Thierry Schuller

A unified model for the prediction of laminar flame transfer functions : comparisons between conical and V-flame dynamics

Transfer functions of premixed laminar flames submitted to incident flow perturbations are envisaged and a unified model is derived analytically. This model, based on a linearization of the G-equation for an inclined flame, includes convective effects of the flow modulations propagating upstream of the flame. It is shown that the flame dynamics is governed by two relevant parameters, a reduced frequency, ω∗, and the ratio of the flame burning velocity to the mean flow velocity, SL/ῡ, or equivalently the flame angle α with respect to the flow direction. In the limit of low driving frequencies, the flame motion is only controlled by ω∗ and the unified model reduces to previous kinematic formulations derived for rim stabilized conical flames and V-flames anchored on a central rod. Flame transfer functions for these flame geometries with the different velocity models proposed are derived and limiting cases are examined. In the conical flame case, the low-frequency model gives a good approximation of the gain, but only a fair approximation of the phase. Convective effects are shown to induce an increasing phase lag, while low-frequency models predict a saturation phenomenon. The convective model derived in this article improves results for the gain and the phase which agree with numerical simulations and experiments. It is shown in particular that 1) the correct transfer function phase trend is retrieved and depends on the flame angle α; 2) the reduced cut-off frequency corresponds to a situation where the convective wavelength along the flame front λ = (ῡ cos α)/f equals the flame length L; and 3) the flame response is weakly affected by the amplitude of these perturbations. In the V-flame case, the low-frequency model yields a good approximation of the phase but does not feature gain values in excess of one found in the simulations. This behavior is correctly predicted by the convective model and is shown to depend on the flame angle α. A V-flame behaves as an amplifier in a certain range of frequencies. It is shown that these types of flames are more susceptible to combustion instabilities than conical flames. The V-flame response is also shown to strongly depend on the amplitude of the fluctuations even for moderate perturbation levels.

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.

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.

Dynamics of and noise radiated by a perturbed impinging premixed jet flame

This paper presents an experimental study of an acoustically excited, premixed, laminar-jet flame impinging on a water-cooled plate. Without excitation, different types of flame shapes are identified. With excitation, a strong noise is produced. The mechanism of flame-induced noise generation is investigated in detail. The results show that the modulation of the flow upstream of the flame produces strong variations in the flames area and also intense noise emission. The amplitude of the sound exceeds by one or two orders of magnitude that associated with a flame under the same flow modulation, but without the plate. The source of sound is identified as the cyclic extinction of flame area as the jet interacts with the cool plate. The link between the far-field pressure, the chemiluminescence from the flame and the flame surface is successfully compared with classical results of combustion-noise theory.

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.

Acoustically perturbed turbulent premixed swirling flames

The dynamics of a turbulent premixed confined swirling flame is investigated using large eddy simulation. The flame response is determined by introducing an external acoustic forcing at two modulation frequencies corresponding to characteristic values of the flame transfer function obtained experimentally. These values were found to give different responses in terms of gain in a previous series of experiments. The underlying physical mechanisms identified experimentally are investigated numerically. Simulations confirm that swirl number fluctuations and vortex roll-up govern the flame response. It is also confirmed that the first mechanism is associated with a mode conversion process taking place when acoustic waves impinge on the swirler unit. The axial acoustic velocity disturbance on the upstream side of the swirler generates an axial acoustic wave and an azimuthal convective disturbance in the downstream flow. These combined disturbances are retrieved in the simulation and their effect on the swirl number is extracted. Calculations also indicate that vortex shedding synchronized by the acoustic forcing takes place at the injector lip outlet. Vortices originating from this region are convected in the jet shear layer, impinge on the flame, and roll-up the flame tip. This process interferes with oscillations in the flame angle induced by swirl number fluctuations. The phasing of the flame angle with respect to the instant of vortex shedding from the injector lips determines the lifetime of the vortex before interaction with the flame and controls the strength of this interaction. When this lifetime is reduced, the vortex cannot fully develop and the flame response remains weak. For larger lifetimes, the vortex can fully develop and produce larger heat release rate perturbations. This process depends on the forcing frequency, which determines the phasing between swirl number fluctuations and vortices generation.

