Dynamics and stability of lean-premixed swirl-stabilized combustion

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

https://hal.archives-ouvertes.fr/hal-00803230/document

Large Eddy Simulations of gaseous flames in gas turbine combustion chambers

The combined dynamics of swirler and turbulent premixed swirling flames

https://hal.archives-ouvertes.fr/hal-00811961/document

Large Eddy Simulation of combustion instabilities in a lean partially premixed swirled flame

LES and experimental studies of cold and reacting flow in a swirled partially premixed burner with and without fuel modulation

Instantaneous heat flux measurements have shown that, in the expansion stroke, heat can flow from the wall into the combustion chamber, even though the bulk gas temperature is higher than the wall temperature. This unexpected result has been explained by modelling of the unsteady flows and heat conduction within the gas side thermal boundary layer. This modelling has shown that these unsteady effects change the phasing of the heat flux, compared with that which would be predicted by a simple convective correlation based on the bulk gas properties. Twelve fast response thermocouples have been installed throughout the combustion chamber of a pent roof, four-valve, single-cylinder spark ignition engine. Instantaneous surface temperatures and the adjacent steady reference temperatures were measured, and the surface heat fluxes were calculated for motoring and firing at different speeds, throttle settings and ignition timings. To make comparisons with these measurements, the combustion system was model...

Unsteady in-cylinder heat transfer in a spark ignition engine: experiments and modelling

Thermo-acoustic characterization of gas turbine combustion systems is crucial for a successful development of new gas turbine engines to meet emission and efficiency targets. In this context, it becomes more and more necessary to predict the limit cycle amplitudes of thermo-acoustic induced combustion instabilities in order to figure out if they can be tolerated or if they are above the critical design limit and will cause damage to the engine. For the prediction of limit cycle amplitudes, the nonlinear flame response of the combustion system is needed, which is represented in this work by the flame describing function (FDF). In this article, the identification of the FDF from a large eddy simulation (LES) is validated. The test case used was a premixed atmospheric swirl flame, for which experimental data on the FDF were available. First a steady reacting LES solution was obtained and compared to experimental data. The simulation was then excited by superimposing a mono-frequency harmonic wave on the velo...

Identification of the Flame Describing Function of a Premixed Swirl Flame from LES

The sound spectrum from a turbulent non-premixed flame as measured in the far field is predicted. A closed form description for the sound spectrum is derived assuming infinitely fast chemistry. The sound spectrum can then be described in terms of the ID turbulent spectrum of the mixture fraction at the flame front. The input values for this turbulent spectrum are derived from a steady state CFD flame calculation. The predictions from the sound spectrum model are compared widi experiments performed in an acoustically one-dimensional combustion chamber. The comparison between measurements and prediction is good for flames in which the mixed-is-burnt approach is valid. For flames, whose burning velocity is limited by chemical kinetics the measured spectrum shows a steeper fall off than the spectrum from the predictions

Jacobus B.W. Kok

Papers

LES and experimental studies of cold and reacting flow in a swirled partially premixed burner with and without fuel modulation

Unsteady in-cylinder heat transfer in a spark ignition engine: experiments and modelling

Identification of the Flame Describing Function of a Premixed Swirl Flame from LES

Sound Generation by Turbulent Non-premixed Flames

Optimizing a steam-methane reformer for hydrogen production