Electrohydrodynamic Effects Research Articles

Effect of EHD on heat transfer enhancement during two-phase condensation of R-134a at high mass flux in a horizontal smooth tube

In this study, effect of electrohydrodynamic (EHD) on the condensation heat transfer enhancement and pressure drop of pure R-134a are experimentally investigated. The test section is a 2.5 m long counterflow double tube heat exchanger with refrigerant flowing in the inner tube and cooling water flowing in the annulus. The inner tube is made from smooth horizontal copper tubing of 9.52 mm outer diameter. The electrode is made from stainless steel wire of 1.47 mm diameter. The test runs are performed at average saturated temperatures ranging between 40 and 60°C, mass flux ranging between 200 and 600 kg/m2 s, heat flux ranging between 10 and 20 kW/m2 and applied voltage at 2.5 kV. For the presence of the electrode, the experimental results indicate that the maximum heat transfer enhancement ratio is around 30% while the maximum increase in pressure drop is about 25%.

Mass Transfer within Electrostatic Precipitators: Trace Gas Adsorption by Sorbent-Covered Plate Electrodes

Varying degrees of mercury (Hg) capture have been reported within the electrostatic precipitators (ESPs) of coal-fired electric utility boilers. There has been some speculation that the adsorption takes place on the particulate-covered plate electrodes. This convective mass transfer analysis of laminar and turbulent channel flows provides the maximum potential for Hg adsorption by the plate electrodes within an ESP under those conditions. Mass transfer calculations, neglecting electrohydrodynamic (EHD) effects, reveal 65% removal of elemental Hg for a laminar flow within a 15-m–long channel of 0.2-m spacing and 42% removal for turbulent flow within a similar configuration. Both configurations represent specific collection areas (SCAs) that are significantly larger than conventional ESPs in use. Results reflecting more representative SCA values generally returned removal efficiencies of <20%. EHD effects, although potentially substantial at low Reynolds numbers, diminish rapidly with increasing Reynolds number and become negligible at typical ESP operating conditions. The present results indicate maximum Hg removal efficiencies for ESPs that are much less than those observed in practice for comparable ESP operating conditions. Considering Hg adsorption kinetics and finite sorbent capacity in addition to the present mass transfer analyses would yield even lower adsorption efficiencies than the present results. In a subsequent paper, the author addresses the mass transfer potential presented by the charged, suspended particulates during their collection within an ESP and the role they potentially play in Hg capture within ESPs.

Determinate and stochastic orientation dynamics of nematic liquid crystals in the cross electric fields with frequencies up to the megahertz range

A theoretical and experimental description is given of the director orientation dynamics in nematic liquid crystals placed in variable electric fields comprising several components and having different directions (crossed fields) and frequencies up to 5 MHz. The calculations show that the system of interest is a physical object governed by non‐linear dynamics. Depending on numerical values of governing parameters, the following director state regimes are obtained: stationary, periodic, quasi‐periodic (multi‐modal), and stochastic strange‐attractor‐type. In the calculations, all the states are obtained by solving a determinate system of two time‐dependent first order non‐linear equations, ignoring the electrohydrodynamic effect. Preliminary verification is performed, and qualitative agreement with the mathematical model under consideration is obtained in the range of frequencies (approximately 100 Hz and higher) that allows electrical conductivity to be ignored. The influence of 2D and 3D rotating electric fields with one frequency, and two‐ or three‐component fields with different frequencies, in the interval from 10 Hz to 5 MHz on the orientation of a nematic liquid crystal is considered.

Mechanisms Leading to Tornado Generation and Sustainment

We are investigating the phenomena leading to the formation of vortex structures in the Earth atmosphere and, in particular, to the conditions under which whirlwinds (tornados) are initiated. Analysis has been performed of the mechanisms of whirl generation and of the models describing the phenomena taking place in a whirl and permitting numerical calculations of the two-phase flows in a developed whirlwind. We have obtained dimensionless criteria for the appearance of a vertical-axis whirl that arises by the loss of stability of ascending axially symmetric thermals when the dominant acting forces are inertia forces of the primary flow or electrohydrodynamic (EHD) effects. For the case where inertia forces dominate, the critical value of the parameter has been established on the basis of the data of observations of the state of the atmosphere prior to the moment a tornado is initiated. For the case where the influence of the EHD mechanism prevails, a theoretical estimate of the critical value of the generation criterion is given. The possibilities of artificial generation of tornado-like whirls and their control are briefly discussed. The possibilities of using artificial tornado-like whirls to localize fallouts resulting from accidents at ecologically dangerous objects and protect objects from destruction are analyzed.

