Paul Palies

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.

Modeling the response of premixed flame transfer functions - Key elements and experimental proofs

A wide variety of analytical models has been proposed to describe the transfer function of premixed flames submitted to flow disturbances. These models generally rely on a kinematic description of the flame reaction surface which is perturbed around its steady state. The response then depends on the type of flow perturbation considered, the mean flow characteristics and flame properties. Systematic comparisons between model predictions and experimental results are however less numerous. This study revisits some of these theoretical descriptions and their underlying hypotheses to delineate their domain of application. The main features of the response of premixed flames to flow disturbances are highlighted. Different mechanisms are identified and their analytical representation is discussed. It is shown that experimental observations provide useful guidelines in the flame transfer function modeling. The cases investigated include laminar and turbulent configurations in conical or inverted V-flame configurations. Effects of swirl, flame root dynamics and confinement are emphasized.

Swirling flame dynamics and describing function

The present article focuses on the dynamics and response of lean premixed confined swirling flames submitted to plane acoustic perturbations. The axial velocity disturbances and the heat release rate are recorded to determine the flame transfer function. Effects of the amplitude of the velocity perturbations are systematically investigated leading to the determination of the flame describing function (FDF) which generalizes the transfer function concept to the case where the gain and phase depend on the input amplitude level. The flame describing function of swirling flame differs from previously determined FDFs as it exhibits a local minimum with a small gain and a maximum response in the same frequency range. In the case studied in this article the minimum is located at 60 Hz while the maximum is reached at 90 Hz. Abel transformed phase conditioned average images are used to described the flame dynamics and explain the different responses. The flame motion and the heat release rate response at these two frequencies are due to a combination of two mechanisms. The first is associated with the vortex roll-up of the flame while the second is associated with swirl number fluctuations. This second mechanism involves the flame response to incoming perturbations which in the case of a swirler comprise an axial acoustic disturbance and a convective azimuthal velocity perturbation.

Swirling Flame Instability Analysis Based on the Flame Describing Function Methodology

Thermoacoustic instabilities are analyzed by making use of a nonlinear representation of flame dynamics based on the describing function. In this framework, the flame response is determined as a function of frequency and amplitude of perturbations impinging on the combustion region. This methodology is applied to confined swirling flames in a laboratory scale setup (2.5 to 4 kW) comprising an upstream manifold, an injection unit equipped with a swirler (swirl number = 0.55) and a cylindrical flame tube. The flame describing function is experimentally determined and is combined with an acoustic transfer matrix representation of the system to provide growth rates and oscillation frequencies as a function of perturbation amplitude. These data can be used to determine regions of instability, frequency shifts with respect to the acoustic eigenfrequencies and they also yield amplitude levels when self-sustained oscillations of the system have reached a limit cycle. This equilibrium is obtained when the amplitude dependent growth rate equals the damping rate in the system. This requires an independent determination of this last quantity which is here based on measurements of the resonance response curve. Results obtained are compared with observations from systematic experiments carried out by varying the test combustor geometry. The demonstration of the FDF framework in a generic configuration indicates that this can be used in more general situations of technological interest.Copyright