Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets
Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets
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DOI：
10.1016/j.combustflame.2023.113003
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发表时间：
2023-11
影响因子：
4.4
通讯作者：
Prabakaran Rajamanickam;Aiden Kelly;J. Daou
中科院分区：
文献类型：
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作者：
Prabakaran Rajamanickam;Aiden Kelly;J. Daou
在非预混微燃烧装置中观察到的扩散火焰街平行于剪切流排列。据观察，如果流动雷诺数或佩克莱特数 Pe 超过临界值，则它们会出现在具有高路易斯数 (Le) 燃料的混合物中。这些观察结果背后的潜在机制尚未完全了解。在本文中，我们将扩散热不稳定性和泰勒色散之间的耦合确定为能够解释上述实验观察结果的机制。该解释主要基于以下事实：泰勒色散增强了流动方向上的所有扩散过程，从而有效地导致各向异性扩散，在流动方向上具有有效（与流动相关）路易斯数，该数与 Pe≫ 1 的 1/Le 成正比。通过研究与狭窄通道中的平面泊肃叶流平行建立的平面扩散火焰的稳定性，在一个简单模型中验证了所确定的机制。线性稳定性分析导致了特征值问题的数值解决，表明当佩克莱特数超过临界值时，高路易斯数燃料会出现细胞（或有限波长）不稳定性。此外，对于低于该临界值的佩克莱特数，可以获得有或没有时间振荡的长波不稳定性。稳定状态图用于说明 Le−Pe 平面中的情况，其中识别了各种不稳定域。最后，线性分析得到时间相关数值模拟的支持和补充，描述了不稳定扩散火焰的演化。模拟证明了稳定细胞结构的存在，并表明长波不稳定性有利于火焰熄灭。
Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number (Le) fuels, provided that the flow Reynolds number, or the Peclet number Pe, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to 1/Le for Pe≫ 1. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a Le− Pe plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.