An AC Electrokinetic Technique for Collection and Concentration of Particles and Cells on Patterned Electrodes

We report an electrohydrodynamic effect arising from the application of alternating electric fields to patterned electrode surfaces. The AC fields were applied to dilute suspensions of latex microspheres enclosed between a patterned silicon wafer and an ITO-coated glass slide in a small chamber. The latex particles became collected in the center of the conductive "corrals" on the silicon wafer acting as bottom electrode. The particle collection efficiency and speed depended only on the frequency and strength of the field and were independent of the material properties of the particles or the electrodes. The leading effect in the particle collection process is AC electrohydrodynamics. We discuss how the electrohydrodynamic flows emerge from the spatially nonuniform field and interpret the experimental results by means of electrostatic and hydrodynamic simulations. The technique allows three-dimensional microfluidic pumping and transport by the use of two-dimensional patterns. We demonstrate on-chip collection of latex particles, yeast cells, and microbes.

A novel valveless micropump with electrohydrodynamic enhancement for high heat flux cooling

Integrated microchannel cooling systems, with micropumps integrated into microchannels, are an attractive alternative to stand-alone micropumps for liquid-cooled microchannel heat sinks. A new micropump design capable of integration into microchannels and especially suited for electronics cooling is presented. It combines induction electrohydrodynamics (EHD) with a valveless nozzle-diffuser micropump actuated using a vibrating diaphragm. A comprehensive numerical model of the micropump has been developed to study the combined effect of EHD and valveless micropumping. The numerical model has been validated using theoretical and experimental results from the literature. The flow rate achievable from the new micropump is presented and the effect of several key parameters on the micropump performance investigated.

Flow Control for Airfoil Wake with Plasma Synthetic Jet Actuator (Wake Characteristics of Strip-Strip Type Actuator)

Flow on an airfoil was investigated with plasma synthetic jet actuator. Plasma synthetic jet actuator is a flow control device, which is composed of two electrodes with a.c. signal, using electrohydrodynamic (EHD) effect and induces flow around the electrodes. Plasma synthetic jet actuator has some important advantages ; for example, miniaturization is possible, mechanical maintenance is almost free. Therefore it is expected to apply for flow control around an airfoil. In this study, to observe behavior of the plasma synthetic jet actuator, experimental study was conducted. A NACA0012 airfoil, in which plasma synthetic jet actuator is embedded, was chosen, and the wake velocity behind airfoil is measured by hot wire anemometer with or without operating the actuator.

NUMERICAL MODELING OF ELECTROHYDRODYNAMIC (EHD) EFFECT ON NATURAL CONVECTION IN AN ENCLOSURE

Mathematical and numerical modeling of electrohydrodynamic (EHD) enhancement of natural convection in enclosures is carried out. An electric current in dielectric liquid is modeled as a directed motion of electrically charged particles injected into a neutral fluid; the electric body force and Joule heat are added to the momentum and energy equations, respectively. Based on this, numerical studies are carried out for EHD effects on natural convection in enclosures. It is found that, at the same electric field intensity, the EHD enhancement of heat transfer is different for different electric density injections; applying a nonuniform electric field offers better EHD enhancement of heat transfer than applying a uniform electric field.

Structure formation in binary colloids.

A theoretical study of the structure formation observed very recently [W. D. Ristenpart, I. A. Aksay, and D. A. Saville, Phys. Rev. Lett. 90, 128303 (2003)] in binary colloids is presented. In our model solely the dipole-dipole interaction of the particles is considered, electrohydrodynamic effects are excluded. Based on molecular dynamics simulations and analytic calculations we show that the total concentration of the particles, the relative concentration, and the relative dipole moment of the components determine the structure of the colloid. At low concentrations the kinetic aggregation of particles results in fractal structures which show a crossover behavior when increasing the concentration. At high concentration various lattice structures are obtained in a good agreement with experiments.

A Continuous Ink Jet Device on the Basis of Electrohydrodynamic Mechanism

A new type of ink jet system is proposed on the basis of electrohydrodynamic (EHD) effect. On the application of high DC electric fields to some insulating fluids, a fluid jet with a velocity of about 1 ms&#x2212;1 is generated from the positive electrode as a bulk flow. By controlling the EHD flow to issue across the free surface, the ink jet device is developed, of which the nozzle unit is simply composed of a needle electrode and capillary. When the fluids are subjected to high electric fields, the ink jet is generated above a critical threshold and the height increases with increasing voltage. The critical voltage strongly depends on the nozzle geometry. The height of ink jet at high voltages increases with decreasing viscosity and conductivity of fluids. The performance of ink jet based on the EHD jet can be improved by a combination of designs of nozzle geometry and material functions.

