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This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.

Lean, premixed combustion has enjoyed increased application due to the need to reduce pollutant emissions. Unfortunately, operating the flame at lean conditions increases susceptibility to thermoacoustic (TA) instability. Self-excited TA instabilities are a result of the coupling of the unsteady heat release rate of the flame with the acoustics of the combustion chamber. The result is large pressure oscillations that degrade performance and durability of combustion systems. Industry currently has no reliable tool to predict instabilities a priori. CFD simulations of full-scale, turbulent, reacting flows remain unrealizable. The work in this paper is part of a study that focuses on developing compact models of TA instabilities, i.e. acoustics and flame dynamics. Flame dynamics are defined as the response in heat release to acoustic perturbations. Models of flame dynamics can be coupled with models of combustor enclosure acoustics to predict TA instabilities. In addition, algorithms to actively control instabilities can be based on these compact models of flame dynamics and acoustics. The work outlined in this thesis aims at determining the flame dynamics model experimentally. Velocity perturbations are imparted on laminar and turbulent flames via a loudspeaker upstream of the flame. The response of the flame is observed through two measurements. Hydroxyl radical (OH*) chemiluminescence indicates the response in chemical reaction rate. Tunable Diode Laser Absorption Spectroscopy (TDLAS), centered over two water absorption features, allows a dynamic measurement of the product gas temperature. The response in product gas temperature directly relates to the enthalpy fluctuations that couple to the acoustics. Experimental frequency response functions of a laminar, flat-flame burner and a turbulent, swirl-stabilized combustor will be presented as well as empirical low-order models of flame dynamics. / Master of Science

Cette thèse a pour but l’étude de la rotation de l’écoulement des grandes échelles dans deux configurations. La première configuration se concentre sur l’effet de la rotation de l’écoulement sur une flamme laminaire. Elle est stabilisée dans le sillage d’un cylindre. La rotation de l’écoulement est introduite en faisant tourner le cylindre autour de son axe. La simulation numérique directe (Direct Numerical Simulation (DNS)) montre que la rotation du cylindre rompt la symétrie des deux branches de la flamme. La fonction de transfert de flamme (Flame Transfer Function (FTF)), obtenue grâce à l’inversion de Wiener-Hopf, indique qu’un faible taux de rotation réduit le gain de la FTF et donc la flamme devient presque insensible aux perturbations acoustiques à une fréquence donnée. De plus, il est démontré que cette diminution du gain est due à une interférence destructive des fluctuations de chaleur produites par les deux branches de la flamme. La fréquence à laquelle le gain de la FTF devient presque nul est ajustable par la vitesse de rotation du cylindre. Cette étude suggère que le contrôle de la symétrie de la flamme pourrait être un outil de contrôle en boucle ouverte des instabilités thermoacoustiques. Dans le cas de la deuxième configuration, la rotation de l’écoulement est induite par une instabilité hydrodynamique, aussi nommée Precessing Vortex Core (PVC) dans un système d’injection de carburant industriel. Des expériences et des simulations aux grandes échelles (Large Eddy Simulation (LES)) montrent que l’écoulement non-réactif dans l’injecteur pri- maire peut être décomposé en une contribution moyenne et un PVC. Cette instabilité hydro- dynamique est étudiée par l’analyse de stabilité linéaire (Linear Stability Analysis (LSA)) en utilisant deux approches différentes (locale et BiGlobale). Les résultats de l’expérience, de la LES et de la LSA démontrent que le montage d’une tige centrale à l’intérieur de l’injecteur stabilise le PVC. De plus, le même injecteur industriel est étudié dans le cas d’un écoule- ment réactif par LES. Les résultats démontrent que la flamme stabilise le PVC. L’analyse de stabilité BiGlobal montre que le gradient de densité dans le front moyen de la flamme a un effet important sur l’amortissement du PVC. Enfin, l’impact de la tige centrale est également étudié pour le cas réactif. La tige centrale impacte marginalement la forme globale de la flamme, mais a un effet positif sur l’accrochage de la flamme dans la zone de combustion pauvre. En comparant deux cas par LSA, celui avec la tige et celui sans la tige, les résultats suggèrent que la tige augmente considérablement l’amortissement du PVC. Cela pourrait causer une