LLean premixed combustion systems, attractive for low NOx performance, are inherently susceptible to thermo-acoustic instabilities - the interaction between unsteady heat release and excited acoustic wave effects. In the present work, a hybrid, coupled Large Eddy Simulation (LES) CFD approach is described, combining the computational efficiency of incompressible reacting LES with acoustic wave effects captured via an acoustic network model. A flamelet approach with an algebraic Flame Surface Density (FSD) combustion model was used. The ORACLES experiments - a perfectly premixed flame stabilised in a 3D sudden expansion - are used for validation. Simulations of the inert flow agree very well with experimental data, reproducing the measured amplitude and distribution of turbulent fluctuations as well as capturing the asymmetric mean flow. With reaction the measured data exhibit a plane wave acoustic mode at 50Hz. The influence of this plane wave must be incorporated into the LES calculation. Thus, a new approach to sensitise the incompressible LES CFD to acoustic waves is adopted. First an acoustic network model of the experimental geometry is analysed to predict the amplitude of the 50Hz mode just before the flame zone. This is then used to introduce a coherent plane wave at the LES inlet plane at the appropriate amplitude, unlike previous LES studies, which have adopted a "guess and adjust" approach. Incompressible LES predictions of this forced flow then show good agreement with measurements of mean and turbulent velocity, as well as for flame shape, with a considerable improvement relative to unforced simulations. To capitalise on the unsteady flame dynamics provided by LES, simulations with varying forcing amplitude were conducted and analysed. Amplitude dependent Flame Transfer Functions (FTFs) were extracted and fed into an acoustic network model. This allowed prediction of the stable/unstable nature of the flame at each forcing amplitude. An amplitude at which the flame changed from unstable to stable would be an indication that this coupled approach was capable of predicting a limit cycle behaviour. With the current simple FSD combustion model almost all cases studied showed a stable flame. Predictions showed considerable sensitivity to the value chosen for the combustion model parameter but specially to the acoustic geometric configuration and boundary conditions assumed showing evidence of limit cycle behaviour for some combinations. Nevertheless, further work is required to improve both combustion model and the accuracy of acoustic configuration and boundary condition specification.

The utilization of lean premixed combustors has become attractive to designers of industrial gas turbines as a means of meeting strict emissions standards without compromising efficiency.  Mixing the fuel and air prior to combustion allows for lower temperature flame zones, creating the potential for drastically reduced nitrous oxide emissions.  While effective, these systems are commonly plagued by combustion driven instabilities.  These instabilities produce large pressure and heat release rate fluctuations due to a resonant interaction between the combustor acoustics and the flame.  A primary feedback mechanism responsible for driving these systems is the propagation of Fuel/Air Ratio (FAR) fluctuations into the flame zone.  These fluctuations are formed inside of the premixing chamber when fuel is injected into and mixed with an oscillating air flow. The research presented here aimed to develop new technology for premixer designs, along with an application strategy, to avoid resonant thermo-acoustic events driven by FAR fluctuations.  A passive fuel control technique was selected for investigation and implementation. The selected technique utilized fuel injections at multiple, strategically placed axial locations to target and inhibit FAR fluctuations at the dominant resonant mode of the combustor.  The goal of this research was to provide an understanding of the mixing response inside a realistic premixer geometry and investigate the effectiveness of the proposed suppression technique. The mixing response was investigated under non-reacting flow conditions using a unique modular premixer.  The premixer incorporated variable axial fuel injection locations, as well as interchangeable mixing chamber geometries.  Two different chamber designs were tested: a simple annular chamber and one incorporating an axial swirler.  The mixing response of the simple annular geometry was well characterized, and it was found that multiple injections could be effectively configured to suppress the onset of an unstable event at very lean conditions. Energy dense flame zones produced at higher equivalence ratios, however, were found to be uncontrollable using this technique. Additionally, the mixing response of the swirl geometry was difficult to predict. This was found to be the result of large spatial gradients formed in the dynamic velocity field as acoustic waves passed through the swirl vanes. / Ph. D.

