In this Ph.D. thesis, we have carried out numerical simulations to study nanosecond repetitively pulsed discharges (NRPD) in a point-to-point geometry at atmospheric pressure in air and in H2-air mixtures. Experimentally, three discharge regimes have been observed for NRPD in air at atmospheric pressure for the temperature range Tg = 300 to 1000 K: corona, glow and spark. To study these regimes, first, we have considered a discharge occurring during one of the nanosecond voltage pulses. We have shown that a key parameter for the transition between the discharge regimes is the ratio between the connection-time of positive and negative discharges initiated at point electrodes and the pulse duration. In a second step, we have studied the dynamics of charged species during the interpulse at Tg = 300 and 1000 K and we have shown that the discharge characteristics during a given voltage pulse remain rather close whatever the preionization level (in the range 109-1011 cm��3) left by previous discharges. Then, we have simulated several consecutive nanosecond voltage pulses at Tg = 1000 K at a repetition frequency of 10 kHz. We have shown that in a few voltage pulses, the discharge reaches a stable quasi-periodic glow regime observed in the experiments. We have studied the nanosecond spark discharge regime. We have shown that the fraction of the discharge energy going to fast heating is in the range 20%- 30%. Due to this fast heating, we have observed the propagation of a cylindrical shockwave followed by the formation of a hot channel in the path of the discharge that expands radially on short timescales (t < 1 _s), as observed in experiments. Then we have taken into account an external circuit model to limit the current and then, we have simulated several consecutive pulses to study the transition from multipulse nanosecond glow to spark discharges. Finally the results of this Ph.D. have been used to find conditions to obtain a stable glow regime in air at 300 K and atmospheric pressure. Second we have studied on short time-scales (t_ 100_s) the ignition by a nanosecond spark discharge of a lean H2-air mixture at 1000 K and atmospheric pressure with an equivalence ratio of _ = 0:3. We have compared the relative importance for ignition of the fast-heating of the discharge and of the production of atomic oxygen. We have shown that the ignition with atomic oxygen seems to be slightly more efficient and has a completely different dynamics.

Dans cette thèse, nous avons étudié des décharges nanosecondes répétitives pulsées dans une géométrie pointe-pointe à la pression atmosphérique dans l’air et dans des mélanges hydrogène-air. Expérimentalement, trois régimes de décharges ont été observés dans l’air à pression atmosphérique entre 300 et 1000 K : couronne, diffus et arc. Pour étudier ces différents régimes, nous avons tout d’abord simulé une décharge ayant lieu pendant un des pulses de tension nanosecondes. Nous avons montré qu’un paramètre clé pour la transition entre les régimes est le rapport entre le temps de connexion entre les décharges positives et négatives initiées aux pointes et la durée du pulse de tension. Dans une seconde étape, nous avons étudié la dynamique des espèces chargées entre les pulses de tension à 300 et 1000 K et nous avons montré que les caractéristiques de la décharge pendant un pulse de tension dépendaient très peu du niveau de préionisation (dans la gamme 109-1011 cm��3) laissé par les décharges précédentes. Nous avons ensuite simulé plusieurs pulses de tensions consécutifs à Tg=1000 K à une fréquence de 10 kHz. Nous avons montré que, en quelques pulses de tension, la décharge atteint un régime diffus "stable", observé dans les expériences. Nous avons ensuite étudié le régime de décharge de type arc nanoseconde. Nous avons montré que la fraction d’énergie de la décharge allant dans le chauffage rapide de l’air est de 20-30 %. A cause de ce chauffage rapide, nous avons observé la propagation d’une onde de choc cylindrique suivie par la formation d’un canal chaud, sur le passage initial de la décharge, qui se dilate radialement sur des temps courts (t 6 1 _s), comme observé dans les expériences. Ensuite nous avons pris en compte un modèle de circuit externe pour limiter le courant et ainsi nous avons simulé plusieurs pulses consécutifs pour étudier la transition entre les régimes diffus et d’arc nanoseconde. Pour finir, les résultats de cette thèse ont été utilisés pour trouver des conditions d’obtention d’un régime diffus stable à 300 K et à la pression atmosphérique. Puis nous avons étudié l’allumage sur des