Intrinsic instabilities in premixed hydrogen flames: Parametric variation of pressure, equivalence ratio, and temperature. Part 1 - Dispersion relations in the linear regime
Intrinsic instabilities in premixed hydrogen flames: Parametric variation of pressure, equivalence ratio, and temperature. Part 1 - Dispersion relations in the linear regime
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DOI：
10.1016/j.combustflame.2021.111935
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发表时间：
2022-01
影响因子：
4.4
通讯作者：
L. Berger;A. Attili;H. Pitsch
中科院分区：
文献类型：
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作者：
L. Berger;A. Attili;H. Pitsch
通过不同当量比[0.4-1.0]、未燃温度[298K-700K]、压力[1bar-20bar]的一系列模拟，研究了稀预混氢火焰的内在燃烧不稳定性的影响。除了达里厄-朗道不稳定性或流体动力学不稳定性外，贫预混氢火焰还容易出现热扩散不稳定性，这将导致显著的火焰锋面起皱以及沿火焰锋面细胞结构形成和破坏的混乱过程。理论模型还不能准确地描述这种火焰的演变，因此，本工作在参数变化中对贫氢火焰发展不稳定性的倾向进行了数值研究。对平面火焰初始暴露于弱谐波扰动下的稳定性进行了分析，研究了火焰的响应。在初始阶段，即线性阶段，观察到初始施加的扰动幅度的增长或减少，而在很长一段时间内，沿着火焰前沿形成混沌的细胞结构，这在本工作的第2部分进行了研究(L.Berger等人，燃烧。火焰,2022)。从初始相位获得的扰动幅度的增长率是固有不稳定性机制强度的度量，并随产生特征色散关系的谐波扰动的波数而变化。等效比的降低、未燃烧温度的降低和压力的增加都能提高材料的生长速度，从而提高材料的内在不稳定性。根据膨胀比、混合物的有效路易斯数和泽尔多维奇数的变化分析了色散关系的变化。对于贫氢火焰，随着压力的增加，截止波数减小，这代表了高波数时色散关系符号的变化。与不受热扩散不稳定性影响的火焰相比，这是相反的趋势。此外，将数值增长率与理论模型进行了比较。结果表明，在与几种燃烧装置相关的条件下，如在稀薄等效比下运行的燃气轮机、高压和高温或在低温和环境压力下运行的家用和工业加热器，贫氢火焰容易产生不稳定性。
The impact of intrinsic combustion instabilities is studied for lean premixed hydrogen flames by means of a series of simulations at different equivalence ratios [0.4-1.0], unburned temperatures [298K-700K], and pressures [1bar-20bar]. In addition to the Darrieus-Landau, or hydrodynamic, instability, lean premixed hydrogen flames are prone to thermodiffusive instabilities, which lead to significant flame front wrinkling and a chaotic process of formation and destruction of cellular structures along the flame front. Theoretical models are not yet capable of accurately describing the evolution of such flames, so the propensity of lean hydrogen flames to develop instabilities is studied numerically in a parametric variation in this work. A stability analysis is conducted, in which planar flames are initially exposed to weak harmonic perturbations and the response of the flame is studied. In the initial phase referred to as linear phase, a growth or decrease of the initially imposed perturbation amplitude is observed, while for long times, chaotic cellular structures are formed along the flame front, which are studied in part 2 of this work (L.Berger et al., Combust. Flame, 2022). The growth rates of the perturbation amplitude that are obtained from the initial phase are a measure of the strength of the intrinsic instability mechanisms and vary with respect to the wave number of the harmonic perturbation yielding characteristic dispersion relations. A decrease of equivalence ratio and unburned temperature and an increase of pressure are found to enhance the growth rates and hence intrinsic instabilities. The variation of dispersion relations is analyzed with respect to variations of the expansion ratio, the effective Lewis number of the mixture, and the Zeldovich number. For the lean hydrogen flames, with increasing pressure a decrease of the cut-off wave number, which represents the change of the sign of the dispersion relation at high wave numbers, is observed. This is the opposite trend compared to flames that are not affected by thermodiffusive instabilities. Further, numerical growth rates are compared to theoretical models. The results show that lean hydrogen flames are prone to develop instabilities at conditions that are relevant to several combustion devices such as gas turbines that operate at lean equivalence ratios, elevated pressures and temperatures or domestic and industrial heaters that operate at low temperatures and ambient pressure.