Showing papers on "Liquid dielectric published in 2008"

Effect of the powder concentration and dielectric flow in the surface morphology in electrical discharge machining with powder-mixed dielectric (PMD-EDM)

Abstract: The addition of powder particles to the electrical discharge machining (EDM) dielectric fluid modifies some process variables and creates the conditions to achieve a higher surface quality in large machined areas. This paper presents a new research work that aims to study the improvement in the polishing performance of conventional EDM when used with a powder-mixed-dielectric (PMD-EDM). The analysis was carried out varying the silicon powder concentration and the flushing flow rate over a set of different processing areas and the effects in the final surface were evaluated. The evaluation was done by surface morphologic analysis and measured through some quality surface indicators. The results show the positive influence of the silicon powder in the reduction of crater dimensions, white-layer thickness and surface roughness. Moreover, it was demonstrated that an accurate control of the powder concentration and flushing flow is a requirement for achieving an improvement in the process polishing capability.

Adaptive dielectric liquid lens.

Abstract: A tunable-focus liquid lens using dielectrophoretic effect is demonstrated. When a voltage is applied to a dielectric liquid droplet, the generated electric field inside the droplet is inhomogeneous. As a result, the liquid bears a dielectric force and its surface profile can be reshaped which causes the focal length to change. Adaptive lenses with different apertures are fabricated and their performances evaluated. In comparison to the patterned-electrode liquid lenses, our lens uses continuous electrode which is much simpler for fabrication.

Tunable-focus liquid microlens array using dielectrophoretic effect.

Abstract: A tunable-focus liquid microlens array based on dielectrophoretic effect was demonstrated. In a lens cell, two immiscible dielectric liquids but with different dielectric constants are sandwiched between electrodes. One electrode has a holed pattern while the other electrode is continuous. The applied voltage generates an inhomogeneous electric field near the hole regions. Owing to such an electric field, the generated dielectric force would separate the low dielectric liquid into many pieces and each piece is pushed to its neighboring hole. After balance, the droplet array is formed in the contracting state surrounded by the high dielectric constant liquid. Each droplet exhibits a lens character. When the voltage is removed, the droplets relax slightly but still keep a certain contact angle. Reactivating the cell will again enforce the droplets to reshape from relaxing to contracting states and, therefore, changing the focal length. Such a lens cell can be fabricated easily and its response time is reasonably fast. Its potential applications in image processing and zoom lens are emphasized.

Effect of electric field distribution uniformity on electrospinning

Abstract: Recent experiments demonstrated that the essential electrospinning mechanism is a rapidly whipping dielectric liquid jet in an electric field. The effects of three kinds of electric field distributions including a very nonuniform electric field, a slightly nonuniform electric field, and a uniform electric field were studied to analyze the influence of the electric field distribution on the electrospinning process and the fiber morphology. The average electric field strength Eav and the electric field nonuniformity coefficient f were used to describe the electric field. The results show that the length range of the straight jet path for stable electrospinning process was a function of the electric field distribution. The length of the jet straight part increased with the decreasing f. The initial angle of the looping envelope and the helical pitch decreased with decreasing f. The results also show that higher voltages can be used at the nozzle for a stable electrospinning when f is small. A higher voltage at the nozzle can produce a larger average electric field strength which will help produce smaller fiber diameters for the large bending frequency.

Natural and Synthetic Ester Liquids as alternative to mineral oil for power transformers

Abstract: The combination of a solid and a liquid insulation is the most frequently used insulating system in high voltage transformers, where components have to be insulated and loss heat has to be dissipated. The requirements on the liquid part of the insulating system are not only the electric and dielectric performance but also the performance regarding environmental requirements and dehydration capability as well as low inflammability. The contribution presents a comparison of the performance of natural and synthetic ester as an alternative to mineral oil.

