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Synthetic esters are potential substitutes for mineral oil in power transformers owing to their biodegradability and excellent dielectric withstand. Furthermore, the addition of certain nanoparticles improves the cooling property of liquids. Both dielectric withstand and cooling are two fundamental characteristics for use in breathable components. Previous studies showed that AC breakdown voltage of synthetic ester could be increased by adding Fe3O4, Al2O3 and SiO2 nanoparticles. This paper deals with the DC breakdown voltage of the same synthetic ester-based nanofluids. Nanofluid samples are characterized using the same techniques to those used for AC breakdown voltage study. This includes a particle size analyzer, scanning electron microscope and energy dispersive x-ray spectroscopy analyzer. Statistical analysis of experimental data using normal and Weibull laws and an estimate of DC breakdown voltage with probabilities of risk of 1% and 50% are performed. The experimental results evidence the existence of an optimum concentration of nanoparticles that gives the highest DC breakdown voltage, except SiO2 nanoparticles which degrade the DC breakdown voltage of pure synthetic ester, regardless of the concentration. For the other two types of nanoparticles, the DC breakdown voltage is improved by approximately 25%, 13%, and 10%, with Al2O3 (13 nm), Al2O3 (50 nm), and Fe3O4 (50 nm) nanoparticles at the optimum concentration that is 0.05 g/L, respectively. It is also shown that the obtained results generally comply with the normal and Weibull distribution laws.

International audience; The characteristics of lighting impulse breakdown voltage (LI-BDV) constitute key performance indicators of insulating materials, especially in insulations based on liquids and liquid-paper for high voltage applications. Encouraged by environmental policies worldwide, new insulating liquids have been developed in recent years. Among these are natural and synthetic esters that are currently considered as potential substitutes for traditional mineral oils due to their environmentally friendly properties. Recent technological advances have shown that the addition of specific nanoparticles (NPs) to insulating liquids can improve both the dielectric withstand voltage and the thermal behavior of liquid insulations. This paper examines the effects of some NPs on the negative LI-BDV of natural and synthetic ester liquids, namely MIDEL 1204 and MIDEL 7131, respectively. The used NPs are Fe3O4, Al2O3 and SiO2. The LI-BDV measurements are executed in accordance with IEC 60060 standard, and the breakdown voltage with 1% probability risk (i.e., the lowest possible breakdown voltage) was also estimated. The experimental results showed that these NPs improved the LI-BDV of both synthetic and natural ester liquids and an optimal concentration of NPs can be determined. Statistical analysis was also performed to check the compliance of the experimental results with the most common probability distributions, normal and Weibull distributions.

International audience; In this paper, we present a new approach for measuring the dielectric liquids relative permittivity over a wideband frequency in the microwave domain. This technique is based on the use of a stripline structure free of substrate as measuring probe. The relative dielectric permittivity (dielectric constant) of the liquids under test is determined from the measured scattering parameters (S parameters) of the developed stripline structure using two different extraction methods where the need to calibrate the network analyzer was not necessary. The obtained results on a vegetable oil sample are in good agreement with results found by other works on the same frequency range using different techniques.

International audience; When an external electric field is applied on a weakly conducting liquid, next to each metallic electrode two layers with a net electric charge of opposing polarity appears. These are called heterocharge layers. The electric field exerts a force on these layers. If the electrodes of different polarity have different geometric characteristics a net electric force is produced, creating a net flow. This is the basis of EHD conduction pumping. This technique has a great number of interesting applications, notably in heat exchange devices to be applied in satellites and aerospace systems. Here we consider a flexible EHD conduction pump. An array of symmetric electrodes is deployed on a flexible non-conducting substrate. This flexibility allows the pump to be installed in conduits of complicated geometries, increasing the applicability of the EHD conduction pumping concept. Specifically, we present the results of numerical simulations with a conic flexible pump with several pairs of electrodes. We discuss the structure of the fluid flow and of the heterocharge layers along the pump.