temps courts (t 6 100 _s) d’un mélange pauvre H2-air par une décharge de type arc nanoseconde à 1000 K et à pression atmosphérique avec une richesse de 0.3. Nous avons comparé les importances relatives pour l’allumage du chauffage rapide et de la production d’oxygène atomique. Nous avons montré que l’allumage par l’oxygène atomique semble être légèrement plus efficace et a une dynamique complètement différente de celle initiée par le chauffage rapide. / In this Ph.D. thesis, we have carried out numerical simulations to study nanosecond repetitively pulsed discharges (NRPD) in a point-to-point geometry at atmospheric pressure in air and in H2-air mixtures. Experimentally, three discharge regimes have been observed for NRPD in air at atmospheric pressure for the temperature range Tg = 300 to 1000 K: corona, glow and spark. To study these regimes, first, we have considered a discharge occurring during one of the nanosecond voltage pulses. We have shown that a key parameter for the transition between the discharge regimes is the ratio between the connection-time of positive and negative discharges initiated at point electrodes and the pulse duration. In a second step, we have studied the dynamics of charged species during the interpulse at Tg = 300 and 1000 K and we have shown that the discharge characteristics during a given voltage pulse remain rather close whatever the preionization level (in the range 109-1011 cm��3) left by previous discharges. Then, we have simulated several consecutive nanosecond voltage pulses at Tg = 1000 K at a repetition frequency of 10 kHz. We have shown that in a few voltage pulses, the discharge reaches a stable quasi-periodic glow regime observed in the experiments. We have studied the nanosecond spark discharge regime. We have shown that the fraction of the discharge energy going to fast heating is in the range 20%- 30%. Due to this fast heating, we have observed the propagation of a cylindrical shockwave followed by the formation of a hot channel in the path of the discharge that expands radially on short timescales (t < 1 _s), as observed in experiments. Then we have taken into account an external circuit model to limit the current and then, we have simulated several consecutive pulses to study the transition from multipulse nanosecond glow to spark discharges. Finally the results of this Ph.D. have been used to find conditions to obtain a stable glow regime in air at 300 K and atmospheric pressure. Second we have studied on short time-scales (t_ 100_s) the ignition by a nanosecond spark discharge of a lean H2-air mixture at 1000 K and atmospheric pressure with an equivalence ratio of _ = 0:3. We have compared the relative importance for ignition of the fast-heating of the discharge and of the production of atomic oxygen. We have shown that the ignition with atomic oxygen seems to be slightly more efficient and has a completely different dynamics.

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No description available.

Interactions between flames and thermal phenomena are the guiding thread of this work. Flamesproduce heat indeed, but can also be affected by it. Large Eddy Simulations (LES) are used hereto investigate these interactions, with a focus on two main topics: wall heat transfer andcombustion control. In a first part, wall heat transfer in a rocket engine sub-scale CH4/O2 burner isstudied. In the context of launchers re-usability and cost reduction, which are major challenges,new propellant combinations are considered and wall heat fluxes have to be precisely predicted.The aim of this work is to evaluate LES needs and performances to simulate this kind ofconfiguration and provide a computational methodology permitting to simulate variousconfigurations. Numerical results are compared to experimental data provided by the TechnischeUniversität München (Germany). In a second part, combustion control by means of NanosecondRepetitively Pulsed (NRP) plasma discharges is studied. Modern gas turbine systems use indeedlean combustion with the aim of reducing fuel consumption and pollutant emissions. Lean flamesare however known to be prone to instabilities and combustion control can play a major role in thisdomain. A phenomenological model which considers the plasma discharges as a heat source isdeveloped and applied to a swirl-stabilized CH4/Air premixed lean burner. LES are performed inorder to evaluate the effects of the NRP discharges on the flame. Numerical results are comparedwith experimental observations made at the King Abdulla University of Science and Technology(Saudi Arabia).

