Development of modern safety standards for hydrogen storage infrastructure requires fundamental insight into the physics of buoyant gas dispersion into ambient air. Also, from a practical engineering stand-point, flow patterns and dispersion of gas originating from orifices in the side wall of circular pipe or storage tank need to be studied. In this thesis, novel configurations were considered to investigate the evolution of turbulent jets issuing from realistic pipeline geometries. First, the effect of jet densities and Reynolds numbers on vertical jets were investigated, as they emerged from the side wall of a circular pipe, through a round orifice. The resulting jet flow was thus issued through a curved surface from a source whose original velocity components were nearly perpendicular to the direction of the ensuing jets. Particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) techniques were employed simultaneously to provide instantaneous and time-averaged flow fields of velocity and concentration. The realistic flow arrangement resulted in an asymmetric flow pattern and a significant deflection from the vertical axis of jets. The deflection was influenced by buoyancy, where heavier gases deflected more than lighter gases. These realistic jets experienced faster velocity decay, and asymmetric jet spreading compared to round jets due to significant turbulent mixing in their near field. In addition to that, horizontal multi-component jets issuing from a round orifice on the side wall of a circular tube were also investigated experimentally by the means of simultaneous velocity and concentration measurements. A range of Reynolds numbers and gas densities were considered to study the effects of buoyancy and asymmetry on the resulting flow structure. The realistic pipeline jets were always exhibited an asymmetry structure and found to deflect about the jet's streamwise axis in the near field. In the far field, the buoyancy dominated much closer to the orifice than expected in the axisymmetric round jet due to the realistic leak geometry along with the pipeline orientation considered in this study. In general, significant differences were found between the centreline trajectory, spreading rate, and velocity decay of conventional horizontal round axisymmetric jets issuing through flat plates and the pipeline leak-representative jets considered in the present study. Finally, the dispersion of turbulent multi-component jets issuing from high-aspect-ratio slots on the side wall of a circular tube were studies experimentally by employing simultaneous PIV and PLIF techniques. Two transversal & longitudinal oblong geometries in respect to the longitudinal axes of the tube , and with an aspect ratio of 10 were considered in this study. Both horizontal and vertical orientations along with broad range of Reynolds numbers and gas densities were considered to investigate the effects of buoyancy and asymmetry on the resulting flow structure. The ensuing jets were found to deflect along the jet streamwise axis, once more, due to the realistic pipeline leak-representative configuration. It was also found that increases in aspect ratio of these realistic jets caused a reduction in the angle of deflection, jet centreline decay rates and the width growth on both velocity and scalar fields compared to their round jets counterparts, most notably in the far field. These findings indicate that conventional jets (those that are issuing through flat surfaces) assumptions are inadequate to predict gas concentration, entrainment rates and, consequently, the extent of the flammability envelope of realistic gas leaks. Thus, extreme caution is required when using conventional jet assumptions to describe the physics of a buoyant jet emitted from realistic geometries. / Graduate

Fundação de Amparo a Pesquisa do Estado de Minas Gerais / Esta dissertação é parte de um amplo projeto de pesquisa, que visa ao desenvolvimento de uma plataforma computacional de dinâmica dos fluidos (CFD) capaz de simular a física de escoamentos que envolvem mistura de várias espécies químicas, com reação e combustão, utilizando um método hibrido Simulação de Grandes Escalas (LES) / Função Densidade Filtrada (FDF) Lagrangiana em malha adaptativa, bloco-estruturada. Uma vez que escoamentos com mistura proporcionam fenômenos que podem ser correlacionados com a combustão em escoamentos turbulentos, uma visão global da fenomenologia de mistura foi apresentada e escoamentos fechados, laminar e turbulento, que envolvem mistura de duas espécies químicas inicialmente segregadas foram simulados utilizando o código de desenvolvimento interno AMR3D e o código recentemente desenvolvido FDF Lagrangiana de composição. A primeira etapa deste trabalho consistiu na criação de um modelo computacional de partículas estocásticas em ambiente de processamento distribuído. Isto foi alcançado com a construção de um mapa Lagrangiano paralelo, que pode gerenciar diferentes tipos de elementos lagrangianos, incluindo partículas estocásticas, particulados, sensores e nós computacionais intrínsecos dos métodos Fronteira Imersa e Acompanhamento de Interface. O mapa conecta informações Lagrangianas com a plataforma Euleriana do código AMR3D, no qual equações de trans- porte são resolvidas. O método FDF Lagrangiana de composição realiza cálculos algébricos sobre partículas estocásticas e provê campos de composição estatisticamente equivalentes aos obtidos quando se utiliza o método de Diferenças Finitas para solução de equações diferenciais parciais; a técnica de Monte Carlo foi utilizada para resolver um sistema derivado de equações diferenciais estocásticas (SDE). Os resultados concordaram com os benchmarks, que são simulações baseadas em plataforma de Diferenças Finitas para solução de uma equação de transporte de composição filtrada. / This master thesis is part of a wide research project, which aims at developing a com- putational fluid dynamics (CFD) framework able to simulate the physics of multiple-species mixing flows, with chemical reaction and combustion, using a hybrid Large Eddy Simulation (LES) / Lagrangian Filtered Density Function (FDF) method on adaptive, block-structured mesh. Since mixing flows provide phenomena that may be correlated with combustion in turbulent flows, we expose an overview of mixing phenomenology and simulated enclosed, ini- tially segregated two-species mixing flows, at laminar and turbulent states, using the in-house built AMR3D and the developed Lagrangian composition FDF codes. The first step towards this objective consisted of building a computational model of notional particles transport on distributed processing environment. We achieved it constructing a parallel Lagrangian map, which can hold different types of Lagrangian elements, including notional particles, particu- lates, sensors and computational nodes intrinsic to Immersed Boundary and Front Tracking methods. The map connects Lagrangian information with the Eulerian framework of the AMR3D code, in which transport equations are solved. The Lagrangian composition FDF method performs algebraic calculations over an ensemble of notional particles and provides composition fields statistically equivalent to those obtained by Finite Differences numerical solution of partially differential equations (PDE); we applied the Monte Carlo technique to solve a derived system of stochastic differential equations (SDE). The results agreed with the benchmarks, which are simulations based on Finite Differences framework to solve a filtered composition transport equation. / Mestre em Engenharia Mecânica

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.