International audience; Nowadays, heat transfer enhancement devices are becoming very essential in many applications. Most of electronic devices, from the simplest to the most sophisticated, contain processors. Advanced processors require efficient cooling for an optimal performance. Due to the growing interest in faster and lighter devices, researchers always seek to innovate and optimize cooling strategies. In spatial applications, the use of typical cooling systems can cause complications due to the vibrations during takeoff and due to zero gravity effects. Electrohydrodynamic (EHD) pumping, which is based on the interaction of a dielectric liquid with an electric field, could present a solution to all these challenges. EHD pumps were considered a breakthrough in the field of cooling since they demonstrate many advantages over other types of pumps. EHD devices are less power consuming, lighter and cheaper. They don't have moving parts and they are suitable for microgravity applications. The hydrofluoroethers (HFE) are dielectric liquids that could be ideally utilized in these pumps for many applications. Being dielectric fluids with eco-friendly properties, HFEs could replace CFCs, HFCs, HCFCs, and PFCs. Due to their promising EHD applications, studies must be done to investigate the variation of their dielectric behavior with electric field and with temperature. This work presents an experimental investigation of the dielectric characteristics of HFE-7000 with temperature variations. Understanding this aspect can help enhance and optimize the performance of EHD systems.

International audience; This study aims to demonstrate the capability of simulating the flow induced by the dissociation of neutral molecules in a weakly conductive liquid in a full 3D channel. We investigate numerically the EHD pumping through pure conduction phenomenon in a 3D rectangular channel. It is important to verify if the assumptions allowing us to consider that the flow remains 2D are always valid and in which circumstances some 3D phenomena may occur? Not very surprisingly it is observed that while increasing the electric Reynolds number in increasing the electric potential difference between the two electrodes makes the flow becoming turbulent. In such situation 2D computations are not anymore valid.

In this paper, we carry out a numerical study of the Coulomb-driven electro-convection flow in a dielectric liquid using the finite volume method (FVM). The system under study consists of an eccentric annulus layer subjected to strong unipolar injection from an inner cylinder. All equations are written in bi-cylindrical coordinate system and are expressed using the vorticity–stream function method. The behavior of the fluid for different system parameters is well studied. The taken parameters are in the range of eccentricity 0 ≤ e ≤ 80%, injection strength in the range of 0.5 ≤ C ≤ 10 and electric Rayleigh number in the range of 30 ≤ T ≤ 1600. The distributions of the electric charge density and stream function are carefully examined. The computations highlight a significant effect of the eccentricity value on the flow topography. Indeed, increasing the eccentricity value leads to a significant reduction in electro-plumes as well as circulation cells number. Furthermore, an intensification in cells rotation, of about 116%, was recorded when eccentricity reaches 80%. Depending on torque values e-T, we observe the existence of multicellular regime made of counter-rotating vortexes.

International audience; The present paper reports on the evidencing of the capacitive effects on the characteristic parameters of the surface discharges propagating over different types of insulators immersed in gases or dielectric liquids, in a point-plane electrode arrangement submitted to different types of voltage waveforms namely AC, DC and lightning impulse voltages. These effects are highlighted through the influence of thickness and dielectric constant of insulators. The investigated insulators are circular samples of different thicknesses issued of various materials of different dielectric constants (namely, PTFE, glass, epoxy, Phenoplast resin (Bakelite) and pressboard); and the considered fluids are mineral oil, vegetable oil, and gases. It is shown that the lengths of discharges and surface density of their branches increase when the thickness of insulator is reduced and/or the dielectric constant of insulator is increased what clearly evidences the role of capacitive effects.

