The present research is an experimental investigation of the atomization of a superheated pressurized liquid jet that is exposed to the ambient pressure due to a sudden depressurization. This phenomena is called flashing and occurs in several industrial environments.<p><p>Liquid flashing phenomena holds an interest in many areas of science and engineering. Typical examples one can mention: a) the accidental release of flammable and toxic pressure-liquefied gases in chemical and nuclear industry; the failure of a vessel or pipe in the form of a small hole results in the formation of a two-phase jet containing a mixture of liquid droplets and vapor, b) atomisation improvement in the fuel injector technology, c) flashing mechanism occurrence in expansion devices of refrigerator cycles etc. The interest in flashing events is especially true in the safety field where any unexpected event is undesirable. In case of an accident, flammable or toxic gas clouds are anticipated in close regions of the release because of the sudden phase change .Due to the non-equilibrium nature of the flow in these near field regions, conducting accurate data measurements for droplet size and velocity is a challenging task resulting in scarce data in the very close area.<p><p>This research has been carried out at the von Karman Institute (VKI) within the 5th framework of European Commission to fulfill the goal of understanding of source processes in flashing liquids in accidental releases. The program is carried out under name of FLIE (Flashing Liquids in Industrial Environments)(Contract no: EVG1-CT-2000-00025). The specific issues that are presented in this thesis study are the following:a) a comprehensive state of art of the jet break up patterns, spray characteristics and studies related to flashing phenomena; b)flashing jet breakup patterns and accurate characterization of the atomized jet such as droplet diameter size, velocity and temperature evolution through carefully designed laboratory-scale experiments; c) the influence of the initial storage conditions on the final atomized jet; d) a physical model on the droplet transformation and rapid evaporation in aerosol jets.<p><p>In order to characterize the atomization of the superheated liquid jet, laser-based optical techniques like Particle Image Velocimetry (PIV), Phase Doppler Anemometry (PDA) are used to obtain information for particle diameter and velocity evolution at various axial and radial distances. Moreover, a high-speed video photography presents the possibility to understand the break-up pattern changes of the simulating liquid namely R-134A jet in function of driving pressure, superheat and discharge nozzle characteristics. Global temperature measurements with an intrusive technique such as thermocouples, non-intrusive measurements with Infrared Thermography are performed. Cases for different initial pressures, temperatures, orifice diameters and length-to-diameter ratios are studied. The break-up patterns, the evolution of the mean droplet size, velocity, RMS, turbulence<p>intensity and temperature along the radial and axial directions are presented in function of initial parameters. Highly populated drop size and velocity count distributions are provided. Among the initial storage conditions, superheat effect is found to be very important in providing small droplets. A 1-D analytical rapid evaporation model is developed in order to explain the strong temperature decrease during the measurements. A sensitivity analysis of this model is provided.<p> / Doctorat en sciences appliquées / info:eu-repo/semantics/nonPublished

[ES] El proceso de atomización primaria es el mecanismo por el cual una vena líquida se disgrega en un ambiente gaseoso. Este proceso está presente en muchas aplicaciones de ingeniería realizando diferentes tareas. En ocasiones es un paso previo antes de ser quemado, como en la industria energética o de propulsión, donde el objetivo es extraer la energía específica del líquido. En otros sectores, como el revestimiento o la extinción de incendios, el objetivo es maximizar el área cubierta por el chorro. Sin embargo, aunque la atomización es una parte fundamental de varios procesos industriales, está lejos de comprenderse por completo. El proceso de atomización es una mezcla de fenómenos de interacción gas-líquido dentro de un campo turbulento que tiene lugar en el campo cercano, que es la región más densa del chorro. Cuando se trata de arrojar luz sobre el proceso de atomización primaria, el problema principal es la falta de teorías físicas definitivas capaces de vincular los complejos eventos de ruptura con la turbulencia. El principal obstáculo que impide investigar el proceso de atomización primaria es la incapacidad de las técnicas ópticas clásicas para proporcionar información de la región densa del chorro. Solo en los últimos años, las nuevas técnicas basadas en rayos X podrían proporcionar nueva información sobre las características de la atomización cerca de la salida de la tobera. Esto también afecta a los modelos computacionales de atomización primaria que, al no disponer de información experimental sobre la región densa, requieren una calibración precisa de sus constantes para proporcionar resultados fiables en el campo lejano. Esta tesis se centra en mejorar el conocimiento del proceso de atomización primaria, especialmente en cómo las condiciones de inyección afectan el desarrollo del chorro en el campo cercano desde dos