La thèse porte sur la simulation de la thermohydraulique et des transferts thermiques dans un faisceau d'aiguilles d'assemblage combustible de réacteur à neutrons rapides à caloporteur sodium.Des premiers calculs ont été réalisés par une approche moyennée de type RANS à l'aide du code industriel STAR-CCM+. De cette modélisation, il ressort une meilleure compréhension des transferts de chaleur opérés entre les aiguilles et le sodium. Les principales grandeurs macroscopiques de l'écoulement sont en accord avec les corrélations. Cependant, afin d'obtenir une description détaillée des fluctuations de température au niveau des fils espaceur, une approche plus détaillée de type LES et DNS est apparue indispensable. Pour la partie LES, le code TRIO_U a été utilisé. Concernant la partie DNS, un code de recherche a été utilisé. Ces approches requièrent des temps de calculs considérables qui ont nécessité des géométries représentatives mais simplifiées.L'approche DNS permet d'étudier l'écoulement à bas nombre de Prandtl, qui induit un comportement très différent du champ thermique relativement au champ hydraulique. Le calcul LES de l'assemblage montre que la présence du fil espaceur génère l'apparition de points chauds locaux (~20°C) en aval de celui-ci par rapport à l'écoulement sodium, au niveau de son contact avec l'aiguille. Les fluctuations de température au niveau des fils espaceur sont faibles (~1°C-2°C). En régime nominal, l'analyse spectrale montre l'absence de grande amplitude d'oscillations de température à basse fréquence (2-10 Hz); les conséquences sur la tenue mécanique des structures devront être analysées. / The thesis focuses on the numerical simulation of sodium flow in wire wrapped sub-assembly of Sodium-cooled Fast Reactor (SFR).First calculations were carried out by a time averaging approach called RANS (Reynolds- Averaged Navier-Stokes equations) using industrial code STAR-CCM+. This study gives a clear understanding of heat transfer between the fuel pin and sodium. The main variables of the macroscopic flow are in agreement with correlations used hitherto. However, to obtain a detailed description of temperature fluctuations around the spacer wire, more accurate approaches like LES (Large Eddy Simulation) and DNS (Direct Numerical Simulation) are clearly needed. For LES approach, the code TRIO_U was used and for the DNS approach, a research code was used. These approaches require a considerable long calculation time which leads to the need of representative but simplified geometry.The DNS approach enables us to study the thermal hydraulics of sodium that has very low Prandtl number inducing a very different behavior of thermal field in comparison to the hydraulic field. The LES approach is used to study the local region of sub-assembly. This study shows that spacer wire generates the local hot spots (~20°C) on the wake side of spacer wire with respect to the sodium flow at the region of contact with the fuel pin. Temperature fluctuations around the spacer wire are low (~1-2°C). Under nominal operation, the spectral analysis shows the absence of any dominant peak for temperature oscillations at low frequency (2-10Hz). The obtained spectra of temperature oscillations can be used as an input for further mechanical studies to determine its impact on the solid structures.

Un mascaret est une vague remontant contre le courant d’un fleuve lorsque la marée se propage dans un estuaire. À son passage, le mascaret induit une forte turbulence et un fort mélange dont les effets sur la vie de l’estuaire sont encore mal quantifiés. Ici, le phénomène est étudié expérimentalement et numériquement en utilisant un modèle d’onde positive se propageant contre un courant permanent.L’étude en laboratoire a permis de mesurer les variations de la surface libre, de la vitesse de l’écoulement ainsi que des échelles de turbulence. Lors de son passage, des fluctuations importantes de la surface libre et de la vitesse de l’écoulement sont observées, ainsi que des variations des échelles de turbulences. Des structures turbulentes semblent se former près du fond sous le front de l’onde et montent dans la colonne d’eau après le passage du front.La simulation numérique fut réalisée à partir de données expérimentales d’onde positive ondulée sur fond lisse. Une validation des méthodes numériques a été réalisée pour différente configuration. Les résultats des simulations d’onde positives donnent une cartographie détaillée de l’écoulement dans tout le canal. De plus, la simulation a permis d’identifier une inversion de la vitesse près des parois lors du passage des crêtes des ondes générant dans certaines configurations des structures turbulentes. / Tidal bores are surge waves propagating upstream rivers as the tide rushes into estuaries. They induce large turbulences and mixing of the river and estuary flow of which effects remain scarcely studied. Herein, tidal bores are investigated experimentally and numerically with an idealised model of positive surges propagating upstream an initially steady flow. The experimental work estimated flow changes and typical turbulent length scale evolution induced by undular bores with and without breaking roller. The bore passage was associated with large free surface and flow velocity fluctuations, together with some variations of the integral turbulent scales. Coherent turbulent structures appeared in the wake of leading wave near the bed and moved upward into the water column during the bore propagation. The numerical simulations were based on previous experimental work on undular bores. Some test cases were realised to verify the accuracy of the numerical methods. The results gave access to the detailed flow evolution during the bore propagation. Large velocity reversals were observed close to the no-slip boundaries. In some configurations, coherent turbulent structures appeared against the walls in the wake of the bore front.

