Impact of acoustic pressure on autoignition and heat release

A feedback mechanism for thermoacoustic pulsations in gas turbine reheat combustors is proposed and investigated, namely the impact of acoustic pressure waves on the reaction kinetics of autoignition. An analytical model framework is developed, which represents the combustor as a plug flow reactor. This analogy allows one to derive modulations of the heat release rate as a result of the history of pressure perturbations during the autoignition process. Numerical studies are conducted on homogeneous reactors with detailed chemistry simulations, in order to assess quantitatively the pressure sensitivity of the reaction kinetics of autoignition. From such sensitivities, flame transfer functions of autoignition flames are derived. The expressions obtained are successfully compared with time-domain simulations in one-dimensional space, and used in acoustic network models for stability predictions. The wider applicability of the model is demonstrated by extending it to nonlinear dynamics, transverse modes and technical premix conditions. The results obtained indicate that in general the feedback mechanism results in a positive contribution to the acoustic source term, in particular at elevated frequencies.