As wireless networks evolve, the frequency bands they exploit multiply. Frequency multiplexing, beamforming and tracking, networks interoperability, those mutations increase the need for agility and tunability in the RF-front end systems. In this thesis, two innovative approaches for the design of tunable RF components are studied. First, a micro-mechanical reconfiguration mechanism is developed. This approach means to control the height of an air gap within the substrate of microstrip components in order to control their electrical dimensions. Considerations for the design and fabrication of a phase-shifter and a frequency tunable patch antenna are made and a low voltage piezoelectric actuation (+/- 30 V) is chosen. The phase-shifter figure of merit reaches up to 312 °/dB which is beyond the state of the art in terms of analogic phase-shifting. Regarding the antenna, the frequency tuning potentially reaches up to 35 \% of a central frequency of 55 GHz. Unlike the classic solidstate or RF-MEMS based solutions, this reconfiguration mechanism does not impact the radiation performance of the antenna whose radiation efficiency is 94 \%. In a second approach, a micro-fluidic solution is studied. Frequency tuning capability is created in different antennas by the flow of successive liquids with different permittivities in integrated micro-channels. A large frequency tuning of 51 \% for a central frequency of 22 GHz is achieved. This study goes along with the search and characterization of several fluids with the objective of increasing both the frequency shift and the radiation performance of those antennas; Il y a à l'heure actuelle un grand besoin en systèmes et composants agiles pour les dispositifs front-end RF. Dans cette thèse, deux approches innovantes sont développées pour la conception de tels dispositifs RF. Dans un premier temps, un mécanisme de reconfiguration micro-mécanique est développé. Cette approche vise à contrôler la hauteur d'un gap d'air inséré dans le substrat de composants microrubans afin d'en modifier les dimensions électriques. Les choix de conception et la fabrication d'un déphaseur à ligne micro-ruban ainsi que d'une antenne accordable en fréquence sont discutés. Une solution d'actionnement piézoélectrique basse tension (+/- 30 V) est retenue. La figure de mérite obtenue dans le cas du déphaseur atteint jusqu'à 313 °/dB ce qui dépasse l'état de l'art en matière de déphasage analogique. Dans le cas de l'antenne, le potentiel d'agilité en fréquence atteint 35 % autour d'une fréquence centrale de 55 GHz. Contrairement aux solutions classiques à base de semiconducteurs ou de RF-MEMS, ce mécanisme de reconfiguration n'impacte pas les performances de l'antenne dont l'efficacité de rayonnement est proche de 94 %. Dans un second temps, c'est un mécanisme micro-fluidique qui est étudié. L'agilité en fréquence d'antennes est créée par l'écoulement successif de liquides de permittivités différentes dans des micro-canaux intégrés au substrat. L'accord en fréquence atteint alors la très large ampleur de 51 % pour une fréquence centrale de 22 GHz. Cette étude s'accompagne de la recherche et caractérisation diélectrique de plusieurs fluides dans l'optique d'augmenter aussi bien la plage d'accord en fréquence que les performances de rayonnement des antennes

International audience; This paper presents a synthetic review of the state of knowledge relative to the pre-breakdown phenomena in dielectric liquids and especially the initiation and propagation of streamer/discharge and their characteristic parameters. The main influencing parameters namely the structure, stopping length, velocity and propagation modes of streamers, current, charge and associated emitted light are developed. These parameters include especially the chemical composition of the liquid (pure or containing specific additives); the applied voltage; the electrode arrangement; the temperature and the hydrostatic pressure. The mechanisms involving in the inception and propagation of streamers are discussed. Recent models enabling to predict the type of streamers that can be initiated and their characteristics when propagating within liquid, are also presented and discussed. It is shown through the experiments and modeling that the pre-breakdown phenomena in dielectric liquids are governed by both electronic and gaseous mechanisms even if one of them is dominant in one case and the second in another.

International audience; This paper describes the procedure to qualify an admittance meter as a permittivity meter and a conductivity meter sigma(sensor) through validation with liquids over a range of relative permittivity epsilon(r) values from 1 to 80 and with operating frequencies f between 1 and 20 MHz. The sensor is a capacitor which consists of two parallel cylinders of 10-cm typical dimension. A circuit-based model of the sensor was previously calibrated against accurate electronic components. A discrepancy of 10% is found between the calibration with components and the validation with liquids. All potential errors have been carefully examined: parasitic impedances of the electronic circuit and leads, modifications of liquid permittivity epsilon(r11q) due to temperature influence, water contamination or relaxation effects, and possible fringing effects with the help of numeric simulations. Forty percent of the discrepancy results from the finite dimensions of the liquid-filled recipient. The fringing effects due to insulating rings at each end of the capacitor seem to be discarded. The analysis of the uncertainty on epsilon(rsensor) and sigma(sensor) shows a relative uncertainty of 3%-5% due in large part to the numerical acquisition of high frequency

This paper is an experimental study of a pulsed filamentary plasma discharge inside liquid water in pin-to-plane electrode configuration. Time-resolved electrical and imaging diagnostics have been performed. The initiation and the propagation of the discharge have been studied for several experimental parameters. The propagation is continuous and is followed by reilluminations at low water conductivity. The measured propagation velocity of the plasma discharge is 30 km s−1for the secondary positive mode. This velocity was found to be surprisingly constant whatever the experimental parameters and especially as a function of the water conductivity.