Le bruit de cavité est un phénomène très fréquent dans le domaine des transports aériens.Il survient notamment lors de l’approche à l’atterrissage, où des interactions entre la cellule de l’aéronef et l’écoulement sont à l’origine de fortes émissions tonales. Il devient dès lors une source de pollution acoustique non-négligeable pour les populations résidant à proximité de zones aéroportuaires. Les études numériques et expérimentales décrites jusqu’à présent dans la littérature abordent essentiellement le cas des cavités rectangulaires. Pourtant, les cavités rencontrées en pratique dans l’industrie aéronautique impliquent des géométries souvent plus complexes. Lorsque ces cavités sont soumises à une excitation de nature aérodynamique, leur spécificité géométrique conduit le plus souvent à des réponses acoustiques assez éloignées des estimations issues de modèles académiques construits sur l’observation de cavités rectangulaires. Quelques travaux seulement abordent le cas des cavités cylindriques.Ce travail est consacré à l’étude aéroacoustique des cavités cylindriques, à l’initiative d’Airbus. Il s’inscrit dans le cadre du projet AEROCAV soutenu par la Fondation de Recherche pour l’Aéronautique & l’espace (FRAE). Son objectif est de déterminer les mécanismes impliqués dans les émissions acoustiques intenses et tonales pour les configurations étudiées.Une première partie présente les résultats expérimentaux issus des campagnes de mesures menées dans la soufflerie anéchoïque du Centre Acoustique du LMFA et de l’école Centrale de Lyon. Un modèle semi-empirique, reposant sur l’hypothèse d’une résonance acoustique pilotée par les instabilités présentes dans la couche de cisaillement à l’ouverture de la cavité,est construit à partir du modèle d’Elder (1978). Le modèle permet d’estimer les fréquences susceptibles de dominer l’acoustique rayonnée en champ lointain à partir de la donnée du champ moyen de vitesse longitudinale, que l’on mesure dans le plan de l’écoulement par Vélocimétrie par Imagerie des Particules (PIV).Une seconde partie est destinée au calcul direct du bruit rayonné par un écoulement laminaire ou turbulent affleurant une cavité cylindrique de référence. Il consiste à calculer le champ acoustique directement à partir de la résolution des équations tridimensionnelles de la mécanique des fluides. Le solver Alesia est présenté dans une version modifiée et adaptée à la mise en oeuvre d’une approche multidomaine d’ordre élevé faisant intervenir plusieurs maillages se recouvrant. Des techniques d’interpolation sont spécifiquement développées en vue d’assurer une communication bidirectionnelle entre les différents maillages, malgré des contraintes géométriques fortes. Un modèle d’excitation de l’écoulement est aussi développé afin de disposer de fluctuations dans l’écoulement incident, pour le cas turbulent. Ces deux points font l’originalité des calculs réalisés.Les simulations, menées sur une cavité de rapport d’aspect géométrique égal à 1 et soumise à un écoulement incident à Mach 0.2, montrent que le rayonnement acoustique peut être fidèlement reproduit numériquement. La couche de cisaillement est caractérisée par la présence de deux larges structures tourbillonnaires s’amplifiant lors de leur convection. Leur présence s’accompagne de fortes fluctuations de vitesse à l’origine d’un débit aérodynamique de fluide à l’ouverture qui excite la cavité acoustiquement. Une résonance forcée s’établit dans celle-ci, excitant la couche de mélange au voisinage du point de séparation. Ce couplage auto-entretenu est à l’origine du rayonnement acoustique intense et fortement tonal de la cavité. Il s’établit à une fréquence proche de la fréquence prédite par le modèle semi-empirique développé. / Cavity noise is a very frequent phenomenon in air transport. It occurs in particular during landing approaches, where airframe–flow interactions are responsible for strong tonal emissions. Accordingly, it turns to be a non negligible source of acoustic pollution for populations living near airport areas. Numerical and experimental studies reported in the literature tackle essentially the case of rectangular cavities. Nevertheless, cavities may often exhibit more complex shapes in practice. When subject to aerodynamic excitations, and because of their geometrical specificity, these cavities may have acoustic responses which can be rather far from estimations resulting from academic models designed for rectangular cavities. Only asmall number of studies tackle the case of cylindrical cavities.The present work requested by Airbus is dedicated to the study of aeroacoustics in cylindrical cavities. This work was been supported by the Fondation de Recherche pour l’Aéronautique& l’Espace (FRAE) under contract reference AEROCAV. It aims at discerning the mechanisms responsible for strong and tonal acoustic emissions for the studied configurations.Experimental data resulting from measurements performed in the anechoic wind-tunnel of the Centre Acoustique at ´Ecole Centrale de Lyon are presented in a first part. A semi-empirical model based on the hypothesis of a shear-layer driven acoustic resonance is constructed from the Elder model (1978). The model provides an estimation of the frequences which are likely to be predominant in the far-field acoustics, given the mean streamwise velocity field, currently measured in the flow plane by Particle Image Velocimetry (PIV).A second part deals with the direct computation of the noise radiated by a laminar or turbulent grazing flow over a standard cylindrical cavity. The method consists in the calculationof the acoustic field directly from the resolution of the tridimensional Navier–Stokes equations. The Alesia solver is presented in a modified form, adapted to the implementationof a high-order chimera method involving several overlapping grids. Interpolation techniques have been specifically developed to achieve a bidirectional communication between the meshes in spite of strong geometrical constraints. A flow excitation model has also been constructed in order to obtain fluctuations into the incoming flow in the turbulent case. These two last points make the present computations original. The simulations, which are performed on a cavity of geometric ratio taken as 1 and subject to a grazing flow of Mach 0.2, reveal that it is possible to retrieve the radiated noise numerically with high fidelity. They indicate the presence of two large amplifying vortices in the shearlayer. These vortices go with strong velocity fluctuations giving rise to an inflow of fluid at the cavity mouth which excites the cavity acoustically. A forced acoustic resonance occurs into the cavity, then destabilises the shear layer near the separation point. This self-sustained coupling is responsible for strong tonal radiations from the cavity. The frequency of the radiated noise is close to the one predicted by the semi-empirical model.

