Novel low voltage EHD spray nozzle for atomization of water in the cone jet mode

Due to the high surface tension and high conductivity, water is unsuitable for electrohydrodynamic (EHD) atomization using a DC electric field in air. The high local electric field, that is required to atomize water, is likely to generate corona discharge and consequently destabilize the atomization process. This study describes a novel low voltage EHD spray nozzle that can be used to atomize water and weak saline solutions in the stable cone jet mode. The properties of the atomization have been investigated together with the generated droplet size distribution. The nozzle operates at very low flow rates (0.5–4.0 μl/min). Due to the high dielectric constant of water and the low flow rate, the atomization takes place outside the applicability range of the scaling laws. The experimental results show that the droplet size is approximately constant when the flow rate is increased from 0.5 to 4.0 μl/min. The atomization of water was numerically simulated using computational fluid dynamics (CFD). The simulation results agree reasonably well with the experimental results with respect to the liquid cone shape and droplet size.     

Current–time characteristic of the transient regime of electrohydrodynamic flow formation

The paper presents the results of computer simulation of the current–time characteristic (CTC) of a cell with low-conducting liquid. The basis of simulation is the complete set of electrohydrodynamic (EHD) equations. The injection and dissociation mechanisms of charge formation as well as field-enhanced dissociation are considered. The simulation is carried out in the needle–plane electrode system. The relation between sections of CTC and stages of EHD flow formation is revealed. The shape of CTC is shown to be dependent on mechanisms of charge formation, the ratio of the initial and steady-state ion concentrations and the mechanisms of charge transport.     

Numerical simulation of electrohydrodynamic (EHD) atomization

The objective of the present work was to use a commercial Computational Fluid Dynamics (CFD) code to simulate the electrohydrodynamic (EHD) atomization process. Although the physics of the atomization and cone formation has been discussed in numerous publications, a comprehensive theory has not been presented. Some of the previous approaches are discussed below. A CFD model can give a unique capability to describe and simulate the liquid cone formation and atomization. The approach in this work was to simultaneously solve the coupled (EHD) and electrostatic equations. The heat conduction equation, solved by the CFD solver, has been modified to solve the electrostatic field equations. From the electrostatic field, the electric body forces have been determined and included in the Navier–Stokes equations. The model does not include any current. The key liquid property for the coupling is the permittivity. The predicted velocity fields for heptane and ethanol and the operating window of heptane were found to be consistent with published results. The model does not include a droplet break-up model. If the jet is cylindrical, the droplet size can be calculated from the jet diameter. The droplet size of ethanol was predicted and compared well with experiments.     

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.     

Formation of conic cusps at the surface of liquid metal in electric field

The formation dynamics is studied for a singular profile of a surface of an ideal conducting fluid in an electric field. Self-similar solutions of electrohydrodynamic equations describing the fundamental process of formation of surface conic cusps with angles close to the Taylor cone angle 98.6° are obtained. The behavior of physical quantities (field strength, fluid velocity, surface curvature) near the singularity is established.     

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.     

A study of vortex shedding induced by dielectric barrier injection

Previous works have proved that a dielectric barrier injection (DBI) device could be used as an electrohydrodynamic (EHD) actuator. Such a device was used to generate a wall jet with an average velocity of 0.14 m/s.In this study, the liquid flow induced in the surface vicinity by a DBI actuator is measured by Particle Image Velocimetry (PIV). The phase analysis of the flow velocity shows that the DBI actuator mainly acts as a vortex generator. The work presented in this paper underline the relation between the vortex structures and the polarity of the injecting electrode.     

Electrohydrodynamics of a Cone–Jet Flow at a High Relative Permittivity

We have proposed a new solution of the electrohydrodynamic equations describing a novel cone–jet flow structure formed at a conductive liquid meniscus in an electric field. Focusing on the liquids characterized by a high relative permittivity and using the slender body approximation, the cone–jet transition profiles and their characteristic radii are predicted in relation to the material parameters. The stable value of the cone angle is obtained using the Onsager’s principle of maximum entropy production. Three different regimes of the cone–jet flow behavior are identified depending on the relative importance of capillary, viscous and inertial stress contributions. The presented complete analytical solutions for the cone–jet transition zone and the far jet region yield several different laws of algebraic decrease for the radius, surface charge, and electric field of the jet.     

EHD sprayings induced by the pulsed voltage superimposed to a bias voltage

Effects of pulsed voltage superimposed on dc bias voltage and meniscus height on electrohydrodynamic (EHD) spraying were investigated. Results show that greater pulsed voltages were associated with jet formation while a dripping mode was apt to appear with a lower pulsed voltage. This is because that with increasing pulsed voltages the energy gain per unit area of the liquid and the tangential electric stress at the meniscus lateral were increased more quickly than the normal electric stress at the apex of the meniscus. Additionally, the increment of meniscus height led to an unchanged tangential electric stress at the meniscus lateral, but a more quickly increased energy gain per unit area of the liquid than the normal electric stress at the apex of the meniscus. For the same pulsed voltage, spraying in Dripping I mode was produced from menisci of smaller heights due to the intensive normal electric stress. A much greater meniscus height, on the other hand, led to spraying in Dripping II mode when the pulsed voltage was insufficiently great. These various modes were determined by contributions of the tangential electric stress, the normal electric stress and the meniscus height.     