This work deals with the pre-breakdown and breakdown phenomena in the liquid dielectrics. It is shown that the initiation of streamers, in a point-plane electrode arrangement, is preceded by the generation of a gaseous phase (bubble) at the point electrode. This bubble results of a current pulse injection into the liquid when the applied voltage reaches a threshold value Us. This latter increases quasi-linearly with the pressure. The addition of a certain concentration of an electronic scavenger compound increases Us ; this indicates that the electronic processes are also present. The analysis of the bubble dynamics shows that two types of deformation of the liquid-bubble interface can occur: (i) an expansion or an implosion and (ii) alternative movements of expansion and/or implosion. According to the disturbance mode of the interface, different types of streamers can be initiated. The dielectric strength of liquid increases when adding a given optimal concentration of electronic scavenger compounds (Iodobenzene or carbon tetrachloride) and/or increasing the pressure. Therefore, while acting on the initiation voltage of streamers, one can improve the dielectric strength of liquid even though the streamer velocity is increased. By increasing the pressure, one can also increase dielectric strength of a liquid. A modeling of streamers by an equivalent electrical network enables to determine the structure of streamers as well as their physical characteristics: currents, electrical charges, power and energy injected, electric fields and potentials at their heads, velocity and mobility of charge carriers within the streamer, local viscosity and temperature. Keywords: Dielectric liquids, pre-breakdown and breakdown, bubble dynamics, streamers, initiation and propagation, dielectric strength, additives and pressure, modeling., Ce travail est dédié aux phénomènes de préclaquage et de claquage dans les diélectriques liquides. Il est montré que dans un système d'électrodes pointe - plan, la génération des streamers est précédée par la formation d'une phase gazeuse (bulle) au voisinage de la pointe, résultant de l'injection d'une impulsion de courant dès que la tension appliquée atteint ou dépasse une certaine valeur seuil Us. Us augmente quasi-linéairement avec la pression. L'addition d'une certaine concentration de composés capteurs d'électrons augmente Us, preuve que les mécanismes électroniques sont également présents. L'analyse de la dynamique de la bulle montre que deux types de déformation de la surface de la bulle sont possibles: (i) une expansion ou une implosion et (ii) des mouvements alternatifs d'expansion et/ou implosion. Selon les modes de perturbation de l'interface, différents types de streamers peuvent prendre naissance. D'autre part, la tension de claquage d'un diélectrique liquide augmente en présence d'une concentration optimale donnée d'additifs capteurs d'électrons (Iodobenzène ou tétrachlorure de carbone). Par conséquent, en agissant sur la tension de génération des streamers, on peut améliorer la tenue diélectrique du liquide même si la vitesse des streamers est augmentée. En augmentant la pression, on peut aussi relever la tenue diélectrique d'un liquide. Une modélisation des streamers par des circuits électriques équivalents et leur résolution par le calcul de réseaux a permis de déterminer la structure des streamers ainsi que leurs paramètres caractéristiques : courants, charges, puissances et énergies injectés, champs électriques et différences de potentiel à leurs têtes, vitesse et mobilité des porteurs de charge dans le streamer, viscosité et température locales.

