Sébastien Ducruix

Combustion Dynamics and Instabilities: Elementary Coupling and Driving Mechanisms

Elementary processes that can be involved in the development of combustion instabilities in gas turbine combustors are described. The premixed mode of combustion is considered more specie cally because it is used in most advanced gas turbine systems. The processes envisaged portray the combustion dynamics of real systems, but they are analyzed in simple laboratory cone gurations. Among the many possible interactions, the most relevant mechanisms are those that generate e uctuations in heat release or induce pressure perturbations. Some typical paths are highlighted to help in the understanding of the multiple links that can exist between elementary processes. Processes involving acoustic/e ame coupling, unsteady strain rates, e ame response to inhomogeneities, interactions of e ames with boundaries, and e ame/vortex interactions are specie cally examined. For each process, a driving or a coupling path is proposed relating heat release e uctuations to acoustic variables in certain cases or leading from acoustic variables to heat release e uctuations in other cases. Stress is also put on characteristic time lags, which are key parameters in the triggering and development of instabilities. Well-controlled experiments illustrate the many possibilities and can serve to guide the modeling effort and to validate computational tools for combustion dynamics.

Theoretical and experimental determinations of the transfer function of a laminar premixed flame

The dynamical behavior of laminar premixed flames is investigated in this article. The flame response to incident perturbations is characterized with a transfer function relating the flow velocity modulations and the heat release fluctuations. This function is obtained using the assumptions introduced in previous studies by Fleifil et al. , but the model is extended to account for any flame angle (i.e., any operating condition). The modeling shows that phenomena can be described using a single control parameter taking the form of a reduced frequency ω* . This quantity is derived as ωR/S L cos α 0 , where ω is the angular frequency, R is the burner radius S L is the laminar burning velocity, and α 0 is the half-cone angle of the steady flame. this parameter may be used to describe the response of the burner to acoustic modulation, knowing its geometry and the flame properties. Two characteristic times have been determined. The first one defines the cut-off frequency of the low-pass filter associated with the flame response. The second one enables the prediction of the time lag between the velocity modulation at the burner exit and the flame heat release the exact transfer function and an approximation in the form of a first-order model are compared with an extensive set of experimental data corresponding to a range of equivalence ratios and two burner diameters. Good agreement is obtained for low values of the reduced frequency. In an intermediate range of frequencies, the experimental phase exceeds the theoretical values by a significant amount, the difference between theory and experiment is due to the simplifying assumptions used in the model.

Modeling Tools for the Prediction of Premixed Flame Transfer Functions

The response of flames to incident perturbations is of central interest in combustion dynamics analysis. The problem can be described in the linear regime by a transfer function. It is shown in this article that the description of the perturbed velocity field incident on the flame front is of crucial importance when dealing with the flame transfer function. This is demonstrated in the special case of a premixed flame anchored on the rim of a burner submitted to flow perturbations. Previous studies of this problem have shown that in the low-frequency range the flame dynamics was governed by a single dimensionless frequency, and that a first-order model described the general behavior of the flame response when the disturbance wave length exceeds the flame height. Complementary experiments reported in this article indicate that this model fails when the modulation frequency is increased. On this experimental basis, a revised formulation of the velocity perturbation incident on the flame is proposed which accounts for the flame cusping phenomenon when more than one wavelength wrinkles the flame front. Combining this more realistic velocity field with a level set approach for the flame dynamics a full numerical integration of the G -equation is carried out. The transfer function is computed over the whole useful range of frequencies. Experimental data are then compared with analytical and numerical predictions. It is shown that the current first-order models underestimate the phase lag between velocity and heat release fluctuations. A constant phase shift is obtained in the high-frequency limit which does not correspond to observations. The new velocity model yields a better representation of the flame response in a wider range of frequencies. It is shown in particular that the modeled phase lag between combustion and flow perturbations increases with frequency, as is effectively observed.