puntos de vista diferentes. Por un lado, con un enfoque computacional usando Direct Numerical Simulations y, por otro lado, experimentalmente usando Near-Field Microscopy. El estudio computacional se centra en variar los números de Reynolds y Weber de inyección. Los resultados muestran que aumentar el número de Reynolds mejora la desintegración del líquido, mostrando un aumento de las gotas generadas y una nube de gotas más fina. Sin embargo, la falta de un perfil turbulento de flujo de entrada completamente desarrollado conduce a comportamientos inesperados en la longitud de ruptura de la vena líquida que también aumenta con el número de Reynolds. El número de gotas también aumenta cuando aumenta el número de Weber, pero los tamaños característicos de las gotas siguen siendo los mismos. La longitud de ruptura no varía, lo que sugiere que las variaciones de la tensión superficial afectan la ruptura de las gotas y los ligamentos, pero no la desintegración del núcleo líquido en sí. Con los resultados obtenidos de ambos estudios, se propone un modelo fenomenológico que predice la distribución del tamaño de gota en función de las condiciones de inyección. Además, también se ha estudiado el efecto de usar toberas elípticas. Se ha obtenido que el número de gotas detectadas aumenta en comparación con el chorro redondo manteniendo ángulos de apertura del chorro similares. Sin embargo, cuando se utilizan toberas extremadamente excéntricas, la disminución de la turbulencia del flujo de entrada contrarresta los beneficios de este tipo de inyectores. En cuanto al análisis experimental, usar Near-Field Microscopy permite magnificar la región densa y analizar las características macroscópicas del chorro. Por lo tanto, se varían las presiones de inyección y descarga, centrándose en el ángulo de apertura del chorro. Se observa el aumento esperado en el ángulo al aumentar tanto la presión de inyección como la de descarga. Sin embargo, adicionalmente, se realiza un análisis de las perturbaciones del contorno del chorro, concluyendo que, al aumentar la presión de inyección, y por lo tanto la turbulencia del flujo de / [CA] El procés d'atomització primària és el mecanisme pel qual una vena líquida es disgrega en un ambient gasós. Aquest procés és present en moltes aplicacions d'enginyeria fent diferents tasques. De vegades és un pas previ abans de ser cremat, com ara en la indústria energètica o de propulsió, on l'objectiu és extraure l'energia específica del líquid. En altres sectors, com ara el revestiment o l'extinció d'incendis, l'objectiu és maximitzar l'àrea coberta pel doll. No obstant això, tot i que l'atomització és una part fonamental de diversos processos industrials, està lluny de comprendre's per complet. El procés d'atomització és una barreja de fenòmens d'interacció gas-líquid dins d'un camp turbulent que té lloc en el camp pròxim, que és la regió més densa del doll. Quan es tracta de donar llum sobre el procés d'atomització primària, el problema principal és la falta de teories físiques definitives capaces de vincular els complexos esdeveniments de trencament amb la turbulència. El principal obstacle que impedeix investigar el procés d'atomització primària és la incapacitat de les tècniques òptiques clàssiques per a proporcionar informació de la regió densa del doll. Només en els últims anys, les noves tècniques basades en raigs X podrien proporcionar nova informació sobre les característiques de l'atomització prop de l'eixida de la tovera. Això també afecta els models computacionals d'atomització primària que, en no disposar d'informació experimental sobre la regió densa, requereixen un calibratge precís de les seues constants per a proporcionar resultats fiables en el camp llunyà. Aquesta tesi se centra a millorar el coneixement del procés d'atomització primària, especialment en com les condicions d'injecció afecten el desenvolupament del doll en el camp pròxim des de dos punts de vista diferents. D'una banda, amb un enfocament computacional usant Direct Numerical Simulations i, d'altra banda, experimentalment usant Near-Field Microscopy. L'estudi computacional se centra a variar els nombres de Reynolds i Weber d'injecció. Els resultats mostren que augmentar el nombre de Reynolds millora la desintegració del líquid, tot mostrant un augment de les gotes generades i un núvol de gotes més fi. No obstant això, la falta d'un perfil turbulent de flux d'entrada completament desenvolupat condueix a comportaments inesperats en la longitud de ruptura de la vena líquida que també augmenta amb el nombre de Reynolds. El nombre de gotes també augmenta quan creix el nombre de Weber, però les grandàries característiques de les gotes continuen sent les mateixes. La longitud de ruptura no varia, la qual cosa suggereix que les variacions de la tensió superficial afecten la ruptura de les gotes i els lligaments, però no la desintegració del nucli líquid en ell mateix. Amb els resultats obtinguts de tots dos estudis, es proposa un model fenomenològic que prediu la distribució de la grandària de gota en funció de les condicions d'injecció. A més, també s'ha estudiat l'efecte d'usar toveres el·líptiques. S'ha obtingut que el nombre de gotes detectades augmenta en comparació amb el doll redó tot mantenint angles d'obertura del doll similars. No obstant això, quan s'utilitzen toveres extremadament excèntriques, la disminució de la turbulència del flux d'entrada contraresta els beneficis d'aquesta mena d'injectors. Quant a