Abstract This paper presents high-fidelity flame structure measurements of premixed methane–air Bunsen flames subjected to extreme levels of turbulence. Specifically, 28 cases were studied with longitudinal integral length scales (Lx) as large as 43 mm, turbulence levels (u′/SL) as high as 246, and turbulent Reynolds (ReT,0) and Karlovitz (KaT) numbers up to 99,000 and 533, respectively. Two techniques were employed to measure the preheat and reaction layer thicknesses of these flames. One consisted of planar laser-induced fluorescence (PLIF) imaging of CH radicals, while the other involved taking the product of simultaneously acquired PLIF images of formaldehyde (CH2O) and hydroxyl (OH) to produce “overlap-layers.” The average preheat layer thicknesses are found to increase with increasing u′/SL and with axial distance from the burner (x/D). In contrast, average reaction layer (i.e. CH- and overlap-layer) thicknesses did not increase appreciably even as u′/SL increased by a factor of ∼ 60. Furthermore, the reaction layer thicknesses (based on the CH images only) did not increase with increasing x/D. The reaction layers are also observed to remain continuous; that is, local extinction events are rarely observed. Although based on a sequence of combined CH–OH PLIF images acquired at a rate of 10 kHz, it is apparent that when instances of local extinction do occur they are the result of cool gas entrainment. The results presented here, as well as those from 12 prior experimental and 9 numerical investigations, do not agree with the predicted Klimov–Williams boundary on the theoretical Borghi Diagram. Thus, a new Measured Regime Diagram is proposed wherein the Klimov–Williams criterion is replaced by a metric that relates the turbulent diffusivity ( D T = u ′ L x ) to the molecular diffusivity within the preheat layer ( D * = S L δ F , L ). Justification for this replacement is based on physical reasoning and the fact that the line defined by DT/D* ≈ 180 accurately separates cases with thin flamelets from those with broadened preheat yet thin reaction layers (i.e. BP-TR flames). Additionally, the results suggest that the BP-TR regime extends well beyond what was previously theorized since neither broken nor broadened reaction layers were observed under conditions with Karlovitz numbers as high as 533, which is five times higher than the theoretical boundary.

中文翻译：

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受极端湍流影响的预混火焰第 I 部分：火焰结构和新的测量状态图

摘要 本文介绍了在极端湍流条件下的预混合甲烷-空气本生火焰的高保真火焰结构测量。具体而言，研究了 28 个案例，纵向积分长度尺度 (Lx) 高达 43 mm，湍流水平 (u'/SL) 高达 246，湍流雷诺数 (ReT,0) 和卡洛维茨 (KaT) 数高达 99,000和 533，分别。采用两种技术来测量这些火焰的预热和反应层厚度。一种包括 CH 自由基的平面激光诱导荧光 (PLIF) 成像，而另一种涉及同时获取甲醛 (CH2O) 和羟基 (OH) 的 PLIF 图像的产物以产生“重叠层”。发现平均预热层厚度随着 u'/SL 的增加和与燃烧器的轴向距离 (x/D) 的增加而增加。相比之下，即使 u'/SL 增加了约 60 倍，平均反应层（即 CH-和重叠层）厚度也没有明显增加。此外，反应层厚度（仅基于 CH 图像）没有随着 x/D 的增加而增加。还观察到反应层保持连续；也就是说，很少观察到局部灭绝事件。尽管基于以 10 kHz 的速率获取的一系列组合 CH-OH PLIF 图像，但很明显，当局部消光确实发生时，它们是冷气体夹带的结果。此处提供的结果以及来自 12 个先前实验和 9 个数值研究的结果与理论 Borghi 图上预测的 Klimov-Williams 边界不一致。因此，提出了一种新的测量模式图，其中 Klimov-Williams 准则被一个度量所取代，该度量将湍流扩散率 (DT = u ' L x ) 与预热层内的分子扩散率 (D * = SL δ F , L ) 联系起来。这种替换的理由是基于物理推理和由 DT/D* ≈ 180 定义的线准确地将具有细火焰的情况与具有加宽的预热但薄的反应层（即 BP-TR 火焰）的情况分开的事实。此外，结果表明 BP-TR 范围远远超出了先前理论的范围，因为在卡洛维茨数高达 533（比理论边界高 5 倍）的条件下，既没有观察到破裂也没有观察到反应层变宽。