Syngas consisting mainly of a mixture of carbon monoxide, hydrogen and other diluents, is an important fuel for power generation applications since it can be obtained from both biomass and coal gasification. Clean coal technologies require stable and efficient operation of syngas-fired gas turbines. The trapped vortex combustor (TVC) is a relatively new gas turbine combustor concept which shows tremendous potential in achieving stable combustion under wide operating conditions with low emissions. In the present work, combustion of low calorific value syngas in a TVC has been studied using in-situ laser diagnostic techniques and numerical modeling. Specifically, this work reports in-situ measurements of mixture fraction, OH radical concentration and velocity in a single cavity TVC, using state-of-the art laser diagnostic techniques such as Planar Laser-induced Fluorescence (PLIF) and Particle Image Velocimetry (PIV). Numerical simulations using the unsteady Reynolds-averaged Navier-Stokes (URANS) and Large Eddy Simulation (LES) approaches have also been carried out to complement the experimental measurements. The fuel-air momentum flux ratio (MFR), where the air momentum corresponds to that entering the cavity through a specially-incorporated flow guide vane, is used to characterize the mixing. Acetone PLIF experiments show that at high MFRs, the fuel-air mixing in the cavity is very minimal and is enhanced as the MFR reduces, due to a favourable vortex formation in the cavity, which is corroborated by PIV measurements. Reacting flow PIV measurements which differ substantially from the non-reacting cases primarily because of the gas expansion due to heat release show that the vortex is displaced from the centre of the cavity towards the guide vane. The MFR was hence identified as the controlling parameter for mixing in the cavity. Quantitative OH concentration contours showed that at higher MFRs 4.5, the fuel jet and the air jet stream are separated and a flame front is formed at the interface. As the MFR is lowered to 0.3, the fuel air mixing increases and a flame front is formed at the bottom and downstream edge of the cavity where a stratified charge is present. A flame stabilization mechanism has been proposed which accounts for the wide MFRs and premixing in the mainstream as well. LES simulations using a flamelet-based combustion model were conducted to predict mean OH radical concentration and velocity along with URANS simulations using a modified Eddy dissipation concept model. The LES predictions were observed to agree closely with experimental data, and were clearly superior to the URANS predictions as expected. Performance characteristics in the form of exhaust temperature pattern factor and pollutant emissions were also measured. The NOx emissions were found to be less than 2 ppm, CO emissions below 0.2% and HC emissions below 700 ppm across various conditions. Overall, the in-situ experimental data coupled with insight from simulations and the exhaust measurements have confirmed the advantages of using the TVC as a gas turbine combustor and provided guidelines for stable and efficient operation of the combustor with syngas fuel.

This thesis aims at improving our knowledge on swirl combustors. The work presented here is based on Large Eddy Simulations (LES) coupled to an advanced combustion model: the Conditional Moment Closure (CMC). Numerical predictions have been systematically compared and validated with detailed experimental datasets. In order to analyze further the physics underlying the large numerical datasets, Proper Orthogonal Decomposition (POD) has also been used throughout the thesis. Various aspects of the aerodynamics of swirling flames are investigated, such as precession or vortex formation caused by flow oscillations, as well as various combustion aspects such as localized extinctions and flame lift-off. All the above affect flame stabilization in different ways and are explored through focused simulations. The first study investigates isothermal air flows behind an enclosed bluff body, with the incoming flow being pulsated. These flows have strong similarities to flows found in combustors experiencing self-excited oscillations and can therefore be considered as canonical problems. At high enough forcing frequencies, double ring vortices are shed from the air pipe exit. Various harmonics of the pulsating frequency are observed in the spectra and their relation with the vortex shedding is investigated through POD. The second study explores the structure of the Delft III piloted turbulent non-premixed flame. The simple configuration allows to analyze further key combustion aspects of combustors, with further insights provided on the dynamics of localized extinctions and re-ignition, as well as the pollutants emissions. The third study presents a comprehensive analysis of the aerodynamics of swirl flows based on the TECFLAM confined non-premixed S09c configuration. A periodic component inside the air inlet pipe and around the central bluff body is observed, for both the inert and reactive flows. POD shows that these flow oscillations are due to single and double helical vortices, similar to Precessing Vortex Cores (PVC), that develop inside the air inlet pipe and whose axes rotate around the burner. The combustion process is found to affect the swirl flow aerodynamics. Finally, the fourth study investigates the TECFLAM configuration again, but here attention is given to the flame lift-off evident in experiments and reproduced by the LES-CMC formulation. The stabilization process and the pollutants emission of the flame are investigated in detail.