The purpose of this dissertation is to — on occasion very literally — shine a light on processes that occur in non-thermal plasmas containing CO2, mostly for CO2 conversion. In particular, the focus lies on the transient behaviour of these discharges: how do these systems evolve over time before they settle in a (non-thermal) equilibrium. In addition to that, it analyses trends in the field of plasma-catalytic CO2 conversion as a whole to evaluate the current state-of-the-art, but also presents a new platform for the community to contribute and collaborate on, to facilitate cross-comparison between disparate experiments. The first part consists of experiments performed on: (a) an atmospheric pressure nanosecond repetitively pulsed (NRP) discharge for CO2 conversion, and (b) a test bed system for a remote CCP plasma source for plasma-enhanced atomic layer deposition (PE-ALD) of trimethylaluminium (TMA). The common theme in these experiments is a focus on the application of time-resolved, in situ diagnostics to study transient behaviour in the systems under investigation. The main diagnostics employed for such measurements are optical emission spectroscopy (OES) and laser induced fluorescence (LIF), which can provide complementary results when used in conjunction. In particular, this work presents the following results: A study of the evolution of emission from an NRP discharge (using OES), establishing both electron densities (by Stark broadening of atomic oxygen and carbon lines) and gas temperatures (by the N2 second positive system) as the discharge evolves from a breakdown phase to a spark phase. It furthermore explores the changes to these properties when operating in burst mode, where a subsequent pulse experiences a memory effect from the preceding one, which has been shown to be conducive to efficient conversion in literature. A study into the effect on energy efficiency of CO2 conversion by alternating the power modulation in an NRP discharge. Crucially, using CET-LIF (collisional energy transfer LIF) and OES it is shown that while power deposition to the discharge occurs in the order of 100 ns in the discharge, CO2 dissociation occurs on a timescale beyond a microsecond. This indicates that instead of direct electron impact, molecular-excitation kinetics play an important role under these conditions for CO2 dissociation. By shortening the time between pulses in a burst (down to 33 us in the work), these mechanisms can be further enhanced, by prolonging the quasi-‘metastable’ state of the system. The application of LIF in a PE-ALD process plasma along with OES, where diffusion profiles were measured close to the substrate surface with local time-resolved measurements of the OH ground state density. These indicate that the investigated surface reactions finish on a timescale of 100 ms, faster than would be indicated by OES which effectively measures emission from the bulk plasma after diffusion of reaction products away from the surface. The second part of this work is an open access database on plasma(-catalytic) CO2 conversion that is instrumental in identifying and verifying trends in experimental data, but also stresses the importance of rigorous reporting of essential parameters in literature. The approach in literature is diverse: some studies focus more on a mechanistic understanding of the fundamental processes, whilst others already focus on process tailoring and optimization for industrial applications. Trends observed in earlier review papers are observed as well and can now be trivially reproduced. The database platform (https://db.co2pioneer.eu) is put forward as a new tool for the community to easily cross-compare and contextualize experimental outcomes and strongly encourages new contributions. Based on the 196 papers included at the time of publication, a number of observations and recommendations can already be made. Chief among those is a clear and present need in the field for a more fundamental understanding of plasma-catalysis interaction, to develop techniques and criteria that are properly suited to test the synergy of both, rather than relying on methods from e.g. traditional thermal-catalysis. Also in this instance, local, time-resolved diagnostics may play a key role, but their implementation will be challenging.

