Structure and Dynamics of Premixed Swirl Flames at Elevated Power Density

Abstract

The Turbomeca gas turbine model combustor was studied to understand the effect of increased thermal power density on the structure and dynamics of premixed swirl flames. The burner was operated under two flame conditions: one stable, and one with self-excited thermoacoustic oscillations. Simultaneous measurements of scalar and three-component velocity fields were acquired at 3 and 6 kHz frequencies. Nonreacting flow measurements were used to approximate the spatial and temporal resolution with respect to the scales of the incoming flow. Time-resolved velocimetry revealed the presence of periodic fluctuations in both flames, occurring at shifted frequencies that did not correspond to a resonant acoustic mode or any corresponding harmonics. Proper orthogonal decomposition analysis revealed stably forced inner shear-layer oscillations that periodically formed and ejected symmetric vortex pairs under stable flame operation. The unstable flame was found to exhibit a single spatiotemporally evolving flow structure consistent with a helical precessing vortex core. A reconstructed time series elucidated the interactions of the reactant jets, the precessing vortex core, and the central recirculation bubble over a thermoacoustic cycle. This coupled interaction, and the resultant modulation of transport within the inner shear layer, were identified as a mechanism by which the precessing vortex core was linked to elevated power density flame dynamics.