diminution de la turbulence dans l’écoulement et empêcher l’extinction de la flamme et donc expliquer l’influence bénéfique de la tige sur la stabilisation de la flamme. / This thesis investigates large scale flow rotation in two configurations. In the first, the effect of flow rotation on a laminar flame is investigated. The flame is anchored in the wake of a cylindrical bluff body. The flow rotation is introduced by turning the cylinder along its axis. It is shown by Direct Numerical Simulation (DNS), that the cylinder rotation breaks the symmetry of both flame branches. Flame Transfer Function (FTF) measurements performed by the Wiener-Hopf Inversion suggest, that low rotation rates lead to deep gaps in the gain and the flame becomes almost insensitive to acoustic perturbation at a specific frequency. It furthermore is demonstrated that this decrease in gain of the FTF is due to destructive interference of the heat release signals caused by the two flame branches. The frequency at which the gain becomes almost zero can be adjusted by tuning the cylinder rotation rate. The study suggests that controlling the symmetry of the flame could be a tool of open-loop control of thermoacoustic instabilities.

Combustor blowout is a very serious concern in modern land-based and aircraft engine combustors. The ability to sense blowout precursors can provide significant payoffs in engine reliability and life. The objective of this work is to characterize the blowout phenomenon and develop a sensing methodology which can detect and assess the proximity of a combustor to blowout by monitoring its acoustic signature, thus providing early warning before the actual blowout of the combustor. The first part of the work examines the blowout phenomenon in a piloted jet burner. As blowout was approached, the flame detached from one side of the burner and showed increased flame tip fluctuations, resulting in an increase in low frequency acoustics. Work was then focused on swirling combustion systems. Close to blowout, localized extinction/re-ignition events were observed, which manifested as bursts in the acoustic signal. These events increased in number and duration as the combustor approached blowout, resulting an increase in low frequency acoustics. A variety of spectral, wavelet and thresholding based approaches were developed to detect precursors to blowout. The third part of the study focused on a bluff body burner. It characterized the underlying flame dynamics near blowout in greater detail and related it to the observed acoustic emissions. Vorticity was found to play a significant role in the flame dynamics. The flame passed through two distinct stages prior to blowout. The first was associated with momentary strain levels that exceed the flames extinction strain rate, leading to flame holes. The second was due to large scale alteration of the fluid dynamics in the bluff body wake, leading to violent flapping of the flame front and even larger straining of the flame. This led to low frequency acoustic oscillations, of the order of von Karman vortex shedding. This manifested as an abrupt increase in combustion noise spectra at 40-100 Hz very close to blowout. Finally, work was also done to improve the robustness of lean blowout detection by developing integration techniques that combined data from acoustic and optical sensors.

Lean, premixed combustion offers a practical approach for reducing nitrogen oxide (NOx) emissions, but increases the risk of lean blowout (LBO) in gas turbines. Active control techniques are therefore sought which can stabilize a lean flame and prevent LBO. The present work has resulted in the development of flame detection, dynamic modeling, blowout margin estimation, and actuation and control techniques. The flame s acoustic emissions were bandpass filtered at select frequencies to detect localized extinction events, which were found to increase in number near LBO. The lean flame was also found to intermittently burst into a transient tornado configuration in which the flame s inner recirculation zone would collapse. The localized extinctions were dynamically linked to the tornado bursts using a linear, first order model. The model was subsequently applied to predict tornado bursts based on optically detected localized extinction events. It was found that both localized extinctions and tornado bursts are by themselves Poisson processes; the exponential distribution of their spacing times could be used to determine blowout probability. Blowout mitigation was achieved by redistributing the fuel flow between the annular swirlers and central preinjection pilot, both of which were premixed. Rule-based and lead-lag control architectures were developed and validated.