Influence of Machining Parameters on Electric Discharge Machining of Maraging Steels - An Experimental Investigation

Abstract: Electric discharge machining is categorized as a thermoelectric process in which heat energy of spark is used to remove material from the work piece. The machining process involves controlled erosion of electrically conducting material by the initiation of rapid and repetitive electrical spark discharges between the tool and workpiece separated by the dielectric medium. The present work is aimed at characterizing the electric discharge machining of maraging steels on EDM. 27 experiments are conducted by varying EDM parameters such as current, pulse-on-time, and duty factor. The performance measures like Material removal rate, surface roughness, and hardness are assessed. It is concluded that metal removal rate, and surface roughness increases with increase in current, duty factor. But, as the pulse-on-time increases MRR and surface roughness decreases. Hardness value increases as the current value increases from 5A to 10A amps and then decreases as the current increases from 10A to 15A. The same effect is observed as in case of duty factor and pulse on-time. Average crack length and recast layer thickness increases with increase in current and duty factor. But, there will be decreasing trend in case of pulse-on-time. is delivered to the electrode at a frequency of several KHz producing sparks of similar frequency between the electrode and the work piece through the dielectric fluid. Intense heat is created in the localized area of spark impact, the metal melts or even vaporizes and gets expelled from the surface of workpiece. The dielectric fluid, which is constantly being circulated, carries away the eroded particles of metal during the off cycle of the pulse and also assists in dissipating the heat caused by the spark.

Quantifying the linear stability of a flowing electrified two-fluid layer in a channel for fast electric times for normal and parallel electric fields

Abstract: Motivated by the destabilization of a two-fluid layer flowing in a microchannel for efficient mixing or droplet formation, we study quantitatively the linear stability of the interface between two liquids subjected to an electric field parallel or normal to the flat interface. In the case of fast electric charge relaxation times, the equations for the perturbation can be significantly reduced [A. K. Uguz, O. Ozen, and N. Aubry, Phys. Fluids 20, 031702 (2008)]. Using a simple argument and without solving the equations, Uguz et al. determined the range of parameters over which the electric field is destabilizing, which is narrower for the parallel compared to the normal electric field. However, the argument of Uguz et al. was not amenable to the calculation of growth rates and neutral stability curves. In this paper, by solving the equations, we not only confirm the previous findings but also determine the quantitative linear stability properties, namely, the growth rates and neutral stability curves. Depen...

Liquid optical fibers with a multistable core actuated by light radiation pressure.

Abstract: We report on spatiotemporal behavior of self-adapted dielectric liquid columns generated and sustained by light radiation pressure. We show that single- or multivalued liquid column diameter depends on the excitation light beam. When the beam diameter is sufficiently small, we observe a well-defined stationary column diameter. In contrast, at a larger beam diameter, the liquid column experiences complex spatiotemporal dynamics whose statistical analysis evidences an underlying multistable structure. Experimental observations are all supported by a full electromagnetic model that accounts for the wave guiding properties of the liquid column viewed as a step-index liquid-core liquid-cladding optical fiber having an optically tunable core diameter.

Effect of electrode surface roughness and dielectric coating on breakdown characteristics of high pressure CO 2 and N 2 in a quasi-uniform electric field

Abstract: SF6 gas has excellent dielectric strength and interrupting capability and is used in various applications such as gas insulated switchgear (GIS) in substations. However, since SF6 has a high global warming potential (GWP), it is imperative to reduce its use and develop recovery technology for its reuse. This paper examined the potential of alternative insulating gases for GIS, determining experimentally the ac and lightning impulse breakdown characteristics of CO2 and N2 gases, which have little environmental impact, in a quasi-uniform electric field. The effect of electrode surface roughness is greater in N2 than in CO2. The impulse ratio in a quasi-uniform electric field was smaller in N2 than in CO2 at low pressure, but this reversed when the pressure reached 1.2 MPa. The dielectric strength of an electrode with a dielectric coating was approximately equal to that of a specular-finish bare electrode in either CO2 or N2 when an ac voltage was applied. When lightning impulse voltage was applied, a significant improvement of dielectric strength was observed if a dielectric coating was applied to the electrode considered to be the initial electron source.

Thermofluid characteristics of two-phase flow in micro-gap channels

Abstract: Microgap coolers provide direct contact between chemically inert, dielectric fluids and the back surface of an active electronic component, thus eliminating the significant interface thermal resistance associated with thermal interface materials and/or solid-solid contact between the component and a microchannel cold plate. This study focuses on the two-phase thermo-fluid characteristics of a dielectric liquid, FC- 72, flowing in an asymmetrically-heated chip-scale micro-gap channel, 10 mm wide by 37 mm long, with channel heights varying from 110 mum to 500 mum and channel wall heat fluxes of 200 kW/m2 (20 W/cm2). The two-phase, area-averaged heat transfer coefficients of FC-72 reached 15.5 kW/m2-K, significantly higher than the single phase FC-72 values, thus providing cooling capability in the range associated with water under forced convection. Data obtained for single phase water yielded very good agreement with predictions for the convective heat transfer coefficients and served to validate the accuracy of the experimental apparatus and measurement technique.

