Premixed flames propagating within small channels show complex combustion phenomena that differ from flame propagation at conventional scales. Available experimental and numerical studies have documented stationary, non-stationary, or asymmetric modes that depend on properties of the incoming reactant flow as well as channel geometry and wall temperatures. This work seeks to illuminate mechanisms leading to symmetry breaking and limit cycle behavior that are fundamental to these combustion modes. Specifically, four cases of lean premixed methane/air combustion—two equivalence ratios (0.53 and 0.7) and two channel widths (2 mm and 5 mm)—are investigated in a 2D configuration with constant channel length and bulk inlet velocity, where numerical simulations are performed using detailed chemistry. External wall heating is simulated by imposing a linear temperature gradient as a boundary condition on both walls. In the 2 mm channel, both equivalence ratios produce flames that stabilize with symmetric flame fronts after propagating upstream. In the 5 mm channel, flame fronts start symmetrically, although symmetry is broken almost immediately after ignition. Further, 5 mm channels produce non-stationary combustion modes with dramatically different limit cycles: in the leaner case (φ = 0.53), the asymmetric flame front flops periodically, whereas in the richer case (φ = 0.7), flames with repetitive extinctions and ignitions (FREI) are observed. To further understand the flame dynamics, reaction fronts and flame fronts are captured and differentiated. Results show that the loss of flame front symmetry originates in a region close to the flame cusp, where flow and chemical characteristics exhibit large gradients and curvatures. Limit cycle behavior is illuminated by investigating flame edges that are formed along the wall, and accompany local or global ignition and extinction processes. In the flopping mode (φ = 0.53), local ignition and extinction in regions adjacent to the wall result in oblique fronts that advance and recede along the wall and redirect the flow ahead of the flame. In the FREI mode, asymmetric flames propagate much farther upstream, where they experience global extinction due to heat losses, and re-ignite far downstream with opposite flame front orientation. In both cases, an interaction of flow and chemical effects drives the asymmetric limit cycles. The lack of instabilities and asymmetries for the 2mm cases is attributed to insufficient wall separation, which is of the same order of magnitude as the flame thickness.

中文翻译：

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狭窄通道内火焰不稳定性的数值分析：层流预混甲烷/空气燃烧

在小通道内传播的预混火焰显示出复杂的燃烧现象，不同于常规规模下的火焰传播。可用的实验和数值研究已记录了取决于进入的反应物流，通道几何形状和壁温的固定，非固定或不对称模式。这项工作试图阐明导致对称性破坏和限制循环行为的机制，这些机制是这些燃烧模式的基础。具体来说，在恒定通道长度和进口速度的二维配置中，研究了四种稀薄的甲烷/空气稀混合预燃烧（两个当量比（0.53和0.7）和两个通道宽度（2 mm和5 mm））的情况，并进行了数值模拟使用详细的化学方法进行。通过在两壁上施加线性温度梯度作为边界条件来模拟外壁加热。在2 mm的通道中，两个当量比产生的火焰在向上游传播后稳定并具有对称的火焰锋。在5毫米通道中，火焰锋面对称开始，尽管对称性几乎在点燃后就被破坏了。此外，5 mm通道产生的非稳态燃烧模式具有不同的极限循环：在较稀疏的情况下（φ= 0.53），非对称火焰前倾周期性地发生，而在较浓密的情况下（φ= 0.7），火焰反复熄灭和熄灭。观察到起火（FREI）。为了进一步了解火焰动力学，捕获并区分了反应前沿和火焰前沿。结果表明，火焰前部对称性的损失起源于靠近火焰尖端的区域，该区域的流动和化学特性表现出较大的梯度和曲率。通过研究沿壁形成的火焰边缘以及局部或全局着火和熄灭过程，可以阐明极限循环行为。在扑灭模式下（φ= 0.53），在靠近壁的区域局部起火和熄灭会导致倾斜的前沿沿着壁前进和后退，并使气流重新导向火焰。在FREI模式下，非对称火焰向上游传播得更远，由于热量的损失，它们会整体熄灭，并以相反的火焰前沿方向再次向下游点燃。在这两种情况下，流动和化学效应的相互作用都会驱动不对称极限循环。