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.

This experimental investigation concerns the effect of imposed axial acoustic velocity fluctuations on the primary atomization of a hollow cone liquid sheet in the presence of a strong air swirl. The atomization dynamics are elucidated by positioning the spray at an acoustic velocity node, antinode, and a mixed point in the standing wave field generated due to the imposed axial acoustic excitation. High-speed shadowgraph images acquired in-sync with dynamic pressure measurements are processed to clarify the unstable behavior observed in the spray dynamics; this was achieved by extracting key parameters such as breakup length, spatial growth rates, phase differences, and by employing Proper Orthogonal Decomposition (POD). A novel method to obtain the breakup length of a hollow cone spray from the position of maximum wave amplitude is presented. The breakup length is the smallest for the mixed point. The phase difference between the left and right half-angle fluctuations shows that the flapping motion of the spray is predominantly observed at the mixed point for different air-to-liquid ratios. Another novel approach is adopted to identify the physical mechanisms corresponding to each POD spatial mode by comparing POD spatial modes obtained from experiments to those generated artificially.

This paper reports an experimental investigation on the effect of imposed acoustic velocity fluctuations on the primary atomization of a hollow cone liquid sheet in the presence of strong air swirl. The atomization dynamics is elucidated by positioning the spray at an acoustic velocity node, antinode, and a mixed point in the standing wave field generated due to the imposed axial acoustic excitation. High speed shadowgraphy images acquired in-sync with dynamic pressure measurements are processed to extract breakup length, spatial growth rates etc. A novel method to obtain the breakup length of a hollow cone spray from the position of maximum wave amplitude is presented. The phase difference between the left and right half-angle fluctuations shows that the flapping motion of the spray is predominantly observed at the mixed point for different air to liquid ratios.