Cooling of an IGBT Drive System with Vaporizable Dielectric Fluid (VDF)

Abstract: The use of a vaporizable dielectric fluid is proposed and demonstrated in a proof-of-concept electrical drive system utilizing medium-range 1200 VAC 450 A IGBT devices. Comparative empirical data is shown for a drive system utilizing production components for a traditional air-cooled extruded aluminum heat sink thermal solution for each IGBT module, versus several water-cooled liquid cold plate solutions and a single-cabinet 750 kW, 1,000-horsepower drive system utilizing low-flow, pumped liquid multiphase cooling. Positive and negative attributes of each thermal solution are described.

Numerical modeling of Joule heating-induced temperature gradient focusing in microfluidic channels.

Abstract: Temperature gradient focusing (TGF) is a recently developed technique for spatially focusing and separating ionic analytes in microchannels. The temperature gradient required for TGF can be generated either by an imposed temperature gradient or by Joule heating resulting from an applied electric field that also drives the flow. In this study, a comprehensive numerical model describing the Joule heating induced temperature development and TGF is developed. The model consists of a set of governing equations including the Poisson-Boltzmann equation, the Laplace equation, the Navier-Stokes equations, the energy equations and the mass transport equation. As the thermophysical and electrical properties including the liquid dielectric constant, viscosity, and electric conductivity are temperature-dependent, these governing equations are coupled, and therefore the coupled governing equations are solved numerically by using a CFD-based numerical method. The numerical simulations agree well with the experimental results, suggesting the valid mathematical model presented in this study.

Vacuum insulator development for the dielectric wall accelerator

Abstract: At Lawrence Livermore National Laboratory, we are developing a new type of accelerator, known as a dielectric wall accelerator, in which compact pulse-forming lines directly apply an accelerating field to the beam through an insulating vacuum boundary. The electrical strength of this insulator may define the maximum gradient achievable in these machines. To increase the system gradient, we use “high-gradient insulators” composed of alternating layers of dielectric and metal for the vacuum insulator. In this paper, we present our recent results from experiment and simulation, including successful testing of a high-gradient insulator in a functioning dielectric wall accelerator cell. Our results indicate that proper high-voltage conditioning of the insulators can delay the onset of flashover, that the observed conditioning consists of both a permanent and a temporary part, and that the insulators’ voltage-holding capability increases with increasing dielectric layer thickness.

Study of the vegetal oil as a substitute for mineral oils in distribution transformer

Abstract: There are so many factors that negatively affect distribution transformer operations, especially its electrical insulation system. Traditionally mineral oil has been used as insulating material, however, in the last years there is a growing interest in using vegetable oil, these fluids are currently being used in the range of small to medium distribution power transformers. The purpose of this work is to determine distribution transformer performance using natural ester dielectric fluid compared to those with standard mineral oil. The most significant factor that affects insulation material deterioration is temperature increments due to overload. We used the ASTM standard test method for analysis of gases dissolved in electrical insulating oil, to assess the aging of transformer insulation paper in natural ester dielectric fluid compared to that in conventional transformer oil. The results showed that distribution transformers using vegetable oil has a useful life longer than others using mineral oil, roughly 42,86 percent.

Discharge Ignition Near a Dielectric

Abstract: Electrical breakdown in noble gas near a dielectric is an important issue in lighting industry. In order to investigate the influence of the dielectric on the ignition process, we perform measurements in argon, with pressure varying from 0.1 to 1 bar, using a pin-pin electrode geometry. Here, we present time-resolved images of ignition process for two different distances from electrodes to the dielectric.

Thermal and mechanical numerical modelling of electric discharge machining process

Abstract: In electric discharge machining (EDM), the heat gradients caused by the electric discharge create a non-uniform local thermal expansion on the level of the surface layers of machined materials from where genesis of thermal stresses takes place. These thermal stresses, if exceeding yield stress, can remain and become residual after the cooling of the part. The modelling of these phenomena, during the heating by the electric discharge and the cooling by the dielectric liquid, requires a heat transfer model, the material behaviour identification, a thermo-mechanical model for the thermal and the residual stress models. This paper presents numerical results concerning the temperature distribution, the thermal and residual stresses of a stable steel material (AISI316L) machined by EDM. Comparison of numerical results with experimental data and numerical results from the literature shows good agreement and is hence quite satisfactory. Copyright © 2008 John Wiley & Sons, Ltd.

