Gradient Trajectory Analysis of the Burning Rate in Turbulent Premixed Jet Flames
Gradient Trajectory Analysis of the Burning Rate in Turbulent Premixed Jet Flames
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DOI：
10.1080/00102202.2020.1811242
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发表时间：
2020-09
影响因子：
1.9
通讯作者：
D. Denker;A. Attili;K. Kleinheinz;H. Pitsch
中科院分区：
文献类型：
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作者：
D. Denker;A. Attili;K. Kleinheinz;H. Pitsch
本文对湍流预混射流火焰在不同雷诺数和近似恒定的Karlovitz数下的燃烧过程进行了直接数值模拟，分析了沿沿着梯度轨迹的燃烧过程。射流雷诺数的变化范围从5600到22400，是通过增加射流的宽度和保持体积速度恒定来实现的，这也意味着跨越火焰的湍流强度的近似恒定值。考虑的火焰名义上是在薄反应区制度的Borghi-Peters图湍流燃烧。增厚的内反应层和它的增强与雷诺数的增加，在以前的作品中观察到的相同的数据库，在这里链接到的极值点的存在下，在温度场（局部最大值在低温侧和最小值在高温侧）在附近或内部的内反应层。因此，湍流火焰速度的增强与湍流对小火焰结构的中断有关，而不是与整个小火焰本身的增厚有关。除了甲醛层的预期的大增厚之外，内部反应区的特征物质层，例如OH和O，显示出显著的增厚，并且在湍流火焰表面之前的几个火焰厚度处观察到这些物质。最后，它示出，在区域中的内反应层增厚，局部燃料消耗率降低相比，层流平面火焰，但在整个火焰结构上集成的总燃烧速率较大，由于反应层的体积增加。这些观察结果突出了评估与高Karlovitz数相关的两种竞争现象之间的平衡的重要性。首先是湍流应变和搅拌对化学反应的影响，通常会降低局部燃烧速率。第二个是对物种场的影响，物种场可能会经历拓扑结构的修改，例如引入局部极值点，以及等值面之间的体积显着增厚。
ABSTRACT An analysis of the combustion process along gradient trajectories is presented for a set of Direct Numerical Simulations of turbulent premixed jet flames at different Reynolds numbers and approximately constant Karlovitz numbers. The variation of the jet Reynolds number, ranging from 5600 to 22400, is achieved by increasing the width of the jet and keeping the bulk velocity constant, which also implies approximately constant values of the turbulence intensities across the flames. The flames considered are nominally in the thin-reaction zone regime of the Borghi-Peters diagram of turbulent combustion. The thickening of the inner reaction layer and its enhancement with increasing Reynolds number, observed in previous works on the same database, is linked here to the presence of extremal points in the temperature field (local maxima in the low temperature side and minima in the high temperature side) in the vicinity or inside the inner reaction layer. Therefore, an enhancement of the turbulent flame speed is linked to the interruption of the flamelet structure by turbulence and not by the thickening of the entire flamelet itself. In addition to the expected large thickening of the formaldehyde layer, the layers of species which are characteristic of the inner reaction zone, such as OH and O, show a significant thickening and these species are observed several flame thicknesses ahead of the turbulent flame surface. Finally, it is shown that, in the regions where the inner reactive layer is thickened, the local fuel consumption rate is reduced in comparison to a laminar planar flame, but the total burning rate integrated over the entire flame structure is larger due to the increased volume of the reactive layer. These observations highlight the importance of assessing the balance between two competing phenomena related to high Karlovitz numbers. The first is the effect of turbulence strain and stirring on the chemistry, which usually decreases the local burning rate. The second is the effect on the species fields, which might experience modifications in their topology, such as the introduction of local extremal points, and significant thickening of the volumes between their isosurfaces.