Thermoacoustic Instabilities and Transient Phenomena in Annular Gas Turbine Combustors

by Guillaume J. J. Fournier, Naman Purwar and Wolfgang Polifke

Motivation

In the coming years, the energy and transportation sectors will face major challenges. Year after year, the demand for energy is growing and air travel is forecasted to increase at a yearly rate of 5%. At the same time, we are facing environmental issues and it is crucial to reduce the environmental footprint. The Advisory Council for Aeronautics Research in Europe (ACARE) has targeted a 50% reduction in CO2 and an 80% reduction in NOx by 2020. The European gas turbine (GT) industry will play a key role in meeting EU objectives and global emissions reduction goals agreed in Paris (COP21).

The combustion efficiency of modern GTs is already very high, so major improvements reductions in NOx, CO, soot, aerosols and particulates can only be achieved if lean-burn combustion concepts are mature enough to enter service. In both land based and aeroengines GTs, modern and future low-emission concepts impose strong adverse consequences on their operating range in the following ways:

• Lean-burn combustors are more prone to self-excited thermo-acoustic combustion instabilities. Repeated exposure to high-pressure levels can lead to structural damages and risks of catastrophic failures.
• Ignition and light round becomes more difficult. Especially, in aeroengines, relight capability at altitude is a critical safety issue.
• Increased danger of lean extinction can occur.

ANNULIGhT is research network funded by the European Commission under Marie Skłodowska-Curie Actions. ANNULIGhT brings together leading universities, research institutes and the gas turbine industry with the goal of tackling these instabilities and transient phenomena in gas turbine combustion with analytical, numerical and experimental approaches. To learn more about the project, you can visit ANNULIGhT webpage.

Objectives

TUM is involved in two work packages. The first one focuses on thermo-acoustic instabilities in annular chambers with the objective of understanding the underlying physics. Thermo-acoustic modes arise from a coupling of the unsteady heat release rate of the flame with the acoustics of the system, resulting in a positive feedback loop. In particular, in such combustors, azimuthal modes will develop it is crucial to understand their structure and their nature. Also new methods and tools to characterize and predict azimuthal modes need to be developed. Large-Eddy Simulations have shown that the nature of these modes, i.e. spinning, standing or mixed, is difficult to predict and changes over time. Recent studies also showed the existence of a new kind of thermo-acoustic instabilities, not involving the acoustic eigenmodes of the system. but intrinsic to the flame [1]. Such instabilities may also develop in annular configurations. Following the approach of Bauerheim [2], low-order network modeling will be used to analyze and understand the thermo-acoustic spectrum of annular combustors at a fundamental level.

The second work package focuses on passive control methods and tools, with emphasis on symmetry breaking. The objective is to understand the effect of multiple parameters on symmetry breaking and develop new method and tools to exploit them and optimize damping strategies. TFD is currently developing a DG-FEM (Discontinuous Galerkin Finite Element Method) solver that allows to compute the Linearized Navier Stokes Equations (LNSE) [3]. Flame sources are added using a Flame Transfer Function (FTF) approach. The framework LNSE-FTF allows for simulations on full annular geometries. Model order reduction techniques will be applied to drastically reduce computational costs.

References

[1] Emmert, T., Bomberg, S., Jaensch, S., Polifke, W., 2017. Acoustic and Intrinsic Thermoacoustic Modes of a Premixed Combustor. Proceedings of the Combustion Institute 36, 3835–3842. https://doi.org/10.1016/j.proci.2016.08.002

[2] Bauerheim, M., Parmentier, J.-F., Salas, P., Nicoud, F., Poinsot, T., 2014. An analytical model for azimuthal thermoacoustic modes in an annular chamber fed by an annular plenum. Combustion and Flame 161, 1374–1389. https://doi.org/10.1016/j.combustflame.2013.11.014

[3] Meindl, M., Albayrak, A., Polifke, W., 2019. A discontinuous Galerkin finite element method for thermoacoustic stability analysis based on the linearized Navier-Stokes equations. Journal of Sound and Vibration. Under review.

Acknowledgment

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