l'anàlisi experimental, usar Near-Field Microscopy permet magnificar la regió densa i analitzar les característiques macroscòpiques del doll. Per tant, es varien les pressions d'injecció i descàrrega, tot centrant-se en l'angle d'obertura del doll. S'observa l'augment esperat en l'angle en augmentar tant la pressió d'injecció com la de descàrrega. No obstant això, addicionalment, es realitza una anàlisi de les pertorbacions del contorn del doll i es conclou que en augmentar la pressió d'injecció, i per tant la turbulència del flux d'entrada, augmenten les pertorbacions en el contorn del ruixat, especialment a pressions de descàrrega mé / [EN] The primary atomisation process is the mechanism by which a liquid vein breaks into droplets in a gaseous ambient. This process is present in many engineering applications accomplishing different tasks. Sometimes it is a previous step before being burned, as in the energy or propulsion industry, where the objective is to extract the specific energy of the liquid. In other sectors, such as the coating or fire extinction, the objective is to maximise the area covered by the droplet cloud. However, although atomisation is a fundamental part of several industrial processes, it is far from fully understood. The atomisation process is a mixture of gas-liquid interaction phenomena within a turbulent field that takes place in the near-field, which is the denser region of the spray. When trying to shed light on the primary atomisation process, the main issue is the lack of definitive physical theories able to link the complex breakup events and the turbulence. The principal impediment that prevents the investigation from breaking through the atomisation process is the inability of the classic optical techniques to provide information from the dense region of the spray. Only in the last years, newer techniques based on X-Ray could provide new information on spray characteristics near the nozzle outlet. This also affects the computational primary atomisation models that, as there is no available experimental information on the dense region, require an accurate calibration of their constants to provide reliable results on the far-field. This thesis focuses on improving the knowledge of the primary atomisation process, especially on how the injection conditions affect the spray development in the near field from two different standpoints. On the one hand, with a computational approach using Direct Numerical Simulations and on the other hand, experimentally using Near-Field Microscopy. The computational study is focused on varying the inflow Reynolds and Weber numbers. Results show that increasing the Reynolds number improves the liquid disintegration, exhibiting an increase of generated droplets and a finer droplet cloud. However, the lack of a fully developed inflow turbulent profile leads to characteristic behaviours on the breakup length of the spray that also increases with the Reynolds number. The number of droplets increases when the Weber number increases, but the characteristic droplet sizes remain the same. The breakup length does not vary, suggesting that the surface tension variations affect the droplet and ligament breakup but not the core disintegration itself. With the results obtained from both studies, a phenomenological model is proposed to predict the droplet size distribution depending on the injection conditions. Additionally, using elliptical nozzles, the number of detected droplets increases compared with the round spray and maintain similar spray apertures. However, when using extremely eccentric nozzles, the inflow turbulence decrease counteracts the elliptical sprays' benefits. Regarding the experimental analysis, the Near-Field Microscopy magnifies the dense region and analyses the macroscopic features on the spray. So the injection and discharge pressure are varied, and the spotlight is put on the spray angle. The expected increase in the spray angle when increasing both the injection and discharge pressure is observed. Nevertheless, additionally, an analysis of the spray contour perturbations is performed, concluding that increasing the injection pressure, and thus the inflow turbulence, increases the perturbations on the spray contour, especially at lower discharge pressures. / González Montero, LA. (2022). Computational and Experimental Study of the Primary Atomisation Process under Different Injection Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/190635

L’application de cette thèse est le contrôle actif de la combustion dans les brûleurs industriels à combustible liquide. Il s’agit d’explorer les possibilités de contrôle d’un spray par des jets gazeux auxiliaires. Deux familles d’actionneurs utilisant ce procédé ont été testées sur un atomiseur coaxial assisté par air. Le premier dispositif est appelé (Dev). Composé d’un unique jet actionneur, il vise à dévier le spray. La seconde configuration, appelée (Sw), est équipée de 4 jets auxiliaires tangents au spray afin de lui conférer un effet de swirl et d’en augmenter le taux d’expansion. Les mesures de granulométrie par PDA et les visualisations du spray par strioscopie démontrent un effet important du contrôle sur l’atomisation et la forme du spray. On observe en outre une déviation pouvant atteindre 30°avec l’actionneur (Dev) et une augmentation du taux d’expansion de 80% dans le cas (Sw). Des simulations du banc expérimental ont de plus été menées avec le code AVBP. L’écoulement