Modeling the Response of Premixed Flames to Flow Disturbances Preetham 178 pages Directed by Dr. Tim Lieuwen Low emissions combustion systems for land based gas turbines rely on a premixed or partially premixed combustion process. These systems are exceptionally prone to combustion instabilities which are destructive to hardware and adversely affect performance and emissions. The success of dynamics prediction codes is critically dependent on the heat release model which couples the flame dynamics to the system acoustics. So the principal objective of the current research work is to predict the heat release response of premixed flames and to isolate the key non-dimensional parameters which characterize its linear and nonlinear dynamics. Explicit analytical solutions of the G- equation are derived in the linear and weakly nonlinear regime using the Small Perturbation Method (SPM). For the fully nonlinear case, the flame-flow interaction effects are captured by developing an unsteady, compressible, coupled Euler-G-equation solver with a Ghost Fluid Method (GFM) module for applying the jump conditions across the flame. The flame s nonlinear response is shown to exhibit two qualitatively different behaviors. Depending on the operating conditions and the disturbance field characteristics, it is shown that a combustor may exhibit supercritical bifurcations leading to a single stable limit cycle amplitude or exhibit sub-critical bifurcations wherein multiple stable solutions for the instability amplitude are possible. In addition, this study presents the first analytical model which captures the effects of unsteady flame stretch on the heat release response and thus extends the applicability of current models to high frequency instabilities, such as occurring during screech. It is shown that unsteady stretch effects, negligible at low frequencies (100 s of Hz) become significant at screeching frequencies (1000 s of Hz). Furthermore, the analysis also yields insight into the significant spatial dependence of the mean and perturbation velocity field induced by the coupling between the flame and the flow field. In order to meaningfully compare the heat release response across different flame configurations, this study has identified that the reference velocity (for defining the transfer function) should be based on the effective normal velocity perturbing the flame and the Strouhal number should be based on the effective residence time of the flame wrinkles.

This thesis focuses on experimentally measuring the response of varying dilatation ratio bluff body flames under harmonic excitation. Such flames are often encountered in jet engine afterburners and are susceptible to combustion instabilities. Previous work has been done modeling such flames, however, only limited experimental data has been obtained at these conditions and is the motivation for this thesis. The focus of this work is to measure the transfer function of longitudinally forced, varying dilatation ratio bluff body flames. The transfer function is obtained by measuring flame position and flame luminosity fluctuations at the forcing frequency. Specifically, the amplitude and phase of the fluctuations are characterized as a function of flow velocity, axial location, and perturbation amplitude. These measurements are also compared to available theoretical predictions, showing that qualitative measured trends are consistent with theory. In addition, a detailed quantitative comparison is performed at one condition, showing good agreement between predictions and measurements in the near and mid-field of the flame response. However, agreement is not obtained in the far-field, indicating that continued theoretical work is needed to understand the flame response characteristics in this region.

