Low swirl premixed methane-air flame dynamics under acoustic excitations

Abstract

In this study, simultaneous particle image velocimetry and planar laser induced fluorescence of hydroxyl radical, chemiluminescence imaging, and hot-wire measurements are utilized to study reacting low swirl flow dynamics under low to high amplitude acoustic excitations. Results show that a temporal weak recirculation zone exists downstream of the flame, which is enlarged in size under acoustic excitations. Investigations show that temporal behaviors of this recirculation zone play a significant role in flame movements and instabilities. As the acoustic wave amplitude increases, the flame lift-off distance changes drastically, resulting in flame instabilities (flashback and blowout) during the excitations. Prior to the flame blowout, although the flame lift-off distance responds periodically to the acoustic perturbations, heat release fluctuations display an aperiodic response. Flame dynamics are further studied by calculated power spectra of acoustic velocity and heat release fluctuations and reconstructed phase portraits of heat release fluctuations. Investigations show that increasing the forcing amplitude results in more deterministic features in the flame dynamics and amplification of the higher harmonic modes in the heat release fluctuations. However, such regular patterns become scattered prior to the flame blowout due to the existence of nonlinearities induced by high amplitude excitations. It is speculated that flame blowout can be a symptom of such nonlinearities. The Rayleigh index is measured to study thermoacoustic couplings. At low amplitude excitations, various coupling patterns occur at the flame. However, such complex patterns are replaced by simple coherent patterns, when the flame is excited by high amplitude acoustic perturbations.