Validity of Particle Image Velocimetry Measurements in Electrohydrodynamic Flows

Control of electrohydrodynamic (EHD) flow by secondary electric potential

New arguments supporting the supposition that the ozone is transported along a corona discharge radical shower (CDRS) reactor by the electrohydrodynamic (EHD) flow are presented. The arguments are based on the analysis of the corona discharge, which is a precursor of the EHD flow in the CDRS reactor, and on the measurements of velocity field of the EHD flow in the CDRS reactor by the particle image velocimetry (PIV). The obtained velocity flow structures and the possible causes of the ozone transport in the CDRS, i.e., diffusion, additional gas flow, EHD flow, and convection by the main flow, were discussed basing on the conservation equations for the EHD flow. The discussion showed that the EHD flow plays a dominant role in the ozone transport. This is also supported by the results of a simple phenomenological model for one-dimensional description of EHD-induced ozone transport in the CDRS reactor. The results of the computer simulation based on this model explained the main features of the measured ozone distribution in the CDRS reactor, establishing the EHD flow as the main cause of the ozone transport from the discharge region upstream, i.e., against the main flow.

The electrohydrodynamic (EHD) flow induced by the corona discharge was experimentally investigated in an electrostatic precipitator (ESP). The ESP was a narrow horizontal Plexiglas box (1300 mm×60 mm×60 mm). The electrode set consisted of a single wire discharge electrode and two collecting aluminum plate electrodes. Particle Image Velocimetry (PIV) method was used to visualize the EHD flow characteristics inside the ESP seeded with fine oil droplets. The influence of applied voltage (from 8 kV to 10 kV) and primary gas flow (0.15 m/s, 0.2 m/s, 0.4 m/s) on the EHD flow transition was elucidated through experimental analysis. The formation and transition of typical EHD flows from onset to the fully developed were described and explained. Experimental results showed that the EHD flow patterns change depends on the gas velocity and applied voltage. EHD flow starts with flow streamlines near collecting plates bending towards the wire electrode, forming two void regions. An oscillating jet forming the downstream appeared and moved towards the wire electrode as voltage increased. For higher velocities (≥0.2 m/s), the EHD transition became near wire phenomenon with a jet-like flow structure near the wire, forming a void region behind the wire and expanding as voltage increased. Fully developed EHD secondary flow in the form of counter-rotating vortices appeared upstream with high applied voltage.

EHD flow in a wide electrode spacing spike–plate electrostatic precipitator under positive polarity

Electrohydrodynamic flow and particle collection efficiency of a spike-plate type electrostatic precipitator

In the present work, results of electrohydrodynamic (EHD) flow field in a wire-plate air-oil droplets electrostatic separator under positive polarity are presented. Using Particle Image Velocimetry (PIV), the structure of EHD flow under fully developed primary laminar gas flow is investigated and corresponding flow patterns are studied. Velocity distribution results show that the transverse velocity induced by EHD flow depends significantly on applied voltage and the cross-section plane position of the separator. Detailed transverse velocity distribution profiles under different ratio between electrostatic force and inertial force of droplets (Ehd/Re2) are presented. Oscillating jets are observed under a relatively low Ehd/Re2 ∼ 4 and counter rotating vortices around the wire for large Ehd/Re2 > 90 are characterized in this experiment. Contrary to EHD flow patterns for fine solid particles, no von Karman vortex was observed downstream of the charged electrode wire.

Electrohydrodynamic (EHD) flow results from the interaction of electric fields with ionized charges, becoming more complex in gas–liquid systems due to interfacial deformation. This study analyzed the interfacial deformation induced by corona discharge in an air–water system using two-phase numerical simulations. A fully coupled model was developed to account for the interactions among charge transport, fluid motion, and temporal interfacial deformation under corona discharge. The model was validated experimentally through particle image velocimetry and discharge current measurements. The effects of applied voltage ranging from 5 to 15) kV and height of water ranging from 4 to 12) mm were analyzed. The results showed that higher voltage increased the EHD flow velocity and interfacial deformation depth. The reduced distance between the discharge electrode and the interface further intensified these effects. The relationship among current, flow velocity, and interfacial deformation could be defined. A distinctive observation of self-sustained and periodic interface deformation under EHD flow conditions was presented with a period of 3 s at 12 mm height and 15 kV applied voltage. A phase lag of 0.4 s was observed between fluctuations in current and flow velocity, while maintaining the periodic oscillation. Current oscillations remained within 3% of the mean value, while flow velocity deviations reached 8.9%, indicating significant temporal fluctuations that should not be neglected. This study offers deeper understanding of the relationship among charge transport, flow structure, and interface deformation in gas–liquid EHD systems with potential applications in areas that require precise control of flow and interfacial behavior.

