Showing papers on "Liquid dielectric published in 1994"

Hydrogen atom yield in RF and microwave hydrogen discharges

Abstract: The hydrogen atom yield in pure-H2 RF and microwave-sustained discharges is investigated both theoretically and experimentally. A particle balance model is developed that provides the concentrations of the H, H2, H+, H 2 + , and H 3 + species. It is also shown that an approximate solution of this model is adequate for calculating the concentration of H atoms (required, for instance, in diamond film deposition) in the 0.1–10 torr range. Next, the validity of the actinometry technique applied to the determination of the H-atom density in pure-H2 discharges is examined. Using this diagnostic, it is observed that the H-atom concentration decreases when the vessel wall temperature increases, owing to the increased efficiency of atomic hydrogen recombination on the wall. To overcome this effect, the discharge tube wall is cooled off with dimethyl polysiloxane, a low-loss dielectric liquid. It improves significantly the H-atom concentration at 2450 MHz provided the pressure is typically below a few torr and the power density is not too high.

Physical mechanisms of instability in a liquid layer subjected to an electric field and a thermal gradient

Abstract: The linear stability of a plane layer of dielectric liquid subjected to an electric field and a thermal gradient is studied. The liquid is supposed to have a non‐negligible residual conductivity and to exhibit an injection of charge from one of the electrodes. Both effects are due to the dissolution of a given salt in the liquid. The ionic mobility and the dielectric constant are temperature dependent. The stability of this configuration is studied with the help of an heuristic model and a precise discussion of the physical mechanisms of instability is made. Complete sets of the stability maps corresponding to different cases are presented.

Basic electrical processes in dielectric liquids

Abstract: The basic processes of electrical conduction in dielectric liquids are reviewed, attention being drawn to the similarities between conductive electrolytes and insulating liquids. The concepts of the electronic amorphous solid state are employed to provide a framework for the review. The conditions at metal electrodes can be incorporated naturally into the scheme, and it is known that the space charge layers occurring on them can control conduction. Although electrical breakdown itself is not considered, the underlying electronic processes which will develop when breakdown electrical fields exist in the liquid are considered. >

Prebreakdown phenomena in mineral oil under step and ac voltage in large-gap divergent fields

Abstract: This paper presents an experimental study of the propagation of prebreakdown phenomena in transformer oil, in large point-plane gaps (5 to 20 cm), in positive polarity under step and ac voltages. The prebreakdown phenomena are characterized via the simultaneous recordings of transient currents, charges, photocurrents, and high-speed photographs of the emitted light. In these experimental conditions, prebreakdown phenomena in oil are basically composed of weakly luminous branched filaments, most of the time not connected to the point electrode, and propagating continuously. Bright luminous fast discharges reilluminate periodically the main branch of the propagating discharge from the point electrode to the extremity of these weakly luminous filaments. Below the breakdown voltage, streamers stop at an average length which increases linearly with the voltage. Above a critical length, they cannot stop any more and lead to breakdown. The main propagation velocity remains constant throughout the propagation. These observations, which present a number of similarities with leader phenomena in gases, are then discussed. >

Experiments on dielectric liquid bridges subjected to axial electric fields

Abstract: Experimental determination of the critical axial electric field needed to hold a nonconducting liquid bridge of given slenderness is presented. The Plateau tank technique is used to minimize gravity forces on earth. Improvement of previous data is achieved by determining the gravitational Bond number. The results are compared to numerical calculations of the stability limits, showing a good agreement.