Lors d’un lancement spatial, le bruit des jets supersoniques chauds, générés par les moteurs-fusées au décollage et en interaction avec le pas de tir, est dommageable pour le lanceur et en particulier sa charge utile. Par conséquent, les acteurs du spatial cherchent à renforcer leur compréhension et leur maîtrise de cette ambiance acoustique, entre autres grâce à des méthodes et outils numériques. Toutefois, ils ne disposent pas d’une approche numérique globale capable de prendre en compte simultanément la génération fidèle du bruit, la propagation acoustique non-linéaire, les effets d’installation complexes et les géométries réalistes, pourtant inhérents aux applications spatiales. Dans cette optique, cette étude consiste à mettre en place et valider une méthodologie de simulation numérique par couplage fort Navier-Stokes − Euler, puis à l’appliquer à des cas réalistes de bruit de jet supersonique. L’objectif est d’affiner les capacités de prévision et de contribuer à la compréhension des mécanismes de génération de bruit dans de tels jets. Le solveur Navier-Stokes repose sur une méthode LES sur maillage non-structuré et le solveur acoustique sur une méthode de Galerkine discontinue d’ordre élevé sur maillage non-structuré. La méthodologie est tout d’abord évaluée sur des cas académiques visant à valider la simulation par couplage fort. Après des calculs préliminaires, la méthodologie est appliquée à la simulation du bruit d’un jet libre supersonique à Mach 3.1. Une méthode de déclenchement géométrique de la turbulence est implémentée sous la forme d’une marche à la paroi de la tuyère. La simulation aboutit à des estimations du bruit très proches des mesures réalisées au banc MARTEL et met en évidence des effets non-linéaires significatifs ainsi qu’un mécanisme singulier de rayonnement des ondes de Mach. Dans une démarche de progression vers des cas toujours plus réalistes, l’ensemble de l’approche numérique est finalement adaptée avec succès à la simulation du bruit d’un jet en présence d’un carneau. À terme, elle pourra être étendue à des configurations multi-jets réactifs, avec injection d’eau, voire à l’échelle 1. / During a space launch, the noise from hot supersonic jets, generated by rocket engines at liftoff and interacting with the launch pad, is harmful to the launcher and in particular its payload. Consequently, space actors are seeking to strengthen their understanding and control of this acoustic environment through numerical methods and tools, among the others. However, they do not dispose of a comprehensive numerical strategy that can simultaneously take into account accurate noise generation, nonlinear acoustic propagation, complex installation effects and realistic geometries, which are inherent to space applications. For this purpose, the present study consists in setting up and validating a numerical simulation methodology using a Navier-Stokes − Euler two-way coupling approach, then applying it to realistic cases of supersonic jet noise in order to improve prediction capabilities and contribute to the understanding of the noise generation mechanisms in such jets. The Navier-Stokes solver is based on an LES method on unstructured mesh and the acoustic solver on a high-order discontinuous Galerkin method on unstructured mesh. The methodology is first assessed on academic cases to validate the use of the two-way coupling. After preliminary computations, the methodology is applied to the simulation of the noise from a supersonic free jet at Mach 3.1. A geometric turbulence tripping method is implemented via a step at the nozzle wall. The computation leads to noise predictions very close to the experimental measurements performed at the MARTEL test bench and highlights significant nonlinear effects as well as a quite particular Mach waves radiation mechanism. Targeting even more realistic cases, the entire numerical approach is finally successfully adapted to the simulation of the noise from a supersonic jet configuration including a flame trench. In the future, it may be extended to configurations with clustered reactive jets, water injection devices or even at full scale.

<p>The main accomplishments of this research are </p> <p>(1) developed a high fidelity computational methodology based on large eddy simulation to capture lean blowout (LBO) behaviors of different fuels; </p> <p>(2) developed fundamental insights into the combustion processes leading to the flame blowout and fuel composition effects on the lean blowout limits; </p> <p>(3) developed artificial intelligence-based models for early detection of the onset of the lean blowout in a realistic complex combustor. </p> <p>The methodologies are demonstrated by performing the lean blowout (LBO) calculations and statistical analysis for a conventional (A-2) and an alternative bio-jet fuel (C-1).</p> <p>High-performance computing methodology is developed based on the large eddy simulation (LES) turbulence models, detailed chemistry and flamelet based combustion models. This methodology is employed for predicting the combustion characteristics of the conventional fuels and bio-derived alternative jet fuels in a realistic gas turbine engine. The uniqueness of this methodology is the inclusion of as-it-is combustor hardware details such as complex hybrid-airblast fuel injector, thousands of tiny effusion holes, primary and secondary dilution holes on the liners, and the use of highly automated on the fly meshing with adaptive mesh refinement. The flow split and mesh sensitivity study are performed under non-reacting conditions. The reacting LES simulations are performed with two combustion models (finite rate chemistry and flamelet generated manifold models) and four different chemical kinetic mechanisms. The reacting spray characteristics and flame shape are compared with the experiment at the near lean blowout stable condition for both the combustion models. The LES simulations are performed by a gradual reduction in the fuel flow rate in a stepwise manner until a lean blowout is reached. The computational methodology has predicted the fuel sensitivity to lean blowout accurately with correct trends between the conventional and alternative bio-jet fuels. The flamelet generated manifold (FGM) model showed 60% reduction in the computational time compared to the finite rate chemistry model. </p> <p>The statistical analyses of the results from the high fidelity LES simulations are performed to gain fundamental insights into the LBO process and identify the key markers to predict the incipient LBO condition in swirl-stabilized spray combustion. The bio-jet fuel (C-1) exhibits significantly larger CH₂O concentrations in the fuel-rich regions compared to the conventional petroleum fuel (A-2) at the same equivalence ratio. It is observed from the analysis that the concentration of formaldehyde increases significantly in the primary zone indicating partial oxidation as we approach the LBO limit. The analysis also showed that the temperature of the recirculating hot gases is also an important parameter for maintaining a stable flame. If this temperature falls below a certain threshold value for a given fuel, the evaporation rates and heat release rated decreases significantly and consequently leading to the global extinction phenomena called lean blowout. The present study established the minimum recirculating gas temperature needed to maintain a stable flame for the A-2 and C-1 fuels. </p> The artificial intelligence (AI) models are developed based on high fidelity LES data for early identification of the incipient LBO condition in a realistic gas turbine combustor under engine relevant conditions. The first approach is based on the sensor-based monitoring at the optimal probe locations within a realistic gas turbine engine combustor for quantities of interest using the Support Vector Machine (SVM). Optimal sensor locations are found to be in the flame root region and were effective in detecting the onset of LBO ~20ms ahead of the event. The second approach is based on the spatiotemporal features in the primary zone of the combustor. A convolutional autoencoder is trained for feature extraction from the mass fraction of the OH ( data for all time-steps resulting in significant dimensionality reduction. The extracted features along with the ground truth labels are used to train the support vector machine (SVM) model for binary classification. The LBO indicator is defined as the output of the SVM model, 1 for unstable and 0 for stable. The LBO indicator stabilized to the value of 1 approximately 30 ms before complete blowout.

