A DNS study of extreme and leading points in lean hydrogen-air turbulent flames-Part I: Local thermochemical structure and reaction rates
A DNS study of extreme and leading points in lean hydrogen-air turbulent flames-Part I: Local thermochemical structure and reaction rates
复制标题
贫氢-空气湍流火焰极值点和主导点的 DNS 研究-第一部分：局部热化学结构和反应速率
DOI：
10.1016/j.combustflame.2021.111716
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发表时间：
2022
影响因子：
4.4
通讯作者：
Lipatnikov Andrei N.
中科院分区：
文献类型：
--
作者：
Lee HsuChew;Dai Peng;Wan Minping;Lipatnikov Andrei N.
摘要采用直接数值模拟（DNS）方法，对具有低Lewis数Le和3 ~ 33种不同Karlovitz数K的一维和平面贫复杂化学氢-空气火焰进行了数值模拟，并通过设置所有种类的分子扩散率等于混合物的热扩散率来模拟相同的复杂化学火焰。模拟结果表明，与等扩散火焰相比，前者（L e< 1）火焰的湍流燃烧速度与层流火焰速度之比显著增加。以峰值（计算域上）燃油消耗率（FCR）或热释放率（HRR）为特征的极端点在每个瞬间被发现。在等扩散火焰中，这种极端FCR和HRR接近其在无扰动层流火焰中的峰值。当L e较低时，由于扩散热效应引起的局部温度、当量比和自由基质量分数的增加，前者的速率明显高于后者。虽然研究的极值点可能出现在距离瞬时火焰刷前缘足够远的地方，但在每个瞬间，在前缘附近观察到的FCR或HRR较低但仍然较高（L e< 1）的极值点。研究了极端点和先导点的各种局部特征（温度、等效比、物种质量分数及其梯度、反应速率等），揭示了高FCR或高HRR区域之间的显著差异。例如，在后一个区域，主要的化学途径发生了变化。此外，极端hrr随时间波动较大，其平均值和均方根值随K a的增加而显著增加，而极端FCR波动较弱，且在不同K a时接近，这意味着在本质上不同的局部燃烧结构下可以达到几乎相同的极端FCR。
Abstract A Direct Numerical Simulation (DNS) study of statistically one-dimensional and planar, lean complex-chemistry hydrogen-air flames characterized by a low Lewis number L e and three different Karlovitz numbers K a ranging from 3 to 33 is performed, with the same complex-chemistry flames being also simulated by setting molecular diffusivities of all species equal to the heat diffusivity of the mixture. The simulations predict a significant increase in a ratio of turbulent burning velocity to the laminar flame speed in the former (L e< 1) flames when compared to the latter (equidiffusive) flames. Extreme points characterized by the peak (over the computational domain) Fuel Consumption Rate (FCR) or Heat Release Rate (HRR) are found at each instant. In the equidiffusive flames, such extreme FCR and HRR are close to their peak values in the unperturbed laminar flame. If L e is low, the former rates are significantly higher than the latter ones due to an increase in the local temperature, equivalence ratio, and radical mass fractions, caused by diffusive-thermal effects. While the studied extreme points may appear sufficiently far from the leading edge of the instantaneous flame brush, leading points characterized by a lower, but still high (L e< 1) FCR or HRR are observed close to the leading edge at each instant. Various local characteristics (temperature, equivalence ratio, species mass fractions and their gradients, reaction rates, etc.) of the extreme and leading points are explored and significant differences between zones characterized by high FCR or HRR are revealed. For instance, in the latter zones, major chemical pathways are changed. Moreover, while the extreme HRRs strongly fluctuate in time, with their mean and rms values being significantly increased by K a, the extreme FCRs fluctuate weakly and are close at different K a, thus, implying that almost the same extreme FCR can be reached in substantially different local burning structures.