In this study, a proposed method for selecting a tracer for particle imaging velocimetry (PIV) measurement in electrohydrodynamics flows was developed. To begin with, several published studies were identified that exploit different tracers, such as oil smoke, cigarette smoke and titanium dioxide (TiO2). An assortment of tracers was then selected based on comparisons with conventional dimensionless numbers; Stokes number (St), Archimedes number (Ar) and electrical mobility ratio (M). Subsequently, an experimental study for testing tracers was developed, which enabled the velocity profile of an ionic wind generated by a needle/ring configuration to be measured. Air velocity measurements carried out with a Pitot tube, considered as the reference measurements, were compared to PIV measurements for each tracer. In addition, the current–voltage curves and the evolution of the current during seeding were measured. All the experimental results show that TiO2, SiO2 microballoons and incense smoke are the ideal tracers in the series of tracers investigated.

We present an image preprocessing method for particle image velocimetry (PIV) measurements of flow around an arbitrarily moving free surface. When performing PIV measurements of free surface flows, the interrogation windows neighboring the free surface are vulnerable to a lack, or even an absence, of seeding particles, which induces less reliable measurements of the velocity field. In addition, direct measurements of the free surface velocity using PIV have been challenging due to the intermittent appearance of the arbitrarily moving free surface. To address the aforementioned limitations, the PIV images with a curvilinear free surface can be treated to be suitable for a structured interrogation window arrangement in a Cartesian grid. The proposed image preprocessing method is comprised of a free surface detection method and an image transform process. The free surface position was identified using a free surface detection method based on multiple textons. The detected free surface points were used to transform PIV images of a curvilinear free surface into images with a straightened free surface using a cubic Hermite spline interpolation scheme. After the image preprocessing, PIV algorithms can be applied to the treated PIV images. The fluid-only region velocities were measured using standard PIV method with window deformation, and the free surface velocities were resolved using PIV/interface gradiometry method. The velocity field in the original PIV images was constructed by inverse transforming that in the transformed images. The accuracy of the proposed method was quantitatively evaluated with two sets of synthetic PIV images, and its applicability was examined by applying the present method to free surface flow images, specifically sloshing flow images.

To analyze the flow pattern inside a typical wire-plate wet electrostatic precipitator (WESP) and its impact on particle trajectories, particle image velocimetry, and the finite element method were applied to visualize inner flow pattern. The flow pattern was primarily influenced by inner water flow along with the collector plates and the electrohydrodynamic (EHD) flow. Particle trajectories under various flow patterns were also studied. The results suggest that the EHD flow has a significant influence on the primary flow by creating vortex structures inside the WESP chamber. The water flow causes the inner flow pattern to become more complicated by generating vertical vortices. The particle trajectories were deformed with the inner flow patterns. Therefore, the collection efficiency can be severely hindered under various conditions. In this article, 16 different configurations were studied. The flow patterns of all configurations were given in two-dimensional images. Flow properties, that is, turbulent kinetic energy, turbulent dissipation rate, pressure, velocity magnitude and shear rate, and collection efficiency of each configuration are given in tabled data.

