Large Eddy Simulations (LES) are used to study a lean swirl-stabilized gas turbine burner where the flow exhibits two stable states. In the first one, the flame is attached to the central bluff body upstream of the central recirculation zone which contains burnt gases. In the second one the flame is detached from the central bluff body downecirculation zone which is filled by cold unburnt gases and dominated by a strong Precessing Vortex Core (PVC). The existence of these two states has an important effect on the dynamic response of the flame (FTF): both gain and phase of the FTF change significantly in the detached case compared to the attached one, suggesting that the stability of the machine to thermoacoustic oscillations will differ, depending on the flame state. Bifurcation diagrams show that the detached flame cannot be brought back to an attached position with an increased fuel flow rate, but it can be re-attached by forcing it at high amplitudes. The attached flame however, behaves inversely: it can be brought back to the detached position by both decreasing or increasing the pilot mass flow rate, but it remains attached at all forcing amplitudes. IntroductionSwirling flows are commonly used to help flame stabilization in gas turbine combustion chambers. They feature several types of vortex breakdown and can exhibit bifurcation phenomena where different states can co-exist and the flow can jump spontaneously from one to another [1,2]. Bifurcations of flames in configurations which are close to real gas turbine chambers have not been investigated so far even though engineers report that they observe these mechanisms and that there is a link between flame states and thermoacoustic instabilities: when the flame changes from one state to another, its acoustic stability characteristics also change.Two dynamic phenomena are usually observed in swirled combustion chambers: (1) a helical flow instability, the so-called precessing vortex core (PVC) and (2) thermo-acoustic instabilites.The PVC is an hydrodynamic instability in swirling flows [3].I t is a large scale structure characterized by a regular rotation of a spiral structure around the geometrical axis of the combustion chamber. It can occur at high Reynolds and swirl number flows [4][5][6][7][8][9][10] and its precession frequency is controlled by the rotation rate of the swirled flow [3]. Several studies show that combustion can suppress the PVC [6,7,11], but other cases also show PVCs which are present in reacting flows [12][13][14][15]. The interaction of PVC with flames has been analyzed for example by Stöhr et al.[16]: they found the PVC to enhance mixing and to increase the flame surface. This was associated to structures in the inner shear layer, whereas Moeck et al. [15] observed the outer shear layer to create most of the flame perturbations. Both researchers as well as Staffelbach [13] using LES evidenced a ''finger-like'' rotating structure at the flame foot around which the PVC is turning. Furthermore, asymmetric fluctuations of the he...

International audienceLarge Eddy Simulations (LES) of a lean swirl-stabilized gas turbine burner are used to analyze mechanisms triggering combustion instabilities. To separately study the effect of velocity and equivalence ratio fluctuations, two LES of the same geometry are performed: one where the burner operates in a "technically" premixed mode (methane is injected by holes in the vanes located in the diagonal passage upstream of the chamber) and the second one where the flow is fully premixed in the diagonal passage. The inlet is acoustically modulated and the mechanisms affecting the dynamic flame response are identified. LES reveals that both cases provide similar averaged (non-)pulsated flame shapes. However, even though the mean flames are only slightly modified, the delays change when mixing is not perfect. LES fields and a simple model for the methane jets trajectories show that mixing in the diagonal passage is not sufficient to damp heterogeneities induced by unsteady fuel flow rate and varying fuel jet trajectories. These mixing fluctuations are phased with velocity oscillations and modify the flame response to forcing. Local fields of delays and interaction indices are obtained, showing that the flame is not compact and is affected by fluctuations of mixing

VeLoNOx™ series (Very Low NOx) burners have been developed by Ansaldo Energia to satisfy the most restrictive emissions limits required by various countries. For these combustion systems the characterization of the field of stable operating conditions is a key design element. In particular, flame transition is of paramount importance for the stability of these combustion systems. Usually, this phenomenon occurs during load ramps and is characterized by a change of the flame shape, occurring in a well-defined load range. Proper flame shape control is essential to achieve more stable operating conditions up to base load. At transition, experiments in a full scale pressure test-rig clearly show both a modification of the flame shape, a change of the measured air side burner effective area, and a reduction of the combustion chamber wall temperatures. LES simulations were conducted using the same boundary conditions as RANS simulations discussed in a previous work [1]. In this paper, we show that both RANS and LES were able to correctly predict flame transition and a RANS-LES-Experiment comparison is performed.

Gas turbines offer a high operational flexibility and a good turn down ratio to meet future requirements of power production. In this context, stable operation over a wide range and for different blends of fuel is requested. Thermoacoustic stability assessment is crucial for accelerating the development and implementation of new combustion systems. The results of nonlinear and linear thermoacoustic stability assessments are compared on the basis of recent measurements of flame describing functions and thermoacoustic stability of a model swirl combustor operating in the fully turbulent regime. The different assessment methods are outlined. The linear thermoacoustic stability assessment yields growth rates of the thermoacoustic instability whereas the limit cycle amplitude is predicted by the nonlinear stability method. It could be shown that the predicted limit cycle amplitudes correlate well with the growth rates of excitation obtained from linear modeling. Hence, for screening the thermoacoustic stability of different design approaches a linear assessment may be sufficient while for detailed prediction of the dynamic pressure amplitude more efforts have to be spent on the nonlinear assessment including the analysis of the nonlinear flame response.

Gas turbines offer a high operational flexibility and a good turn down ratio to meet future requirements of power production. In this context stable operation over a wide range and for different blends of fuel is requested. Thermoacoustic stability assessment is crucial for accelerating the development and implementation of new combustion systems. The results of nonlinear and linear thermoacoustic stability assessments are compared on basis of recent measurements of flame describing functions and thermoacoustic stability of a model swirl combustor operating in the fully turbulent regime. The different assessment methods are outlined. The linear thermoacoustic stability assessment yields growth rates of the thermoacoustic instability whereas the limit cycle amplitude is predicted by the nonlinear stability method. It could be shown that the predicted limit cycle amplitudes correlate well with the growth rates of excitation obtained from linear modeling. Hence for screening the thermoacoustic stability of different design approaches a linear assessment may be sufficient while for detailed prediction of the dynamic pressure amplitude more efforts have to be spent on the nonlinear assessment including the analysis of the nonlinear flame response.