Liquid Immersion Cooling of a Substrate-Mounted Protrusion in a Three-Dimensional Enclosure: The Effects of Geometry and Boundary Conditions

Abstract: A three-dimensional computational study of steady natural convection cooling of a substrate-mounted protrusion (chip) in a rectangular enclosure filled with dielectric liquid is described. Energy is generated in the chip at a uniform rate Q. Conduction within the chip and substrate are accounted for in the model, as is the coupled natural convection in the surrounding liquid. The nondimensional governing equations with the appropriate boundary conditions have been solved in the primitive variable form for Ra=10 8 using a fully implicit finite volume formulation. Baseline computations have been performed for a cubical enclosure with a centrally placed silicon chip on a vertical alumina substrate, which forms one enclosure wall

Breakdown and prebreakdown phenomena in liquids under positive impulse voltages

Abstract: We examine the breakdown and prebreakdown behavior of two insulating liquids: benzyl-toluene and mineral oil in a nonuniform electric field at large distances (up to 20 cm) under a positive impulse voltage. We evaluate and compare the breakdown strengths of both liquids in these conditions as a function of interelectrode distance via Weibull type plots, a method commonly used when dealing with insulating gases and solids. The comparative behaviors of the liquids change greatly when the cumulative probability of breakdown is decreased from 100%. The second part of this study concerns streamer stopping lengths and the corresponding current and light emission signals. >

Measurement of the 2-dimensional electric field vector in dielectric liquids

Abstract: A new measurement technique for two-dimensional electric field vectors (electric field intensity and its direction) in liquid dielectrics has been developed in this study. The theoretical equation for measuring the electrical field vector is derived for the optical system which consists of a He-Ne laser, polarizer, quarter-wave plate, test cell with nonuniform electric field, nonpolarizing beam splitter, two analyzers and two photo detectors. The effectiveness of the technique is confirmed by comparing the theoretical values and experimental results. Furthermore, the nonuniform electric field vector distributions of the transformer oil in a compound insulation system of liquids and solids are measured. >

Laser triggering of electric breakdown in liquids

Abstract: Electric breakdown in semihomogeneous geometries has been studied in n-hexane and transformer oil using a laser triggering method for fields far below the spontaneous breakdown field. The method uses a laser-produced plasma to initiate the breakdown. Strong self focusing in the liquids was limited by increasing the numerical aperture of the laser beam to >0.20 prior to focusing. Electrical breakdown could be triggered at fields as low as 50 kV/cm in transformer oil and 25 kV/cm for n-hexane. The time lags are between 30 and 200 /spl mu/s, depending on voltage, liquid, and gap size. Current and light emission measurements of the prebreakdown phenomena are presented and a model for the triggering process is proposed. >

Natural Convection Heat Transfer From an Array of Rectangular Protrusions in an Enclosure Filled With Dielectric Liquid

Abstract: An experimental investigation of natural convection liquid immersion cooling of a three by three array of rectangular protrusions in an enclosure is presented. The heated elements geometrically simulated 20 pin dual-inline electronic packages and were mounted on a plexiglass substrate, which formed one vertical wall of a dielectric liquid filled rectangular enclosure. The remaining vertical boundaries of the enclosure were insulated, while the top and bottom were maintained at prescribed temperatures using individual heat exchanger plates. Protrusion surface temperatures in steady state are reported for a range of power dissipation levels for three fluorinert liquids spanning a Prandtl number range from about 20 to 1400. The influence of enclosure top and bottom surface boundary conditions and its width on element temperatures is investigated. Non-dimensional heat transfer results are empirically correlated. Changes in component temperatures due to partial powering of the array are also measured.

Effect of an Electric Field on the Behavior of a Drop Moving in a Quiescent Liquid

Abstract: A mathematical model for predicting the behavior of a dielectric liquid drop moving in a medium of another dielectric liquid in the presence of a uniform electric field has been developed. It is found that the terminal velocity of the drop is not strongly affected by the strength of electric field at small Reynolds numbers. A wake appears behind the drop due to the hybrid circulations induced by both the gravity and the electric force even at small Reynolds numbers and thus the drop is deformed asymmetrically about the equatorial plane.

