Flame fingers and interactions of hydrodynamic and thermodiffusive instabilities in laminar lean hydrogen flames
Flame fingers and interactions of hydrodynamic and thermodiffusive instabilities in laminar lean hydrogen flames
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DOI：
10.1016/j.proci.2022.07.010
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发表时间：
2023-06-07
影响因子：
3.4
通讯作者：
Pitsch, Heinz
中科院分区：
文献类型：
--
作者：
Berger, Lukas;Grinberg, Michael;Pitsch, Heinz
贫氢/空气火焰容易产生流体动力学和热扩散不稳定性。在这项工作中，通过使用不同的扩散模型和状态方程对层流平面贫氢/空气火焰进行详细模拟，分别量化了每种不稳定机制的贡献，以选择性地抑制流体动力或热扩散不稳定机制。从模拟初始阶段的分析来看，热扩散不稳定性在火焰动力学中起主导作用。如果微分扩散，因此，热扩散不稳定性被抑制，火焰的特点是不稳定性增长率的强烈降低，而如果存在，由于微分扩散的强烈不稳定性质，观察到大范围的不稳定波数。当不稳定性充分发展时，稀薄氢/空气火焰的特征是形成小尺度的细胞结构和大尺度的火焰指。虽然已知前者的大小接近线性稳定性分析中最不稳定的波长，但这项工作表明，火焰指也源于热扩散不稳定，最值得注意的是，与两种不稳定机制的相互作用无关。它们相对于外部扰动是稳定的，并具有增强火焰传播的特点，因为在它们的尖端形成一个中心尖端，使得两个具有高反应性的强弯曲前缘共存。热扩散不稳定性对火焰的消耗速度有显著影响，而流体动力不稳定性对火焰消耗速度的增强作用要小得多。此外，如果缺少两种不稳定机制中的一种，由于起皱而增加的表面积将大大减少。这与两种机制之间的协同作用有关，因为由于火焰指前方流线的扩大，流体动力不稳定性的存在增强了火焰指的传播。&副本;2022年燃烧研究所。Elsevier Inc.出版。版权所有。
Lean hydrogen/air flames are prone to hydrodynamic and thermodiffusive instabilities. In this work, the con-tribution of each instability mechanism is quantified separately by performing detailed simulations of laminar planar lean hydrogen/air flames with different diffusivity models and equations of state to selectively suppress the hydrodynamic or thermodiffusive instability mechanism.From the analysis of the initial phase of the simulations, the thermodiffusive instability is shown to domi-nate the flame dynamics. If differential diffusion and, hence, the thermodiffusive instability is suppressed, the flame features a strong reduction of the instability growth rates, whereas if present, a wide range of unstable wave numbers is observed due to the strong destabilizing nature of differential diffusion. When instabili-ties are fully developed, lean hydrogen/air flames feature the formation of small-scale cellular structures and large-scale flame fingers. While the size of the former is known to be close to the most unstable wave length of a linear stability analysis, this work shows that flame fingers also originate from the thermodiffusive in-stability and most noteworthy, are not linked to an interaction of the two instability mechanisms. They are stable with respect to external perturbations and feature an enhanced flame propagation as the formation of a central cusp at their tip enables the co-existence of two strongly curved leading edges with high reac-tivity. The thermodiffusive instability is shown to significantly affect the flames' consumption speed, while the consumption speed enhancement caused by the hydrodynamic instability is significantly smaller. Further, the surface area increase due to wrinkling is strongly diminished if one of the two instability mechanisms is missing. This is linked to a synergistic interaction between the two mechanisms, as the propagation of flame fingers is enhanced by the presence of the hydrodynamic instability due to a widening of the streamlines ahead of the flame fingers.& COPY; 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.