Experimental Study on the Effect of Swirler Geometry and Swirl Number on Flame Describing Functions

This paper deals with the response of swirling flames submitted to acoustic velocity disturbances when the rotation of the flow is produced by an axial or a radial swirler. The objective is to compare responses obtained in these two cases. The response is characterized in terms of the flame describing function (FDF), which generalizes the classical flame transfer function concept by considering not only the frequency but also the amplitude of the velocity disturbances. Results indicate that for both types of swirlers, the dynamics is essentially similar for the gains and the phases of the FDF. It is also found that the swirl number value markedly influences the gain response. The characteristic shape of the FDF, with a local minimum and maximum, are found in both cases and these features correspond to mechanisms already described previously: swirl number fluctuations and vortex rollup of the flame. Swirl number fluctuations are induced by the interaction of the incident acoustic disturbances with the swirler. This generates in the two cases a transmitted acoustic wave and a convective vorticity wave. This last wave is characterized by azimuthal velocity perturbations. The mode conversion process giving rise to the latter type of disturbance was already demonstrated in the case of an axial swirler. It is here examined in the radial swirler geometry. It is shown that the mode conversion processes in the two geometries are quite similar and that they produce similar effects on the flame dynamics and response.

Effect of Nanosecond Repetitively Pulsed Discharges on the Dynamics of a Swirl-Stabilized Lean Premixed Flame

The effects of Nanosecond Repetitively Pulsed (NRP) plasma discharges on the dynamics of a swirl-stabilized lean premixed flame are investigated experimentally. Voltage pulses of 8-kV amplitude and 10-ns duration are applied at a repetition rate of 30 kHz. The average electric power deposited by the plasma is limited to 40 W, corresponding to less than 1 % of the thermal power of 4 kW released by the flame. The investigation is carried out with a dedicated experimental setup that allows for studies of the flame dynamics with applied plasma discharges. A loudspeaker is used to perturb the flame acoustically, and the discharges are generated between a central pin electrode and the rim of the injection tube. Velocity and CH* chemiluminescence signals are used to determine the flame transfer function assuming that plasma discharges do not affect the correlation between CH* emission and heat release rate fluctuations. Phase-locked images of the CH* emission were recorded to assess the effect of the plasma on the oscillation of the flame. The results show a strong influence of the NRP discharges on the flame response to acoustic perturbations, thus opening interesting perspectives for combustion control. An interpretation of the modifications observed in the transfer function of the flame is proposed by taking into account the thermal and chemical effects of the discharges. It is then demonstrated that by applying NRP discharges at unstable conditions, the oscillation amplitudes can be reduced by an order of magnitude, thus effectively stabilizing the system.Copyright

Experimental and Numerical Investigation on the Laminar Flame Speed of CH4/O2 Mixtures Diluted With CO2 and H2O

The effects of CO2 and H2 O addition on premixed oxy-fuel combustion are investigated with experiments and numerical simulations on the laminar flame speed of CH4 /O2 /CO2 /H2 O(v) and CH4 /O2 /N2 /H2 O(v) mixtures, at atmospheric pressure and for a reactants inlet temperature Tu = 373 K. Experiments are conducted with steady laminar conical premixed flames over a range of operating conditions representative of oxy-fuel combustion with flue gas recirculation. The relative O2 -to-CO2 and O2 -to-N2 ratios, respectively defined as O2 /(O2 +CO2 ) (mol.) and O2 /(O2 +N2 ) (mol.), are varied from 0.21 to 1.0. The equivalence ratio of the mixtures ranges from 0.5 to 1.5, and the steam molar fraction in the reactive mixture is varied from 0 to 0.45. Laminar flame speeds are measured with the flame area method using a Schlieren apparatus. Experiments are completed by simulations with the PREMIX code using the detailed kinetic mechanism GRI-mech. 3.0. Numerical predictions are found in good agreement with experimental data for all cases explored. It is also shown that the laminar flame speed of CH4 /O2 /N2 mixtures diluted with steam H2 O(v) features a quasi-linear decrease when increasing the diluent molar fraction, even at high dilution rates. Effects of N2 replacement by CO2 in wet reactive mixtures are then investigated. A similar quasi-linear decrease of the flame speed is observed for CH4 /O2 /CO2 H2 O-diluted flames. For a similar flame speed in dry conditions, results show a larger reduction of the burning velocity for CH4 /O2 /N2 /H2 O mixtures than for CH4 /O2 /CO2 /H2 O mixtures, when the steam molar fraction is increased. Finally, it is observed that the laminar flame speed of weakly (CO2 , H2 O)-diluted CH4 /O2 mixtures is underestimated by the GRI-mech 3.0 predictions.© 2010 ASME

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.