The characteristics of the flame structure, stabilization, and extinction of counterflow nonpremixed flames of CH₄/He versus air at low strain rates are investigated by performing a series of experiments and two-dimensional (2-D) numerical simulations. By adopting an experimental methodology using He curtain flow, we can locate the flames near the center of the counterflow burner and measure the critical He mole fraction in the fuel stream, $X_{\text{He},\text{cr}}$, for flame extinction at very-low strain rates. $X_{\text{He},\text{cr}}$ obtained from 2-D numerical simulations in normal and zero gravity show a good agreement with those from the experiments, which substantiates that the experimental methodology can effectively reduce the buoyancy effect at low strain rates. It is found from various steady and unsteady 2-D numerical simulations that the dynamics of flame edge plays a critical role in determining the flame stabilization and extinction, and the edge flame is stabilized at a location where negative edge flame propagation speed, $S_{\mathrm{e}}$, balances positive local flow velocity, $U_{\mathrm{e}}$. The transport budget analysis reveals that despite the negative $S_{\mathrm{e}}$ by the diffusive loss of heat and radicals, the edge flame can survive by the help of the convective gain of heat and radicals from the trailing diffusion flame. It is also found that the counterflow flame can survive the increase of He mole fraction in the fuel stream, $X_{\text{He}}$, by shrinking its flame length since the local chemical reaction at the flame edge is enhanced with decreasing the flame length. However, as $X_{\text{He}}$ exceeds $X_{\text{He},\text{cr}}$, a slight inward movement of the edge flame induces a large magnitude of negative $S_{\mathrm{e}}$ compared to positive $U_{\mathrm{e}}$ such that the counterflow flame is totally extinguished by the shrinkage of the outer edge flame toward the flame center.

通过执行一系列实验和二维 (2-D) 数值模拟，研究了在低应变率下 CH ₄ /He 与空气的逆流非预混火焰的火焰结构、稳定性和熄灭特性。通过采用氦气帘流的实验方法，我们可以将火焰定位在逆流燃烧器中心附近，并测量燃料流中的临界氦摩尔分数，$X_{\text{他},\text{铬}}$，用于在非常低的应变率下熄灭火焰。 $X_{\text{他},\text{铬}}$法向和零重力下二维数值模拟结果与实验结果吻合较好，证明该实验方法可以有效降低低应变率下的浮力效应。从各种稳态和非稳态二维数值模拟中发现，火焰边缘的动力学在决定火焰稳定和熄灭方面起着关键作用，边缘火焰稳定在负边缘火焰传播速度的位置，${秒}_{\mathrm{电子}}$，平衡正局部流速， ${你}_{\mathrm{电子}}$. 交通预算分析表明，尽管负${秒}_{\mathrm{电子}}$通过热量和自由基的扩散损失，边缘火焰可以借助来自尾随扩散火焰的热量和自由基的对流增益而存活下来。还发现逆流火焰可以承受燃料流中 He 摩尔分数的增加，$X_{\text{他}}$，因为火焰边缘的局部化学反应会随着火焰长度的减少而增强，从而缩短其火焰长度。然而，作为$X_{\text{他}}$ 超过 $X_{\text{他},\text{铬}}$，边缘火焰的轻微向内运动会引起大量的负 ${秒}_{\mathrm{电子}}$ 与正面相比 ${你}_{\mathrm{电子}}$ 使外缘火焰向火焰中心收缩，使逆流火焰完全熄灭。