de gaz est calculé par simulation aux grandes échelles (SGE ou LES en Anglais). L’approche lagrangienne est utilisée pour simuler la phase dispersée. Une attention particulière a été portée aux conditions d’injection du gaz et des gouttes dans le calcul. Ceci a abouti au développement d’une nouvelle condition limite caractéristique non réfléchissante (VFCBC) destinée à l’injection d’écoulements turbulents en LES compressible. Les résultats de LES présentent un bon accord avec les mesures expérimentales. Les effets du contrôle sur la dynamique des gouttes et sur la topologie du spray (forme, déviation, expansion) sont correctement décrits. / The present work focuses on active control of two-phase combustion in industrial burners. The generic method explored in this thesis consists in controlling the injected fuel spray with transverse air jets. Two families of these jet actuators are tested on a coaxial airblast atomizer. The first system (Dev) is used to modify the trajectory of the spray, while the second one (Sw) introduces swirl into the spray to modify its spreading rate and mixing with the surrounding air. Experimental characterisations of the controlled flow with Schlieren visualisations and Phase Doppler Anemometry (PDA) show that actuators induce important effect on the spray. The deviation angle reaches 30° for the actuator (Dev) and the expansion rate increases of 80 % in the swirl case (Sw). Simulations of the experiment are then performed with the CFD code AVBP. The gas flow is computed with Large Eddy Simulation (LES). A Lagrangian formulation is used to simulate droplets trajectories. A particular attention is given to the injection of the gas flow and the droplets in the calculations. Therefore, a new non-reflecting characteristic boundary condition (VFCBC) has been derived to inject turbulent flows in compressible LES. A good agreement is observed between simulation and experiment. Control effects on the spray topology ( features, deviation, spread rate) and on the droplets velocities and diameters are correctly described by the Lagrangian LES.

The main problem encountered in this thesis is the lack of research and knowledge of selective laser melted-printing with Uddeholm Dievar®. This absence of information could cause issues regarding quality and properties of the alloy as well as uncertainty regarding an appropriate heat treatment cycle. This thesis mainly focuses on observing the changes that occur in the microstructure when Uddeholm Dievar® is manufactured through the additive manufacturing (AM) method known as selective laser melting (SLM). The SLM- method consists of a high-power laser that melts together thin layers of powder, one layer at a time, until a three-dimensional product is created according to selected drawings. The methodology on which this thesis is based on is the execution of a theoretical study, scientific experiments and thermodynamic calculations. Analysis of the microstructure is performed using a scanning electron microscope with techniques such as Energy-dispersive X-ray spectroscopy (EDS) and Electron backscatter diffraction (EBSD). The purpose of the methods are to map the constituent elements of the alloy and observe the orientation of the crystallographic phases in the atomic lattice respectively. The results show that the powder, both before and after printing, mainly consists of martensite with a low amount of residual austenite. The amount of primary carbides is relatively low and has been classified as MC (V-rich) and/or M6C (Mo- rich) type. The remaining residual austenite could be explained by the segregation of constituent alloying elements, where the carbon content is a dominant factor to why the MS -temperature lowers significantly causing the presence of retained austenite even though SLM has a cooling rate that varies between 103 and 108 [K/s]. / Det huvudsakliga problemet som denna avhandling behandlar är bristen på forskning och kunskap inom selective laser melting (SLM) 3D-printing med Uddeholm Dievar®. Avsaknaden kan leda till sämre kvalité och produktegenskaper hos legeringen. Det kan även leda till ovisshet gällande val av lämplig värmebehandling. Arbetet fokuserar på att dokumentera utformningen av stålets mikrostruktur när Uddeholm Dievar® tillverkas med den additiva tillverkningsmetoden SLM. Tillverkningsprocessen består av en högeffektslaser som detaljerat smälter samman tunna lager pulver, ett lager i taget, tills att en tredimensionell produkt skapats utefter valda ritningar. Använda metoder är; utförandet av en teoretisk studie, vetenskapliga experiment och thermodynamiska beräkningar. Analys av mikrostrukturen genomförs med hjälp av svepelektronmikroskåp där teknikerna Energy-dispersive X-ray spectroscopy (EDS) och Electron backscatter diffraction (EBSD) används. Syftet med EDS är att kartlägga de ingående elementen i legeringen, syftet med EBSD är att se orientering av de kristallografiska faserna i atomgittret. Resultaten visar på att legeringen, både före och efter printing, till största del består av martensit med en låg mängd restaustenit. Mängden primärkarbider är relativt låg och har klassifiserats som typen MC (V-rik) och/eller M6C (Mo- rik). Den kvarstående restausteniten kan möjligen förklaras av segringen av ingående legeringsämnen där kolhalten är en dominerande faktor som sänker MS-temperaturen. Detta gör att restaustenit förekommer trots den höga kylhastigheten som varierar mellan 103 och 108 [K/s] i SLM.