Les contraintes économiques, environnementales et sociétales de ces vingt dernières années notamment dans les domaines de l’énergie et des transports ont débouché sur le développement de nouvelles technologies faisant intervenir la combustion pauvre et prémélangée. Ce mode de combustion à partir d'un mélange homogène conduit à des températures de flamme plus faibles qui permettent de réduire les émissions d'oxydes d'azote tout en limitant la production d'oxydes de carbone. Pour autant, la combustion pauvre prémélangée présente le désavantage d’être sensible à toute forme de couplage notamment acoustique, menant à des instabilités de combustion. Ces instabilités sont largement étudiées, mais restent très difficiles à prévoir car elles font intervenir de nombreux phénomènes physiques multi-échelles. Dans la plupart des cas les oscillations résultent d’un couplage résonant entre la dynamique de la combustion et l’acoustique du système. Les conditions aux limites acoustiques du système déterminent la structure du champ de pression dans l’installation, ainsi que les flux acoustiques entrants et sortants. Malgré son importance, l’influence des conditions aux limites n’est pas toujours bien comprise et prise en compte et elle ne fait pas l’objet d’études systématiques. Les conditions aux limites acoustiques ne sont pas faciles à déterminer expérimentalement sur des configurations pratiques et leur contrôle est rarement envisagé. L’objectif de ce travail est donc de répondre à ce manque d’information, en étudiant sur un banc de combustion turbulente (CTRL-Z) l’influence de la condition acoustique d’entrée sur les oscillations de combustion auto-entretenues qui apparaissent dans la chambre de combustion. Un système de contrôle a été développé pour piloter l’impédance du système de prémélange de façon passive, sans modification des conditions de fonctionnement ou de la géométrie du brûleur. Ce système de contrôle d’impédance (ICS, « Impedance Control System ») s’appuie sur une utilisation de plaques perforées faiblement poreuses, au travers desquelles circule un écoulement. Un piston mobile permet de piloter la profondeur de la cavité résonante formée en amont des plaques, et ainsi de piloter leurs impédances. L’impédance de ces plaques perforées a été étudiée pour de faibles et de forts niveaux d’excitation acoustique, et un critère de transition entre les régimes linéaire et non-linéaire a été déterminé. L’ICS a été optimisé pour permettre un contrôle du module du coefficient de réflexion de 0 à 1 sur une large plage de fréquences (100 à 1000 Hz) et de niveaux d’amplitude de perturbations (100 à 150 dB) couvrant ainsi la gamme des instabilités thermoacoustiques classiques. L’ICS est utilisé pour contrôler l’impédance d’entrée du système de prémélange du banc CTRL-Z, en regard de la zone de combustion. L’analyse spectrale des fluctuations de pression et de dégagement de chaleur en fonction de l’impédance d’entrée démontre qu’il est possible d’obtenir un amortissement de l’instabilité principale pouvant atteindre 20 dB. Ces résultats sont confirmés par une estimation au premier ordre d’un bilan d’énergie acoustique prenant en compte le terme source dû à la combustion ainsi que les flux acoustiques en amont et aval de la zone de flamme. Ce bilan démontre par ailleurs l’importance du flux d’énergie transmis vers l’amont, du même ordre de grandeur que le terme source, et souligne la nécessité de prendre en compte ces flux pour déterminer correctement le taux de croissance de l’énergie. Finalement, une analyse acoustique de l'installation a été menée pour déterminer la nature des modes d'instabilités observés et pour examiner les conditions nécessaires au bon fonctionnement de l'ICS. / Combustion instabilities induced by a resonant flame-acoustic coupling are commonly observed in most applications of combustion from gas turbines to domestic or industrial boilers. These oscillations are detrimental by nature, and are still very difficult to predict at the design stage of a combustor. They imply numerous physical phenomena at multiple scales. They mainly result from a resonant coupling between the unsteady combustion and the acoustics of the system. The basic driving and coupling mechanisms have been extensively studied: acoustics in complex geometries and combustion dynamics of turbulent swirled flames are now reasonably well understood. However the effects of the acoustic boundary conditions on the system stability are less well documented, as they are not easy to access or to control in practical systems. They are however of prime importance as they determine the acoustic fluxes at the inlets and outlets of the combustor, as well as the preferential eigenfrequencies of the system. The main objective of this study is to investigate experimentally the influence of the inlet boundary condition of a generic turbulent burner on the observed self-sustained thermoacoustic oscillations. To carry out this investigation, a passive control solution has been developed. An innovative use of perforated panels with bias flow backed by tunable cavities allows to control the acoustic impedance at the inlet of a lean swirled-stabilized staged combustor (CTRL-Z facility). This impedance control system (ICS) has been