Response of a laminar premixed V-flame to a high-frequency transverse acoustic field

• Published in: Combustion and flame Vol. 161; no. 5; pp. 1247 - 1267
• Main Authors: Baillot, Françoise, Lespinasse, Florian
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier Inc 01-05-2014; Elsevier
• Subjects: Acoustics; Amplitudes; Applied sciences; CH∗ emission; Combustion; Combustion instability; Combustion. Flame; Energy; Energy. Thermal use of fuels; Engineering Sciences; Exact sciences and technology; Fluctuation; Helical; Instability; Miscellaneous; Mode conversion; Modulation; Reactive fluid environment; Straightening; Subharmonic frequency bifurcation; Theoretical studies. Data and constants. Metering; Transverse acoustic pressure field; V-flame; Combustion instability; Transverse acoustic pressure field; V-flame; Subharmonic frequency bifurcation; Mode conversion; CH∗ emission; Laminar flame; Acoustic wave; CH emission; Bifurcation; Emission; Premixed flame; Pressure; Transverse wave; High frequency
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2013.11.009

This work presents new results describing the action of an acoustic transverse standing wave on a methane–air premixed V-flame located at a pressure antinode. Investigations of the jet highlight a mode conversion from a transverse cavity mode to a longitudinal mode, involving a “plugging” flow-rate modulation. This essential mechanism generates vortical structures convected toward the flame stabilized on a vertical rod introduced inside the burner and aligned with its axis. Depending on acoustic conditions, they develop through one of the following patterns: a pairing process, a multiple-vortex interaction in the jet outer layer, or a helical mode in the inner layer behind the rod. The flame responses are arranged in the physical space defined by the acoustic pressure amplitude (P∼ref) and the Strouhal numbers, St characteristic of the dominant (outer or inner) shear-layer. A harmonic flame response at the forcing frequency, f0, is not the only possible behavior. The other behaviors observed are an asymmetrical “elongated wrinkled flame” due to the helical mode; an “aperiodic fluctuating flame”; and a “subharmonic rolled-up flame” due to the pairing process, which induces strong CH∗ emission fluctuations at f0/2. Several flame dynamics evolutions are noted when P∼ref increases. The heat release rate (h.r.r.) always begins to fluctuate at f0. Then, depending on St, it can undergo the nonlinear frequency bifurcation; and next, it may fluctuate at f0 again in addition to f0/2. Another scenario shows the h.r.r. losing any kind of ordered modulation before it eventually fluctuates at f0. The flame stabilization is described at blowout via the leading edge behavior. Foot-displacement amplitude, and foot-width thinning and straightening are complementary dynamics and morphology features that characterize flame destabilization. The different patterns, observed as P∼ref increases, of successive ordered flame responses, sometimes alternating with aperiodic responses, appear as the main elements in understanding the processes involved in thermoacoustic instabilities.