Modern combustion systems benefit from constant technological advanceswhich aim at reducing the emissions of chemical pollutants and at wideningregimes of stable operation. Further progress in the combustion field requiresa better understanding and modelling of the combustion dynamics. In thesesystems, the combustible is often injected as a liquid polydisperse spray. Experimentaldata are thus required to validate simulation tools in configurationswith flames interacting with controlled structures in multi-phase flows.This thesis aims at studying some of these fundamental interactions in wellcontrolledlaminar flows submitted to upstream modulations. Two experimentalconfigurations are investigated comprising counterflow flames and free inertjets, fed with gaseous or liquid combustibles. The flows may be submittedto upstream velocity modulations to reproduce effects of unsteadiness. Dependingon the pulsation frequency, vortices of controlled sizes are shed fromthe burner lips and convected with the flow, while interacting with the sprayand the flame.In the first part of this thesis, the dynamics of a premixed stretched flameis analysed in a stagnation flow. The study focuses on determining the flowand flame structures under upstream modulations, and principally on studyingthe dynamics of flame/vortex interactions. Different responses of the flameare identified and analysed relative to the size of the vortex ring generated atthe burner outlet. Two propagation modes for the velocity perturbations areidentified, corresponding to a bulk oscillation of the entire reaction zone orto a flame perturbed only at its periphery. This leads to a discussion on thechoice of velocity boundary conditions to conduct 1D simulations of theseconfigurations. Comparisons between simulations and measurements of thevelocity field illustrate these conclusions. Flame transfer functions betweenheat release rate and velocity perturbations imposed at the burner outlet areestablished for different flow conditions. These measurements relying on localand global chemiluminescence of the flame show again a distinct behaviourof the emission originating from the flame region close to the burner axis andthe whole flame. Mechanisms of sound production by partially and perfectlypremixed flames are also identified and analysed relative to flame/vortex interactions.In the second part, the dynamics of a spray convected by a free inert jet or impinginga diffusion flame submitted to velocity modulations is analysed. Theoriginality of this work consists in characterizing the flow and spray dynamicsusing a set of advanced diagnostics. Phase-conditioned images at different instantsin the modulation cycle are used to analyse the interactions between thegaseous phase and the spray. The spatial distribution of combustible vapourand liquid phases is determined using Laser Induced Exciplex Fluorescence(LIEF). Velocities and sizes distribution of droplets from the spray are determinedlocally by Phase Doppler Anemometry (PDA) and in a plane by InterferometricParticle Imaging (IPI). Laser Doppler Velocimetry (LDV) andParticle Image Velocimetry (PIV) are also used to determine the response ofgaseous phase. These phase-conditioned analysis highlight some interactionsbetween the gaseous and liquid phases and constitute an interesting databasefor detailed simulation of these two-phase flows.

La réduction des émissions de polluants et l’augmentation du rendement des moteurs ont conduit à une large utilisation de régimes de combustion pauvres en carburant dans les foyers de type moteurs aéronautiques et turbines à gaz. Des phénomènes de bruit et d’instabilités de combustion peuvent alors apparaître. Des fluctuations cycliques auto-entretenues de la pression au sein d’un foyer peuvent conduire à une limitation des régimes de fonctionnement ou une usure rapide et indésirable des installations et dans certains cas une destruction du système. L’objectif de ce travail de thèse est d’étudier les mécanismes responsables du bruit de combustion et des instabilités dans un foyer turbulent prémélangé swirlé. L’étude repose sur une analyse du champ de pression au sein du foyer, de la dynamique de la combustion et une caractérisation détaillée des conditions limites en amont, aval et dans les lignes d’alimentation en combustible et en comburant. Le banc expérimental CESAM ("Combustion Étagée Swirlée Acoustiquement Maîtrisée") est utilisé au cours de ce travail. Basée sur des observations expérimentales, une étude théorique de l’acoustique du foyer est tout d’abord réalisée grâce à un modèle à deux cavités couplées qui modélisent le tube de prémélange et la chambre de combustion de ce banc. Les fréquences et les structures spatiales des modes propres du foyer sont examinées, et des comparaisons sont menées avec les résultats expérimentaux. La condition limite au fond du tube de prémélange est mesurée, et utilisée comme entrée dans le modèle. L’effet de cette condition sur la prévision des fréquences des modes propres est examiné. Par la suite, le code de calcul AVSP est utilisé pour valider les résultats obtenus avec le modèle couplé. L’interaction entre ces modes acoustiques et la flamme est mise en évidence en caractérisant la dynamique de l’écoulement réactif. La vélocimétrie par images de particules (PIV) à haute cadence est utilisée. Une