Influence of Composition of Dielectric Liquid Mixtures on Electrostatic Charge Tendency and Physicochemical Parameters

Abstract: The paper presents research results on ECT (electrostatic charge tendency) of hexane and toluene mixtures, that is dielectric liquids of a simple chemical structure and high purity (Pro analysis). The research tests are carried out in a spinning disk system, where the factors affecting the value of the electrification current registered are a varying composition of the mixture, rotational speed and a disk diameter. In the next stage physicochemical parameters are measured in order to determine the relationship between the properties of the mixtures under study and the electrification current generated. The research results show that the ECT of the mixture depends, to a large extent, on the proper selection of particular components.

Hermetically sealed motor lead tube

Abstract: An electrical submersible pump assembly has a motor cooperatively engaged with a pump for driving the pump and a motor lead extending to the motor for supplying power to the motor. The motor lead has an electrical conductor enclosed within a tube. The tube is sealed from an exterior environment of the motor and contains a dielectric fluid. The motor may be filled with a dielectric lubricant and the interior of the tube of the motor lead may be in fluid communication with the dielectric lubricant. Alternatively, the motor may filled with a dielectric lubricant and the dielectric fluid within the interior of the tube of the motor lead may be sealed from the dielectric lubricant.

Liquid-gap electrostatic hydraulic micro actuators

Abstract: A liquid-gap electrostatic hydraulic micro actuator is provided that produces higher displacement (in and out of plane) and larger force than typical electrostatic actuators by utilizing a non-conducting liquid as its dielectric material. This new class of actuators utilizes the liquid dielectric for hydraulic amplification and force transfer. The liquid electrostatic actuator consists of two chambers each forming a parallel-plate capacitor, filled with a non-conducting incompressible liquid. One chamber is compressed by pulling down a flexible membrane using electrostatic actuation, thus forcing the liquid under it to transfer into the other chamber. Such movement causes the other chamber's membrane to expand out of plane.

Theory of small-signal ac response of a dielectric liquid containing two groups of ions

Abstract: The analysis of Macdonald for electrolytes is generalized to the case in which two groups of ions are present. We assume that the electrolyte can be considered as a dispersion of ions in a dielectric liquid, and that the ionic recombination can be neglected. We present the differential equations governing the ionic redistribution when the liquid is subjected to an external electric field, describing the simultaneous diffusion of the two groups of ions in the presence of their own space charge fields. We investigate the influence of the ions on the impedance spectroscopy of an electrolytic cell. In the analysis, we assume that each group of ions have equal mobility, the electrodes perfectly block and that the adsorption phenomena can be neglected. In this framework, it is shown that the real part of the electrical impedance of the cell has a frequency dependence presenting two plateaux, related to a type of ambipolar and free diffusion coefficients. The importance of the considered problem on the ionic characterization performed by means of the impedance spectroscopy technique was discussed.

Microscopic fields in liquid dielectrics.

Abstract: We present the results of an analytical theory and numerical simulations of microscopic fields in dipolar liquids. Fields within empty spherical cavities (cavity field) and within cavities with a probe dipole (directing field) and the field induced by a probe dipole in the surrounding liquid (reaction field) are considered. Instead of demanding the field produced by a liquid dielectric in a large-scale cavity to coincide with the field of Maxwell's dielectric, we continuously increase the cavity size to reach the limit of a mesoscopic dimension and establish the continuum limit from the bottom up. Both simulations and analytical theory suggest that the commonly applied Onsager formula for the reaction field is approached from below, with increasing cavity size, by the microscopic solution. On the contrary, the cavity and directing fields do not converge to the limit of Maxwell's dielectric. The origin of the disagreement between the standard electrostatics and the results obtained from microscopic models is traced back to the failure of the former to account properly for the transverse correlations between dipoles in molecular liquids. A new continuum equation is derived for the cavity field and supported by numerical simulations. Experimental tests of the theoretical results are suggested.

Numerical study on the splitting of a vapour bubble in the process of EDM.