Effect of barbed tubular electrode corona discharge EHD flow on submicron particle collection in a wide-type ESP

The electrohydrodynamic flow associated with a fluid drop in an electric field is a consequence of the tangential electric stress at the fluid interface. The tangential viscous stress due to the electrohydrodynamic flow arises to just balance the tangential electric stress at the fluid interface so that the traction boundary condition is satisfied. Influenced by both the local electric stress and viscous stress, the drop interface may exhibit various shapes. The presence of fluid flow also leads to charge convection phenomena. The relative significance of the charge convection effect is usually measured in terms of the electric Reynolds number, Re E , defined as the ratio of the timescales of charge convection by flow and that for charge relaxation by ohmic conduction. This work presents a quantitative analysis of the charge convection effects in a framework of the leaky dielectric model at finite Re E , which has not been considered in previous investigations. Axisymmetric steady flows driven by an applied uniform electric field about a deformable fluid drop suspended in an immiscible fluid are studied by computational means of the Galerkin finite–element method with supplementary asymptotic analysis. The results of finite–element computations are in general agreement with the prediction by the asymptotic analysis for spherical drops at vanishingly small Re E . A common effect of charge convection is found to reduce the intensity of electrohydrodynamic flow. As a consequence, oblate drops are predicted to be less deformed in an electric field when charge convection is taken into account. The prolate drops are often associated with an equator–to–pole flow, which convects charges toward the poles to form a charge distribution resembling that in a highly conducting drop immersed in an insulating medium. Therefore, charge convection tends to enhance the prolate drop deformation. In many cases, charge convection effects are found to be significant even at apparently small Re E , corresponding to the charge relaxation time–scale about 10 −3 s, suggesting that many experimental results reported in the previous publications could have been influenced by charge convection effects.

Electrohydrodynamic (EHD) flows emerge in dielectric liquids under the action of the Coulomb force and underlie energy-efficient techniques for heat and mass transfer. The key issue in the phenomena is the way how the net charge is created. One of the most promising, yet poorly studied charge formation mechanisms is the field-enhanced dissociation (or the Wien effect). So the paper studies an EHD flow caused solely by the effect by virtue of both experiment and computer simulation. To preclude the competing mechanism of charge formation—the injection—a new EHD system of a special design was examined. Its main feature is the use of solid insulation to create the region of the strong electric field far from the electrode metal surfaces. The experimental study used the particle image velocimetry technique to observe velocity distributions, whereas the computations were based on the complete set of electrohydrodynamic equations employing the commercial software package COMSOL Multiphysics. Spatial distributions of key quantities (including the ion concentrations, the total space charge density, and the velocity) and the acting forces were obtained in the computer simulation and were analyzed. The experimental flow structure was observed for a number of voltages up to 30 kV. The comparison of the numerical and experimental results yielded a good quantitative agreement for strong electric fields though some overshoot was observed for weak ones. The results allow concluding on the applicability of the Onsager theory of the field-enhanced dissociation in the context of EHD flows.

This work was aimed at measurements of the electrohydrodynamic (EHD) secondary flow in a non-thermal plasma reactor using three-dimensional particle image velocimetry (3D PIV) method. The wide-type non-thermal plasma reactor used in this work was an acrylic box with a wire discharge electrode and two plate collecting electrodes. The positive DC voltage was applied to the wire electrode through a 10 MΩ resistor. The collecting electrodes were grounded. The voltage applied to the wire electrode was 28 kV. Air flow seeded with a cigarette smoke was blown along the reactor duct with an average velocity of 0.6 m/s. The 3D PIV velocity fields measurements were carried out in four parallel planes stretched along the reactor duct, perpendicularly to the wire electrode and plate electrodes. The measured flow velocity fields illustrate complex nature of the EHD induced secondary flow in the non-thermal plasma reactor.

This paper is an experimental study on electroconvective flows produced by an asymmetric electric field in a cavity. It is part of a scientific project aiming to develop electrohydrodynamic (EHD) flow actuator in order to enhance heat transfer and mixing efficiency in fluidic and microfluidic systems. In this study, the EHD flow is obtained by applying a dc voltage between two electrodes (cylinder/plane) immersed in a working liquid. The produced velocity fields strongly depend on the applied voltage. Whatever may be the applied voltage, the fluid goes from the cylinder toward the plate or the other way round. The velocity fields obtained by particle image velocimetry for different potentials are analyzed in both cases with the usual mechanisms (injection and conduction) at the origin of the motion of the liquid. A discussion on the transition between injection and conduction phenomena is finally presented.