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.

在目前的实验工作中，我们研究了在直径为 4 和 6 毫米的管（略大于淬火直径）内传播的甲烷火焰的热声耦合。选择了多个管子的长度（275 到 900 毫米之间），因此典型系统的基频和谐波频率范围在 190 到 600 Hz 之间。获得了化学计量和贫甲烷-空气混合物的实验结果。结果表明，即使在这种两端开口的小直径管中，热声不稳定性仍然会发展并影响火焰传播动力学。我们表明，根据管子长度（声学），火焰要么完全传播，要么熄灭。在 350 Hz（短管）下，化学计量和稀薄火焰以振荡模式在管的整个长度上传播，而在 190 Hz（长管）下，火焰总是在距离管入口很短的距离内熄灭，即使在 6 毫米直径的管内也是如此。因此，这些结果表明，甲烷火焰的熄灭直径在很大程度上取决于管子的自然声学特性。此外，在 250 Hz（中长管）下，我们确定了一个过渡状态，其中火焰传播是一个随机事件：燃烧、壁传热和声学在该状态下处于激烈的竞争中。我们还观察到，在纵横比为 95 的管中，火焰形状反转类似于郁金香火焰。在此过渡状态中，还观察到从 CH* 发射强度峰值的基频 (250 Hz) 到次谐波频率 (125 Hz) 的模式转移。这种模式转移被认为是在郁金香火焰尖头结构坍塌时发生的，