L’objectif de cette étude est de valoriser des coproduits végétaux tout en proposant une alternative biosourcée, économique et performante aux tensioactifs classiques. Des émulsions de type huile-dans-eau sont fabriquées à partir de poudres végétales finement broyées comme seul matériel tensioactif. Les systèmes dispersés qui en résultent, stabilisés par des particules solides, sont des émulsions dites de Pickering. La première partie de ce manuscrit décrit l’influence du procédé d’émulsification (rotorstator en régime turbulent, ultrasons et homogénéisation haute pression) et des paramètres de formulation sur les propriétés d’émulsions stabilisées par de la poudre de cacao. Le rôle des différentes fractions de la poudre est déterminé et le phénomène de coalescence limitée propre aux émulsions de Pickering est étudié. Nous mettons en évidence une évolution structurale des particules (« dépliement ») sous l’effet du cisaillement appliqué lors de l’émulsification, permettant d’augmenter leur performance en tant qu’agents stabilisants. L’influence de la nature végétale est explorée en élargissant l’étude à des poudres de compositions différentes issues de tourteaux de colza et de lupin. La stabilité de ces émulsions face à l’élimination de la phase continue est ensuite évaluée au regard de la technique de séchage, par lyophilisation et atomisation. Des émulsions sèches riches en huile et redispersables dans l’eau sont ainsi obtenues. Enfin, le pouvoir antioxydant des poudres végétales avant et après séchage des émulsions est caractérisé. / This study aimed at valorizing vegetal byproducts while offering a bio-sourced, cheap and efficient alternative to conventional surfactants. Oil-in-water emulsions were obtained from finely ground plant powders as the only surfactant material. The resulting dispersed systems, stabilized by solid particles, are so-called Pickering emulsions. The first part of this manuscript describes the influence of the emulsification process (rotor-stator in turbulent regime, sonication and high-pressure homogenization) and of the formulation parameters on the properties of emulsions stabilized by cocoa powder. The role of the different fractions of the powder was determined and a phenomenon characteristic of Pickering emulsions, namely limited coalescence, was identified. We evidenced a structural evolution of the particles ("unwrapping") under the effect of the shear applied during the emulsification process, allowing to increase their performance as stabilizing agents. The influence of the vegetal origin was explored by extending the study to powders deriving from rapeseed and lupin oilcakes. The stability of these emulsions with respect to the elimination of the continuous phase was evaluated using 2 drying techniques, freeze-drying and spray-drying. Dry emulsions rich in oil and redispersable in water were obtained. Finally, the antioxidant properties of vegetable powders before and after the drying process were characterized.