initially designed and tested in a high load impedance tube. This facility also allowed to develop a robust impedance measurement technique, along with experimental protocols to measure acoustic velocities and fluxes. The acoustic response of perforates in both linear and nonlinear regimes was investigated as function of the plate porosity, bias flow velocity, back-cavity depth and incident pressure wave amplitude and frequency. The transition between the linear regime and the detrimental nonlinear regime has been linked to the perforates geometrical and operational parameters. As a result the ICS enables control of its acoustic reflection coefficient from 1 to 0 in a wide frequency range, 100 to 1000 Hz, for low and large incident pressure amplitudes (from 100 to 150 dB). The ICS, once implemented on the CTRL-Z facility, allowed to passively control the inlet boundary condition of the combustion rig. The impedance measurement technique was successfully used in harsh combustion situations, with high noise levels, to obtain in-situ measurements of the ICS impedance. Spectral analysis of the pressure and heat-release rate fluctuations demonstrated damping of the main self-sustained oscillation by up to 20 dB. A quantitative estimation of the acoustic energy balance was then obtained, highlighting the importance of the inlet acoustic flux. In this configuration, this term is of the same order of magnitude as the driving Rayleigh source term. Finally, an acoustic analysis of the combustion rig was led to determine the nature of the observed combustion instabilities modes and examine conditions required for an effective use of the ICS.

Les instabilités thermo-acoustiques causent des problèmes récurrents dans les chambres de combustion pour une large gamme d'applications industrielles, allant des chaudières domestiques aux turbines à gaz, en passant par les moteurs fusées. Ces phénomènes résultent d’un couplage résonant entre la dynamique de la combustion et les modes acoustiques du foyer, et peuvent donner lieu à de fortes vibrations, un vieillissement prématuré des composants de la chambre, voire des dommages structurels. Les mécanismes physiques mis en jeu sont complexes et difficiles à modéliser, ainsi les oscillations thermo-acoustiques ne sont pas facilement prévisibles au stade de la conception d’une chambre de combustion. Dans de nombreux foyers, des systèmes d’amortissement passifs sont installés pour augmenter la dissipation d’énergie acoustique et empêcher le développement de ces instabilités. Dans ce travail, des systèmes d’amortissement basés sur des plaques perforées couplées à une cavité résonante et traversées par un écoulement moyen sont analysés. Les principaux objectifs sont : (i) d’améliorer et de simplifier la conception de systèmes d’amortissement robustes en maximisant leurs propriétés d’absorption acoustique en régime linéaire, (ii) d’analyser l’effet de l’amplitude des ondes sonores incidentes sur la réponse acoustique des plaques perforées et (iii) de développer des modèles capables de reproduire cette réponse aux hautes amplitudes. Tout d’abord, deux régimes asymptotiques intéressants sont identifiés où le système fonctionne à faibles et forts nombres de Strouhal respectivement. Dans ces régimes la conception d’un système d’amortissement maximisant l’absorption acoustique est grandement simplifiée, puisque les calculs de la vitesse optimale de l’écoulement et de la taille de la cavité sont découplés. Il est démontré qu’à faible nombre de Strouhal le système se comporte comme un résonateur quart d’onde, et dispose d’une bande d’absorption très large. À fort nombre de Strouhal, le système fonctionne comme un résonateur de Helmholtz, comportant une cavité de taille plus réduite, mais une bande d’absorption beaucoup plus étroite que dans le régime précédent. Ces prévisions sont confirmées par des mesures réalisées dans les différents régimes identifiés sur un dispositif expérimental dédié. L’évolution des propriétés acoustiques d’une plaque perforée lorsque l’amplitude de forçage augmente est ensuite examinée par le biais de simulations directes. Il est montré que la transition du régime linéaire au régime non linéaire se produit lorsque l’amplitude de la vitesse acoustique dans l’orifice est comparable à la vitesse de l’écoulement moyen dans les trous. Pour des amplitudes élevées, une inversion périodique de l’écoulement est observée dans l’orifice. Des anneaux tourbillonnaires sont alternativement éjectés en amont et en aval de l’orifice à une vitesse de convection qui augmente avec l’amplitude de la perturbation acoustique. Ces mécanismes influencent profondément l’absorption acoustique des plaques perforées dans le régime non linéaire. Deux nouveaux modèles décrivant la réponse non linéaire de ces systèmes sont ensuite développés en exploitant la trajectoire des vortex (modèle VC), et une approche quasi-stationnaire (modèle IDF). Les prévisions de ces modèles sont confrontées à des mesures effectuées dans le tube à impédance et aux résultats de simulations directes. Les résultats obtenus au cours de ces travaux peuvent être utilisés pour guider la conception de systèmes d’absorption robustes, capables de fonctionner dans des environnements difficiles avec des