This paper presents recent progress in the field of thermoacoustic combustion instabilities in propulsion engines such as rockets or gas turbines. Combustion instabilities have been studied for more than a century in simple laminar configurations as well as in laboratory-scale turbulent flames. These instabilities are also encountered in real engines but new mechanisms appear in these systems because of obvious differences with academic burners: larger Reynolds numbers, higher pressures and power densities, multiple inlet systems, complex fuels. Other differences are more subtle: real engines often feature specific unstable modes such as azimuthal instabilities in gas turbines or transverse modes in rocket chambers. Hydrodynamic instability modes can also differ as well as the combustion regimes, which can require very different simulation models. The integration of chambers in real engines implies that compressor and turbine impedances control instabilities directly so that the determination of the impedances of turbomachinery elements becomes a key issue. Gathering experimental data on combustion instabilities is difficult in real engines and Large Eddy Simulation (LES) has become a major tool in this field. Recent examples, however, show that LES is not sufficient and that theory, even in these complex systems, plays a major role to understand both experimental and LES results and to identify mitigation techniques.

https://hal.archives-ouvertes.fr/hal-01502419/document

Prediction and control of combustion instabilities in real engines

A review of acoustic dampers applied to combustion chambers in aerospace industry

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 usua

Flame Dynamics and Combustion Noise: Progress and Challenges

https://www.cerfacs.fr/~cfdbib/repository/TR_CFD_09_8.pdf

Acoustic modeling of perforated plates with bias flow for Large-Eddy Simulations

Thermoacoustic instability of a laminar premixed flame in Rijke tube with a hydrodynamic region

Despite extensive efforts, controlling combustion instabilities in modern gas turbines remains a challenge at the design stage. The strong coupling between unsteady combustion and acoustics that leads to the growth of such instabilities is not yet fully mastered even though acoustics in complex geometries and combustion dynamics of turbulent swirled flames are now reasonably well understood. Comparatively, the effects of the acoustic boundary conditions on the system stability are much less studied. They are nonetheless of prime importance when a global acoustic energy balance is to be written in a combustor, as they determine the acoustic fluxes at the inlets and outlets of the combustor. The present study describes a reliable solution to efficiently control the acoustic properties of a boundary condition upstream of the combustion zone, like the premixer manifold or the inlet of a combustor. Effects of the acoustic reflection coefficient on self-sustained combustion oscillations are investigated and a passive control solution is proposed, using perforated plates with bias flow. Performances of this system are characterized on an existing turbulent swirl-stabilized combustion facility which exhibits strong unstable regimes. Tuning the upstream reflection coefficient leads to strong damping of the main resonant modes of the combustion instabilities, while modifications of the initial geometry and flow operating conditions are minimal. The combustion facility and the design of the control system are first described. Efficient control of the reflection coefficient is then assessed in an impedance tube, with large amplitudes of pressure fluctuations, typical of those encountered in practical systems. The influence of this control method on unstable regimes in the turbulent combustor facility is then presented. Finally the acoustic energy budget is examined and discussed at limit cycles for different values of the burner inlet reflection coefficient: ∣R∣ = 0.2 to 0.8.

Damping combustion instabilities with perforates at the premixer inlet of a swirled burner

Modeling the damping properties of perforated screens traversed by a bias flow and backed by a cavity at low Strouhal number

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.

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

With the growing development of lean premixed combustion, prediction of combustion instabilities becomes a key issue. Amongst the numerous problems to overcome, the influence of the acoustic boundary conditions on the development of instabilities remains a difficult issue. These boundaries are usually not known and difficult to measure in reacting systems. However, they are a key element in the coupling since they strongly influence both the unsteady pressure and flow fields inside the combustion chamber. In the present study, an original passive control method of the upstream acoustic boundary condition is proposed. Our aim is to provide a tunable and predictable acoustic boundary condition, where the reflection coefficient can be adjusted from0 to 1 at each frequency in the range below 1 kHz to study the influence of the acoustic boundary condition on self-sustained oscillations, without any external flow forcing devices. Acoustic properties of this system were first tested in a cold gas setup. Results for the reflection coefficient measured with a three microphones technique are in good agreement with predictions. This system was then integrated in a combustor and it was shown that combustion instabilities could be strongly damped using the adapted set of parameters.

Analysis and control of combustion instabilities by adaptive reflection coefficients

The efficiency of perforated panels inserted in the injection line of a swirled turbulent burner is 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 noise levels. This paper focuses on the implementation of this technology in the injection line of a burner. The system is used to control the inlet acoustic reflection coefficient of the burner 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 in the injection line. 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). However it is shown that their behavior largely differs when facing large pressure fluctuations levels (non linear regime) typical of those encountered during self-sustained combustion oscillations. Design criteria are given to control the reflection coefficient of perforated panels submitted to high pressure fluctuations levels and damp thermo-acoustic oscillations. While developed on a laboratory scale swirled combustor, this method is more general and could easily be adapted to practical combustors.Copyright © 2008 by Laboratoire EM2C - Ecole Centrale Paris

Nicolas Tran

Papers

Damping combustion instabilities with perforates at the premixer inlet of a swirled burner

Modeling the damping properties of perforated screens traversed by a bias flow and backed by a cavity at low Strouhal number

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

Analysis and control of combustion instabilities by adaptive reflection coefficients

Passive Control of the Inlet Acoustic Boundary of a Swirled Turbulent Burner