Camporeale, et al.. Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments. Combustion and Flame, Elsevier, 2017, 184, pp.(D. Laera).merical procedure combining the Flame Describing Function (FDF) framework with a Helmholtz solver to analyze azimuthal instabilities:• Specific iterative algorithms are developed to simulate the dynamics of spinning and standing modes within the FDF framework. • This is tested first on a system represented by an ideal flame response and results of recent analytical investigations are fully retrieved validating this methodology.where ψ ( )/ ψ max is the normalized azimuthal eigenmode structure. The numerical implementation of this model in the

Ignition is of importance in many combustion applications and raises fundamental and practical issues. The lightround process corresponding to the flame spreading phase in the ignition of annular combustors is examined in this article by performing experiments in a model scale configuration "MICCA-Spray". This system features 16 swirling injectors each comprising a hollow cone pressurized injector. Experiments are carried out with premixed gases as well as n-heptane and dodecane sprays. The flow, spray and flame are first characterized in a single injector configuration. Propagation from the initial kernel created by a spark plug is then observed using high speed light emission imaging. This provides flame structures at various times during the process and gives access to the time delays for flame merging. With n-heptane and dodecane fuel injection, it is found that the light-round process is similar to the one observed under fully premixed propane/air experiments but the duration of the process is augmented especially for the less volatile fuel. It is also confirmed that the delay is notably influenced by thermal conditions prevailing in the chamber at the moment of ignition, injection process and fuel composition. Making use of a flamelet like model of the combustion process, the relative changes in light-round time delay are found to be, to the first order, proportional to the relative changes in laminar burning velocity induced by the fuel spray in the air flow.

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The light-round is defined as the process by which the flame initiated by an ignition spark propagates from burner to burner in an annular combustor, eventually leading to a stable combustion. Combining experiments and numerical simulation, it was recently demonstrated that under perfectly premixed conditions this process could be suitably described by large eddy simulation (LES) using massively parallel computations. The present investigation aims at developing light-round simulations in a configuration that is closer to that found in aero-engines by considering liquid nheptane injection. The large-eddy simulation of the ignition sequence of a laboratory scale annular combustion chamber comprising sixteen swirled spray injectors is carried out with a mono-disperse Eulerian approach for the description of the liquid phase. The objective is to assess this modeling approach of the two-phase reactive flow during the ignition process. The simulation results are compared in terms of flame structure and light-round duration to the corresponding experimental images of the flame front recorded by a high-speed intensified CCD camera and to the corresponding experimental delays. The dynamics of the flow is also analyzed to identify and characterize mechanisms controlling flame propagation during the light-round process.

The present article reports original experiments carried out in the MICCA-Spray combustor developed at EM2C, CNRS and CentraleSupélec. This system comprises 16 swirl spray injectors. Liquid n-heptane is injected by hollow cone simplex atomizers. The combustion chamber is formed by two cylindrical quartz tubes allowing full optical access to the flame region and it is equipped with eight pressure sensors recording signals in the plenum and chamber. A high speed camera provides images of the flames and photomultipliers record the light intensity from different flames. For certain operating conditions, the system exhibits well defined instabilities coupled by the first azimuthal mode of the chamber at a frequency of about 750 Hz. These instabilities occur in the form of bursts with a moderate level of growth. Examination of the pressure and the light intensity signals gives access to the acoustic energy source term. Analysis of the phase between the two signals during the instability bursts (growth, limit cycle, decay) is carried out using cross-spectral analysis. At limit cycle, large amplitude of pressure oscillations are reached with peak values around 5000 Pa (or 5% of the mean pressure in the chamber), and these levels persist over a finite period of time. Detailed analysis of the signals using the spin ratio indicates that the standing mode is predominant. The chamber can exhibit a spinning mode but with a lower amplitude of acoustic fluctuation. Analysis of the flame dynamics at the pressure anti-nodal line reveals a strong longitudinal pulsation with heat release rate oscillations in phase and increasing linearly with the acoustic pressure even at the highest oscillation levels. At the pressure nodal line, the flames are subjected to large transverse velocity fluctuations leading to a transverse motion of the flames and partial blow-off. Scenarios and modeling elements are developed to interpret these features. To the best of our knowledge, this is the first time that azimuthal instabilities are characterized in a well-controlled annular combustor with swirled spray flames.

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A successful ignition in an annular multi-injector combustor follows a sequence of steps. The first injector is ignited; two arch-shaped flame branches nearly perpendicular to the combustor backplane form; they propagate, igniting each injection unit; they merge. In this paper, characterization of the propagation phase is performed in an annular combustor with spray flames fed with liquid n-hepane. The velocity and the direction of the arch-like flame branch are investigated. Near the backplane, the flame is moving in a purely azimuthal direction. Higher up in the chamber, it is also moving in the axial direction due to the volumetric expansion of the burnt gases. Time-resolved particle image velocimetry (PIV) measurements are used to investigate the evaporating fuel droplets dynamics. A new result is that, during the light-round, the incoming flame front pushes the fuel droplets in the azimuthal direction well before its leading point. This leads to a decrease in the local droplet concentration and local mixture composition over not yet lit injectors. For the first time, the behavior of an individual injector ignited by the passing flame front is examined. The swirling flame structure formed by each injection unit evolves in time. From the ignition of an individual injector to the stabilization of its flame in its final shape, approximately 50 ms elapse. After the passage of the traveling flame, the newly ignited flame flashbacks into the injector during a few milliseconds, for example, 5 ms for the conditions that are tested. This could be detrimental to the service life of the unit. Then, the flame exits from the injection unit, and its external branch detaches under the action of cooled burnt gases in the outer recirculation zone (ORZ).

Annular combustors of aero-engines and gas turbine are often affected by thermo-acoustic combustion instabilities coupled by azimuthal modes. Previous experiments as well as theoretical and numerical investigations indicate that the coupling modes involved in this process may be standing or spinning but they provide diverse interpretations of the occurrence of these two types of oscillations. The present article reports a numerical analysis of instability coupled by a spinning mode in an annular combustor. This corresponds to experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global experimental flame describing function (FDF) and it is considered that the flames are sufficiently compact to interact with the mode without mutual interactions with adjacent burning regions. A harmonic balance nonlinear stability analysis is carried out by combining the FDF with a Helmholtz solver to determine the system dynamics trajectories in a frequency-growth rate plane. The influence of the distribution of the volumetric heat release corresponding to each burner is investigated in a first stage. Even though the 16 burners are all compact with respect to the acoustic wavelength considered and occupy the same volume, simulations reveal an influence of this volumetric distribution on frequencies and growth rates. This study emphasizes the importance of providing a suitable description of the flame zone geometrical extension and correspondingly an adequate representation of the level of heat release rate fluctuation per unit volume. It is found that these two items can be deduced from a knowledge of the heat release distribution under steady state operating conditions. Once the distribution of the heat release fluctuations is unequivocally defined, limit cycle simulations are performed. For the conditions explored, simulations retrieve the spinning nature of the self-sustained mode that was identified in the experiments both in the plenum and in the combustion chamber.