International audience; Our purpose is to present a critical review of the current understanding of streamer propagation in dielectric liquids in order to help define the direction of future research. We show that the molecular structure has a significant effect on streamer propagation. The main parameter affecting propagation is the electronic affinity of the liquid molecules

Insulating liquids represent a weakness in high voltage devices in that their failure to maintain a high applied voltage between component électrodes generally means the breakdown. Much research has gone into attempting to explain the fundamental processes (electronic and hydrodynamic processes) behind this. The clarification of thèse mechanisms, especially the hydrodynamic processes is one of the main tasks of our work, We présent an expérimental study about the évolution of the hydrodynamic phenomena (bubble and shock wave) as a function of injected energy (0,01-lOOnJ), liquid pressure in a large range, from 0.1 to lOMPa, liquid properties, etc. At first, we hâve evidenced the existence of the bubble largely above the critical pressure of the liquids. For the purpose of making the mechanism of cavitation clear the chronology of the events was also discussed. The observed physical phenomena are similar to those obtained by laser induced breakdown in water. The différence is mainly revealed in the dimensions and characteristic times of the phenomena. An analysis of the bubble rebounds and dissipatives processes is also given. Moreover, our expérimental study of the bubble dynamics in viscous liquids allows us to clearly separate two différent hydrodynamic régimes namely inertial and viscous régimes. We hâve shown that viscosity plays an important rôle in putting a lower limit on the size of cavitating bubbles and is the prédominant damping factor for small bubbles. In conséquence, bubble lifetime is much higher than the Rayleigh time and rebounds of the bubble are practically undetectable. Numerical results of the Rayleigh-Plesset model are in good agreement with expérimental observations, Les liquides isolants représentent une faiblesse des dispositifs haute tension, car leur incapacité à maintenir une tension appliquée élevée entre les électrodes des composants signifie généralement une panne. De nombreuses recherches ont été consacrées à expliquer les processus fondamentaux (processus électroniques et hydrodynamiques) à l’origine de ce phénomène. La clarification de ces mécanismes, en particulier des processus hydrodynamiques, est l’une des tâches principales de notre travail. Nous présentons une étude expérimentale sur l’évolution des phénomènes hydrodynamiques (bulle et onde de choc) en fonction de l’énergie injectée (0,01 à 100 nm). ), la pression du liquide dans une large gamme, allant de 0,1 à lOMPa, ses propriétés, etc. Nous avons d’abord mis en évidence l’existence de la bulle bien au-dessus de la pression critique des liquides. Afin de clarifier le mécanisme de cavitation, la chronologie des événements a également été discutée. Les phénomènes physiques observés sont similaires à ceux obtenus par la décomposition en eau induite par laser. La différence est principalement révélée par les dimensions et les moments caractéristiques des phénomènes. Une analyse des processus de rebond de bulle et de dissipation est également présentée. De plus, notre étude expérimentale de la dynamique des bulles dans les liquides visqueux nous permet de séparer clairement deux régimes hydrodynamiques différents, à savoir les régimes inertiel et visqueux. Nous avons montré que la viscosité joue un rôle important en limitant la taille des bulles cavitantes et constitue le facteur d’amortissement prédominant pour les petites bulles. En conséquence, la durée de vie de la bulle est beaucoup plus longue que celle de Rayleigh et les rebonds de la bulle sont pratiquement indétectables. Les résultats numériques du modèle de Rayleigh-Plesset sont en bon accord avec les observations expérimentales