première étude est menée sur les champs de vitesse moyens et fluctuants puis on s’intéresse à l’analyse spectrale des champs de vitesse instantanés, rendue possible par la haute cadence du diagnostic. Un post-traitement faisant intervenir une méthode de détection des tourbillons est ensuite mis en oeuvre en utilisant le critère _2. Des structures cohérentes sont convectées le long du front de flamme à la fréquence du second mode instable du foyer. Le chapitre précédent ayant permis de montrer que ce mode acoustique était essentiellement associé au tube de prémélange, le mécanisme de couplage est clairement identifié. Par la suite, un traitement en moyenne de phase est appliqué aux champs de vitesse axiale. Des mouvements de battements des bras de la flamme dans les directions longitudinale et transverse sont mis en évidence aux fréquences des modes instables. L’émission naturelle de la flamme est également mesurée avec une caméra rapide. Une analyse spectrale et un traitement en moyenne phase avec transformée d’Abel sont appliqués aux images pour caractériser les régions de la flamme présentant une forte réponse aux fréquences des modes acoustiques du foyer. Les mécanismes à l’origine du bruit sont analysés en corrélant les mesures optiques et acoustiques. Au cours de cette étude, des fonctions de transfert de flamme FTF sont également caractérisées aux fréquences des modes propres du foyer, liant perturbations amont et réponse de flamme. La vitesse acoustique est reconstruite dans le tube de prémélange à partir des mesures des microphones. La FTF est calculée grâce aux mesures de vitesse par PIV, à l’émission des radicaux OH* et CH* et à l’émission naturelle de la flamme obtenue par caméra rapide. La caractérisation et la modélisation du système composé du tube de prémélange et de la chambre de combustion montrent qu’il est nécessaire de s’intéresser à l’influence des conditions aux limites sur les propriétés de la flamme et la stabilité du brûleur. / Lean premixed combustion is widely used to limit pollutant emissions and improve efficiency. However in this situation combustion instabilities and associated noise may occur. The growth of self-sustained pressure fluctuations within the combustor may limit the operating conditions and eventually damage the installation. The objective of this work is to study the mechanisms induced in combustion noise and instabilities in a turbulent premixed swirled burner. The study is based on a detailed analysis of the pressure field of the combustor, the flame dynamics and a characterization of the upstream and downstream acoustic boundary conditions and in the air and fuel feeding lines. Based upon experimental investigations, a theoretical study of the burner acoustics is carried out using a low-order model with two coupled cavities. The eigenfrequencies and spatial distribution of the pressure field are obtained, allowing comparisons with experimental results. The impact of the inlet acoustic impedance on the prediction of the eigenmodes is examined through the use of the measured impedance in the model. Thereafter calculations with the AVSP Helmholtz code are carried out to confirm the results obtained with the loworder model. The interactions between the burner acoustic modes and the flame are investigated and the reacting flow dynamics is characterized, using High Speed Particle Image Velocimetry HSPIV at 15 kHz. A first analysis concerns the mean and fluctuating velocity fields and a spectral analysis of the collection of instantaneous velocity fields is carried out. Then a method based on the _2 criterion is used to detect vortices, showing that coherent structures are convected through the flame front at the frequency of the second unstable combustor mode. It is shown in the previous chapter that this mode is essentially associated with the premixer acoustics, allowing a clear coupling scenario between the acoustics and the flame. A phase locked averaging method is applied to the axial velocity fields. Flapping motions of the flame branches are highlighted in longitudinal and transverse directions at the unstable modes frequencies. The natural light emission from the flame is also measured using a fast camera. Spectral analysis and phase locked averaging with Abel transform are applied to images in order to determine the flame regions where a strong response is visible at the acoustic modes. Mechanisms underlying combustion noise are analyzed by correlating the optical and acoustic measurements. Flame transfer functions FTF are also characterized between upstream disturbances and the flame response at the combustor eigenfrequencies. Acoustic velocity is reconstructed in the premixer using microphones measurements. The FTF is calculated using PIV velocity fields, OH* or CH* intensities and flame natural light emissions measurements. Measurements and modeling show that boundary conditions play a crucial role in the burner stability. The acoustic impedance at the premixer inlet can be modified using an impedance control system (ICS). Thus, the pressure field and flame dynamics are characterized for different boundary conditions imposed by the ICS. The acoustic boundary conditions in the feeding lines are characterized using an Impedance Measurement Device (IMD) equipped with microphones and mounted within the supplies.