niveaux sonores élevés, comme ceux rencontrés lors d’instabilités thermo-acoustiques. / Thermo-acoustic instabilities are of primary concern in combustion chambers for a wide range of industrial applications, from domestic boiler to gas turbines or rocket engines. They are the consequence of a resonant coupling between the flame dynamics and the acoustic modes of the combustor, and can result in strong vibrations, early aging of combustor components and structural damage. The physical mechanisms involved are complex and difficult to model, thus thermo-acoustic oscillations are not easily predictable at the design stage of a combustor. In many combustors, passive dampers are implemented to increase the acoustic energy dissipation of the system and to hinder detrimental flame-acoustics interactions. In the present work, passive damping systems based on perforated screens backed by a resonant cavity and traversed by a bias flow are investigated. The main objectives are: (i) to improve and simplify the design of these dampers by maximizing their acoustic absorption properties in the linear regime, (ii) to analyze the effect of the sound wave amplitude on the acoustic response of these systems and (iii) to develop models capable of capturing absorption at high oscillation amplitudes. First, two interesting asymptotic regimes are identified where the plate operates at low and high Strouhal numbers respectively. In these regimes the design of a damper maximizing absorption is greatly simplified, since the choice of the optimal bias flow velocity and back cavity size can be decoupled. It is shown that at low Strouhal numbers the damper behaves as a quarter-wave resonator, and features a wide absorption bandwidth. At high Strouhal numbers, the system operates as a Helmholtz resonator, featuring shorter optimal back cavity sizes but narrower absorption bandwidths. These predictions are compared to measurements in a dedicated experimental setup for the different operating regimes identified. The dependence of the acoustic properties of a perforated plate on the forcing amplitude is then examined by means of direct numerical simulations. It is shown that transition from linear to nonlinear regimes occurs when the acoustic velocity amplitude in the orifice is comparable to the mean bias flow velocity. At high amplitudes, periodic flow reversal is observed within the perforation, vortex rings are alternatively shed upstream and downstream of the hole and convected away at a velocity which is increasing with the forcing amplitude. These mechanisms greatly influence the acoustic absorption of the perforate in the nonlinear regime. Two novel models capturing this nonlinear response are then inferred based on an analysis of the vortex trajectory (VC model), and on a quasi-steady description of the flow (IDF model). Their predictions are finally compared to measurements conducted in an impedance tube, and to results from numerical simulations. The results obtained in this work can be used to ease the design of robust dampers capable of operating in harsh environments with high sound levels, such as those found during self-sustained thermo-acoustic instabilities.

The evolution of injection processes toward more fuel efficient and less polluting combustion systems tend to make them more prone to critical events such as Thermo-Acoustic Instabilities, Blow-Off and Flash-Back. Moreover, the addition of Di-Hydrogen as a secondary or as the main fuel is in discussion by aeronautical engines manufacturers. It drastically modifies the stability of the system and thus raise several interrogations concerning the multiplicity of its use. Being able to predict critical phenomena becomes a necessity in order to efficiently operate a system without having to pre-test every configuration and without sacrificing the safety of the user. Based on Deep Learning techniques and more specifically Speech Recognition, the following study presents the steps to develop a tool able to successfully detect and translate precursors of instability of an aeronautical grade swirled injector confined in a tubular combustion chamber. The promising results obtained lead to proposals for future transpositions to real-size systems. / Utvecklingen av injektionsprocesser mot mer bränsleeffektiva och mindre förorenande förbränningssystem, tenderar att göra dem mer benägna att utsättas för kritiska händelser som Thermo-Acoustic Instabilities, Blow-Off och Flash-Back. Dessutom diskuterar flygmotorkonstruktörer möjligheten att använda Dihydrogen som sekundärt eller som huvudbränsle. Det modifierar drastiskt systemets stabilitet och det väcker frågan hur man kan använda det effektivt. Att kunna förutsäga kritiska fenomen blir en nödvändighet för att använda ett system utan att behöva att på förhand testa varje konfiguration och utan att reducera användarens säkerhet. Baserat på Deep-Learning-tekniker och Speech-Recognition-tekniker, presenterar följande studie stegen för att utveckla ett verktyg som kan upptäcka och översätta föregångare till instabilitet hos en swirled flygmotorerinsprutningspump som är innesluten i en förbränningskammare. De lovande resultaten leder till idéer om hur man kan anpassa det här verktyg till ett system i verklig storlek.

