Exploration of combustion instability triggering using Large Eddy Simulation of a multiple injector Liquid Rocket Engine

This article explores the possibility of analyzing combustion instabilities in liq- uid rocket engines by making use of Large Eddy Simulations (LES). Calculations are carried out for a complete small-scale rocket engine, including the injection manifold thrust chamber and nozzle outlet. The engine comprises 42 coaxial injectors feeding the combustion chamber with gaseous hydrogen and liquid oxy- gen and it operates at supercritical pressures with a maximum thermal power of 80 MW. The objective of the study is to predict the occurrence of transverse high-frequency combustion instabilities by comparing two operating points fea- turing different levels of acoustic activity. The LES compares favorably with the experiment for the stable load point and exhibits a nonlinearly unstable trans- verse mode for the experimentally unstable operating condition. A detailed analysis of the instability retrieves the experimental data in terms of spectral features. It is also found that modifications of the flame structures and of the global combustion region configuration have similarities with those observed in recent model scale experiments. It is shown that the overall acoustic activity mainly results from the combination of one transverse and one radial mode of the chamber, which are also strongly coupled with the oxidizer injectors

Study of flame response to transverse acoustic modes from the LES of a 42-injector rocket engine

The Large-Eddy Simulation of a reduced-scale rocket engine operated by DLR has been conducted. This configuration features 42 coaxial injectors fed with liquid oxygen and gaseous hydrogen. For a given set of injection conditions the combustor exhibits strong transverse thermo-acoustic oscillations that are retrieved by the numerical simulation. The spatial structure of the two main modes observed in the LES is investigated through 3D Fourier analysis during the limit cycle. They are respectively associated with the first transverse and first radial resonant acoustic modes of the combustion chamber. The contributions of each individual flame to the unsteady heat release rate and the Rayleigh index are reconstructed for each mode. These contributions are in both cases low in the vicinity of velocity anti-nodes and high near pressure anti-nodes. Moreover it is noticed that these pressure fluctuations lead to large velocity oscillations in the hydrogen stream. From these observations, a driving mechanism for the flame response is proposed and values for the gain and phase of the associated flame transfer function are evaluated from the LES

Dynamics and control of premixed combustion systems based on flame transfer and describing functions

This article describes recent progress on premixed flame dynamics interacting with acoustic waves. Expressions are derived to determine the stability of combustors with respect to thermoacoustic oscillations. The validity of these expressions is general, but they are illustrated in laminar systems. Laminar burners are commonly used to elucidate the response of premixed flames to incoming flow perturbations, highlight the role of acoustic radiation in their stability, identify modes associated with thermoacoustic intrinsic instabilities and decipher the leading mechanisms in annular systems with multiple injectors. Many industrial devices also operate in a laminar premixed mode such as, for example, domestic gas boilers and heaters equipped with matrix burners for material processing in which unconfined flames are stabilized at one extremity of the system. This article proposes a systematic approach to determine the stability of all these systems with respect to thermoacoustic oscillations by highlighting the key role of the burner impedance and the flame transfer function (FTF). This transfer function links in frequency space incoming flow perturbations to heat release rate disturbances. This concept can be used in the turbulent flame case as well. Weakly nonlinear stability analysis can also easily be conducted by replacing the FTF by a flame describing function in the expressions derived in this work. The response of premixed flames to harmonic mixture compositions and flow-rate perturbations is then revisited and the main parameters controlling the FTF are described. A theoretical framework is finally developed to reduce the system thermoacoustic sensitivity by tailoring the FTF

Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments

This article reports a numerical analysis of combustion instabilities coupled by a spinning mode or a standing mode in an annular combustor. The method combines an iterative algorithm involving a Helmholtz solver with the Flame Describing Function (FDF) framework. This is applied to azimuthal acoustic coupling with combustion dynamics and is used to perform a weakly nonlinear stability analysis yielding the system response trajectory in the frequency-growth rate plane until a limit cycle condition is reached. Two scenarios for mode type selection are tentatively proposed. The first is based on an analysis of the frequency growth rate trajectories of the system for different initial solutions. The second consid- ers the stability of the solutions at limit cycle. It is concluded that a criterion combining the stability analysis at the limit cycle with the trajectory analysis might best define the mode type at the limit cy- cle. Simulations are compared with experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global FDF and it is considered that the spacing between burners is such that coupling with the mode takes place without mutual interac- tions between adjacent burning regions. Depending on the nature of the mode being considered, two hypotheses are made for the FDFs of the burners. When instabilities are coupled by a spinning mode, each burner features the same velocity fluctuation level implying that the complex FDF values are the same for all burners. In case of a standing mode, the sixteen burners feature different velocity fluctua- tion amplitudes depending on their relative position with respect to the pressure nodal line. Simulations retrieve the spinning or standing nature of the self-sustained mode that were identified in the exper- iments both in the plenum and in the combustion chamber. The frequency and amplitude of velocity fluctuations predicted at limit cycle are used to reconstruct time resolved pressure fluctuations in the plenum and chamber and heat release rate fluctuations at two locations. For the pressure fluctuations, the analysis provides a suitable estimate of the limit cycle oscillation and suitably retrieves experimental data recorded in the MICCA setup and in particular reflects the difference in amplitude levels observed in these two cavities. Differences in measured and predicted amplitudes appear for the heat release rate fluctuations. Their amplitude is found to be directly linked to the rapid change in the FDF gain as the velocity fluctuation level reaches large amplitudes corresponding to the limit cycle, underlying the need of FDF information at high modulation amplitudes

Combustion dynamics of annular systems.

New results on the dynamics of annular combustors during ignition and combustion instabilities will be reviewed. Ignition dynamics is consid- ered first by examining experiments carried out in a system comprising a plenum feeding premixed gaseous reactants through multiple swir- ling injectors and an annular combustor formed by two concentric transparent quartz walls allowing full optical access to the flame. The analysis focuses on the “light-round” process during which the flame spreads from one injector to the next eventually leading to established flames on each injector. The transparent lateral walls allow a full view of the flame propagation from a spark igniter located in the neighborhood of one injector. High speed imaging is used to examine flame displace- ment and deduce the ignition delay yielding a full light around of the annular combustor. Changes associated to operation with spray flames are then discussed. The second part of this article is concerned with combustion instabilities of annular systems coupled by azimuthal modes. This type of oscillation has received considerable attention in recent years because the underlying coupling is often observed in the advanced premixed combustion architectures used in modern gas tur- bines. Recent studies have allowed a detailed examination of the dynamics of annular devices comprising multiple swirling injectors. Experiments on annular systems and single sector configurations pro- vide new insight on the coupling process between acoustics and unsteady combustion. Results for self-sustained combustion oscillations coupled by azimuthal modes are presented for operation with gaseous premixed reactants and with spray flames

Nonlinear thermoacoustic mode synchronization in annular combustors

Nonlinear coupling between azimuthal and axisymmetric modes in annular combustors is studied analyti- cally. Based on the thermoacoustic wave equation, a model featuring three nonlinearly coupled oscillators is derived. Two oscillators represent the dynamics of an azimuthal mode, and the third accounts for the axisym- metric mode. A slow-time system for the evolution of the mode amplitudes and phases is obtained through the application of the method of averaging. The averaged system is shown to accurately reproduce the solu- tions of the full oscillator model. Analysis of this five-dimensional dynamical system shows that a standing azimuthal mode may synchronize with an axisymmetric mode, provided that their individual resonance fre- quencies and growth rates are similar. This phase-coupled two-mode oscillation corresponds to the so-called slanted mode, observed in recent experiments involving an annular model combustion chamber. Quantitative conditions for the occurrence of mode synchronization are derived in terms of the growth rate ratio and a frequency detuning parameter. The analysis results are found to be consistent with experimental observations of the slanted mode

Flame and spray dynamics during the light-round process in an annular system equipped with multiple swirl spray injectors

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 combus- tor 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 azi- muthal 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 igni- tion of an individual injector to the stabilization of its flame in its final shape, approxi- mately 50ms 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