Ce travail traite de la dynamique des flammes turbulentes prémélangées confinées et swirlées soumises à des perturbations de vitesses acoustiques. L'objectif général est d'acquérir une compréhension des mécanismes régissant la réponse de ces flammes et d'en tirer des méthodes de prévision des instabilités de combustion. Les écoulements swirlés sont d'abord examinés en termes de nombre de swirl et de nouvelles expressions sont données pour cette quantité. On traite notamment des effets de perturbations de vitesse et une expression est proposée qui tient compte des fluctuations de vitesses dans l'écoulement. Le système utilisé pour l'étude expérimentale comprend une cavité amont, un injecteur équipé d'un swirler et un tube à flamme transparent permettant la visualisation directe du mouvement de la flamme. Deux points de fonctionnement sont étudiés correspondant à des vitesses débitantes différentes. La cavité amont et le tube à flamme du brûleur peuvent être facilement changés pour étudier plusieurs configurations différentes. L'acoustique du brûleur est également analysée au moyen d'une approche de cavités couplées pour déterminer les fréquences de résonance du système en configuration non-réactive. Des expériences sont menées pour mesurer les fréquences propres du système et l'estimation du coefficient d'amortissement est réalisée à partir de la réponse du système à une modulation externe. Un critère de découplage des mode acoustiques est proposé. La dynamique de l'écoulement est examinée en termes de conversion de modes au niveau de la vrille (swirler) ou dans une grille d'aubes. Cette partie du travail, effectuée au moyen de simulations numériques montre que lorsqu'une grille ou une vrille sont soumis à une onde acoustique, le swirler donne naissance à une onde azimutale convective en plus de l'onde acoustique axiale transmise. Les deux types de swirlers, axial et radial, donnent lieu à ce mécanisme, un fait confirmé par des expériences. Il est montré que ce processus de conversion de mode a un impact important sur la dynamique de la flamme swirlée. La dynamique de la combustion est ensuite analysée en mesurant la fonction de transfert généralisée ainsi que les distributions de taux de dégagement de chaleur au cours du cycle d'oscillation. La fonction de transfert est utilisée pour déterminer la réponse de la flamme à des perturbations acoustiques se propageant dans l'écoulement en amont de la flamme. Il est aussi montré que le nombre de Strouhal est un groupe sans dimensions qui permet de caractériser la réponse de la flamme. La dynamique est également analysée au moyen d'un ensemble de diagnostics comprenant des sondes de pression, un photomultiplicateur et un vélocimètre laser Doppler. Un modèle pour la fonction de transfert linéaire de la flamme est dérivé théoriquement à partir d'une description de la flamme au moyen de l'équation pour une variable de champ G. Les mécanismes physiques de la réponse de la flamme sont identifiés : enroulement tourbillonnaire et fluctuations du nombre de swirl. L'enroulement tourbillonnaire est associé à l'onde acoustique transmise en aval du swirler et qui pénètre dans la chambre de combustion. Tandis que les fluctuations du nombre de swirl sont directement liées aux mécanismes de conversion de mode au swirler qui induit différentes vitesses pour les perturbations axiales et azimutales. L'enroulement tourbillonnaire enroule l'extrémité de la flamme tandis que les fluctuations du nombre de swirl agissent sur l'angle de la flamme. Ces deux mécanismes en compétition se combinent de manière constructive ou destructive conduisant à des gains faibles ou élevés dans la réponse de la flamme en fonction de la fréquence. Ces mécanismes sont retrouvés par simulation aux grandes échelles (LES). / This work is concerned with the dynamics of premixed confined turbulent swirling flames submitted to acoustic velocity disturbances. The general objective is to gain an understanding of the mechanisms governing the response of these flames and to derive predictive methods for combustion instabilities. Swirling flows are first reviewed in terms of swirl numbers and novel expressions for them are given. Perturbed form of the swirl number are suggested taking into account acoustic disturbances in the flow. The experimental system comprises an upstream manifold, an injector equipped with a swirler and a transparent flame tube allowing direct visualization of the flame motion. Two operating points are investigated corresponding to different bulk velocities. The upstream manifold and the flame tube of the burner can be easily change to test several configurations. The burner acoustic is also analyzed in term of coupled cavities approach to determined the resonant frequencies of the system in non reactive cases. Experiments are carried out to measure the system eigen frequencies and the estimate damping coefficient of the various burners arrangements. A criterion for decoupling acoustic mode is suggested. The flow dynamics is examined in terms of mode conversion occurring at the swirler or downstream an airfoil cascade. This part of the work, carried out with numerical simulations, shows that when submitted to an acoustic wave, a swirler gives rise to an azimuthal convective wave in addition to the transmitted acoustic wave. Both axial and radial swirlers are prone to this mechanism as confirmed by experiments. It is found that this mode conversion process has a strong impact on the flame dynamics in swirling flames combustors. Combustion dynamics is then analyzed by measuring