Thermoacoustic instability drivers and mode transitions in a lean premixed methane-air combustor at various swirl intensities

Abstract

In this study, an in-house developed fully compressible solver utilizing high-order numerical discretization schemes is used to study the effects of swirl intensity on thermoacoustic instabilities in a lean premixed swirl stabilized combustor. Turbulent combustion modeling is achieved using large eddy simulations along with dynamically thickened flame combustion model. Obtained results reveal that there are two critical burner swirl numbers in which the combustor dynamics change drastically. Results show that the combustor is stable at low swirl numbers (S ≤ 0.5). However, as the swirl intensity increases, the combustor experiences thermoacoustic instabilities. The first critical swirl number lies within the range of 0.5–0.6, in which a transition occurs in the combustor dynamics from a state of stable operation to thermoacoustic instabilities. The second critical swirl number (S = 0.9) corresponds to the transition of different thermoacoustic instability modes. The combustor experiences limit cycle instabilities coupled with the first tangential acoustic mode of the combustor for 0.6 ≤ S < 0.9, while for S > 0.9 the instabilities are burst like and coupled with the second tangential acoustic mode of the combustor. The combustor experiences instabilities coupled with both the first and the second tangential modes, when the burner swirl number is 0.9. In order to evaluate key parameters inducing the thermoacoustic instabilities, heat release fluctuations are calculated through hydrodynamic flow features (central and side recirculation zones, coherent structures, and precessing vortex core). Investigations show that all the above features contribute almost equally in inducing heat release fluctuations in-phase with the acoustic perturbations at low swirl numbers. However, as the swirl number increases, side recirculation zone and coherent structures play the key role in producing heat release fluctuations in-phase with the acoustic perturbations, while central recirculation zone has the least effects on driving the instabilities.