Influence of Nonlinear Flame Models on Thermoacoustic Instabilities in Combustion Chambers

Abstract

Thermoacoustic phenomena observed in gas turbine combustion chambers are usually related to a dependence of the heat release fluctuations to the velocity fluctuations at the injection point. Linear models for this dependence give information about stability, while nonlinear models can predict also limit cycles amplitude and frequency. This nonlinear dependence can be obtained by means of experiments or numerical tools by measuring the flame describing function (FDF). Analytical expressions have been proposed in the years to obtain nonlinear flame models to be introduced into simplified tools. Through these nonlinearities, information about limit cycle amplitudes, bifurcation diagrams and hysteresis can be achieved. The importance of this information is related to the possibility of managing the variation of certain operating parameters to keep the combustor in a safe operating zone. Additionally, from the bistable zone information about the operational margin can be obtained in order to stay far from the unstable condition. The aim of this work is to describe a simplified procedure to track combustors bifurcation diagrams using nonlinear flame models implemented in a 3D thermoacoustic tool based on the finite element methods (FEM) in the frequency domain. First flame describing functions (FDF) for not swirled and swirled burners from the literature are considered and applied to simplified configurations. Then, FDFs are introduced into an industrial configuration and the bifurcation diagrams are tracked. For both cases, the paper describes the influence of various FDFs parameters and shapes on the predictions of combustors stability boundaries.