Further Characterization of the Disturbance Field in a Transversely Excited Swirl-Stabilized Flame

Abstract

This paper describes an analysis of the unsteady flow field in swirl flames subjected to transverse acoustic waves. This work is motivated by transverse instabilities in annular gas turbine combustors, which are a continuing challenge for both power generation and aircraft applications. The unsteady flow field that disturbs the flame consists not only of the incident transverse acoustic wave, but also longitudinal acoustic fluctuations and vortical fluctuations associated with underlying hydrodynamic instabilities of the base flow. We show that the acoustic and vortical velocity fluctuations are of comparable magnitude. The superposition of these waves leads to strong interference patterns in the velocity field, a result of the significantly different wave propagation speeds and axial phase dependencies of these two disturbance sources. Vortical fluctuations originate from the convectively unstable shear layers and absolutely unstable swirling jet. We argue that the unsteady shear layer induced fluctuations are the most dynamically significant, as they are the primary source of flame fluctuations. We also suggest that vortical structures associated with vortex breakdown play an important role in controlling the time-averaged features of the central flow and flame spreading angle, but do not play an important role in disturbing the flame at low disturbance amplitudes. This result has important implications not only for our understanding of the velocity disturbance field in the flame region, but also for capturing important physics in future modeling efforts.

Azimuthal instabilities in annular combustors: standing and spinning modes

“…At velocity antinodes, the symmetry of the perturbation breaks: the heat release is found to be approximately in anti-phase on the two sides of the flame, in the direction of the transverse velocity. In O'Connor & Lieuwen [16], the same break of symmetry happens for vorticity disturbances, which are asymmetric at velocity antinodes. This means that the spatially averaged heat release fluctuation of an injector is smaller at velocity antinodes.…”

Measurement of Flame Frequency Response Functions Under Exhaust Gas Recirculation Conditions

“…This effect is a common aspect of theoretical fiame sheet models, which exhibit a flame-wrinkle response to acoustic excitation along with an explicitly introduced vortical disturbance [26][27][28]. O'Connor and Lieuwen [24], along with Acharya et al [30] also describe the impact of interference related to convective perturbations when considering transverse, rather than axial, acoustic excitations. A theoretical modeling effort by the authors and coworkers attempts to provide a framework in which to distinguish the effects interference due to spatial effects and multiple perturbations based on convective analysis [29].…”

An analytical model for azimuthal thermoacoustic modes in an annular chamber fed by an annular plenum

“…First, the combustor is decomposed into N sectors (Fig. 4) by assuming that every sector can be studied separately and that no flame-to-flame interaction occurs between neighboring sectors, a question which is still open today [32,33]. Staffelbach et al [31] have shown that this was the case in LES of azimuthal modes.…”