Pulse measurements using LED in dielectric liquids under impulse voltages

Abstract: Current pulses, in the form of partial discharges, are known to be precursors of electrical breakdown in dielectric liquids. The precise measurement of these pulses is of great value for the understanding of the breakdown mechanism. The measurement of current pulses under impulse voltage is very difficult when using conventional electrical measurement systems. To overcome this difficulty, a novel measuring system using fiber optics and a light emitting diode (LED) has been developed. In this paper, the improvements in time response and charge sensitivity of such a LED current-pulse measurement system are described. Using this system and a high speed shadow-graph assembly under impulse conditions, the current pulse and the streamer initiation in dielectric liquids are measured simultaneously. >

Thermal bubble breakdown in liquid nitrogen under nonuniform fields

Abstract: The electrical forces acting on thermally induced bubbles in nonuniform field gaps with/without fins and the correlation between bubble behavior and dielectric characteristics in liquid nitrogen are studied. The results show that the gradient force and Maxwell stress strongly affect the bubble dynamics and bubble shape in the gap. Especially the pronounced gradient force near the fin tip reduces the growth of bubble there. These electrical forces acting on the bubble result in a lesser effect of thermal bubble on the breakdown voltage if the fins are formed to guide a bubble trapped in the groove from a higher field region to a lower one. >

Galerkin analysis of electrothermal instabilities in plane liquid layers

Abstract: The stability of a horizontal layer of poorly conducting dielectric liquid subjected to a unipolar injection of ions and a thermal gradient is studied. Ions arising from injections and dissociation are supposed to have the same nature. Ionic mobility and electric permittivity are allowed to vary linearly with temperature. The linear stability analysis has been carried out by means of a Galerkin method with one and two trial functions. A single trial function provides useful analytical relations between parameters for the neutral stability surface. A second trial function is necessary to account for the existence of overstability in the absence of a thermal gradient and to achieve better accuracy in the solution. >

Solutions for the electrical interfacial problem between a dielectric liquid and a metallic wall: applications to industry

Abstract: This paper deals with the diffuse layer present at the interface between a liquid and a solid in a cylindrical geometry. After having set out the governing equations of the phenomenon in the most general case, we present the real solution, asymptotic solutions and an approach solution. The first one has been obtained numerically and the last one comes from an empirical approach. In the last part of the paper we compare the different profiles of space charge density obtained by the different ways which are mentioned above. We also compare the corresponding convected mean space charge densities in terms of the radius of the pipe and the space charge density at the wall. This leads to determination of the cases in which we can use the empirical solution which is of a great interest for industry. >

Bubble formation in synthetic insulating liquids in a pulsed divergent electric field

Abstract: This is a study of high field conduction phenomena in two aromatic insulating liquids (monobenzyltoluene and dibenzyltoluene) in a point-plane electrode geometry. These liquids behave qualitatively like saturated hydrocarbons; current instability (electron avalanche) for high enough fields near the negative point giving rise to a current pulse regime, which causes the formation of bubbles by local heating. The generation and dynamics of these bubbles as a function of various parameters (injected energy, hydrostatic pressure and the nature of the liquid) are presented and discussed. >

Liquid helium breakdown as function of temperature and electrode roughness

Abstract: The high dielectric dc strength of liquid helium can be explained by the effect of an energetic barrier against germinal vapor bubble elongation and runaway up to vapor discharge. The barrier normally rises in front of any cathode asperity where a germinal bubble may be produced from a collapsing vacuous cavity. After reaching a peak, the barrier changes into a decreasing tail, caused by significant vapor ionization. This so-called 'antibubble barrier' can be overcome at a sufficiently high field strength. For temperatures above the normal boiling point, the barrier peak must be lower than the threshold limit. At lower temperatures, the germinal bubbles may extend already into the falling tail, which favors runaway considerably. Nominal breakdown field strengths have been calculated assuming various surface roughness condition in the temperature range 2.5 to 4.5 K. A simple engineering formula for the roughness effect estimation is given. >

