The Contribution of Intrinsic Thermoacoustic Feedback to Combustion Noise and Resonances of a Confined Turbulent Premixed Flame

The influence of Intrinsic ThermoAcoustic (ITA) feedback on the noise spectrum produced by a confined, turbulent, premixed, swirl flame is investigated. Sound generation due to unsteady heat release is understood to result from inherent turbulent fluctuations on the one hand, and from the flame response to incoming acoustic perturbations on the other. The former are described by a source term for unsteady heat release, the latter by the flame transfer function. Both quantities are identified from time series data for fluctuating velocity and heat release rate, generated with Large Eddy Simulation (LES) of a combustion test rig. Source term and transfer function are then introduced in an acoustic network model of the test rig in order to compute the sound pressure level at a determined location. Results for the noise spectrum are in good agreement with experiment, showing a broadband component and well-defined peaks. The frequencies of the peaks correspond to either acoustic cavity or ITA resonances. The acoustic network model is used for parametric studies, where the acoustic reflection coefficient at the combustor exit is varied. Remarkably, it is found that the magnitude of the ITA peak increases with decreasing values of the acoustic reflection coefficient, and vice versa. Furthermore, the influence of combustion chamber length on resonance frequencies is explored. It is observed that the frequency of the ITA resonance is insensitive to combustor length. This behaviour is observed qualitatively also in experiments.     «

The influence of Intrinsic ThermoAcoustic (ITA) feedback on the noise spectrum produced by a confined, turbulent, premixed, swirl flame is investigated. Sound generation due to unsteady heat release is understood to result from inherent turbulent fluctuations on the one hand, and from the flame response to incoming acoustic perturbations on the other. The former are described by a source term for unsteady heat release, the latter by the flame transfer function. Both quantities are identified fro...     »