Augmentation of Laminar Forced Convective Heat Transfer of an Oil Flow in an Enhanced Tube by EHD Effect

Heat transfer enhancement using an applied electric field has fascinating application perspective since it can realize a good enhancement effect at the expense of only a little cost of energy. Many fundamental studies have been carried out in this field and the emphasis of the study has been gradually turning to application study. The electrohydrodynamic ~EHD! enhancement of the forced convective heat transfer refers to the coupling of an electric field with the fluid field in a dielectric fluid medium: The net effect is production of electrically induced secondary motions that destabilize the thermal boundary layer near the heat transfer surface. The effect of electrohydrodynamics on the phase-change heat transfer has been under investigation for the past two decades. On the other hand, the studies of the convective heat transfer enhancement by the EHD effect are relatively little. The experimental results reported by Franke and Hogue @1# and Hasegawa et al. @2# denoted that the strong electric field could disrupt the laminar thermal boundary layer of air or refrigerants in-tube and shift the flow conformation from the laminar flow to the turbulent flow. Compared with air and refrigerants, oil is a so viscous fluid that it keeps still the laminar flow when the external electric field is applied although the secondary flow may be aroused. It is quite meaningful to reinforce the forced convective heat transfer of oil in-tube, since it has a low heat transfer coefficient, but a widely practical perspective. In general, it is difficult to enhance the laminar forced convective heat transfer of oil in-tube. Common enhancement techniques use mainly various surface-worked tubes such as the two-dimensional or three-dimensional extended surfaces tubes ~see, for example, @3#! and the helical groove tubes ~see, for example, @4#!, the smooth tube inserted swirl-flow de-

Electrohydrodynamic Effect on Phase Separation Morphology in Polymer Blend Films

We have investigated the effect of electrohydrodynamic (EHD) convection on the domain structure in a polystyrene (PS)/polyvinyl acetate (PVA) blend film to demonstrate the feasibility of using the EHD effect as a means of mixing and morphology control in a polymer blend film prepared by solvent evaporation. Here, polymers-toluene solutions were spread on a glass substrate with patterned electrodes to apply a dc electric field, and well-defined structures of EHD convection were formed in the polymer solutions. As a result, regular patterns were formed in the PS/PVA polymer blend film in which PVA-rich domains were confined within each unit of patterned electrodes, i.e., between positive and negative electrodes, at an appropriate electric voltage. In addition, it was demonstrated that such novel morphology is not due to the wetting/dewetting effect of polymer components to the Pt electrodes deposited on the glass substrate, by experiments with a SiO2-covered substrate.

Theoretical Model of Electric Field Effects on The Enhancement of Critical Heat Flux

Critical heat flux enhancement by the electrohydrodynamic (EHD) effect has been analyzed quantitatively based on the increased frequency of liquid-vapor interface oscillations around the edge of the bubble. The majority of heat transfer occurs when the liquid film thickness becomes less than 50 μm, which only occurs once per period. The main mechanism of heat flux enhancement induced by the EHD effect would be a result of an increase in surface tension due to the effect of electric lines of force. By representing the terms of the forces for a change in curvature and the surface tension resulting from the electric lines of force, the equation of the liquid-vapor instability was obtained and analyzed. Experimentally it has been shown that as the applied voltage increased, the periodic time interval of the thickness change was shortened. This effect reduces the potential for dryout of the liquid film by making the minimum thickness time period shorter. By measuring the pressure oscillation on the boiling surface, the change of the thin liquid film thickness and the dynamic shape of bubbles, the relationship among the pressure, the liquid film thickness and the bubble shape was clarified. Consequently, this model successfully explains the relationship between the applied voltage and the enhancement of the critical heat flux.

Electrorheology of dielectric liquids

Dielectric liquids that show striking electrorheological (ER) effects are formulated by controlling the conductivity. Although the viscosity is increased on the application of a d.c. field, the flow of electrified fluids is Newtonian in the plain electrodes with smooth surfaces. When the liquids are sandwiched between the electrodes with flocked fabrics, the viscosity behavior is converted from Newtonian to shear-thinning flow. In electric fields, the convective flow is induced over the system due to the electrohydrodynamic(EHD) effect. The interactions between EHD convection and external shear give rise to the additional energy dissipation and in turn the increase in viscosity. The ER effects of simple liquids are very attractive in application to new fluid devices.