the flame describing function (FDF) of this burner. This FDF is used to determine the response of the flame to acoustic velocity disturbances propagating on the upstream flow. It is shown that the Strouhal number is a suitable dimensionless group to characterize the swirling flame response. The flame dynamics is also analyzed with an ensemble of diagnostics including pressure probes, photomultipliers and laser Doppler velocimeter (LDV). A model for the linear swirling flame transfer function is derived theoretically. The physical mechanisms driving the response of the flame are identified : vortex rollup and swirl number fluctuations. The vortex rollup is associated to the acoustic wave transmitted downstream of the swirler and entering in the combustor while the swirl number fluctuations are directly linked to the mode conversion mechanisms downstream the swirler which induced different axial and azimuthal speeds upstream the flame. The rollup phenomena acts at the extremity of the flame while swirl number fluctuations act on the flame angle. These competiting mechanisms act constructively or destructively leading to low or high gains in the flame response depending on the frequency. These mechanisms are retrieved by large eddy simulations of the flame dynamics. Finally, an instability analysis is carried out by combining the experimental flame describing function (FDF) and an acoustic model of the combustor to determine the frequency and the amplitude of the velocity disturbances at the limit cycle. A good agreement between predictions and experiments is obtained in most cases indicating that the method is suitable subject to further developments.

The efficient and enhanced mixing of heat and incoming reactants is achieved in modern gas turbine systems by employing swirling flows. This is realized by a low velocity region (internal recirculation zone -IRZ) zone resulting from vortex breakdown phenomenon. Besides, IRZ acts as effective flame holder/stabilization mode. Double concentric swirling jet is employed in plethora of industrial applications such as heat exchange, spray drying and combustion. As such, understanding essential features of vortex breakdown induced IRZ and its acoustic response in swirling flow/flame is important in thermo-acoustic instability studies. The key results of the present experimental investigation are discussed in four parts. In the first part, primary transition (sub-critical states) from a pre-vortex breakdown (Pre-VB) flow reversal to a fully-developed central toroidal recirculation zone (CTRZ) in a non-reacting, double-concentric swirling jet configuration is discussed when the swirl number is varied in the range 0.592 S 0.801. This transition proceeds with the formation of two intermediate, critical flow regimes. First, a partially-penetrated vortex breakdown bubble (VBB) is formed that indicates the first occurrence of an enclosed structure resulting in an opposed flow stagnation region. Second, a metastable transition structure is formed that marks the collapse of inner mixing vortices. In this study, the time-averaged topological changes in the coherent recirculation structures are discussed based on the non-dimensional modified Rossby number (Rom) which appears to describe the spreading of the zone of swirl influence in different flow regimes. The second part describes a secondary transition from an open-bubble type axisymmetric vortex breakdown (sub-critical states) to partially-open bubble mode (super-critical states) through an intermediate, critical regime of conical sheet formation for flow modes Rom ≤ 1 is discussed when the swirl number (S) is increased beyond 0.801. In the third part, amplitude dependent acoustic response of above mentioned sub and supercritical flow states is discussed. It was observed that the global acoustic response of the sub-critical VB states was fundamentally different from their corresponding super-critical modes. In particular, with a stepwise increase in excitation amplitude till a critical value, the sub-critical VB topology moved downstream and radially outward. Beyond a critical magnitude, the VB bubble transited back upstream and finally underwent radial shrinkage at the threshold excitation amplitude. On the other hand, the topology of the super-critical VB state continuously moved downstream and radially outwards and finally widened/fanned-out at threshold amplitude. In the final part, transition in time-averaged flame global flame structure is reported as a function of geometric swirl number. In particular, with a stepwise increase in swirl intensity, primary transition is depicted as a transformation from zero-swirl straight jet flame to lifted flame with blue base and finally to swirling seated flame. Further, a secondary transition is reported which consists of transformation from swirling seated flame to swirling flame with a conical tailpiece and finally to highly-swirled near blowout ultra-lean flame. For this purpose, CH* chemiluminescence imaging and 2D PIV in meridional planes were employed. Three baseline fuel flow rates through the central fuel injection pipe were considered. For each of the fuel flow cases (Ref), six different co-airflow rate settings (Rea) were employed. The geometric swirl number (SG) was increased in steps from zero till blowout for a particular fuel and co-airflow setting. A regime map (SG vs Rea) depicting different regions of flame stabilization were then drawn for each fuel flow case. The secondary transformation is explained on the basis of physical significance of Rom.