Camporeale, et al.. Flame Describing Function analysis of spinning and standing modes in an annular combustor and comparison with experiments. Combustion and Flame, Elsevier, 2017, 184, pp.(D. Laera).merical procedure combining the Flame Describing Function (FDF) framework with a Helmholtz solver to analyze azimuthal instabilities:• Specific iterative algorithms are developed to simulate the dynamics of spinning and standing modes within the FDF framework. • This is tested first on a system represented by an ideal flame response and results of recent analytical investigations are fully retrieved validating this methodology.where ψ ( )/ ψ max is the normalized azimuthal eigenmode structure. The numerical implementation of this model in the

The present article investigates the correlation between flame macrostructures and thermoacoustic combustion instabilities in stratified swirling flames. Experiments are carried out in a laboratory scale longitudinal test rig equipped with the Beihang Axial Swirler Independently-Stratified (BASIS) burner, a

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Lean premixed combustion chambers fuelled by natural gas and used in modern gas turbines for power generation are often affected by combustion instabilities generated by mutual interactions between pressure fluctuations and heat oscillations produced by the flame. Due to propagation and reflection of the acoustic waves in the combustion chamber, very strong pressure oscillations are generated and the chamber may be damaged. This phenomenon is generally referred as thermoacoustic instability, or humming, owing to the cited coupling mechanism of pressure waves and heat release fluctuations.Over the years, several different approaches have been developed in order to model this phenomenon and to define a method able to predict the onset of thermoacoustic instabilities. In order to validate analytical and numerical thermoacoustic models, experimental data are required. In this context, an experimental test rig is designed and operated in order to characterize the propensity of the burner to determine thermoacoustic instabilities.In this paper, a method able to predict the onset of thermoacoustic instabilities is examined and applied to a test rig in order to validate the proposed methodology. The experimental test is designed to evaluate the propensity to thermoacoustic instabilities of full scale Ansaldo Energia burners used in gas turbine systems for production of energy.The experimental work is conducted in collaboration with Ansaldo Energia and CCA (Centro Combustione e Ambiente) at the Ansaldo Caldaie facility in Gioia del Colle (Italy).Under the hypotheses of low Mach number approximation and linear behaviour of the acoustic waves, the heat release fluctuations are introduced in the acoustic equations as source term. In the frequency domain, a complex eigenvalue problem is solved. It allow us to identify the frequencies of thermoacoustic instabilities and the growth rate of the pressure oscillations.The Burner Transfer Matrix (BTM) approach is used to characterize the influence of the burner characteristics. Furthermore, the influence of different operative conditions is examined considering temperature gradients along the combustion chamber.

Annular combustors of aero-engines and gas turbine are often affected by thermo-acoustic combustion instabilities coupled by azimuthal modes. Previous experiments as well as theoretical and numerical investigations indicate that the coupling modes involved in this process may be standing or spinning but they provide diverse interpretations of the occurrence of these two types of oscillations. The present article reports a numerical analysis of instability coupled by a spinning mode in an annular combustor. This corresponds to experiments carried out on the MICCA test facility equipped with 16 matrix burners. Each burner response is represented by means of a global experimental flame describing function (FDF) and it is considered that the flames are sufficiently compact to interact with the mode without mutual interactions with adjacent burning regions. A harmonic balance nonlinear stability analysis is carried out by combining the FDF with a Helmholtz solver to determine the system dynamics trajectories in a frequency-growth rate plane. The influence of the distribution of the volumetric heat release corresponding to each burner is investigated in a first stage. Even though the 16 burners are all compact with respect to the acoustic wavelength considered and occupy the same volume, simulations reveal an influence of this volumetric distribution on frequencies and growth rates. This study emphasizes the importance of providing a suitable description of the flame zone geometrical extension and correspondingly an adequate representation of the level of heat release rate fluctuation per unit volume. It is found that these two items can be deduced from a knowledge of the heat release distribution under steady state operating conditions. Once the distribution of the heat release fluctuations is unequivocally defined, limit cycle simulations are performed. For the conditions explored, simulations retrieve the spinning nature of the self-sustained mode that was identified in the experiments both in the plenum and in the combustion chamber.