As wind energy technology continues to mature, farm sizes grow and wind farm layout design becomes more difficult, in part due to the number of design variables and constraints. Wind farm layout optimization is typically approached using gradient-free methods because of the highly multi-modal shape of the wind farm layout design space. Gradient-free method performance generally degrades with increasing problem size, making it difficult to find optimal layouts for larger wind farms. However, gradient-based optimization methods can effectively and efficiently solve large-scale problems with many variables and constraints. To pave the way for effective and efficient wind farm layout optimization for large-scale wind farms, we have worked to overcome the primary barriers to applying gradient-based optimization to wind farm layout optimization. To improve model/algorithm compatibility, we adjusted wake and wind farm models, adding more realistic curvature and smoothness to enable optimization algorithms to travel through areas in the design space where they had previously gotten stuck. We reduced the number of function calls required for gradient-based wind farm layout optimization by over three orders of magnitude for large farms by using algorithmic differentiation to compute derivatives. We reduced the multi-modality of the wind farm layout design space using wake expansion continuation (WEC). We developed WEC to work with existing optimization algorithms, enabling them to get out of local optima while remaining fully gradient-based. Across four case studies, WEC found results with lower wake loss, on average, than the other methods we tested. To resolve concerns about optimization algorithms exploiting model inaccuracies, we compared the initial and optimized layouts to large-eddy simulation (LES) results. The simple models predicted an AEP improvement of 7.7% for a low-TI case, and LES predicted 9.3%. For a high-TI case, the simple models predicted a 10.0% improvement in AEP and LES predicted 10.7%. To resolve uncertainty regarding relative solution quality for gradient-based and gradient-free methods, we collaborated with seven organizations to compare eight optimization methods. Each method was managed by researchers experienced with them. All methods found solutions of similar quality, with optimized wake loss between 15.48 % and 15.70 %. WEC with SNOPT was the only purely gradient-based method included and found the third-to-best solution.

[ES] La creciente preocupación sobre el efecto de la emisión de gases nocivos provenientes de motores de combustión interna alternativos (ICE) a la atmósfera ha llevado a los gobiernos a lo ancho del planeta a limitar la cantidad de dichas emisiones, particularmente en Europa a través de las normas EURO. La dificultad en cumplir dichas limitaciones ha llevado a la industria automovilística a cambiar el foco de motores de encendido por compresión (CI) o provocado (SI) hacia la electrificación o los combustibles libres de carbono. Sin embargo, esta transición no se puede llevar a cabo de manera sencilla en el corto y medio plazo, mientras que combustibles libres de carbono como el Hidrógeno (H2 ) o el Amoniaco (NH3 ) siguen produciendo algunos contaminantes como los Óxidos de Nitrógeno (NOx ), con los cuales hay que lidiar. Estas emisiones pueden ser particularmente dañinas para el ser humano ya que incrementan el riesgo de cáncer de pulmón. La Reducción Catalítica Selectiva (SCR) ha demostrado ser una tecnología eficaz para la reducción de este contaminante en particular. A través de una inyección de una Solución de Urea-Agua, junto con la energía térmica de los gases de escape, se genera una cantidad suficiente de NH 3 capaz de neutralizar los indeseados NOx en un catalizador de reducción. Con la inclusión de los SCR en automóviles ligeros además de su presencia tradicional en automóviles pesados, los SCR han sido el foco de la comunidad científica para mejorar el entendimiento de su principio de actuación, y mejorar su eficiencia en un entorno legislativo en el que los limites de emisión se han estrechado enormemente. Esta Tesis intenta ser parte de ese esfuerzo científico en caracterizar el proceso de inyección de UWS en su totalidad a través de un entorno computacional. El presente estudio tiene como objetivo proveer de un mejor entendimiento del proceso de atomización y degradación sufrido por los chorros de UWS. Las dinámicas no estacionarias que se dan lugar en la zonas cercana del chorro, añadido a la gran influencia de las características internas del inyector sobre el desarrollo del spray hacen que los métodos experimentales sean complicados para poder entender dicho proceso. Por otro lado, la Mecánica de Fluidos Computacional (CFD) presenta una alternativa. Para el propósito de esta Tesis, el CFD ha sido utilizado para caracterizar los sprays de SCR. Se intenta desarrollar y seleccionar los modelos más apropiados a chorros de baja velocidad, y establecer un conocimiento Una vez adquiridos dichos métodos, los mecanismos principales de rotura del chorro y de degradación de la urea se han analizan. En ese sentido, el uso de técnicas experimentales podrían ser sustituídos en el futuro para esta aplicación. Los métodos CFD son validados tanto en el campo cercano como en el lejano. Para el campo cercano, el tratamiento multi-fase se lleva a cabo a través de métodos de Modelo de Mezclas, o el método Volume-Of-Fluid. A través de ellos, la caracterización hidráulica de dos reconstrucciones del inyector de UWS se lleva a cabo. Subsiguientes análisis se llevan a cabo sobre las dinámicas de rotura de la vena líquida, descubriendo que mecanismos rigen el proceso. El estudio de campo lejano usa un