Drug administration through the pulmonary route is an ancient technique that evolved from inhaling the smoke of certain leaves as a medicine. The optimum droplet diameter for the pulmonary system deposition has been identified to be in the range from 2 to 3.5 μm, with potential deposition rates of up to 80% of this size range. Currently, the most used aerosol generator methods are the pressurized metered dose inhalers. However, they generally exhibit low deposition efficiency with less than 20 % of the spray reaching the target area of the lungs as most of the drug deposited in the upper airways. This is for the most part due to the droplet size polydispersity that is inherent in these systems. The droplets of the biggest diameter will deposit in the upper airways, and then the deposited medicine will be swallowed and absorbed in the gastrointestinal tract. This can produce adverse medical side effects. Electrospray (ES) or electrohydrodynamic atomization (EHDA) is a promising atomization process due to its ability to produce a spray with monodisperse droplet size. The current study will investigate the feasibility of using electrospray in a pulmonary drug delivery system. Assessments, selection and characterization of suitable biocompatible solvents that can be used as a lung obstruction relief drug were carried out. Tests to identify the electrospray setup necessary to produce droplet sizes in the appropriate range for deposition in the lungs were carried out. The study found that both stable and pulsating cone jet modes can produce the required droplet size and the pulsating mode can produce at least four times higher flow than stable cone jet mode. A low-cost image analysis technique developed for this work gave satisfactory results that could be compared to droplet size scaling laws from the literature. However, it proved to be relatively time consuming and further automation of this technique would make it more suitable for large-scale studies. The image analysis results show a correlation between the cone length, cone angle and the applied voltage. The droplet scaling laws discrepancies such as the solution flow rate exponent and the constant that is used by some scaling laws may be attributed to the droplet evaporation time which is quite short for the water/ ethanol solutions. The emitter diameter and the conductivity effect on the I(Q) power law and the sensitivity of the onset voltage (Vonset) to the liquid flow rate (Q), were demonstrated for solutions of triethylene-glycol (TEG), and for an ethanol-water mixture solution.

La thèse porte sur l'étude d'un système de contrôle de la pression au sein de réservoirs d’ergols cryogéniques (dihydrogène ou dioxygène) dans le cadre de missions spatiales de longue durée. Ce système de contrôle doit permettre d’éviter la perte excessive d’ergols associée à un contrôle basique de la pression consistant en l’évacuation directe d’une fraction du fluide. Le système alternatif étudié, dit de contrôle thermodynamique, repose sur la réinjection d’un spray sous-refroidi permettant d’abaisser température et pression dans un réservoir soumis à une chauffe (en pratique, le rayonnement solaire). Nous avons analysé les performances de ce système en développant en parallèle un banc d'essai adapté aux conditions du laboratoire, et un modèle théorique de type 0D, à base de bilans globaux, de l’effet du spray sous-refroidi sur les caractéristiques thermodynamiques de l’enceinte. La confrontation des mesures et des calculs a permis de valider l’outil de modélisation théorique. Les caractéristiques du système réel (ensemble des circuits d'injection et de refroidissement) ont ensuite été introduites dans le modèle théorique afin de quantifier de façon réaliste les gains offerts par le système de contrôle thermodynamique, i.e. en prenant en compte la pénalité en masse associée à ces circuits. Des solutions optimales de contrôle de la pression au sein de réservoirs d’ergols cryogéniques lors de missions spatiales de longue durée ont pu alors être proposées. / This PHD thesis deals with the study of a pressure control system inside a cryogenic propellant tank for long duration space missions. This system must be able to reduce propellant losses induced by direct venting, which is the simplest pressure control system. The alternative system which has been studied, called Thermodynamic Vent System (TVS), is based on reinjecting subcooled spray to make the pressure and temperature decrease in a heated tank. The system performance has been analysed developing simultaneously an experimental setup, adapted to laboratory environment, and a theoretical 0D-modelling of subcooled spray impact on tank's thermodynamic characteristics. Facing experimental and theoretical results has permitted to validate the 0D-modelling tool. Inputing the real system characteristics in theoretical modelling has enabled to assess the effective gains of thermodynamic vent system. Finally, optimal solutions to control pressure inside a cryogenic propellant tank for long duration space missions have been proposed.