Flame Dynamics and Combustion Noise: Progress and Challenges

This article proposes a review of the state of knowledge in the field of combustion noise. The survey comprises an initial discussion of indirect and direct noise sources and their general characteristics, a summary of expressions devised to estimate combustion noise from turbulent flames, a discussion of the fundamental equations describing sound emission from a reactive region and an evaluation of scaling methods for combustion noise. An account is provided of a set of experiments on noise radiation from perturbed laminar flames. Sources of intense radiation of sound are identified and theoretical expressions of the pressure field are compared with detailed measurements from well controlled experiments. These experiments indicate that flame dynamics determine to a great extent the radiation of sound from flames. This is further demonstrated with experiments dealing with effects of confinement. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects are usually treated separately but they are manifestations of similar processes. Much of the current effort in the field of combustion noise focuses on numerical estimation techniques using modern computational tools. The state of the art is less advanced than in computational aeroacoustics (CAA) but it is possible to foresee that computational combustion acoustics (CCA) will progressively evolve into a well established scientific field.

Large eddy simulation and experimental study of flashback and blow-off in a lean partially premixed swirled burner

Lean premixed prevaporized (LPP) combustion is a widely used concept for reducing pollutant emissions in gas turbines. In LPP systems, a mixing tube is added between the injector and the combustion chamber to promote mixing and combustion efficiency. These devices are efficient to reduce pollutant emissions but can be sensitive to complex transient phenomena such as blow-off or flashback which are still beyond the prediction capabilities of most numerical tools. The present study describes a joint experimental and numerical study to evaluate the capacities of large eddy simulation (LES) for the prediction of flame dynamics in a swirl-stabilized LPP burner operated with propane. Combustion regimes are first identified experimentally: compact flames (the normal regime for LPP and also flashback regimes where the flame is stabilized in the mixing tube) as well as lean blow-off situations are encountered. LES is then used to investigate each regime as well as the transition and the hysteresis phenomena betwee...

High-Frequency Transverse Acoustic Coupling in a Multiple-Injector Cryogenic Combustor

High-frequency combustion oscillations are investigated experimentally. The combustor fed by cryogenic propellants operates under elevated pressure conditions (p c = 0.9 MPa) and is equipped with three coaxial injectors fed by liquid oxygen and gaseous methane. Injection parameters are in the typical range used in rocket engines. This experiment simulates on a model scale conditions prevailing in such systems, but full similarity is not achieved. The chamber exhibits a set of resonant modes with eigenfrequencies above 1 kHz. The study focuses on high-frequency dynamics resulting from a strong coupling between one of the transverse modes and combustion. The combustor is forced with an external actuator. The eigenmodes are identified with a linear frequency sweep, and then the system is modulated at the first transverse resonant frequency. The flame motion and response are observed with a high speed and two intensified charge-coupled-device cameras recording phase-conditioned images. In a set of experiments carried out on the multiple-injector combustor, operating conditions were changed systematically to determine parameter ranges leading to combustion sensitivity to transverse excitation. Strong coupling is observed in this way with a spectacular modification of the flame spread. Emission from the three flames is notably intensified when this coupling occurs, whereas thermocouples placed on the lateral walls detect a rapid increase in temperature. The OH* emission intensity that can be linked to the heat-release rate is increased. A phase analysis indicates that the pressure and OH* emission oscillate transversally and in phase at the modulation frequency. This behavior is also observed with the high-speed camera, which also features enhanced reactive vortices convected in the downstream direction at a lower frequency.

