Thermoacoustic coupling of premixed flames in mesoscale diameter tubes

In the present experimental work, we investigate the thermoacoustic coupling of methane flames propagating inside 4 and 6 mm diameter tubes (slightly larger than the quenching diameter). Multiple tube’s lengths were selected (between 275 and 900 mm) so typical system’s fundamental and harmonic frequencies ranged between 190 and 600 Hz. Experimental results were obtained for stoichiometric and lean methane-air mixtures. The results show that even in such small diameter tubes, open at both ends, thermoacoustic instability still develops and impacts the flame propagation dynamics. We show that depending on the tube length (acoustics) the flame either fully propagates or extinguishes. At 350 Hz (short tubes) both stoichiometric and lean flames propagated over the entire length of the tube in an oscillatory mode, whereas at 190 Hz (long tubes) the flames always quenched within a short distance from the tube’s entrance, even inside the 6 mm diameter tube. These results thus show that the quenching diameter of methane flames can be highly dependent on the tube’s natural acoustics. Moreover, at 250 Hz (medium length tubes), we identified a transition regime where the flame propagation is a stochastic event: combustion, wall-heat transfer and acoustics are in fierce competition in this regime. We have also observed, in tubes with an aspect ratio of 95, a flame shape inversion resembling the tulip flame. In this transition regime, a mode transfer from the fundamental (250 Hz) to subharmonic frequency (125 Hz) of the peak of CH* emission intensity was also observed. This mode transfer is believed to occur when the tulip-flame cusped structure collapses, promoting the mixture of burnt gases with fresh gases, and the consequent increase of the ignition delay and therefore the delay of the peak of CH* emission intensity.