was removed. It was found that the decrease in conduction current was related to the decrease of heterocharge in LDPE     

Charging and behavior of a spherically conducting particle on a dielectrically coated electrode in the presence of electrical gradient force in atmospheric air

This paper discusses the charging and behavior of a spherically conducting particle on a dielectrically coated electrode in non-uniform dc and ac electric fields between non-parallel plane electrodes in atmospheric air. The charging mechanism of a particle resting on a dielectrically coated electrode is investigated by observing light emissions from partial discharges and by utilizing the dust figure technique. The charge acquired by a particle is estimated from the results of particle motion onset voltage measurement and direct measurement with a Faraday cage. The basic particle movement as well as particle-triggered breakdown characteristics under non-uniform electric field distribution along the electrode surface is also investigated. The results suggest that the effect of the electrical gradient force/dielectro-phoretic force on particle motion on a dielectrically coated electrode should be adequately considered in the design of gas insulated systems (GIS). This is because a particle can move laterally on the coated electrode toward increasing electric field regions by the action of the electrical gradient force which is independent of the charge on the particle. Also the possibility of the particle charging and lifting at high field regions or reaching and adhering onto an insulating spacer further increases the risk of breakdown.     

Field-Filter Probes and the Measurement of Alternating-Current Corona

The principles of the field-filter probe, which is capable of measuring both field changes and conduction current at an electrode in the presence of space charge, are described. Design characteristics are given for a simple field filter and for two further types which can be used for bipolar (reversing) fields and currents. Measurements are given for the discharge characteristics at the plane electrode of rod-plane gaps in the range 50-300 mm subjected to alternating voltages up to 100 kV root mean square (rms).     

Influence of humidity on current waveform and light emission of alow-frequency discharge controlled by a dielectric barrier

The aim of this work is to determine the effect of humidity on a silent discharge in air. A point to plane configuration has been used. Current and emission spectra of the discharge have been recorded in dry and humid air for short and long inter-electrode gaps and with the plane electrode coated by a silicone rubber plate alone or under a polypropylene film. Results show the typical corona discharge regimes. For a long gap, humidity decreases the number of breakdown streamer pulses. For a short gap, the behavior observed depends on the surface conduction of the dielectric in contact with the discharge. These results show that the presence of humidity in the discharge cell not only modifies the gas but also the surface states of the dielectrics coating the electrodes. The surface charge decay appears as a major parameter, since it is related to the local field value which results from the applied field and the reverse field induced by charge accumulation during previous discharges