This study deals with the inception and the propagation of prebreakdown phenomena ("streamers") in some hydrocarbon liquids, in point (anode) to plane geometry, under step voltage.Two propagation modes are observed: the first is a slow one (velocity < 500 m/s) for which streamer shape is "bush-like", and a fast one for which they hâve a filamentary shape (velocity > 1 km/s).A detailed study of positive streamers inception versus expérimental parameters allow us to évidence the existence of an inception field (which dépends only on the point tip radius) and a threshold propagation voltage (depending only on the gap distance). The threshold propagation voltage may be measured only under well defined expérimental conditions (short voltage puise rise time, small tip radius, very good électrode coupling).The study of the dynamics of expansion and collapse of positive streamer filaments évidences their gaseous nature. The dynamics of the filaments is mainly controlled by the liquid inertia and is independent of the applied voltage. An estimation of the energy required for their création shows that they resuit from the vaporization of the liquid. The influence of the gaseous nature of streamers on the propagation phase is also studied.Modélisation of the transit currents detected during the propagation of a single filament (at atmospheric pressure) indicates that streamers are rather good conductors (small voltage drop on the filaments)., Le travail présenté concerne la génération et la propagation des phénomènes prédisruptifs ("streamers") dans quelques hydrocarbures, en géométrie d'électrodes pointe anode - plan, sous créneau de tension.Deux modes de propagation des streamers positifs sont observés: un mode lent (vitesse < 500 m/s) dans lequel ils ont une forme touffue en "buisson", et un mode rapide (vitesse > 1 km/s) pour lequel leur aspect est filamentaire.Une étude détaillée de la génération des streamers positifs en fonction de divers paramètres expérimentaux permet de mettre en évidence l'existence d'un champ seuil de génération (ne dépendant que du rayon de courbure de l'électrode pointe) et d'une tension seuil de propagation des streamers filamentaires (qui ne dépend que delà distance inter-électrodes). La tension seuil de propagation ne peut être mesurée que dans des conditions expérimentales parfaitement définies (créneau de tension de temps de montée rapide, pointes fines, influence électrostatique parfaite entre les électrodes).L'étude de la dynamique d'expansion et d'implosion des filaments composant les streamers positifs rapides à permis de mettre en évidence leur nature gazeuse. La dynamique est déterminée par l'inertie du liquide et est indépendante de la tension appliquée. Une estimation de l'énergie nécessaire à leur création indique qu'ils proviennent de la vaporisation du liquide. L'influence de la nature gazeuse des streamers sur la phase de propagation est également étudiée.La modélisation des courants transitoires détectés durant la propagation de streamers uni-filamentaires (à pression atmosphérique) indique que les streamers sont bons conducteurs (faible chute de potentiel le long des filaments)

International audience; Spectral analysis of the light emitted by streamers during their propagation in liquid dielectrics reveals the presence of the Halpha line and C2 Swan bands as well as a continuum which corresponds to dissociation and fragmentation of liquid molecules and recombination of molecular fragments. Spectra of the emitted light vary according to polarity and streamer velocities. The more energetic the streamer, that is, the faster it is, the higher the characteristic lines emerging from the continuum. The above results confirm those obtained by gas chromatography conducted in a blend of mono- and dibenzyltoluene (M/DBT) and phenylxylylethane (PXE). Indeed, the latter shows the presence of decomposition products such as H-2, CH4, C2H2, C2H4 and C2H6. However, hydrogen and acetylene are predominant. These gases may result from vaporization for slow bushlike streamers, whereas for fast filamentary ones the dominant mechanisms may be dissociation and ionization in the liquid phase.

This work concerns the study of the mechanisms of breakdown of insulating mineral oils for transformer. In liquid media, the appearance of an electric arc is immediately preceded by the generation and propagation in the liquid phenomena generally having the appearance of a tree, called in English "streamer". In this study, an experimental rapid visualization device was developed, with the aim of highlighting the existence of streamers under experimental conditions close to the industrial practice of mineral oils. The electric field conditions necessary for the generation and propagation of streamers have been studied in various electrode systems, ranging from very divergent geometries (tip-plane) to quasi-uniform (sphere-plane) with distances up to 25mm . The results obtained are then analyzed with the aim of establishing a general diagram of the breakdown of oils in various experimental situations, particularly at great distances., Ce travail concerne l'étude des mécanismes de claquage des huiles minérales isolantes pour transformateur. Dans les milieux liquides, l'apparition d'un arc électrique est immédiatement précédé par la génération et la propagation dans le liquide de phénomènes présentant généralement l'aspect d'un arbre, appelés en anglais "streamer". Dans cette étude, un dispositif expérimental de visualisation rapide a été mis au point, dans le but de mettre en évidence l'existence de streamers dans des conditions expérimentales proches de la pratique industrielle des huiles minérales. Les conditions de champ électrique nécessaires à la génération et à la propagation de streamers ont été étudiées dans divers système d'électrodes, allant de géométries très divergentes (pointe-plan) à quasi-uniforme (sphère-plan) avec des distances maximum de 25mm. Les résultats obtenus sont ensuite analysés dans le but d'établir un shéma général du claquage des huiles dans diverses situations expérimentales, particulièrement à grande distance.