Abstract: Electrical discharge machining (EDM) is a powerful technique for machining of hard and brittle materials. In this process, because of electrical discharge, a vapour bubble is generated in the dielectric liquid between the tool and the workpiece. The growth and collapse phases of the vapour bubble have significant effect on the hydrodynamic behaviour of the dielectric liquid domain between the tool and the workpiece and cause molten material to escape from the crater. Previous numerical studies on the dynamics of an electrical discharge-generated vapour bubble have simulated the growth and collapse of the bubble until it has taken the shape of an hour-glass. This is necking phenomenon which is followed by splitting of the bubble into two parts. In this paper dynamics of an electrical discharge-generated vapour bubble between the tool and the workpiece after its splitting are investigated by using the boundary integral equation method. Development of a liquid jet on the boundary of the each of the upper and lower parts of the bubble and the impingement of the liquid jets on the nearby rigid surfaces are sought. This paper consists of two parts. In part one, the vapour bubble is initially located between the tool and the workpiece. Consequently the dynamic behaviour of the two parts of the bubble in the absence of the buoyancy forces are symmetric with respect to a horizontal axis through the mid-point between the tool and the workpiece. In part two, the elrecrical discharge-generated vapour bubble is initially located in the vicinity of the workpiece. Therefore during the necking phenomenon the upper part of the bubble is smaller than its lower part. Consequently the dynamic behaviour of the two parts of the bubble after its splitting are significantly different.

Onset of electroconvective instability of Oldroydian viscoelastic liquid layer in Brinkman porous medium

Abstract: The problem of the onset of electrohydrodynamic instability in a horizontal layer of Oldroydian viscoelastic dielectric liquid through Brinkman porous medium under the simultaneous action of a certical ac electric field and a vertical temperature gradient is analyzed. Applying linear stability theory, we derive an equation of eight order. Under somewhat suitable boundary conditions, this equation can be solved exactly to yield the required eigenvalue relationship from which various critical values are determined in detail. Both the cases of stationary and oscillatory instabilities are discussed if the liquid layer is heated from below or above. The effects of the porosity of porous medium, the medium permeability, the Prandtl number, the ratio of retardation time to relaxation time, the elastic number, in the presence or absence of Rayleigh number are shown graphically for both cases. Some of the known results are derived as special cases. The electrical force has been shown to be the sole agency causing instability of the considered system since it is much more important than the buoyancy force even if the medium is porous.

Dielectric Properties of Diala D, MIDEL 7131 and THESO Insulating Liquids

Abstract: Insulating oils can be used as dielectric materials in a wide range of high voltage applications. Most notable amongst these liquids are naphthenic mineral oils, which are used in elements of pulsed power systems, such as switches and capacitors. Dielectric liquids are also used as energy storage media in pulsed power systems. In this application the use of liquids with higher dielectric permittivities will allow high-density energy storage systems to be developed. A dielectric oil with an unusually high, static relative permittivity of ~15, THESO oil, has been recently developed by Tetra Corp. This paper compares the measured dielectric properties of three types of insulating liquids: Diala D (Shell Ltd), ester liquid MIDEL 7131 and the high permittivity oil THESO. The study includes measurements of their relative permittivities, their electrical conductivities and their impulse dielectric strength. The data obtained on the dielectric characteristics of these liquids will be important for the optimisation of their applications in high voltage and pulsed power systems.

Numerical Simulation of Liquid-Solid Two-Phase Flow Field in Discharge Gap of High-Speed Small Hole EDM Drilling

Abstract: The flow field characteristics have a significant effect on the machining stability in high-speed small hole EDM drilling. Thus, Lagrangian discrete phase model (DPM) has been developed to simulate the gap liquid-solid two-phase flow field. The numerical calculation is based on the standard k-e turbulent model, and the SIMPLEC algorithm is used in the simulation. All the governing equations are solved by software Fluent 6.2. Through numerical simulation, the pressure distribution, the velocity distribution of the dielectric liquid, traces of debris particles, and the debris particle concentration were obtained. The flow field characteristics under different pressures and drilling depths were obtained through simulations. Finally, experiments were carried out to investigate the effects of the flush velocity at exit obtained through simulation on material removal rate (MRR).

Electrohydrodynamical characteristics of a dielectric liquid flow induced by charge injection

Abstract: 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 particle density on the current is measured. In this work, experiments were investigated done on a typical two-dimensional charged plume flow produced between a blade and a flat plate.