Jean-François Bourgouin

Characterization and Modeling of a Spinning Thermoacoustic Instability in an Annular Combustor Equipped With Multiple Matrix Injectors

Oscillations in fully annular systems coupled by azimuthal modes are often observed in gas turbine combustors but not well documented. One objective of the present study is to characterize this type of oscillation in a laboratory scale system, allowing detailed pres- sure measurements and high speed visualization of the flame motion. The experiment is designed to allow detailed investigations of this process at a stable limit cycle and for an extended period of time. Experiments reported in the present article are carried out in the MICCA facility which was used in our previous work to analyze instabilities arising when the chamber backplane was equipped with multiple swirling injectors (Bourgouin et al., 2013, “Self-Sustained Instabilities in an Annular Combustor Coupled by Azimuthal Acoustic Modes,” ASME Paper No. GT2013-95010). In the present study, these units are replaced by a set of matrix injectors. The annular plenum feeds 16 such devices confined by two cylindrical quartz tubes open to the atmosphere. The multiple flames formed by the matrix injectors are laminar and have a well documented describing function. This constitutes an ideal configuration allowing systematic investigations of thermo-acoustic oscillations coupled by longitudinal or azimuthal modes while avoid- ing complexities inherent to swirling turbulent flames studied previously. Optical access to the chamber allows high speed imaging of light emission from the flames providing instantaneous flame patterns and indications on the heat release rate fluc- tuations. Eight waveguide microphones record the pressure signal at the combustor injection plane and in the plenum. Among the unstable modes observed in this setup, this analysis focuses on situations where the system features a spinning azimuthal mode. This mode is observed at a frequency which is close to that associated with the 1A mode of the plenum. A theoretical analysis is then carried out to interpret the angular shift between the nodal lines in the plenum and chamber, and the meas- ured flame describing function (FDF) is used to quantify this shift and determine the linear growth rate.

Self-Sustained Instabilities in an Annular Combustor Coupled by Azimuthal and Longitudinal Acoustic Modes

Annular combustors may give rise to various types of combustion instabilities. Some of the resulting oscillations coupled by transverse acoustic modes are commonly observed in practice and their suppression or reduction is an important issue which needs to be considered. The present study is carried out in a system comprising an annular plenum feeding 16 swirling injectors confined by two cylindrical quartz tubes opened to the atmosphere. Calculations based on a Helmholtz solver provide a suitable estimate of frequencies observed experimentally and reveal the modal structure corresponding to the longitudinal and transverse oscillations. High speed images obtained under reactive conditions are then processed to extract the structure of heat release rate perturbations and match this structure with that of the coupling acoustic mode. It is found that the transverse instability is coupled by a first azimuthal mode which is characterized by a time varying spin ratio. This index gives the respective levels of rotating components in the azimuthal mode. Another instability arising at a lower frequency is coupled by a longitudinal acoustic mode giving rise to high-amplitude oscillations in heat release rate in which most of the flames (but not all) are synchronized and in phase with the pressure perturbation.© 2013 ASME

Ignition sequence of an annular multi-injector combustor

Ignition is a critical process in combustion systems. In aeronautical combustors, altitude relight capacities are required in case of accidental extinction of the chamber. A simultaneous study of light-round ignition in an annular multi-injector combustor has been performed on the experimental and numerical sides. This eort allows a unique comparison to assess the reliability of Large-Eddy Simulation (LES) in such a conguration. Results are presented in uid dynamics videos.

Simulation of the Ignition Process in an Annular Multiple-Injector Combustor and Comparison With Experiments

Ignition is a problem of fundamental interest with critical practical implications. While there are many studies of ignition of single injector configurations, the transient ignition of a full annular combustor has not been extensively investigated, mainly because of the added geometrical complexity. The present investigation combines simulations and experiments on a complete annular combustor. The setup, developed at EM2C laboratory, features sixteen swirl injectors and quartz walls allowing direct visualization of the flame. High speed imaging is used to record the space time flame structure and study the dynamics of the light-round process. On the numerical side, massively parallel computations are carried out in the LES framework using the Filtered Tabulated (F-TACLES) flamelet model. Comparisons are carried out at different instants during the light-round process between experimental data and results of calculations. It is found that the simulation results are in remarkable agreement with experiments provided that the thermal effects at the walls are considered. Further post-processings indicate that the flame burning velocity and flame front geometry are close to those found in the experiment. This analysis confirms that the LES framework used for these calculations and the selected combustion model are adequate for such calculations but that further work is needed to confirm that ignition prediction can be used reliably over a range of operating parameters.Copyright

Investigation of Precessing-Vortex-Core–Flame Interaction Based on Tomographic Reconstruction Techniques

Unsteady helical flow structures, such as the precessing vortex core (PVC), are often observed in swirling flows with vortex breakdown. Although this type of flow is of high relevance for industrial combustors, the role of these flow instabilities in reacting systems, in particular their effect on flame stabilization and combustion instabilities, remains poorly understood. The three-dimensional structure of the interaction between the helical mode and the flame is difficult to assess with common measurement techniques, such as chemiluminescence imaging, due to the non-axisymmetry of the oscillation pattern. In the present work, a novel method is proposed to determine the full field of the heat release rate perturbation associated with the helical mode. This method requires only line-of-sight integrated information from a single camera. Tomographic reconstruction techniques are used, exploiting the fact that the helical mode is a rotating structure. Reconstruction algorithms are presented that are tailored to the specific spatio-temporal structure of the oscillation pattern, and it is shown that these techniques outperform standard methods. The proposed methodology is applied in a turbulent swirl-stabilized model combustor with significant PVC oscillations. Images from an intensified high-speed camera are used for the reconstruction. The analysis shows that the helical mode perturbs the flame in the inner and the outer shear layers of the annular jet and thereby creates helical traveling waves. The perturbation in the outer shear layer grows significantly in downstream direction and causes strong heat release rate fluctuations when impinging on the combustor wall.Copyright