Self-protected immersed polyphase electrical transformer

Abstract: This transformer, with a sealed tank filled with a dielectric cooling liquid, is provided with equipment for self protection against the risk of explosion of the tank, consisting of: * an immersed unit for protection against internal breakdowns degenerating relatively slowly and comprising: - an immersed sensor 1, sensitive to changes of a characteristic of the dielectric liquid which surrounds it, chosen from among those, such as pressure, which react at the site of the sensor practically upon the appearance, at any site in the liquid mass, of a degenerative defect, this sensor delivering an "on or off" signal in response to this alteration. - a non-rearmable polyphase contact breaker 5 between the phases of the power supply and the corresponding primary winding of the transformer. - and a trigger 6 for control of the opening of the contact breaker 5 driven by the signal delivered by the sensor 1, when the amplitude of the said signal exceeds a preestablished safety threshold value: * and a unit for protection against sudden current overloads drawn by the transformer from the power supply means, comprising, on each of the phases of the power supply, an array of fuses 26, 27 mounted in cascade and including staged cutoff thresholds.

Modeling of dielectric fluid solidification with charged particles in electric fields and reduced gravity

Abstract: A mathematical model and an explicit finite-difference iterative integration algorithm for two-dimensional laminar steady flow and solidification of an incompressible, viscous, electrically conducting but neutrally charged melt containing electrically charged panicles and exposed to an externally applied electrostatic field were developed. The system of governing electrohydrodynamic equations was derived from a combination of Maxwell's equations and the Navier-Stokes equations, including thermally induced buoyancy, latent heat release, and Joule heating, while accounting for the mushy region. Physical properties were treated as arbitrarily temperature-dependent. Numerical results demonstrate the existence of strong electrothermoconvective motion in the melt and quantify its influence on the amount of accrued solid, deposition pattern of the electrically charged particles inside the accrued solid, and the melt/solid interface shape.

Pulsed electrical breakdown in liquid helium in the /spl mu/s range

Abstract: Information about breakdown time-lags under pulsed voltage stress is prerequisite to the insulation design of superconducting devices. Experiments have been carried out using a pulse generator which provides a step voltage with 200 ns rise time and a long decay time constant of more than 1 ms. The present experiments may bridge the information gap between time lag measurements in the ns range using extremely high field strengths, and existing data found with usual lightning impulse voltage. The test device is described in the paper, and some typical results are discussed in detail. >

Single-Phase, Liquid Jet Impingement Cooling of High-Performance Chips

Abstract: For computer chips configured in isolation or as part of an array, a consistent trend has been one of progressively increasing power dissipation. In many applications chip heat fluxes are approaching 100 W/cm2, and it is not unlikely that fluxes of 200 W/cm2 will be realized before the turn of the century. One of the most promising options for dissipating such heat fluxes, while maintaining chip surface temperatures at acceptable levels, involves single phase convection cooling via liquid jet impingement. By eliminating intervening conduction resistances between the chip and the liquid and by providing highly effective convection cooling at the chip surface, chip-to-coolant thermal resistances as low as 0.3°C·cm2/W may be achieved using a dielectric liquid with a modest coolant flowrate of 1 1pm. After considering pertinent hydrodynamic conditions for both free-surface and submerged jets, this paper reviews the results of heat transfer studies which have been performed for simulated chip conditions. The review considers both circular and rectangular (slot) jets, single and multiple chip configurations, the use of extended surfaces for heat transfer enhancement, and the effects of jet confinement and outflow conditions. The results confirm the suitability of using impinging liquid jets for high performance cooling and provide a useful knowledge base for cooling system design.

Fundamentals of Nucleate Pool Boiling of Highly-Wetting Dielectric Liquids

Abstract: Direct cooling with inert, dielectric liquids may well become the technique of choice for the thermal management of future electronic systems. Due to the efficiency of phase-change processes and the simplicity of natural circulation, nucleate pool boiling is of great interest for this application.