This last decade experienced a sudden increase of interest in the control of thermo-acoustic instabilities, in particular through the use of fuel modulation techniques. The primary goal of this research was to design, construct and characterize a high bandwidth proportional fuel injection system, which could be used to study the effect of specific levels of fuel modulation on the combustion process and the reduction of thermo-acoustic instabilities. A fuel injection system, incorporating the use of a closed loop piston and check valve, was designed to modulate the primary fuel supply of an atmospheric liquid-fueled swirl stabilized combustor operating at a mean volumetric fuel flow rate of 0.4 GPH. The ability of the fuel injection system to modulate the fuel was examined by measuring the fuel line pressure and the flow rate produced during operation. The authority of this modulation over the combustion process was investigated by examining the effect of fuel modulation on the combustor pressure and the heat release of the flame. Sinusoidal operation of the fuel injection system demonstrated: a bandwidth greater that 800 Hz, significant open loop authority (averaging 12 dB) with regards to the combustor pressure, significant open loop authority (averaging 33 dB) with regards to the unsteady heat release rate and an approximate 8 dB reduction of the combustor pressure oscillation present at 100 Hz, using a phase shift controller. It is possible to scale the closed loop piston and check valve configuration used to create the fuel injection system discussed in this work to realistic combustor operating conditions for further active combustion control studies. / Master of Science

Thermoacoustic instabilities are a problem in modern pre-mixed combustors causing reduced performance and leading in the extreme to combustor failure from excessive pressure cycles. Much work has been done using linear controllers to eliminate these instabilities. Many experimenters in the field have used pulsed and subharmonic fuel controllers to eliminate these instabilities, but very little investigative work has been done on these controllers. The goal of this work is to explain the mechanism of control behind pulsed controllers. It is shown that the combustion system can be treated as a linear system, thus meaning that frequency components of the control signal at the desired instability frequency are the dominant means of control, with nonlinear effects only serving to slightly reduce the gain necessary for control. Fourier analysis is thus performed on pulsed signals and the components analyzed, showing that there will indeed be a component of a pulsed signal at the frequency of the instability, aside from a few select cases. It is then proven that this frequency component is largely responsible for control of the thermoacoustic system using proportional height pulse train signals, which will change pulse height based on the amplitude of the instability. This analysis is then used to predict the height of instabilities resulting from the use of fixed height pulse control signals. Finally, numerical simulations and experimental observations support the analytical constructs. Acoustic control is mainly used for these experiments, although some preliminary work with liquid fuel controllers is also presented. / Master of Science

Le travail présenté concerne l'étude des instabilités thermo-acoustiques apparaissant dans une chambre de combustion aéronautique. Le montage expérimental permet de faire varier continument les fréquences de résonances et de passer pour une même condition d'écoulement, d'un régime de combustion stable à un régime instable. La caractérisation complète d'un cas instable comprend une mesure des conditions acoustiques aux limites du banc, une analyse du comportement des phases liquide et évaporée, de celui du dégagement de chaleur instationnaire et une mesure de la fonction de transfert de la flamme. Ces travaux ont mis en évidence trois phénomènes jouant sur l’injection cyclique de carburant liquide. Les temps caractéristiques des différents phénomènes intervenant dans le couplage thermo-acoustique et une distribution spatiale de l'indice de Rayleigh sont présentés. / The purpose of this experimental study was to further our understanding of the fuel spray behavior during combustion instability phenomena in combustion chambers. An aeronautical injection system with dual kerosene lines was mounted on the LOTAR setup, which was equipped with an adjustable exhaust length. Stability maps were generated by varying the global equivalent ratio and the fuel split parameter, for two Inner Exhaust Lengths (IEL). A non-unique multiphase flow condition was found to produce stable and unstable combustion for different IELs. Each configuration was fully characterized. Acoustic boundary conditions were measured using the 2-microphone technique. Different optical techniques were used toanalyze the unsteady behavior of the liquid phase, fuel vapor, and heat release. Moreover, two techniques were exploited to study the Flame Transfer Function using velocity measurement supstream and downstream of the injection device. Altogether, these results highlighted three atomization phenomena occurring during the cycles of thermo-acoustic instabilities. The phase-averaged analysis applied on the different measurements permitted to determine thetime scales associated with each process appearing in the thermo acoustic coupling. This cyclic injection of liquid fuel into the chamber was followed by a vapor phase increase corresponding to a wave of equivalent ratio. The delay between the two phenomena was of10°. In addition, OH* emissions showed a cyclic behavior following these waves. The delay between the wave of equivalent ratio and the unsteady heat release was approximately of 25°.Finally, spatial distribution of the Rayleigh index revealed that the inner recirculation zone contributed to sustain the combustion instability.