Discrete Droplet Model (DDM) para lidiar con las fases líquidas y gaseosas. En él, la evaporación del agua y el proceso de termólisis de la urea han sido considerados y comparados con resultados experimentales con el fin de obtener una metodología fiel para su caracterización. Todo el conocimiento adquirido se aplica más tarde a un Close-Coupled SCR, en el cual condiciones de trabajo realista han sido consideradas. Además, una herramienta llamada Maximum Entropy Principle (MEP) es presentada. Por tanto, esta Tesis aporta una metodología valiosa capaz de predecir tanto el campo cercano como el lejano de chorros de UWS de una manera precisa. / [CA] La creixent preocupació sobre el efecte de l'emissió de gasos nocius provenients the motors de Combustió Interna Alternatius (ICE) a l'atmosfera ha dut als governs de tot el planeta a limitar la quantitat d'aquestes emisions, particularment a Europa mitjant les normes EURO. La dificultat de complir aquestes limitacions ha portat a l'industria automovilística a cambiar el focus de motors d'encedut per compresió (CI) o provocat (SI) cap a la electrificació o els combustibles lliures de carbó. No obstant això, aquesta transició no es pot dur a terme de manera senzilla , mentres que els combustibles lliures de carbó como l'Hidrogen (H2 ) o l'Amoniac (NH3 ) seguirien produint contaminants como els Óxids de Nitrogen (NOx ), amb els quals n'hi ha que bregar. Estes emissions poden ser particularment nocives per a l'esser humà ja que incrementen el risc de càncer de pulmó. La Reducció Catalítica Selectiva (SCR) ha demostrat ser una tecnología eficaç per a la reducció d'este contaminant en particular. Mitjançant una injecció d'una Solució D'Urea i Aigua, junt a l'energía térmica dels gasos d'fuita, es pot generar una quantitat suficiente de NH 3 capaç de neutralitzar els indesitjats NO x a un catalitzador de reducció. Amb l'inclusió dels SCR en automòvils lleugers a més de la seua tradicional presència en automòvils pesats, els SCR han segut el foc per a mijorar l'enteniment del seu principi d'actuació, i mijorar la seua eficiencia. Este estudi té como a objectiu proveir d'un mijor entenement del procés d'atomizació y degradació patit pels dolls de UWS. Les dinàmiques no estacionaries que es donen lloc en la zona propenca al doll, afegit a la gran influència de les característiques internes del injector sobre el desentroll de l'esprai, fan que els métods experimentals siguen complicats d'aplicar per entendre dit procés. Per un altre costat, la Mecànica de Fluïts Computacional (CFD) supon una alternativa que té certes avantatges. Per al propòsit d'esta Tesi, el CFD ha sigut utilitzat com la principal metodología per a caracteritzar elsesprais de SCR. Per mitjà de dits métodes, la Tesi vol desentrollar i seleccionar els models més apropiats que mitjos s'adapten a sprays de baixa velocitat, i establir un coneiximent per a posteriors estudis desentrollats sobre la mateixa temàtica. Una volta adquirits dits métodes, els mecanismes principals de trencament del doll, així com els de degradació de l'urea en amoníac s'analitzaran. En aquest sentit, l'us de técniques experimentals podría no ser utilitzat més en el futur per aquesta aplicació.Els métods CFD son aplicats i validats tant el el camp propenc com en el llunyà. Per al camp propenc, el tractament multi-component es porta a terme a través de métodes Eulerians-Eulerians, com el Model de Mescles, o el métode Volume-Of-Fluid. La caracterització hidràulica de dos reconstruccions de l'injector es porta a terme, els resultats del qual són comparats amb resultats experimentals. Subsegüents anàlisis es porten a terme sobre les dinàmiques de trencament de la vena líquida, descobrint qué mecanismes regeixen el procés. L'estudi de camp llunyà usa un Discrete Droplet Model (DDM) per a bregar en la fase líquida i gaseosa. En ell, l'evaporació del aigua y el procés de termòlisis de l'urea han sigut considerats i comparats amb el resultats experimentals amb la finalitat d'obtindre una metodología fidel per a la seua caracterització. Tot el coneixement obtingut s'aplica més tard a un Close-Coupled SCR, en el qual condicions de treball realistes han sigut considerades. Dels resultats obtinguts dels distints estudis, una ferramenta adicional anomenada Maximum Entropy Principle (MEP),capaç de predir el fenomen d'atomització dels doll de UWS sense la necessitat de realitzar simulacions del camp propenc, es presentat. Per tant, esta Tesi aporta una metodología capaç de predir tant el camp proper como el llunyà d'una manera precisa. / [EN] The increasing awareness of the effect of emitting harmful gases from Internal Combustion Engines (ICE) into the atmosphere has driven the governments across the globe to limit the amount of these emissions, par ticularly in Europe through the EURO norms. The difficulty to meet such limitations has driven the automotive industry to shift from traditional Compression Ignited (CI) or Spark Ignited (SI) engines toward electrification or carbon-free fuels. Nonetheless, this transition will not be easily done in the short and medium time frames, while carbon-free fuels such as Hydrogen (H2 ) and Ammonia (NH3 ) will keep producing certain pollutants such as Nitrogen Oxides (NOx ) which need taking care of. These emissions can be particularly hazardous for humans, increasing the risk of developing lung cancer. Selective Catalytic Reduction (SCR) is an effective technology for reducing this specific ICE contaminant. An injection of a Urea-Water Solution (UWS), together with the thermal energy of the combustion gases can generate a sufficient amount of NH 3 capable of neutralizing the unwanted NO x in a catalyst. With the fitting of SCR systems within light-duty