Amplitude-Dependent Flow Field and Flame Response to Axial and Tangential Velocity Fluctuations

Abstract

Lean premixed combustion is prone to thermoacoustic instabilities. These mostly self-excited instabilities are caused by a feedback mechanism between the acoustic field, hydrodynamic structures, and the heat release rate in the flame. While various modeling tools are available for the linear analysis of thermoacoustic systems, a detailed knowledge of the governing nonlinearities, responsible for the saturation in the flame response, is still missing. The fundamental understanding of the flow field–heat release interaction is of crucial importance for the prediction of the pressure oscillation amplitude. To improve the understanding of these interaction processes, the current paper investigates the nonlinear interaction of the flow field and the unsteady heat release rate and the role of swirl fluctuations. The test rig that is used in the present work consists of a generic swirl-stabilized combustor fed with natural gas and equipped with a high-amplitude forcing device. The influence of the phase between axial and azimuthal velocity oscillations is assessed on the basis of the amplitude and phase relations between the velocity fluctuations at the inlet and the outlet of the burner. These relations are determined in the experiment with the Multi-Microphone-Method and a two component laser-Doppler velocimeter. Particle image velocimetry and OH*-chemiluminescence measurements are conducted to study the interaction between the flow field and the flame. For several frequency regimes, characteristic properties of the forced flow field and flame are identified, and a strong amplitude dependence is observed. It is found that the convective time delay between the swirl generator and the flame has an important influence on swirl-number oscillations and the flame dynamics in the low-frequency regime. For mid and high frequencies, significant changes in the mean flow field and the mean flame position are identified for high forcing amplitudes. These affect the interaction between coherent structures and the flame and are suggested to be responsible for the saturation in the flame response at high forcing amplitudes.