Thermodiffusively-unstable lean premixed hydrogen flames: Phenomenology, empirical modelling, and thermal leading points
Thermodiffusively-unstable lean premixed hydrogen flames: Phenomenology, empirical modelling, and thermal leading points
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DOI：
10.1016/j.combustflame.2023.112811
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发表时间：
2023-07
影响因子：
4.4
通讯作者：
T.L. Howarth;E. F. Hunt;A. Aspden
中科院分区：
文献类型：
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作者：
T.L. Howarth;E. F. Hunt;A. Aspden
采用有限速率化学动力学方法对热扩散不稳定贫燃预混氢火焰进行了三维直接数值模拟。已经进行了大量的模拟研究反应物的条件（压力，温度和当量比）对自由传播和湍流火焰中的热扩散响应的影响。反应条件的特点是使用不稳定性参数ω 2，这是最近被证明可以很好地抑制自由传播的火焰在2D。自由传播的火焰速度和厚度被发现与ω 2使用相同的功能形式在2D中发现（与较大的模型常数），和不同的相关性需要在ω 2空间中的最不稳定的表面的两侧。自由传播的值被证明是更适合于表征湍流火焰比相应的一维层流火焰值。湍流和热扩散不稳定性带来了类似的火焰响应（增加曲率，反应速率和温度），因此从强烈的热扩散不稳定的条件下开始限制了潜在的湍流响应;经验模型，局部火焰速度和厚度，将这种依赖性。联合概率密度函数被用来关联局部消耗的火焰速度与曲率，应变率和拉伸与单一和独立的Markstein数。一个简单的曲率为基础的模型与经验火焰速度模型相结合，发现产生合理的结果。主曲率被用来划分火焰表面分为六个类别，允许在火焰表面的不同部分的条件分析。分数的贡献表明，大部分的燃料消耗发生在平坦的区域和前缘，从前者转移到后者，增加不稳定性和/或湍流。此外，平坦火焰区域经历超过参考值的速度，尽管与热扩散不稳定性的常规预期相反，在这些区域中缺乏通过优先扩散的燃料聚焦。我们提出了对这些现象的热前沿点解释：正如预期的那样，前沿点/边缘的强正曲率导致燃料的扩散聚焦，增加反应速率，导致超绝热温度;这些前沿点/边缘留下的高温然后支持火焰表面相对平坦区域的反应速率高于预期。
Thermodiffusively-unstable lean premixed hydrogen flames are investigated using three-dimensional direct numerical simulation with finite-rate chemical kinetics. A large number of simulations have been performed to investigate the influence of reactant conditions (pressure, temperature, and equivalence ratio) on thermodiffusive response in freely-propagating and turbulent flames. Reactant conditions are characterised using an instability parameter ω 2, which was recently shown to characterise freely-propagating flames well in 2D. Freely-propagating flame speeds and thicknesses are found to correlate with ω 2 using the same functional form found in 2D (with larger model constants), and different correlations are required either side of the most-unstable surface in ω 2-space. The freely-propagating values are demonstrated to be more appropriate for characterising turbulent flames than the corresponding 1D laminar flame values. Both turbulence and thermodiffusive instability bring about a similar flame response (increasing curvature, reaction rates and temperature), and so starting from strongly thermodiffusively-unstable conditions limits the potential turbulent response; an empirical model for local flame speed and thickness is provided incorporating this dependence. Joint probability density functions are used to correlate local consumption-based flame speed with curvature, strain-rate and stretch with single and independent Markstein numbers. A simple curvature-based model coupled with the empirical flame speed model is found to yield reasonable results. Principal curvatures are used to partition the flame surface into six classifications, allowing for conditional analysis in different parts of the flame surface. Fractional contributions show that the bulk of the fuel consumption occurs in flat regions and leading edges, shifting from the former to the latter with increasing instability and/or turbulence. Furthermore, the flat flame regions experience speeds in excess of the reference value despite the lack of focussing of fuel by preferential diffusion in these regions, contrary to the conventional expectations of thermodiffusive instability. We propose a thermal leading point interpretation of these phenomena: as expected, strong positive curvature in the leading points/edges result in diffusive focussing of fuel, increasing the reaction rates, resulting in superadiabatic temperatures; these high temperatures left behind the leading points/edges then support higher-than-expected reaction rates in regions where the flame surface is relatively-flat.