Dans la première partie de cette thèse de doctorat une méthodologie est présentée qui permet de prédire les niveaux de suies produits dans des flammes laminaires monodimensionnelles, ou un modèle semi-empirique de suies est utilisé en combinaison avec une chimie complexe et un solveur radiatif détaillé. La méthodologie est appliquée au calcul de suies dans une série de flammes de diffusion à contre-courant d'éthylène/air. Plusieurs modèles d'oxydation de suies sont testés et les constantes du modèle sont ajustées afin de retrouver un meilleur accord avec les expériences. L'effet des pertes thermiques radiatives sur la formation de suies et la structure des flammes est évalué. Finalement, la performance du modèle de suies est évalué sur des flammes prémélangées monodimensionnelles, ou une expression alternative du terme de croissance de surface est proposée pour reproduire les résultats expérimentaux. Dans la deuxième partie de cette thèse, des outils de Simulation aux Grandes Échelles (SGE) et d'analyse acoustique sont appliqués à la prédiction des oscillations de cycle limite (OCL) d'une instabilité thermo-acoustique qui apparaît dans un brûleur académique partiellement prémélangé de méthane/air à pression atmosphérique. La SGE prédit bien l'apparition et le développement des OCL est un bon accord est trouvé entre simulations et expériences en termes d'amplitude et fréquence des OCL. La simulation permet de révéler certains aspects clés responsables du comportement instable de la flamme. Ensuite, une analyse préliminaire de la quantification des incertitudes est fait, ou l'effet des paramètres tels que l'impédance des entrées, le degré de raffinement du maillage ou les pertes thermiques sur les caractéristiques des OCL est évalué. Aussi, la SGE prédit bien la dépendance de la stabilité de la flamme du point d'opération et de la géométrie du brûleur / In the first part of the present PhD. thesis a methodology is presented that allows to predict the soot produced in one-dimensional academic flames, where a semi-empirical soot model is used in combination with a complex chemistry and a detailed radiation solver. The methodology is applied to the computation of soot in a set of ethylene/air counterflow diffusion flames. Several oxidation models are tested and the constants of the model were adjusted to retrieve the experimental results. Also, the effect of radiative losses on soot formation and the flame structure is evaluated. Finally, the performance of the soot model is evaluated on 1D premixed flames, where an alternative expression for the surface growth term is proposed to better reproduce the experimental findings. In the second part of the thesis, Large-Eddy Simulation (LES) and acoustic analysis tools are applied to the prediction of limit cycle oscillations (LCO) of a thermo-acoustic instability appearing in a partially premixed methane/air academic burner operating at atmospheric pressure. The LES captures well the appearance and development of the LCO and a good agreement is found between simulations and experiments in terms of amplitude and frequency of the LCO. Some light is shed on the mechanisms leading to the existence of such instability. Then, a preliminar uncertainty quantification (UQ) analysis is performed, where the effect on the features of the LCO of several computational parameters such as the inlets impedances, mesh refinement or heat losses is assessed. Also, the LES captures well the flame stability behaviour dependence on the operating point and the burner geometry