High-Frequency Instabilities of Combustion Processes Stabilised by Auto-Ignition
by M. Zellhuber and Wolfgang Polifke
Motivation and background
The constant effort to improve both efficiency and pollutant emissions of industrial gas turbines tends to move the combustor operating conditions out of the usual range into regimes involving auto-ignition. This leads to a strong change in both stationary and dynamic flame behaviour when compared to conventional propagating flames.
In sequential combustors like Alstom's SEV system, which is already in use in the GT24/GT26 product line, the flame stabilisation is strongly influenced by auto-ignition due to the high oxidiser inlet temperature. Such a combustor serves as a practical example for the present research project, in which the thermoacoustic stability of auto-ignition flames is investigated. A focus is given to the stability at higher frequencies, since pulsations of this kind had been observed previously in experiments under certain operating conditions [1].
However, a thorough physical understanding of the mechanisms leading to such instabilities has not been achieved yet, and shall therefore be developed in the present research project.
Work approach and objectives
A schematic sketch of a lab-scale sequential combustor is given in Fig. 1. It mainly consists of a jet of fuel penetrating into the main flow of hot combustion pro\-ducts. The mixing of the streams and the high oxidiser temperature lead to the initiation of chemical reactions and the accumulation of radical species, which subsequently ends with the ignition of the mixture and the heat release. Thus, auto-ignition significantly contributes to the flame stabilisation. However, the presence of recirculation zones will lead to an additional contribution of flame propagation in the developing shear layer.
From this basic description of the involved physical effects, one can already derive the three main hypothesis concerning the feedback mechanisms that might lead to instabilities in sequential combustors:
- The modulation of heat release through coherent flow structures,
- Fluctuations of the equivalence ratio caused by acoustic perturbations of the mixing process,
- Alteration of the ignition time delay through pressure fluctuations.
This numerical study shall therefore investigate the relevance of these effects for varying operating conditions, mainly using Large Eddy Simulations (LES). A separate analysis of each effect is made possible through a targeted definition of generic geometries and ad-hoc-modifications of the simulation models. Eventually, it should be possible to explain the stability behaviour of sequential combustors at machine level and to provide valuable numerical tools for stability prediction.
Current project status
From the very beginning of the project, high priority was addressed to the development of a suitable combustion model that allows to perform compressible calculations of sequential combustors including both auto-ignition and propagation effects. This task was achieved in close collaboration with R. Kulkarni in a partner project on SEV combustion modeling. To the present point, a partially validated combustion model is functional, which allows to perform first LES-based investigations of the thermoacoustic stability behaviour.
The combustion model is based on perfectly stirred reactor calculations with detailed chemistry. These were also used to define kinetic sensitivity factors which quantify the influence of pressure and mixture variations on radical build-up during ignition delay. These factors are at the heart of a new theoretical formulation for auto-ignition flame dynamics, based on the previous ideas of Ni et al [1]. It yields quantitative expressions for flame transfer functions regarding pressure and mixture influences that could be validated through the use of reactor sequence calculations.
Furthermore, compressible LES calculations with the new combustion model led to the appearance of a non-planar, self-excited acoustic mode (see Fig. 2) for an auto-ignition flame sitting in a straight tube. The investigation regarding the feedback mechanism is still ongoing; first results indicate that instability growth ought to be due to perturbations of flame propagation in the near-wall region. Future work will include the elaboration of non-reflecting boundary conditions adapted to the damping of such non-planar waves, in order to be able to identify the flame dynamics from LES timeseries.
Project Partners
This project is part of the research initiative "Kraftwerke des 21. Jahrhunderts" (KW21), with the overall goal of more efficient, economic and enviromentally friendly power plants. It is funded within the framework of KW21 by Alstom Power and the Bavarian State, whose support is gratefully acknowledged.
References
[1] A. Ni, W. Polifke, F. Joos, Proceedings of the ASME Turbo Expo (2000) 2000-GT- 0103.