Dynamic instability of an excited cylindrical interface supporting surface charges and admitting mass and heat transfer

Abstract: The linear stability of two dielectric inviscid fluids separated by a cylindrical interface is investigated. The interface allows mass and heat transfer. The system is stressed by a radial periodic electric field that allows the presence of surface charges at the interface. The standard normal-modes approach is utilized. A general dispersion equation valid for all (axisymmetric and non-axisymmetric) modes of disturbances is derived. Some previous studies are compared using appropriate data. Parametric excitation of electrohydrodynamic surface waves is obtained in the case of Rayleigh-Taylor instability. The transition curves are obtained by means of Whittaker's technique and the results are confirmed numerically. Contrary to the case of uniform electric field, the periodic one frequently shows the effect of mass and heat transfer on the conditions of stability. The presence of surface charges made the radial electric field play a dual role in the stability criterion, which shows some analogy to the nonlinear theory of stability.

Dielectric material containing dipolar molecules

Abstract: Dielectric materials having a high dipole moment density are provided. The high dipole moment density is achieved by the use of a substituted α-amino acid, such as N-trimethyl glycine and related homologues having non-polar sidechain groups, with three alkyl substitutions on the nitrogen of the α-amino group. These molecules have a high dipole moment and small volume, allowing rapid rotation around the α-amino, carboxyl dipole. N-trimethyl glycine alone or in combination with one or more homologues is preferably provided as a solute in a liquid dielectric solution. Alternatively, these molecules are provided as substituents on a linear polymer.

Evaluation of dielectric liquids by measurement of their partial discharge characteristics

Abstract: Measurements of discharge signatures under AC differentiate between various liquids. The signatures are heavily influenced by the condition of the liquid (i.e gas and moisture content). The radius of the needle electrode also determines which types of positive discharges develop. Measurement of light emission is a sensitive alternative to conventional quasi integrating electrical measurement. The risk for breakdown is greatest for the positive discharges. The positive discharges occur at low rates, probably depending on field enhancement at the needle electrode due to negative space-charges from numerous sub-pC negative discharges.

Effect of electrode ice layer on prebreakdown current in liquid nitrogen

Abstract: The effect of ice on dc prebreakdown events was investigated using a needle-to-plane electrode system in liquid nitrogen at 77.3 K. It was found that H/sub 2/O ice has marked effects on prebreakdown current, corona onset voltage and breakdown voltage. The breakdown voltage at atmospheric pressure depends on microscopic cracks in H/sub 2/O ice. >

Electrical discharge machine with pulses of both polarities

Abstract: An electrical discharge machine for applying a pulsed voltage to a machining gap between an electrode and a workpiece disposed opposite thereto. An insulating dielectric fluid, comprising an oil or water with compound additive, is disposed in the gap. A positive polarity voltage and an opposite polarity voltage are applied via switches and respective impedances to the machining gap in order to generate a discharge for machining the workpiece. The impedance values may be selectively set and the switching of the voltages controlled such as to apply a positive polarity voltage at the stop time of an opposite polarity pulsed voltage, whereby the average voltage approximates a predetermined value, preferably zero, during the stop time. Voltage control during the stop time will result in high speed and finish, especially for control in connection with a jump operation.

Relation between power loss factor and insulation resistance on measuring transformers in on site measurement

Abstract: Prophylactic measurements on insulating system of high voltage measuring transformers are of great importance for reliable operation of electrical utilities and for safety of the personnel. These measurements are preferably made on site. For this reason, the High Voltage Laboratory of the Faculty of Electrical Engineering in Skopje, Macedonia, a few years ago began systematic on site measurements on the measuring transformers for voltage level of 110 kV. They are with paper-oil insulation. The following two measuring methods were applied: 1. Power Loss Factor (tg/spl delta/) measurements of the entire insulating system between the high voltage winding and ground; 2. Insulation Resistance measurements between the high voltage winding and ground. Both measuring methods have difficulties for on site application. The main reason is the very small capacitance of the measured objects and the presence of strong electrostatic interference from the high voltage conductors in the vicinity of measuring arrangement. This problem is solved in both measuring methods. The measuring technique used is fully described. The results obtained with both methods are compared with each other and good correlation is found. The results are very useful for discussing criteria for insulation evaluation. >