Les plasmas froids d’air à pression atmosphérique sont très utiles pour un grand nombre d’applications grâce à leur chimie hors-équilibre et leur souplesse d’utilisation. Leur intérêt réside dans la production de certaines espèces réactives ou chargées avec un coût énergétique plus avantageux que la chimie à l’équilibre. L’objectif de cette thèse est de combiner les décharges nanosecondes répétitives pulsées (NRP) avec une géométrie micrométrique. Par cette combinaison, nous souhaitons palier au chauffage excessif des étincelles qui génèrent pourtant des fortes densités d’espèces. Notre étude se concentre en trois points principaux. Dans un premier temps la phase de claquage est étudiée ; c’est pendant cette étape que l’énergie est déposée et que les espèces sont produites. La combinaison des diagnostics électriques et de spectroscopie d’émission optique montrent que l’air est presque complètement dissocié et ionisé. Ensuite, nous nous intéressons à la phase de recombinaison qui conditionne la durée de vie de ces espèces. Les résultats mettent en évidence une réaction à trois corps comme mécanisme de recombinaison principal. Et enfin, le dernier point concerne le transport des espèces vers un substrat conducteur. En lui appliquant une tension, celui-ci nous permet de générer un écoulement de vent ionique provenant de la décharge. L’écoulement est étudié par vélocimétrie d’images de particules et imagerie Schlieren. Ce travail a permis de démontrer la capacité des NRP micro-plasmas dans la production contrôlée d’espèces réactives et chargées, mais aussi dans leur transport vers une surface par panache électro-aérodynamique. / Non-thermal plasmas generated in air at atmospheric pressure have numerous potential applications due to their non-equilibrium chemistry and ease of use. Their main advantages lie in the cost-efficient production of reactive and charged species compared to that of equilibrium chemistry. The aim of this thesis is to combine nanosecond repetitively pulsed discharges (NRP) with a microscale geometry. Using this combination, we seek to reduce the excessive heat release of NRP sparks, while nonetheless reaching high densities of reactive species and electrons. This work is comprised of three main parts. Our first goal is to study the breakdown phase, in which energy is deposited and charged species are produced. We employ both electrical characterization and optical emission spectroscopy in order to show that the NRP microplasma fully ionizes and dissociates the gas. The second part consists of the study of the recombination phase, in which the produced species recombine or survive. Results show that three-body recombination can explain the electron lifetime in this phase. Finally, we study the transport of plasma chemical species from the microplasma to a DC-biased conductive plate representing a substrate. By applying a voltage to this third electrode, we drive an electro-thermal plume via an ionic wind from the microplasma to the plate. This flow is investigated mainly by particle image velocimetry as well as Schlieren imaging. This work shows the capability of NRP microplasmas to produce high densities of reactive and charged species and transport them to a surface using an electrohydrodynamic plume.

Ce travail propose d’interroger la constitution du lien thérapeutique avec des enfants présentant des troubles autistiques ou psychotiques. Ces pathologies narcissiques-identitaires considérées comme troubles majeurs de l’intersubjectivité soumettent nos dispositifs de soins à de constants ajustements, pour accueillir ce que ces sujets nous livrent avec insistance, et qui prend la forme de comportement répétitifs. Nous sommes dans une pratique clinique extrême, aux confins du langage qui nous oblige à repenser notre cadre interne, à redéfinir nos postures, pour soutenir un travail de symbolisation. Retirés dans une sensorialité réconfortante ou absorbés dans des activités ritualisées, ces sujets nous engagent psychiquement et corporellement à « trouver-créer » comment leur faire signe pour établir le contact et relancer les processus de réflexivité entravés. La trajectoire vers l’autre, détour essentiel à la subjectivation des éprouvés corporels est à construire. Ce travail est une modélisation de cette trajectoire du retrait devant l’objet à l’attrait pour l’objet. Deux notions sont développées, celle de l’effacement et celle de l’oscillatoire, qui viennent figurer la dynamique de la rencontre, faite de rapprochements et d’éloignements. Les « états de présence » du thérapeute sont un rouage essentiel de ce processus d’attraction, dont l’accomplissement sera « le partage d’affects ». / This work presents an investigation of the therapeutic relation to children with autistic or psychotic problems. These narcissistic-identity pathologies, considered as a major problem of intersubjectivity, submit our instruments of care to constant adjustment; in order to harvest what these subjects insistently present in the form of repetitive behaviors. We find ourselves in an extreme of clinical practice, at the limits of language, which forces us to reconsider our internal framework and redefine our positions with the aim of supporting a task of symbolization. Withdrawn into a comforting sensoriality or absorbed in ritualized activities, these subjects engage us psychically and corporally to "find-create" how to draw attention so as to establish contact and relaunch the inhibited processes of reflection.The trajectory towards the other, an essential detour towards the subjectivizing of corporeal feelings, is in need of construction. This work models that trajectory from withdrawal before the object to the attraction towards the object. Two concepts are developed, that of erasure and of the oscillatory. They come to fashion the dynamics of interaction, composed of advances and retreats. The "states of presence" of the therapist are an essential cog in the process of attraction, for which attainment becomes "affective sharing".

