Response of a swirl-stabilized flame to transverse acoustic excitation

This work addresses the issue of transverse combustion instabilities in annular gas turbine combustor geometries. While modern low-emissions combustion strategies have made great strides in reducing the production of toxic emissions in aircraft engines and power generation gas turbines, combustion instability remains one of the foremost technical challenges in the development of next generation combustor technology. To that end, this work investigates the response of a swirling flow and swirl-stabilized flame to a transverse acoustic field is using a variety of high-speed laser techniques, especially high-speed particle image velocimetry (PIV) for detailed velocity measurements of this highly unsteady flow phenomenon.;Several important issues are addressed. First, the velocity-coupled pathway by which the unsteady velocity field excites the flame is described in great detail. Here, a transfer function approach has been taken to illustrate the various pathways through which the flame is excited by both acoustic and vortical velocity fluctuations. It has been shown that while the direct excitation of the flame by the transverse acoustic field is a negligible effect in most combustor architectures, the coupling between the transverse acoustic mode in the combustor and the longitudinal mode in the nozzle is an important pathway that can result in significant flame response. In this work, the frequency response of this pathway as well as the resulting flame response is measured using PIV and chemiluminescence measurements, respectively.;Next, coupling between the acoustic field and the hydrodynamically unstable swirling flow provides a pathway that can lead to significant flame wrinkling by large coherent structures in the flow. Swirling flows display two types of hydrodynamic instability: an absolutely unstable jet and convectively unstable shear layers. The absolute instability of the jet results in vortex breakdown, a large recirculation zone along the centerline of the flow. Experiments in this study showed that high amplitudes of acoustic forcing could alter both the time-average and dynamical characteristics of this structure, although very little effect was measured at low amplitude forcing. The convectively unstable shear layers, however, displayed significant response to the acoustics, even at low levels of acoustic forcing, and are responsible for the majority of the flame wrinkling and resultant heat release fluctuation of the flame. The modal structure of the transverse acoustic field played a large role in the characteristics of the response of these structures.;The two major contributions of this work are the development of a detailed velocity-coupled pathway for transversely forced flames, as well as a methodology based on the principles of hydrodynamic stability theory by which to assess the response of complex combusting flows to acoustic fields. Additionally, a large archival data set has been produced with measurements of velocity and flame behavior for future modeling and analysis.