Static electrification by laminar flows through artificiallyroughed pipes

The mechanism of static electrification of hydrocarbon liquids flowing through artificially roughed metallic pipes is analyzed. Only the case of a laminar flow is considered. The equations of motion of the liquid are computed in the case of circular pipes with periodic roughness of rectangular cross section in the axial direction. The results reveal the formation of eddies, which depend on the size and the period of the roughness and on the Reynolds number. After determining the charge convected in a smooth pipe and the velocity profile in a rough one, it is possible to predict the convected space charge in terms of the roughness characteristics. The experimental results obtained with different rough pipes and several Reynolds numbers are in good agreement with the predictions, making it possible to explain the observations made during previous experiments on static electrification in rough pipes     

Experimental study and modeling of static electrification in powertransformers

For the last 20 years, static electrification has been suspected to he responsible for failures in power transformers (electric “tree” paths, “worm holes,” etc.). The phenomenon consists in a preferential adsorption of negative ions from the oil (impurities) into the pressboard. This yields, on the one hand, a space charge in the oil which can relax in contact with grounded metallic walls and, on the other hand, a space charge in the pressboard which can accumulate depending on the leakage paths. As part of a research program by Electricite de France in the field of static electrification in transformers, an experiment has been carried out, at the Universite de Poitiers, Poitier, France, to study the most constrained parts in transformers with regard to this phenomenon: the insulated pressboards close to the oil inlet. For this, the experimental loop consists in impinging an immersed oil jet on a pressboard target in which annular insulated electrodes have been inserted to determine the charge in the pressboard. Considering the scale effect, the equipment involves taking oil from an operating transformer and making it possible to study the influence of the flow velocity and the length of the free jet. From a first modeling, compared to the authors' experimental results, it seems that the local wall current coming from the pressboard is proportional to the local wall shearing stress. Considering the magnitude of the process, it finally turned out that this new configuration increases greatly the charge generation process in comparison to a flow parallel to the pressboard     

Analysis of flow electrification in fuel distribution systems

The authors examine flow electrification in typical automotive fuel distribution systems consisting of a fuel tank, pump, insulating fuel lines, and filter. Factors influencing the degree of electrification and the failure mechanisms of these fuel systems are discussed. Various experimental techniques that may be used to characterize flow electrification in any fuel distribution system are reviewed. An analysis methodology that allows use of charge density data to quantitatively assess the probability of an external electrical discharge or dielectric breakdown of the pipe wall is presented     

PIV Measurements on Charged Plumes-Influence of SiO2 Seeding Particles on the Electrical Behavior

It is well known that an isothermal dielectric liquid can be driven by electroconvection. It has been demonstrated that two phenomena could generate space charge in isothermal dielectric liquids and then induce a fluid movement. These two electroconvective effects are bulk conduction and ion injection. In order to improve the performance of electroconvective devices, the velocity of the flows must be recorded as accurately and precisely as possible. In this paper, the particle image velocimetry (PIV) method that was originally developed in the field of experimental fluid mechanics is adapted to electroconvective flow measurements. The choice and the size of seeding particles are discussed. The influence of the seeding particles density on the current is measured. In this work, experiments were investigated on a typical two-dimensional charged plume flow produced between a blade and a flat plate. Both negative and positive polarities are also investigated.     

Streaming Electrification Current Density Distribution inside Pipes Assuming Overcharged Boundary Layer

In this paper the distribution of electrification current flowing across a pipe, for alternating charge near a surface, is presented. A distribution of charge density was arbitrarily assumed compatible to the Stern theory. Assuming that the velocity distribution is parabolic (in laminar regime) or logarithmic (in turbulent regime), the convective current density can be obtained. The distribution of current density shows changes of sign at different distances from the boundary. This may be the reason for the change of total electrification current sign that is often observed during experimental measurements.     

