Effect of charge mobility on dielectric liquid flow driven by EHD conduction phenomenon

Electrohydrodynamic (EHD) conduction phenomenon takes advantage of the electrical Coulomb force exerted on a dielectric liquid generated by externally applied electric field and dissociated charges from electrolytes. EHD conduction generated flow relies primarily upon the asymmetry of the electrodes where the flow is always directed toward the broken symmetry regardless of the electrodes polarity. This paper studies the effects of unequal positive and negative charge mobilities on the heterocharge layer structure and generated flow with symmetric and asymmetric electrode designs. The numerical simulations are conducted for a 2-D rectangular channel with the electrodes embedded against the channel wall.     

One model of electric conduction and electric field distributions in a liquid insulator

The electric properties of the liquid insulator have been studied in this work. We have described the time-dependent model of the conduction processes in the liquid insulator under high voltages. Our model is a generalization of the Frenkel conduction model. (The last deals only with stationary conduction processes). Calculation of the electric field has also been made for two types of problems: (1) the external electric field applied to a liquid, is created by plane electrodes. (2) A spherical electrode of a small size creates it. The electrodes are assumed to be under constant high voltage. There is no electron emission or ion injection from the electrodes. But the field intensity influences on the molecular dissociation rate, and this fact has been taken into account. We have shown that the space charge, which arises under conditions mentioned above gives rise to changes in the electric field distribution. The model on hand is also used for calculating the hydrodynamic phenomena under high non-uniform electric field intensities. It has also been shown that the velocity of the strong EHD flows is proportional to the value of direct current or the squared value of applied voltage.     

A fundamental characteristic and image analysis of liquid flow in an AW type EHD pump

Non-intrusive two-phase fluid pumping based on an electrohydrodynamically (EHD) induced flow phenomenon with free liquid surface exposed to gas-phase corona discharges is experimentally investigated. Dielectric liquid flow generated near a corona discharge electrode progresses toward an inclined plate electrode, and then climbs up the surface against the gravitational force for an air-wave (AW) type EHD pump. The AW type EHD pump is operated on ionic wind field along the inclined plate electrode. The pumping performance of time-averaged liquid flow rate and the liquid-phase flow motion are characterized. The liquid flow characteristics related to a dimensionless parameter of corona discharge fields are presented.     

Mechanism of electrohydrodynamically induced flow in a wire-non-parallel plate electrode type gas pump

An experimental investigation and one-dimensional modeling have been conducted to study the mechanism of net flow direction induced by electrohydrodynamic (EHD) forces in a wire-non-parallel plate electrode type EHD gas pump. The experiments were conducted with various different locations of corona wire electrode for negative and positive applied voltage from 0 to 14 kV at atmospheric pressure and room temperature, where air was used as the working fluid. A one-dimensional cross-sectional averaged model based on mass and momentum conservation as well as Poisson electric field and ion transport equations was also developed. The results show that the net flow direction of electrohydrodynamically induced gas flow in a wire-non-parallel plate electrode system significantly depends on the location of the corona wire electrode relative to the grounded electrode position. The effect of conversion angle of non-parallel plate electrode on the net flow direction and pressure drop also was investigated and discussed in detail.     

Direct numerical simulation of electrohydrodynamic plumes generated by a hyperbolic blade electrode

This paper presents a numerical study of two-dimensional EHD flow occurring between a hyperbolic blade and a plate electrode. The whole set of coupled equations is solved: Navier–Stokes equations, Poisson equation and charge conservation equation. A finite volume approach designed for non-orthogonal structured grid is used to discretize all governing equations. An efficient numerical procedure based on total variation diminishing (TVD) scheme is implemented to compute the distribution of charge density. Two different injection laws are considered: a simple autonomous one and a non autonomous which relates the charge injected by the blade and the local electric field. The flow structure which results in an EHD plume analogous to a thermal plume, has also been successfully characterized numerically by the temporal evolution of the charge density distribution. Preliminary results indicate that the flow is characterized by two different regimes according the value of the applied voltage. The critical Reynolds number for which the transition between the steady and unsteady regimes occurs has been determined to be within the range Re = [1000, 1100].     

Dielectrophoresis flow control in microchannels

The control of flow in microscale is one of the most important problems in microfluidic devices, which in particular, are used as micro heat exchangers. The use of electric field is one of the efficient methods of control of dielectric liquid flow in microscale. The electric field influences liquid flow by the EHD force which affects liquid behaviour in terms of the flow rate and pressure.The EHD force consists of three components: the first is the electrostatic force due to free charges present in the liquid, the next one is the force due to the gradient of permittivity of material, and the third one is caused by the change in the electric field intensity.The EHD force is used also in many commercial devices, for example EHD pumps or dielectrophoretic separators. An own approach to apply the EHD force to control the liquid flow rate is presented in this paper. Authors paid a close attention to the dielectrophoresis effect. Dielectric liquid in a non-uniform electric field tends to drift/migrate towards the region of high electric field intensity. With decreasing the electrode dimensions, the dielectrophoresis force becomes relatively stronger. For the dimensions under 400 μm the dielectrophoresis phenomenon can be used for control and actuation of the liquid flow in microchannels. The originally developed design of such flow controller is presented in this paper. The experimental investigations covered flow rate measurement of 2-propanol in microchannel flow controller with application of AC field. It was showed that the dielectrophoresis phenomenon could effectively control the flow. The results for distilled water are also comparatively discussed in the paper.     

