Effect of Transverse Acoustic Excitation Over a Partially-Premixed Swirling Flame

This is the authors' abstract. We don't add key points for this paper.

In this work, we investigate the effect of transverse acoustic excitation on a partially premixed swirling flame. Azimuthal instabilities in annular gas turbine combustors are one of the major challenges. Thermoacoustic instabilities are driven by azimuthal acoustic modes. We have designed and fabricated a multi-nozzle linear array combustor to simulate the flow conditions of an annular combustor. The nozzle features an axial swirler with a centerbody acting as a fuel injector. Simultaneous high-speed chemiluminescence (OH*) and high-frequency pressure measurements are conducted to measure the time-averaged flame shape and the chamber acoustics respectively. These measurements are taken at multiple forcing frequencies and amplitude under in-phase (IP) and out-of-phase (OP) conditions. It is observed that, at one of the combustor modes (580Hz), during OP forcing, the flame transitions from a V-shape to an M-shape as the forcing amplitude is increased. However, with IP forcing the flame continues to remain in V-shape with the increase in forcing amplitude. Spectral proper orthogonal decomposition (SPOD) analysis of OH* images show that for OP forcing dominant mode shapes are present mostly at 0.5D away from the center-line, but for IP forcing the dominant mode shapes indicate combined axial and radial oscillations in the OH* intensity field. Furthermore, cross-wavelet transform (XWT) analysis shows a strong correlation between pressure fluctuations and OH* fluctuations around the forcing frequency for IP forcing, and a weak or intermittent correlation for OP forcing. Based on the acoustic simulation, instantaneous time snapshots of OH* chemiluminescence and existing literature, we hypothesize that the transition from V-shape to M-shape can be due to acoustically induced velocity and vortical fluctuations near burner lip during OP forcing. This causes the V-flame to stretch, roll outward, and stabilize in the outer recirculation zone (ORZ).