Influence of pressboard physico-chemical composition on static electrification in power transformers

As part of a research program in the field of flow electrification in power transformers, some fundamental studies have been undertaken on the physical chemistry of this phenomenon. The two main constituents involved in the process of charge generations are oil and pressboard. As oil composition is difficult to deal with, the choice has been made to understand the overall chemistry through the role played by the pressboard, whose composition is better mastered. In order to identify the chemical species in the pressboard responsible for the charge generation, in view of perfecting additives to neutralize the phenomenon, the French Institute of Pulp and Paper carried out modifications on commercial pressboards, and manufactured some others. Charge accumulation, measured from capacitive currents on insulated electrodes facing the pressboard ducts, has been observed for modified and manufactured pressboards with regard to oil flow rate and temperature. All the experiments have led to various behaviors in terms of kinetics and magnitude of the charge accumulation. The obtained results tend to prove that chemical composition of pressboards, with regard to their content in chemical functional groups (hydroxyl or carboxyl groups) or radical nature (lignin), as well as board manufacturing parameters (pulp refining, layer composition) and surface properties seem to be at the origin of the electrification phenomenon.     

Effect of streaming potential on heat transfer in transformer oil

The results of an experimental study of the effects of streaming electrification on the heat transfer of transformer oil (Voltesso 35) in Plexiglas and paper/pressboard pipe models are reported. The magnitude and the polarity of the charge in the oil entering the test section was varied by using two different filters (Millipore cellulose and Sartorius fiber glass) in the upstream of the oil flow. An enhancement in heat transfer was observed with both types of filters in the Plexiglas model and with cellulose filter in the paper/pressboard model; whereas a reduction in heat transfer was observed with fiber glass filter in paper/pressboard model under similar flow rates and temperatures. With uncharged oil, heat transfer was either inhibited or no enhancement was noticed in both models. The results have been discussed considering the temperature gradients, the flow conditions and the electrical forces due to static charge in the liquid     

变压器油流带电数学模型的研究及其应用 第一部分：数学模型的推导

由油流带电引发的变压器事故或故障引起了各国电力部门的广泛关注，许多国家纷纷开展变压器油流带电的研究，大量的油流带电的机理模型也就应运而生。本文在国外学者提出的流动作用机理和电动作用机理两种机理模型的基础上，推导出变压器油流带电的数学模型，与目前存在的几种典型的油流带电计算模型相比，本模型的特点在于它可综合反映起电的流动作用和电动作用，并可基本反映油流流速、油品电导率、外加交流电场强度和系统温度等因素对油流带电的影响。关于本模型的应用实例及与实验的比较见第二部分。     

Charge convected in highly turbulent flows. Experiments on tankerunloading

It is a known fact that very high potentials can be generated during the flow of dielectric liquids, especially for hydrocarbons and liquefied gas. For such liquids the flow is highly turbulent. In the first part of this study, the mechanism of charge formation is analysed by taking the effect of the eddy diffusivity into account. These are two-phase flows and the influence of the ratio liquid/gas on the charge generated is explored. The second part of the study is experimental. The electrical potentials generated during the various phases of the unloading of a tanker are measured. A potential of ~3 kV is observed inside the pipe at the beginning of the unloading process     

Electrification of polymer surface caused by sliding ultrapurewater

It has been thought that the conductivity of pure water is high enough to neglect the streaming electrification. However, with the recent advancement of semiconductor industries, ultrapure water appeared to cause electrification problems at the washing and the rinsing processes of semiconductor wafers. The investigation on the electrification phenomenon of polymer (PTFE) surface with ultrapure water is described in this article. Water droplets are always charged positively after sliding on the PTFE surface, and the surface potential on PTFE is mostly negative and varies along the droplet path becoming positive downstream. As the conductivity of water becomes closer to that of ideal water, the amount of droplet charge increases drastically. This electrification phenomenon is also enhanced by decreasing the thickness of the PTFE plate. It suggests that the Coulombic force between the charge on the PTFE surface and the mirror image charge is an important factor. We have proposed a model which is based on the electric double layer similar to conventional streaming electrification for insulating liquid, assuming that the charged droplet leaves the excess positive charge on the polymer surface     