Electroconvection between coaxial cylinders of arbitrary ratio subjected to strong unipolar injection

This study deals with the electroconvection phenomenon that takes place in a dielectric liquid layer placed between two annular electrodes and subjected to the action of an electric field.The full set of governing equations describing the combined Electro-Hydro-Dynamic (EHD) flow is directly solved with the finite volume method.The development of electroconvective motion is investigated in details in the case of strong injection when the emitter electrode is the inner cylinder.We first examine the stability of such flow. As in the plane–plane configuration we have highlighted the existence of a linear and non-linear critical electric Rayleigh numbers giving rise to a hysteresis loop. The flow structure under the effect of the electric field is analyzed and the distribution of electric charge density is presented. In particular we investigate the effect of various parameters involved in this configuration, such as the radius ratio, injection strength and electric Rayleigh number.A multicellular convective pattern is particularly observed and it is shown that the number of cells is increased when the annular gap is narrower. Finally a significant increase of the rotational speed of the vortex with the electric Rayleigh number is recorded.     

Formation of electrohydrodynamic flows in strongly nonuniform electric fields for two charge-formation modes

The main features of the formation of electrohydrodynamic (EHD) flows are analyzed for two basic regimes of electrization of weakly conducting liquids: injection from the electrode surface and dissociation in the bulk. Analysis is carried out on the basis of the results of computer simulation of EHD flows in a strongly nonuniform electric field in the needle-plane electrode system. This system creates favorable conditions for the injection as well as dissociation mechanisms of charge formation. Typical features are revealed for each model of charge formation. The current-time characteristics of the transient process of stabilization of EHD flows are calculated.     

Effect of emitting electrode number on the performance of EHD gas pump in a rectangular channel

Previous studies have shown that electric field in the form of corona wind can be used for gas pumping. It has also been shown that the maximal volume flow rate can be achieved by an optimal design and arrangement of electrode(s) involved. In this study, the number of emitting electrodes has been considered for its effects on the pump performance. To seek the relation between the electrode number and pump performance, an EHD gas pump with three configurations (4, 12, and 28 emitting electrodes) is critically evaluated by experimental measurements and numerical simulations.     

Study of the transition from conduction to injection in an electrohydrodynamic flow in blade-plane geometry

A dielectric fluid can be set into motion with the help of electric forces, mainly Coulomb force. This phenomenon, called electroconvection, can be induced by electrohydrodynamic conduction, injection, and induction. Conduction is based on the dissociation/recombination phenomenon, generates heterocharge layers, and occurs for low electric field values. Injection produces homocharge layers in the electrode vicinity and requires stronger electric fields to be initiated. This study is an experimental observation of the transition from conduction to injection of a dielectric liquid in blade-plane geometry using Particle Image Velocimetry. In addition, the electric current is measured to completely understand the flow behavior.     

A semi-analytical approach using artificial neural network for dielectrophoresis generated by parallel electrodes

In this paper, we present a semi-analytical approach to obtain the DEP force generated by parallel electrodes. By solving the electric potential equation using the separation of variables method, a solution was found in the form of a Fourier series with unknown coefficients. The unknown coefficients were determined by training a linear artificial neural network with the appropriate data satisfied on the boundary. This results of calculated electric field and DEP force for both planar electrode system and 3D electrode system are validated by comparison with the numerical results obtained using the commercial software CFD-ACE+.     

Numerical investigation of using various electrode arrangements for amplifying the EHD enhanced heat transfer in a smooth channel

Forced convection heat transfer enhancement with electrohydrodynamic (EHD) technique of turbulent flow inside a smooth channel has been numerically investigated. A two dimensional numerical approach has been chosen to evaluate the local and average heat transfer coefficient. In addition, the swirling flow pattern in the presence of an electric field has been studied. To achieve higher enhancement while using multiple electrodes, variety of electrode arrangements have been examined for specified values of Reynolds number, applied voltage, and wire radius. The results demonstrate that different electrode arrangements cause significant improvement of the heat transfer coefficient.     

等离子体激励翼型分离流动控制数值模拟

文章利用CFD软件FLUENT中的自定义函数接口, 将等离子体对中性气体的激励作用模型化为体积力引入Navier-Stokes方程, 研究了等离子体气动激励诱导的平板射流, 以及介质阻挡放电(dielectric barrier discharge, DBD)等离子体激励对NACA0015翼型大迎角分离流的控制作用.计算分析表明, 多对电极等离子体激励器可以有效控制NACA0015翼型大迎角分离流动.      