Ce travail porte sur la modélisation de la formation et la dispersion d'un nuage de gouttes, par déconfinement d'un liquide: agression extérieure ou situation accidentelle. Le but est la construction d'un modèle apte à reproduire simultanément les conditions génératrices de la formation du nuage et l'évolution de ce nuage dans le temps (dispersion). La principale difficulté réside en la différence des modèles adaptés à la description d'écoulements caractérisant chaque étape du phénomène global : modèle d'écoulement multiphasique à phases compressibles (milieux continus) initialement, puis fragmentation et formation du nuage de gouttes dispersées dans une phase porteuse (modèle d'écoulements dilués). En l'absence de modèle analytique unique apte à décrire l'ensemble de ces processus, on propose une approche originale pour réaliser un couplage effectif entre ces deux modèles. La problématique de formation et de dispersion de liquide implique la prise en compte de plusieurs phénomènes physiques: fragmentation, transferts de chaleur et de masse ainsi que la traînée entre les phases. Ces différents phénomènes sont introduits dans le modèle global via des termes d'interactions présents dans les systèmes d'équations. La construction de ce modèle complet à permis la réalisation de calculs décrivant la formation et la dispersion d'un nuage de gouttes pouvant intervenir lors de situations accidentelles sur des sites industriels par exemple. / This work focuses on modeling the formation and the dispersion of a cloud of droplets, induced by ejection of a liquid, resulting from an external aggression or an accidental situation. The goal is to build a model able to reproduce simultaneously the conditions which generate the cloud formation and the cloud evolution in time (dispersion). The main difficulty lies in the differences between the already existing models adapted to the description of flows which are able to characterize each stage of the global phenomenon: initially a multiphase flow model with compressible phases (Continuum), then the atomization and the formation of a cloud of droplets dispersed in a carrier phase (dilute flow model). We propose a new approach to achieve an effective coupling between these two models. The problem of the formation and the dispersion of the liquid requires to take into account several physical phenomena: atomization, heat and mass transfers and drag between phases. These phenomena are included in the global model through interaction terms involved in the systems of equations. The construction of this model has permited the realization of calculations describing the formation and dispersion of a cloud of droplets which may occur during, for axample, in accidental situations at industrial sites.

The work presented here pertains to investigations on gas-on-liquid type of impinging injectors with a generic approach with prospective applications in several areas, and at places with particular emphasis on cryogenic or semi-cryogenic liquid propellant rockets. In such rockets, one of the components arrives at the injector in a gaseous phase after passing through the regenerative coolant passages or a preceding combustion stage. Most often, the injectors in such systems are of shear coaxial type. The shear coaxial injectors suffer from several disadvantages like complexity in design, manufacture and quality control. Adoption of impinging jet configuration can alleviate these problems in addition to providing further benefits in terms of cost, robustness in high temperature environment and manifolding. However, there is very little literature on gas-on-liquid injectors either in this context or in any other Even for the simplest form of impinging injectors such as like-on-like doublets, literature provides no conclusive direction at describing a spray from the theoretical models of physical mechanisms. Empirical approach is still the prime mode of obtaining a proper understanding of the phenomena. Steady state spray characterization includes mainly of describing the spatial distribution of liquid mass and drop size distribution as a function of geometric and injection parameters. The parameters that are likely to have an impact on spray characteristics are orifice diameter, ratio of orifice length to diameter, pre-impingement length of individual jets, inter orifice distance, impingement angle, jet velocity and condition of the jet just before impingement. The gas-on- liquid configuration is likely to experience some qualitative changes because of the expansion of the gas jet. The degree to which each one of the above variables influences the drop size and mass distribution having implication to combustion performance forms the core theme of the thesis. A dedicated experimental facility has been built, calibrated and deployed exhaustively. While spray drop size measurement is done largely by a laser diffraction instrument, some of the cases warranted an image processing technique. Two different image processing algorithms are developed in-house for this purpose. The granulometric image processing method developed earlier in the group for cryogenic sprays is modified and its applicability to gas-on-liquid impinging sprays are verified. Another technique based on the Hough transform which is feature extraction technique for extracting quantitative information has also been developed and used for gas-on-liquid impinging injectors. A comparative study of conventional liquid-on-liquid doublet with gas-on-liquid impinging injectors are first made to establish the importance of studying gas-on-liquid impinging injectors. The study identifies the similarities and differences between the two types and highlights the features that make such injectors attractive as replacements to coaxial configuration. Spray structure, drop-size mass distributions are quantified for the purpose of comparison. This is followed by a parametric study of the gas-on-liquid impinging injectors carried out using identified control variables. Though momentum ratio appeared to be a suitable parameter to describe the spray at any given impingement angle, the variations due to impingement