Streaming electrification of thin insulating pipes under electricfield

Streaming electrification under dc and ac electric fields was investigated by using test pipes and oil circulation apparatus. We used two different test pipes, one made of pressboard, and the other of insulating paper. At first, the amount of static charges which flowed into the test pipes was measured without electric field. For the experiment using an insulating paper pipe, when the oil temperature increased, the measured current curve shows a peak. When the velocity increased, the current increased proportionally from the 2nd to 4th power of velocity. For the experiment using a pressboard pipe, the current increased proportionally from the 3rd to 5th power of temperature, and by the 1st power of velocity. Then, we applied ac and dc electric fields on the test pipes and the current from a relaxation tank to ground was measured. The ions in oil carry the charges, so an electric field affects ions in the oil. It makes the mathematical approach to streaming electrification difficult. We know that as the electric field intensity increased, the current due to selective adsorption of the test pipe was increased. The current vs. velocity curve under ac electric field had a peak point in velocity     

气化炉洗涤冷却室内气液两相湍流流动特性的数值模拟

建立分别描述气相和液相湍流的两相湍流欧拉-欧拉双流体模型,模拟新型水煤浆气化炉洗涤冷却室内气体穿越液池过程中的气液两相流动特性。通过与试验结果的对比,表明该模型能对气化炉洗涤冷却室内的气液流动进行较好的预测。研究发现,气体在反折向上流动过程中,在冷却管外壁面聚集并主要沿冷却管边缘上升,且具有较大的上升速度;气液湍动能主要发生在冷却管出口及冷却管外壁面附近;洗涤冷却室内分隔板的存在促进了气体的扩散,使气相在液池内的分布更广,能有效抑制液面的波动,有利于装置的稳定运行。     

Development and calibration of a streaming electrification modelfor a cellulose duct

Streaming electrification occurs in large power transformers where insulating oil circulated for cooling purposes acts to separate charge from the interfacial double layer present on a multitude of cellulose insulation surfaces. In this paper, the electrification phenomenon is addressed from a microscopic viewpoint. An empirically calibrated analytical model is provided for calculating the amount of charge separated in a core-form transformer duct. The model incorporates a boundary layer approach in approximating the wall charge distribution which provides a critical length scale for the analysis. This length scale is used to specify the appropriate model for charge development under turbulent flow conditions. The duct model considers both laminar and turbulent flows subject to ac electric fields and varying temperature. This concept forms the elemental building block of a network-based methodology for examining electrification in a complete transformer geometry     

Influence of dc and dc/ac composite fields on the flow electrification characteristics of transformer oil

Flow electrification characteristics of transformer oil were investigated by applying dc and dc/ac composite fields to both bare and covered coaxial cylinder electrode systems. For the bare electrode system, flow electrification characteristics were almost the same between positive and negative dc voltage applications and negative charges were generated for both dc and dc/ac composite voltage applications at high dc fields. For the covered electrode system, the apparent flow electrification characteristics differed between positive and negative dc voltage applications. For the former, negative charges were generated at high dc fields for both dc and dc/ac composite voltage applications, and for the latter, positive charges were always generated for both voltage applications. However, negative charge generation for the positive voltage application was attributed to the influence of negative charge generation at the bare part of the covered electrode end; the fundamental charge generation characteristics of the covered electrode showed an increase of positive charge generation by dc voltage application, and positive charge generation was accelerated by a composite ac field. © 1999 Scripta Technica, Electr Eng Jpn, 130(2): 10‐17     