Dynamics of cholesteric structures in an electric field.

Motivated by Lehmann-like rotation phenomena in cholesteric drops we study the transverse drift of two types of cholesteric fingers, which form rotating spirals in thin layers of cholesteric liquid crystal in an ac or dc electric field. We show that electrohydrodynamic effects induced by Carr-Helfrich charge separation or flexoelectric charge generation can describe the drift of cholesteric fingers. We argue that the observed Lehmann-like phenomena can be understood on the same basis.

Electrohydrodynamic effects on characteristic of isolated bubbles in the nucleate pool boiling regime

Application of electric fields demonstrates great promise as an active heat transfer enhancement technique. Here, we study experimentally some characteristics of isolated bubbles such as bubble departure diameter, nucleation rate (frequency), and density of nucleation sites under the influence of an electric field In the nucleate pool boiling regime, isolated bubbles were recorded with a high-speed video camera at a rate of 1000 frames/s. The quantitative and qualitative analyses of bubble characteristics, and latent heat transfer concluded that at a constant heat flux condition in the isolated bubble regime, the density of nucleation sites was a very sensitive boiling parameter when electric field varied. The latent heat contribution varied proportional with the density of nucleation sites when heat flux and/or electrode voltage varied. This further simplifies the complex task of modeling of the electrohydrodynamic enhanced pool boiling heat transfer in the isolated bubble regime.

Band-broadening effects in preparative free-flow zone electrophoresis.

Recently, we proposed a novel preparative free flow zone electrophoresis cell with extremely short residence time, which does not require external cooling (Poggel, M., Melin, T., Electrophoresis 2001, 22, 1008-1015). Within the new cell the smallest chamber dimension is not orientated perpendicular but in direction of the electric field. This alteration provides straight forward scale up opportunities. In this paper, new experimental results are reported, from which the limits of stable flow can be determined. The data suggest that not density differences but electrohydrodynamic effects are responsible for the disturbance of the laminar flow pattern, which is observed above a critical field strength. To demonstrate the efficiency of the new system, a three-component mixture consisting of bovine serum albumin (BSA), myoglobin and cytochrome c is processed, resulting in relatively high recovery and purity values of the different proteins, although a complete separation is not achieved.

Enhancement of condensation heat transfer by means of EHD condensate drainage

The problem of electrohydrodynamic (EHD) condensation enhancement has been studied very recently. The electric field is commonly generated in such case by means of rod and mesh electrodes. The EHD method is suitable for dielectric media used in refrigeration and heat pump devices. The strong positive EHD effect has been obtained so far for vertical and horizontal bare tubes. However it was found that this effect for horizontal integral-fin tubes, commonly used in heat exchangers, is negligible small. The mechanism of the EHD condensation enhancement is discussed in the paper and a novel arrangement of the tube-electrode system is proposed. It consists of a horizontal finned tube with a rod type electrode placed beneath the tube. The experimental investigations have been carried out for this tube-electrode arrangement using HCFC-123 as a working medium. The obtained results confirmed the expectations since the application of EHD method for the new tube-electrode configuration increased the heat transfer coefficient from 27% to 110%, depending on the electrode potential. A simple model of the EHD condensate drainage has been proposed for the new tube-electrode arrangement.

NUCLEATE POOL BOILING UNDER DC ELECTRIC FIELD

This article deals with an experimental study of the influence of a DC uniform electric field on the nucleate boiling heat transfer. Electrohydrodynamic (EHD) effects on heat transfer coefficients for dielectric liquids are quantitatively investigated by performing experiments on various liquids with different properties. In these experiments, n-pentane, R-113, and R-123 are used as working fluids and the boiling phenomenon takes place on a horizontal plane copper surface. The experimental results have shown: (1) a threefold increase of nucleate pool boiling heat transfer coefficients, (2) a threefold increase of the critical heat flux (CHF), and (3) the disappearance of the hysteresis phenomenon. For nucleate pool boiling and CHF regimes, heat transfer laws based on dimensionless numbers are proposed. The results obtained by the proposed EHD model are in good agreement with the experimental results.

Electrohydrodynamic instability of a charged membrane.

The stability of shape fluctuations of a flat charged membrane immersed in a fluid is analyzed using a linear stability analysis. A displacement of the membrane surface causes a fluctuation in the conterion density at the surface. This in turn causes an additional contribution to the force density in the momentum equation for the fluid, which results in a normal stress at the surface which is opposite in direction to the stress caused by surface tension. This electrohydrodynamic effect destabilizes fluctuations when the surface potential exceeds a critical value.