Les interactions entre les flammes et les phénomènes thermiques sont le fil conducteur de ce travail. En effet, les flammes produisent de la chaleur, mais peuvent aussi être affectées par des transferts ou des sources de chaleur. La Simulation aux Grandes Echelles (SGE) est utilisée ici pour étudier ces interactions, en mettant l’accent sur deux sujets principaux: le transfert de chaleur aux parois et le contrôle de la combustion. Dans un premier temps, on étudie le transfert de chaleur aux parois dans un modèle de brûleur CH4/O2 de moteur-fusée. Dans un contexte deréutilisabilité et de réduction des coûts des lanceurs, qui constituent des enjeux majeurs, de nouveaux couples de propergols sont envisagés et les flux thermiques à la paroi doivent êtreprécisément prédits. Le but de ce travail est d’évaluer les besoins et les performances des SGEpour simuler ce type de configuration et de proposer une méthodologie de calcul permettant desimuler différentes configurations. Les résultats numériques sont comparés aux donnéesexpérimentales fournies par la Technische Universität München (Allemagne). Dans un deuxième temps, le contrôle de la combustion au moyen de décharges de plasma de type NRP (en anglaisNanosecond Repetitively Pulsed) est étudié. Les systèmes de turbines à gaz modernes utilisent en effet une combustion pauvre dans le but de réduire la consommation de carburant et les émissions de polluants. Les flammes pauvres sont connues pour être sujettes à des instabilités et le contrôle de la combustion peut jouer un rôle majeur dans ce domaine. Un modèle phénoménologique qui considère les décharges de plasma comme une source de chaleur est développé et appliqué à un brûleur pauvre avec prémélange CH4/Air stabilisé par un swirler. LesSGE sont réalisées afin d’évaluer les effets des décharges NRP sur la flamme. Les résultats numériques sont comparés aux observations expérimentales faites à la King Abdulla University ofScience and Technology (Arabie Saoudite) / Interactions between flames and thermal phenomena are the guiding thread of this work. Flamesproduce heat indeed, but can also be affected by it. Large Eddy Simulations (LES) are used hereto investigate these interactions, with a focus on two main topics: wall heat transfer andcombustion control. In a first part, wall heat transfer in a rocket engine sub-scale CH4/O2 burner isstudied. In the context of launchers re-usability and cost reduction, which are major challenges,new propellant combinations are considered and wall heat fluxes have to be precisely predicted.The aim of this work is to evaluate LES needs and performances to simulate this kind ofconfiguration and provide a computational methodology permitting to simulate variousconfigurations. Numerical results are compared to experimental data provided by the TechnischeUniversität München (Germany). In a second part, combustion control by means of NanosecondRepetitively Pulsed (NRP) plasma discharges is studied. Modern gas turbine systems use indeedlean combustion with the aim of reducing fuel consumption and pollutant emissions. Lean flamesare however known to be prone to instabilities and combustion control can play a major role in thisdomain. A phenomenological model which considers the plasma discharges as a heat source isdeveloped and applied to a swirl-stabilized CH4/Air premixed lean burner. LES are performed inorder to evaluate the effects of the NRP discharges on the flame. Numerical results are comparedwith experimental observations made at the King Abdulla University of Science and Technology(Saudi Arabia).