Flexoelectric origin of oblique rolls with helical flow in electroconvective nematics under DC excitation

We present a 3-dimensional, linear analysis of electrohydrodynamic (EHD) roll instability in a nematic liquid crystal under DC excitation. It is shown that the flexoelectric effect leads to a new symmetry of the flow pattern, viz. ahelical flow inoblique rolls. Our experimental observations agree with the theoretical predictions.     

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.     

Anomalous electrohydrodynamic phenomena on the surface of liquid tungsten

We have discovered that Taylor cone formation on the surface of liquid tungsten in air occurs in anomalously weak electric fields. This phenomenon is observed exclusively on tungsten and only in the presence of oxygen in the surrounding atmosphere. This anomaly could be due to the low surface tension of liquid tungsten in an oxygen-containing atmosphere. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 8, 583–587 (25 April 1996)     

Compact polymer multi-nozzles electrospray device with integrated microfluidic feeding system

Electrohydrodynamic (EHD) atomization consists in using an electric field for spraying a liquid flowing through a capillary. The applications are: mass spectrometry, colloid thrusters and more recently medicine nebulization processes. EHD atomization provides the ability to control the generated droplets size by adjusting electrospray parameters. It is however essential to manufacture the emitters into arrays because flow through a stable cone-jet mode electrospray can only be maintained at low flow rate and most applications require a high throughput. We propose a new design of a multiple electrospray system involving an innovative nozzle shape and flow restrictor system. Nozzles and microfluidic restrictor system are manufactured on the same polycarbonate sheet using the excimer laser technology and thus allowing a high compactness of this system.     

Modeling of the EHD effects on hydrodynamics and heat transfer within a flat miniature heat pipe including axial capillary grooves

A numerical model for an electrohydrodynamic (EHD) grooved Flat Miniature Heat Pipe (FMHP) is developed. Two microchannel shapes are considered as axial capillary structures: square and triangle grooves. For both groove shapes, the electric field affects the liquid-vapor radius of curvature which decreases in the condenser and increases in the evaporator under the action of the electric field. The liquid and vapor velocities are also affected by the EHD effects. The electric field effects on the velocities depend on the FMHP zone. It is also demonstrated that the electric field increases the vapor pressure drop; however, it decreases the liquid pressure drop. The liquid-wall and vapor-wall viscous forces as well as the shear liquid-vapor forces are affected by the electric field. The analysis of the electric forces shows that the dielectrophoretic forces which act on the liquid-vapor interface are predominant and their order of magnitude is much higher than the Coulomb forces. Finally, it is also demonstrated that the capillary limit increases with the electric field for both groove shapes.     

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].     

Electrohydrodynamic cone-jet bridges: Stability diagram and operating modes

An electrohydrodynamic cone-jet bridge is formed when two opposing Taylor cones are bridged by a liquid jet. We used high-speed video imaging to systematically investigate the operating regimes of the cone-jet bridge established between a nozzle and a liquid pool that were closely separated. There was a stability island for the cone-jet bridge in the voltage-flow rate operating diagram, and the stable bridge could only be formed above a minimum flow rate and at an intermediate range of voltages. In the vicinity of the stability island, the cone-jet bridge broke up via a thinning or beading mode.     

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.     

Laser flow visualization and velocity fields by particle image velocimetry in an electrostatic precipitator model

Although improving electrostatic precipitator (ESP) collection of fine particles (micron and submicron sizes) remains of interest, it is not yet clear whether the turbulent flow patterns caused by the presence of electric field and charge in ESPs advance or deteriorate fine particle precipitation process. In this paper, results of the laser flow visualization and Particle Image Velocimetry (PIV) measurements of the particle flow velocity fields in a wire-to-plate type ESP model with seven wire electrodes are presented. Both experiments were carried out for negative and positive polarity of the wire electrodes. The laser flow visualization and PIV measurements clearly confirmed formation of the secondary flow (velocity of several tens of cm/s) in the ESP model, which interacts with the primary flow. The particle flow pattern changes caused by the strong interaction between the primary and secondary flows are more pronounced for higher operating voltages (higher electrohydrodynamic numbernehd) and lower primary flow velocities (lower Reynolds number Re). The particle flow patterns for the positive voltage polarity of the wire electrodes are more stable and regular than those for the negative voltage polarity due to the nonuniformity of the negative corona along the wire electrodes (tufts).     

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.     

Flow control with plasma-based actuator in different velocity regimes: Momentum and heat transfer effects

In this study, control of the airflow by the direct current (DC) electrical discharge with bare electrodes has been investigated in different velocity regimes. The discharge characteristics of the plasma model are obtained numerically. An induced electrohydrodynamic (EHD) force on neutral flow was characterized based on momentum transfer from charged particles. The change in the incident flow parameters was studied by applying Navier–Stokes (N-S) equations, considering source terms arising from a weakly ionized plasma. The effect of the discharge on the low- and high-speed flow was simulated in this study. It was concluded that the changes of the velocity profile, airflow pressure, and oblique shock wave could be attributed to the EHD force from a nonthermal plasma to the incoming airflow. It was seen that the incident airflow is accelerated also by the induced EHD force. Our results show that the most important mechanism in the plasma-based flow control is the momentum transfer from the electrical discharge to the incident flow and that the gas heating has no significant role.