angle had to be factored in. It was found that normal gas momentum to liquid mass is an apt parameter to generalize the spray characteristics. It was also found that using identical nozzles for desired mass ratio could lead to rather large deflection of the spray which may not be acceptable in combustion chamber design. One way of overcoming this is to work with unequal orifice sizes for gas and liquid. It was found that using smaller gas orifice for a given liquid orifice resulted in lower SMD (Sauter Mean Diameter of the spray) for constant gas and liquid mass flow rates. This is attributable to the high dynamic pressure of gas in the case of smaller gas orifices for the same mass flow rate. The impinging liquid jets with unequal momentum in the doublet configuration would result in non-uniform mass and mixture ratio distribution within the combustion chamber which may have to operate under varying conditions of mass flow rates and/or mixture ratio. The symmetrical arrangement of triplet configuration can eliminate this problem at the same time generating finely atomized spray and a homogeneous mixture ratio. In view of the scanty literature available in this field, the atomization characteristics of the spray generated by liquid centered triplet jets are examined in detail. It was found that as in the case of gas-on-liquid impinging doublets, normal gas momentum to liquid mass is an ideal parameter in describing the spray. Variants of this configuration are studied recently for many other applications too. As done in the case of doublets, efforts have also been made to compare gas centered triplet to liquid-liquid triplet. It was found that the trend of SMD of gas centered triplet is different from that of liquid-liquid triplets, thus pointing to a different mechanism in play. The SMD in the case of liquid-liquid triplets decreases monotonically with increasing specific normal momentum. It is to be noted that specific normal momentum is an ideal parameter for describing the spray characteristics of liquid-liquid triplets and doublets. In the case of gas centered triplet the SMD first increases and then decreases with specific normal momentum, the inversion point depends on the gas mass flow rate for a constant specific normal momentum. The thesis concludes with a summary of the major observations of spray structures for all the above injector configurations and quantifies the parametric dependencies that would be of use to engineering design

Cette thèse présente une extension de l'approche stochastique de l'atomisation primaire de type air assisté près d'un injecteur. Cette approche avait déjà été introduite dans les publications de Gorokhovski et al. Dans le cadre de la simulation des grandes échelles, la zone d'atomisation primaire est simulée comme un corps immergé avec une structure stochastique. Ce dernier est défini par la simulation stochastique de la position et de la courbure de l'interface entre le liquide et le gaz. La simulation de la position de l'interface est basée sur l'hypothèse de symétrie d'échelle pour la fragmentation. La normale extérieure à l'interface est modélisée en supposant une relaxation statistique vers l'isotropie. Les statistiques de la force du corps immergé servent de conditions aux limites pour le champ de vitesse issu de la LES ainsi que pour la production des gouttes de l'atomisation primaire. Celles-ci sont ensuite transportées par une approche lagrangienne. Les collisions entre les gouttes dans la zone d'atomisation primaire sont prises en compte par analogie avec l'approche standard de la théorie cinétique des gaz. Une fermeture est proposée pour la température statistique des gouttelettes. Cette approche est validée par des comparaisons avec les mesures expérimentales de la thèse de Hong. Les résultats numériques pour la vitesse et de la taille des gouttes dans le spray à différentes distances du centre du jet et de l'orifice de la buse sont relativement proches des résultats expérimentaux. Différentes conditions d'entrée pour la vitesse sont testées et comparées aux résultats expérimentaux. Par ailleurs, le rôle spécifique de la zone de recirculation devant le dard liquide est soulignée par le battement du dard liquide et la production de gouttelettes. / This thesis introduced an extension to stochastic approach for simulation of air-blast atomization closely to injector. This approach was previously proposed in publications of Gorokhovski with his PHD students. Our extension of this approach is as follows. In the framework of LES approach, the contribution of primary atomization zone is simulated as an immersed solid body with stochastic structure. The last one is defined by stochastic simulation of position-and-curvature of interface between the liquid and the gas. As it was done previously in this approach, the simulation of the interface position was based on statistical universalities of fragmentation under scaling symmetry. Additionally to this, we simulate the outwards normal to the interface, assuming its stochastic relaxation to isotropy along with propagation of spray in the down-stream direction. In this approach, the statistics of immersed body force plays role of boundary condition for LES velocity field, as well as for production of primary blobs, which are then tracked in the Lagrangian way. In this thesis, the inter-particle collisions in the primary atomisation zone are accounted also by analogy with standard kinetic approach for the ideal gas. The closure is proposed for the statistical temperature of droplets. The approach was assessed by comparison with measurements of Hong in his PHD. The results of computation showed that predicted statistics of the velocity and of the size in the spray at different distances from the center plane, at different distances from the nozzle orifice, at different inlet conditions (different gas velocity at constant gas-to-liquid momentum ratio, different gas-to-liquid momentum ratio) are relatively close to measurements. Besides, the specific role of recirculation zone in front of the liquid core was emphasized in the flapping of the liquid core and in the droplets production.