Abstract The mechanisms of flame blowout under pressure gradient effects are explored for a bluff-body stabilized flame. The blowout process is induced through equivalence ratio reduction from a lean stabilized flame to complete blowout. Simultaneous high-speed particle image velocimetry (PIV) and C2*/CH* chemiluminescence imaging diagnostics are used to obtain the instantaneous flame structure, vorticity field, equivalence ratio, and local strain rate during the extinction process. The goal is to elucidate the effect of flame-generated vorticity on lean flame extinction. Three test-sections configured as a nozzle, a rectangular duct, and a diffuser, are used to alter the downstream pressure gradient yielding high, nominal, and low magnitudes of flame-generated baroclinic torque. For all three configurations, the flame brush narrows and the shear layer vorticity expands in the transverse direction resulting in flame-shear interactions and extinction. The flame-shear layer interaction increases the strain rate along the flame; however, the strong flame-generated vorticity for the nozzle case delayed the strain rate increase by keeping the flame away from the shear layer the longest. The sharp increase in the Karlovitz number above unity caused by the sudden increase in the strain rate corresponds to the time of flame brush contraction and shear layer width expansion. It is shown that the downstream pressure gradient can either augment or attenuate the time required for the Karlovitz number to reach a critical value of unity, which is associated with local extinctions along the flame. In all of the test-section configurations, the flame-generated vorticity has a weak influence on the Benard von Karman (BVK) instability mode and its harmonics. The Strouhal number during blow-out remained relatively constant in all of the cases showing greater sensitivity to the shear layer length than to the BVK frequency.

中文翻译：

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压力梯度对钝体火焰熄灭机制的影响

摘要 研究了钝体稳定火焰在压力梯度效应下的火焰爆裂机理。熄火过程是通过当量比从稀薄的稳定火焰到完全吹灭而引起的。同时采用高速粒子图像测速 (PIV) 和 C2*/CH* 化学发光成像诊断技术，获取消光过程中的瞬时火焰结构、涡量场、当量比和局部应变率。目的是阐明火焰产生的涡流对贫火焰熄灭的影响。三个测试部分配置为喷嘴、矩形管道和扩散器，用于改变下游压力梯度，产生高、标称和低量级的火焰产生的斜压扭矩。对于所有三种配置，火焰刷变窄，剪切层涡度在横向上扩大，导致火焰-剪切相互作用和消失。火焰-剪切层相互作用增加了沿火焰的应变率；然而，喷嘴情况下强烈的火焰产生的涡流使火焰远离剪切层的时间最长，从而延迟了应变率的增加。由应变率的突然增加引起的高于单位的卡洛维茨数的急剧增加对应于火焰刷收缩和剪切层宽度膨胀的时间。结果表明，下游压力梯度可以增加或减弱卡洛维茨数达到统一临界值所需的时间，这与沿火焰的局部消光有关。在所有测试段配置中，火焰产生的涡度对 Benard von Karman (BVK) 不稳定性模式及其谐波有微弱的影响。在所有情况下，爆裂期间的 Strouhal 数保持相对恒定，显示对剪切层长度的敏感性高于对 BVK 频率的敏感性。