Contributions of hydrodynamic features of a swirling flow to thermoacoustic instabilities in a lean premixed swirl stabilized combustor

Abstract

A comprehensive study on influences of hydrodynamic features of a swirling flow on thermoacoustic instabilities in a swirl stabilized combustor is performed by using large eddy simulations along with the dynamically thickened flame combustion model. The governing equations in full compressible form are solved by an in-house developed high-order numerical solver. The combustor is simulated in six different equivalence ratios to assess effects of equivalence ratio on the contributions of hydrodynamic features in inducing thermoacoustic instabilities. The obtained results show that the combustor suffers from combustion instabilities at equivalence ratios of 0.55, 0.6, 0.75, and 0.8, while it is stable at the midrange equivalence ratios (0.65 and 0.7). The results indicate that the instabilities are the result of the lock-in mechanism between heat release fluctuations induced by hydrodynamic features and the mixed first tangential and quarter wave longitudinal mode of the combustor. Investigations are carried out to evaluate contributions of central and side recirculation zones, precessing vortex core, and coherent structures in heat release fluctuations. The results show that contributions of hydrodynamic features highly depend on the combustor operating condition. At low equivalence ratios (0.55 and 0.6), coherent structures and side and central recirculation zones are the key features to induce heat release fluctuations in phase with the acoustic perturbations, while at equivalence ratios of 0.75 and 0.8, coherent structures and precessing vortex core play the main role in inducing combustion instabilities.