The effect of the electrode gap on breakdown in liquid dielectrics

It is a well known fact that the breakdown voltage of an insulating liquid increases nonlinearly with increasing electrode gap. Under nonuniform field dc conditions two breakdown voltages are determined depending on the electrode polarity. Using point-plane geometries, with gaps of 5 mm or larger, one finds that typical transformer oils have higher breakdown voltages when the point is negative than when it is positive. Recently, perfluorinated polyethers have been found to produce opposite results when using average gap sizes of 5 to 10 mm. To elucidate this situation a study of the effect of gap size on breakdown voltage has been undertaken. The results show that for the polyethers at larger gaps the sequence is reversed. These observations are explained in terms of the respective streamer developments in these liquids and the relative stability of their negative ions. In the case of the conventional hydrocarbon type oils, the positive streamer usually grows slightly faster than the negative one, but the streamer inception voltages of both are rather similar. In the case of perfluorinated polyethers the positive streamers move at least 10× faster than the negative ones, but they require much higher inception voltages than negative ones. At gaps ≳12 mm and the associated higher applied voltages this breakdown at negative polarity occurs at higher voltages than at the positive one. The negative ions of the perfluoro compounds are much more stable than those of conventional hydrocarbons. The discharge of the negatively charged perfluoro ions requires much higher fields than do those of conventional hydrocarbons. The implications of these observations are discussed in some detail     

大型循环流化床锅炉炉膛颗粒速度的光学测量

循环流化床锅炉炉膛内的颗粒速度分布对炉内燃烧、传热和受热面磨损均具有重要影响。在一台330 MW循环流化床锅炉炉膛水冷壁上开设测孔,利用水冷激光光纤测枪结合互相关计算对炉膛内的颗粒轴向速度进行了测量,研究了炉膛近水冷壁区的颗粒轴向速度水平分布、“环-核”边界层厚度、颗粒团贴壁下滑速度以及气固相滑移速度。结果表明,炉膛贴壁面位置颗粒轴向速度为负值,随着与壁面距离的增加,颗粒轴向速度逐渐转为正向,颗粒由下降流过渡到上升流动。颗粒“环-核”边界层厚度在0.1~0.2 m,且炉膛下部测点的边界层边界厚度略大于炉膛上部测点,颗粒团贴壁下滑速度均小于1 m/s。炉膛中心区气固相滑移速度为2.05~2.73 m/s,大于单颗粒终端速度。     

Flow electrification in power transformers. Explanation of thewall-current measurements

The aim of our work is to understand better the mechanisms of flow electrification due to oil circulating inside a rectangular pressboard channel. This research has been done because of technical incidents in power transformers in which oil is flowing past pressboard with a velocity in the range of several meters per second. A possible explanation of the wall current observed along the channel is proposed     

Experimental study of the effect of spacer grid on the flow structure in fuel assemblies of the AES 2006 reactor

Results from an experimental study of the local hydrodynamic structure of liquid flow in a 37-cell model simulating a fuel assembly used in the AES-2006 reactor are presented. Special attention is paid to the effect of spacer grid on flow hydrodynamics. Data on variations of the local and integral values of the liquid axial velocity and friction stress on the fuel rod simulator’s wall with distance from the grid are given.     

不同湍流模型在管道流动阻力计算中的应用和比较

应用几种不同的湍流模型,模拟计算水力光滑管的沿程阻力系数。将计算结果与尼古拉茨试验结果进行比较,可见Spalart-Allmaras湍流模型与试验结果吻合得最好,即在5个不同雷诺数(Re)下,其计算结果的最大误差为1.65%。研究还表明,无论圆管内流体流动是层流还是湍流,流动阻力的大小主要由流体在壁面附近的速度分布决定。     

The Electrohydrodynamic Origin of Turbulence in Electrostatic Precipitators

The functioning of an electrostatic precipitator in the light of previous studies on various regimes of electroconvection in both parallel and divergent electric fields is examined. Coupling between velocity and charge density fluctuations for both ions and charged particles is discussed. It is shown that for a certain diameter range of the particles, their nondimensional mobility parameter takes values similar to those characterizing ions in liquids. An experimental simulation using insulating liquids is proposed.