A numerical model of streamlines in coplanar electrodes induced by non-uniform electric field

A new model for investigating the non-uniform electric field and potential distribution of fluid flow and streamlines induced by non-uniform electric field with the induced charge in the electrical double layer on the electrode surfaces is presented. Accurate computation of the non-uniform electric field is a pre-requisite for observing fluid flow and streamlines. The electric field distribution is obtained from Laplace's and Neumann's equations. Finite Element Methods is adopted for this work. The simulation results has been compared with available experimental observations of the fluid flow profile obtained by superimposing images of particle movement in a plane normal to the electrode surface. A good agreement is found between the numerical and experimental streamlines.     

Induced soap-film flow by non-uniform alternating electric field

Fluid flows generated on soap films by non-uniform alternating electric fields are studied. Two parallel metal rods subjected to an AC voltage are placed perpendicular to the soap film, which is anchored in a dielectric frame. The fluid flow is generated by electrohydrodynamic induction. At very low signal frequencies there is induced surface charge, but there is no tangential electric field at the surface, so there is no force and no flow. Fluid flow is observed increasing the frequency, when there are both surface charge and tangential electric field. The flow velocity increases with decreasing thickness of the soap film.     

Analysis of the Electrohydrodynamic Flow in a Symmetric System of Electrodes by the Method of Dynamic Current–Voltage Characteristics

We have solved the problem of injection-type through electrohydrodynamic (EHD) flow in a closed channel. We have considered a model of a liquid with four types of ions. It is shown that a through EHD flow without internal vortices in the electrode gap is formed for the ratio 2 : 1 of the initial injection current from the electrodes in the channel. The structure of the flow in different parts of the channel and the integral characteristics of the flow have been analyzed. It is shown that for a quadratic function of injection at the electrodes, the current–voltage characteristic of the flow is also quadratic.     

A charge-conservative approach for simulating electrohydrodynamic two-phase flows using volume-of-fluid

In the present study we propose a charge-conservative scheme to solve two-phase electrohydrodynamic (EHD) problems using the volume-of-fluid (VOF) method. EHD problems are usually simplified by assuming that the fluids involved are purely dielectric (insulators) or purely conducting. Gases can be considered as perfect insulators but pure dielectric liquids do not exist in nature and insulating liquids have to be approximated using the “Taylor–Melcher leaky dielectric model” [1], [2] in which a leakage of charge through the liquid due to ohmic conduction is allowed. It is also a customary assumption to neglect the convection of charge against the ohmic conduction. The scheme proposed in this article can deal with any EHD problem since it does not rely on any of the above simplifications. An unrestricted EHD solver requires not only to incorporate electric forces in the Navier–Stokes equations, but also to consider the charge migration due to both conduction and convection in the electric charge conservation equation [3]. The conducting or insulating nature of the fluids arise on their own as a result of their electric and fluid mechanical properties. The EHD solver has been built as an extension to Gerris, a free software solver for the solution of incompressible fluid motion using an adaptive VOF method on octree meshes developed by Popinet [4], [5].     

Control of adiabatic two-phase dielectric fluid flow distribution with EHD conduction pumping

Electrohydrodynamic (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 dielectric liquid/vapor flow distribution between two parallel branch lines utilizing an EHD conduction pump at a select mass flux level under adiabatic condition.     

Electrohydrodynamic gas flow in a positive polarity wire-plate electrostatic precipitator and the related dust particle collection efficiency

In this paper, the results of the particle image velocimetry measurements of the flow velocity fields in an intermediate spacing wire-to-plate type electrostatic precipitator (ESP) with a single positive polarity wire electrode are presented. The observation plane was placed perpendicular to the wire electrode at its half-length. The investigation showed significant influence of the electric field and charge on the flow patterns in the intermediate spacing ESP under an extreme large electrohydrodynamic (EHD) number. The EHD forces cause the formation of strong vortex pairs in the upstream and downstream ESP regions for Ehd/Re²>1.     

Effect of nonequilibrium near-electrode layers on the structure of EHD flows in the three-ions model of a dielectric liquid

An electrohydrodynamic (EHD) flow is a spontaneous flow of a liquid in the electrode gap under the action of a strong electric field. Most experimental data from an investigation of the velocity field of EHD flows were obtained in the wire-over-plane electrode configuration. For this system, the flow can be treated as a 2D flow. We report on the results of a computer simulation of the complete system of electrohydrodynamics equations in the three-ion model of a dielectric liquid. The structure of nonequilibrium dissociation–recombination layers and their effect on the structure of EHD flows have been analyzed based on the results of the computer simulation of EHD flows in liquids with different low-voltage conductivities for the wireover- plane electrode system.