applications, in addition to their traditional presence on heavy-duty usage, SCR has been on the focus to understand their working principle and improve their efficiency . This Thesis tries to become part of that scientific ensemble by characterizing the whole UWS injection process within a computational framework. The present research aims to provide a better understanding of the atomizing and degradation processes undergone by the UWS sprays. The transient dynamics taking place in the near-field region, added to the great influence of the inner-injector characteristics on the development of the spray make experimental approaches on such sprays challenging in providing such knowledge. Computational Fluid Dynamics (CFD) provide an alternative that has certain advantages. For this Thesis they have been adopted as the main methodology on characterizing SCR sprays. The Thesis tries to develop and select the appropriate models that best suit low-velocity sprays. With the suitable methods that best predict these sprays, the main jet breakup mechanisms, together with the urea-to-ammonia transformation will have their behavior analyzed. In that way, experimental techniques could be avoided for such applications. CFD is applied and validated both in the near-field and far-field regions. For the near-field, multi-component flows are treated through Eulerian-Eulerian such as the Mixture Model or the Volume-Of-Fluid method. Through them, a hydraulic characterization on two recon structions of the UWS injector is performed, with results compared with experimental data. Further analysis is done on the jet-to-droplet dynamics, assessing which mechanisms drove the process. The far-field analy sis uses a Discrete Droplet Model (DDM) for dealing with the gas and liquid phases. In it, the evaporation of water and the thermolysis process of the urea have been considered and again compared with experimental results to have a faithful methodology for its characterization. All the acquired knowledge has been later applied to a commercial Close-Coupled SCR, in which real-working conditions have been considered. From the results obtained from several studies, an additional tool called Maximum Entropy Principle (MEP), capable of predicting the UWS spray atomization phenomenon without the need to perform near-field simulations, has been provided. Accordingly, this Thesis provides a valuable methodology capable of predicting the near-field and far-field dynamics accurately thanks to its validation against experimental results from literature. Additionally, the MEP tool can be used independently for computational and experimental works to predict the performance of UWS atomizers.The work carried out presents a significant leap in the application of CFD tools in predicting low-velocity sprays. / Javier Marco Gimeno has been founded through a grant from the Government of Generalitat Valenciana with reference ACIF/2020/259 and financial support from the European Union. These same institutions, Government of Generalitat Valenciana and The European Union, supported through a grant for pre-doctoral stays out of the Comunitat Valenciana with reference CIBEFP/2021/11 the research carried out during the stay at Energy Systems, Argonne National Laboratory, United States of America. / Marco Gimeno, J. (2023). Computational study of Formation and Development of Liquid Jets in Low Injection Pressure Conditions. Focus on urea-water solution injection for exhaust gas aftertreatment [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/198699

[ES] El alto impacto del sector transporte respecto a las emisiones globales de CO2 y su efecto en el cambio climático ha llevado a que éste transite hacia tecnologías más eficientes y medioambientalmente sostenibles. Sin embargo, el ritmo de transformación es lento en relación a lo que se necesita para frenar el calentamiento global existente. En este sentido, en los últimos tiempos los caminos hacia la transformación se han diversificado; el concepto de "defossilization" ha surgido como alternativa a la descarbonización, ya que destaca la posibilidad de incluir una mayor cantidad de combustibles sintéticos y renovables, con los cuales se pueden obtener resultados igualmente efectivos. Dentro de estos, destacan los combustibles Polioximetileno dimetil éter (OMEn), su carácter oxigenado y no poseer enlaces carbono-carbono, los hace prometedores respecto a la formación de hollín. Además, presentan grandes similitudes y compatibilidades con el diésel convencional, lo cual posibilita el uso de la flota de vehículos con motores de combustión interna existente a nivel mundial, acelerando así la transición y siendo una alternativa con alcance global. La presente tesis tiene como objetivo llevar a cabo un estudio fundamental sobre el proceso de combustión y la estructura de la llama de chorros tipo Diésel cuando se utilizan combustibles tipo OMEn. Para la consecución de dicho objetivo, la metodología planteada es eminentemente computacional, encontrando aquí las mayores brechas en la literatura. Se lleva a cabo un estudio de la cinética química y el efecto de la difusión en los combustibles estudiados mediante configuraciones canónicas, como reactores homogéneos y flamelets de contraflujo. Posteriormente, se estudia detalladamente el proceso de combustión y la estructura de la llama mediante el uso extensivo de Dinámica de fluidos computacional (CFD, en inglés), con modelos de turbulencia RANS y LES, en conjunto con un modelo de combustión avanzado basado en el concepto de flamelets, denominado UFPV. Todos los casos estudiados están definidos siguiendo las directrices de la Engine Combustion Network (ECN), los cuales representan chorros inyectados