Identification of Combustion Noise and Flame Dynamics of Confined Turbulent Flames

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Combustion noise is an undesirable by-product of combusion processes, generated by unsteady heat release rate of the flame. For confined turbulent flames, two contributions can be distinguished: On the one hand, turbulent fluctuations of velocity or fuel concentration generate as a source term broad-band fluctuations of the heat release. On the other hand, the flame may be disturbed by acoustic waves impinging upon the flame. This acoustic-flame interaction is generally described by the flame transfer function (FTF) of the system, or by the corresponding scattering matrix. In general it is not possible to determine accurately the source terms without knowledge of the flame dynamics (as expressed in terms of the scattering matrix or the FTF), and vice versa. Furthermore, the sound pressure levels in a combustion chamber are determined not only by the noise sources, but also the flame dynamics. Thus comprehensive thermo-acoustic characterization of confined, turbulent flames requires to determine both the combustion noise source as well as the scattering matrix or FTF. The overall objective of the proposed project is to develop methods that allow the concurrent determination of the combustion noise source and the flame dynamics by combining advanced system identification (ASI) techniques with experiment or large eddy simulation. The main idea consists in modelling both flame dynamics and combustion noise by a polynomial approach based on the Box-Jenkins structure. This model may be considered as an extension of Finite Impulse Response (FIR) methods. This will make possible to predict combustion noise of confined, turbulent flames even in situations where acoustic-flame interactions are important -- a case that cannot be handled well by existing methods. Furthermore, ASI will increase the accuracy and robustness of FTF or scattering matrix identification in situations where combustion noise levels are high.