Intermittency as a Transition State in Combustor Dynamics: An Explanation for Flame Dynamics Near Lean Blowout

The dynamic transitions preceding lean blowout were investigated experimentally in a laboratory scale turbulent combustor by systematically varying the flow Reynolds number . Previous studies on combustor dynamics have shown that the onset of large-amplitude, combustion-driven oscillations is, at times, presaged by intermittent bursts of high-amplitude periodic pressure pulsations. These intermittent bursts appear in a near random fashion amidst regions of aperiodic low-amplitude fluctuations, provided the underlying flow-field is turbulent. In the present study, we show that intermittent burst oscillations are also observed in combustors close to the lean blowout limit. We show that such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of pressure oscillations. The formation of such orbits points to the complex nature of the interaction between the hydrodynamics and acoustic subsystems, which operate over a range of different time scales. High-speed flame images reveal that the intermittent states observed prior to lean blowout correspond to aperiodic detachment of the flame from the bluff-body lip. These findings are consistent with other reports of possibly intermittent states in the literature.本研究通过系统改变流动雷诺数，在实验室尺度湍流燃烧室中实验探究了贫油熄火（lean blowout）前的动态转变过程。既往关于燃烧室动力学的研究表明，大振幅燃烧驱动振荡的起振过程，有时会以间歇性的高振幅周期压力脉动爆发作为前兆。当底层流场为湍流时，这类间歇性爆发会准随机地出现在非周期低振幅脉动区域内。本研究发现，在接近贫油熄火极限的燃烧室中同样可观测到间歇性爆发振荡，且此类间歇性振荡通过压力振荡相空间中的同宿轨道（homoclinic orbits）形成而涌现。这类轨道的形成揭示了流体动力学与声学子系统之间相互作用的复杂本质，二者的特征时间尺度跨度较大。高速火焰成像结果显示，贫油熄火前观测到的间歇性状态对应火焰从钝体唇口发生非周期脱离。本研究结果与文献中关于间歇性状态的其他相关报道一致。