Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames
Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames
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DOI：
10.1016/j.combustflame.2022.112254
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发表时间：
2022
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
在这项工作中，我们通过大规模直接数值模拟 (DNS) 研究了热扩散不稳定性和湍流的相互作用。使用详细的化学机理，在槽式燃烧器配置中，在相同的喷射雷诺数 Re= 11, 000 和卡洛维茨数 Ka≈ 15 下，进行了两种湍流预混合稀氢/空气火焰的 DNS 测试。在一种情况下采用了现实的输运模型，该模型具有热扩散不稳定火焰的特征模式，例如放热和超绝热温度的强烈变化。在另一种情况下，所有物质的扩散率均设置为等于热扩散率（统一路易斯数假设），因此热扩散不稳定性受到抑制。没有热扩散不稳定性的湍流火焰的局部燃烧类似于未拉伸的层流火焰，并且湍流火焰速度仅由于火焰表面积的增加而增加，这与之前对类似条件下火焰的研究一致。相比之下，热扩散不稳定火焰的特点是湍流火焰速度显着增强，这不仅是由火焰起皱引起的，而且由于局部反应速率的显着变化而大大增加。这是由于氢的不同扩散导致的局部当量比的变化引起的。与相同条件下的热扩散不稳定层流火焰的比较表明，由于湍流引起的较高的曲率波动和增强的平均应变率，湍流火焰中局部当量比和局部反应速率的变化显着增强。因此，湍流和热扩散不稳定性表现出协同效应，这反映在单位火焰表面积的燃料消耗率明显更高。两种情况下，由切向应变和弯曲火焰段中的火焰传播控制的火焰表面积的产生也不同。最值得注意的是，切向应变率由湍流火焰中的最小湍流结构决定，并且不受热扩散不稳定机制的影响。然而，热扩散不稳定性导致在凸形弯曲的火焰段中产生火焰表面积，其特征是形成渗透到未燃烧气体中的舌状结构，这在具有统一路易斯数的湍流火焰中不存在。这与热扩散不稳定火焰中火焰位移速度随曲率的增强有关，而在不存在不稳定的情况下，观察到火焰位移速度随曲率增加而降低，导致火焰表面积的破坏。这些发现表明，热扩散不稳定性在湍流中持续存在，甚至表现出与湍流的协同相互作用，这需要在湍流燃烧模型中加以考虑。
Interactions of thermodiffusive instabilities and turbulence have been investigated by large-scale Direct Numerical Simulations (DNS) in this work. Two DNS of turbulent premixed lean hydrogen/air flames have been performed in a slot burner configuration at the same jet Reynolds number of R e= 11, 000 and Karlovitz number of K a≈ 15 using a detailed chemical mechanism. Realistic transport models are employed in one case, which features the characteristic patterns of thermodiffusively unstable flames, such as strong variations of the heat release and super-adiabatic temperatures. In the other case, the diffusivities of all species are set equal to the thermal diffusivity (unity Lewis numbers assumption) and thermodiffusive instabilities are therefore suppressed. The local burning of the turbulent flame without thermodiffusive instabilities is similar to an unstretched laminar flame and the turbulent flame speed increases only due to the increase of flame surface area in agreement with previous studies for flames at similar conditions. In contrast, the thermodiffusively unstable flame features a strong enhancement of the turbulent flame speed, which is not only caused by flame wrinkling, but is greatly increased due to significant variations of the local reaction rates. These are caused by variations of the local equivalence ratio due to the differential diffusion of hydrogen. A comparison with a thermodiffusively unstable laminar flame at the same conditions reveals that the variations of the local equivalence ratio and local reaction rates are significantly enhanced in the turbulent flame due to higher fluctuations of curvature and an enhanced average strain rate induced by turbulence. Thus, turbulence and thermodiffusive instabilities show synergistic effects, which are reflected in a significantly higher fuel consumption rate per flame surface area. The flame surface area generation, which is governed by the tangential strain and the flame propagation in curved flame segments, is also different in the two cases. Most noteworthy, the tangential strain rate is shown to be determined by the smallest turbulent structures in both turbulent flames and to be unaffected by the thermodiffusive instability mechanism. However, thermodiffusive instabilities lead to a production of flame surface area in convexly curved flame segments, featuring the formation of tongue-like structures that penetrate into the unburned gas, which do not exist in the turbulent flame with unity Lewis numbers. This is linked to an enhancement of the flame displacement speed with curvature in the thermodiffusively unstable flame, while in the absence of instabilities, a reduction of the flame displacement speed with increasing curvature is observed, leading to a destruction of flame surface area. These findings suggest that thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence, which needs to be accounted for in turbulent combustion models.