Low swirl premixed methane-air flame dynamics under acousticexcitations

• Published in: Physics of fluids (1994) 2019-09, Vol.31 (9)
• Main Authors: Farshchi, M, Chakravarthy, S R, Chakraborty, A, Aravind, I B
• Format: Article
• Language: English
• Subjects: Acoustic velocity; Acoustic waves; Acoustics; Amplitudes; Chemiluminescence; Couplings; Downstream effects; Excitation; Flame stability; Flashback; Fluid dynamics; Hydroxyl radicals; Liftoff; Organic chemistry; Particle image velocimetry; Physics; Planar laser induced fluorescence; Power spectra; Variation
• ISSN: 1070-6631; 1089-7666
• DOI: 10.1063/1.5118826

In this study, simultaneous particle image velocimetry and planar laser inducedfluorescence of hydroxyl radical, chemiluminescence imaging, and hot-wire measurements areutilized to study reacting low swirl flow dynamics under low to high amplitude acousticexcitations. Results show that a temporal weak recirculation zone exists downstream of theflame, which is enlarged in size under acoustic excitations. Investigations show thattemporal behaviors of this recirculation zone play a significant role in flame movementsand instabilities. As the acoustic wave amplitude increases, the flame lift-off distancechanges drastically, resulting in flame instabilities (flashback and blowout) during theexcitations. Prior to the flame blowout, although the flame lift-off distance respondsperiodically to the acoustic perturbations, heat release fluctuations display an aperiodicresponse. Flame dynamics are further studied by calculated power spectra of acousticvelocity and heat release fluctuations and reconstructed phase portraits of heat releasefluctuations. Investigations show that increasing the forcing amplitude results in moredeterministic features in the flame dynamics and amplification of the higher harmonicmodes in the heat release fluctuations. However, such regular patterns become scatteredprior to the flame blowout due to the existence of nonlinearities induced by highamplitude excitations. It is speculated that flame blowout can be a symptom of suchnonlinearities. The Rayleigh index is measured to study thermoacoustic couplings. At lowamplitude excitations, various coupling patterns occur at the flame. However, such complexpatterns are replaced by simple coherent patterns, when the flame is excited by highamplitude acoustic perturbations.