Experimental and numerical investigation of thermoacoustic sources related to high-frequency instabilities

Flame dynamics related to high-frequency instabilities in gas turbine combustors are investigated using experimental observations and numerical simulations. Two different combustor types are studied, a premix swirl combustor (experiment) and a generic reheat combustor (simulation). In both cases, a very similar dynamic behaviour of the reaction zone is observed, with the appearance of transverse displacement and coherent flame wrinkling. From these observations, a model for the thermoacoustic feedback linked to transverse modes is proposed. The model splits heat release rate fluctuations into distinct contributions that are related to flame displacement and variations of the mass burning rate. The decomposition procedure is applied on the numerical data and successfully verified by comparing a reconstructed Rayleigh index with the directly computed value. It thus allows to quantify the relative importance of various feedback mechanisms for a given setup.