Transfer function measurements in a model combustor: Application to adaptive instability control

Due to its good performance in terms of low pollutant emissions, the lean premixed, prevaporized combustor design has received a special interest in the field of gas turbines. This article reports an experimental study of a laboratory-scale, premixed, prevaporized, swirl-stabilized combustor operated with preheated air and fed with liquid heptane as fuel. Under certain operating conditions, it exhibits well-established combustion oscillations at frequencies near 400 Hz. To understand and control these instabilities, it is useful to determine the burner response to external modulations. A novel actuator relying on secondary air modulation was designed and integrated in the injector to allow measurements of the combined transfer function of the burner and actuation system. Two different techniques were used for this determination: direct measurement between command and pressure on photo-multiplier signals based on spectral and cross-spectral densities, or identification through a finite impulse response numerical filter. The two techniques are in good agreement. This identification is then successfully applied to active control of combustion instabilities. The mean pressure oscillation amplitude is reduced by about 50% (from 650 Pa to 400 Pa, the noise level being 130 Pa). This modest reduction is due to the fact that the actuator operates at a frequency that is slightly greater than its cutoff frequency and that its effectiveness is therefore degraded. This highlights the difficulties associated with the bandwidth limitations of practical actuators.

Experiments on collapsing cylindrical flames

This article is concerned with the effect of curvature on laminar flame dynamics. This topic is of fundamental interest and it has practical implications in turbulent combustion. It is shown that highly curved premixed flames may be obtained by operating a standard axisymmetric burner in a specific pulsed mode. Collapsing cylindrical flames are observed by submitting the burner to suitably tuned plane pulsations of the flow velocity. The cylindrical flame pattern collapses in an essentially radial motion. In these circumstances the flame cylinder separates unburned inner gases from the burned outer flow. The temporal evolution of the flame may be monitored using schlieren images while the inner flow velocity is determined from particle image velocimetry (PIV). It is shown that these data yield the stretched laminar burning velocity up to very small radii. Models of the stretched laminar burning velocity as a function of curvature are compared to experimental data and Markstein lengths are deduced. These experiments indicate how laminar flames respond to large curvature values. The data gathered may be used to guide modeling efforts in the area of turbulent combustion.

Dynamics of free jets submitted to upstream acoustic modulations

The response of low Reynolds number air jets to acoustic modulations is investigated with systematic experiments. Flow transfer functions, power spectral densities of the perturbations plotted in the form of space-frequency maps, instantaneous jet images recorded using laser tomography, and phase-conditioned velocity fields serve to identify the different regimes of motion. Results indicate that five regions can be distinguished in a diagram where the jet Strouhal number StD serves as a horizontal coordinate while the shear layer Strouhal number Stθ constitutes the vertical coordinate. Boundaries of the different regions are defined by the characteristic values of these two dimensionless groups at preferred mode and neutral wave conditions, corresponding, respectively, to a maximum of amplification and a vanishing rate of growth. The forcing frequencies imparted to the flow can be ordered with respect to these characteristic parameters. The flow transfer functions at the excitation frequency indicate that...

Passive Control of the Inlet Acoustic Boundary of a Swirled Burner at High Amplitude Combustion Instabilities

Perforated panels placed upstream of the premixing tube of a turbulent swirled burner are investigated as a passive control solution for combustion instabilities. Perforated panels backed by a cavity are widely used as acoustic liners, mostly in the hot gas region of combustion chambers to reduce pure tone noises. This paper focuses on the use of this technology in the fresh reactants zone to control the inlet acoustic reflection coefficient of the burner and to stabilize the combustion. This method is shown to be particularly efficient because high acoustic fluxes issued from the combustion region are concentrated on a small surface area inside the premixer. Theoretical results are used to design two types of perforated plates featuring similar acoustic damping properties when submitted to low amplitude pressure fluctuations (linear regime). Their behaviors nonetheless largely differ when facing large pressure fluctuation levels (nonlinear regime) typical of those encountered during self-sustained combustion oscillations. Conjectures are given to explain these differences. These two plates are then used to clamp thermoacoustic oscillations. Significant damping is only observed for the plate featuring a robust response to increasing sound levels. While developed on a laboratory scale swirled combustor, this method is more general and may be adapted to more practical configurations.