en ambientes quiescentes con toberas monoorificio. Particularmente, se evalúan los Sprays A y D, y el impacto de variar la temperatura ambiente. Como conclusión general, se puede afirmar que estos modelos CFD predicen correctamente el desarrollo de la combustión bajo las condiciones analizadas, y que estos combustibles son capaces de desarrollar diferentes estructuras de llama altamente dependientes de las condiciones de contorno impuestas. / [CA] L'alt impacte del sector del transport respecte a les emissions globals de CO2 i el seu efecte en el canvi climàtic ha portat a que aquest transite cap a tecnologies més eficients i mediambientalment sostenibles. No obstant això, el ritme de transformació és lent en relació amb el que es necessita per frenar l'escalfament global existent. En aquest sentit, en els últims temps els camins cap a la transformació s'han diversificat; el concepte de "defossilització" ha sorgit com a alternativa a la descarbonització, ja que destaca la possibilitat d'incloure una major quantitat de combustibles sintètics i renovables, amb els quals es poden obtenir resultats igualment efectius. Dins d'aquests, destaquen els combustibles tipus polioximetilen dimetil èters (OMEn), el seu caràcter oxigenat i al no posseir enllaços carbó-carbó, els fa prometedors respecte a la formació de sutge. A més, presenten grans semblances i compatibilitats amb el dièsel convencional, la qual cosa possibilita l'ús de la flota de vehicles amb motors de combustió interna existent a nivell mundial, accelerant així la transició i essent una alternativa amb abast global. La present tesi té com a objectiu dur a terme un estudi basic sobre el procés de combustió i l'estructura de la flama de dolls tipus Dièsel quan s'utilitzen combustibles tipus OMEn. Per a la consecució d'aquest objectiu, la metodologia plantejada és eminentment computacional, trobant ací les majors mancances en la literatura. Es realitza un estudi de la cinètica química i l'efecte de la difusió en els combustibles estudiats mitjançant configuracions canòniques, com ara reactors homogenis i flamelets de contraflux. Posteriorment, s'estudia detalladament el procés de combustió i l'estructura de la flama mitjançant l'ús extensiu de dinàmica de fluids computacional (CFD, en anglés), amb models de turbulència RANS i LES, conjuntament amb un model de combustió avançat basat en el concepte de flamelets, anomenat UFPV. Tots els casos estudiats estan definits seguint les directrius de l'Engine Combustion Network (ECN, en anglés), els quals representen dolls injectats en ambients quiescents amb toveres mono-orifici. Particularment, s'avaluen els Sprays A i D, i l'impacte de variar la temperatura ambient. Com a conclusió general, es pot afirmar que aquests models CFD prediuen correctament el desenvolupament de la combustió sota les condicions analitzades, i que aquests combustibles són capaços de desenvolupar diferents estructures de flama altament dependents de les condicions de contorn imposades. / [EN] The significant impact of the transportation sector on global CO2 emissions and its effect on climate change has led to a shift towards more efficient and environmentally sustainable technologies. However, the pace of this transformation is slow relative to what is needed to mitigate existing global warming. In this regard, pathways toward transformation have diversified recently, with the concept of defossilization emerging as an alternative to decarbonization. Defossilization emphasizes the possibility of incorporating a greater variety of synthetic and renewable fuels, which can yield equally effective results. Among these alternatives, Polyoxymethylene dimethyl ether ($OMEn$) fuels stand out due to their oxygenated character and absence of carbon-carbon bonds, making them promising in reducing soot formation. Furthermore, their similarities and compatibilities with conventional diesel enable the utilization of the existing global fleet of internal combustion engine vehicles, thus potentially accelerating the transition on a global scale. This thesis aims to conduct a fundamental study on the combustion process and flame structure of Diesel-like sprays when OMEn-type fuels are utilized. To achieve this objective, the proposed methodology is eminently computational, addressing significant gaps in the existing literature. A study of chemical kinetics and diffusion effects in the fuels under investigation uses canonical configurations such as homogeneous reactors and counterflow flamelets. Subsequently, the combustion process and flame structure are examined in detail through extensive Computational fluid dynamics (CFD) simulations, employing RANS and LES turbulence models in conjunction with an advanced combustion model based on the flamelet concept, UFPV. All studied cases are defined according to the Engine Combustion Network (ECN) guidelines, representing sprays injected into quiescent environments with single-hole nozzles. Specifically, Spray A and D are evaluated, along with the impact of varying ambient temperatures. In conclusion, it can be affirmed that the CFD models accurately predict combustion development under the analysed conditions, and these fuels can develop different flame structures highly dependent on the imposed boundary conditions. / The respondent wishes to acknowledge the financial support received through a grant from Vicerrectorado de Investigación of Universitat Politècnica de València with reference FPI UPV SUBP2 (PAID-01-20) / De León Ceriani, D. (2024). Evaluation by Detailed CFD Modelling of the Effect of Renewable Fuels on the Flame Structure under Compression Ignition Engine Conditions [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/207008