Flame front identification and analysis in premixed turbulent combustion often relies on some scalar iso-surface as a flame marker, and the quest for a suitable scalar, and an appropriate flame-identifying iso-value, then becomes important. In an effort to contribute to this field, we study the suitability of various flame front identification approaches using the concept of ridge analysis. This serves to mark the central region of a given scalar field. In combustion, an essential scalar field is the heat release rate (HRR). Ridge analysis applied to the HRR-field therefore delivers a central region of combustion, which we identify with the flame front. Similar to iso-surfaces, ridges can define sheet-like objects that track the flame front in a given combustion data set. Ridges bear the advantage of being universal, intrinsic flame identifiers, because they do not depend on user-defined parameters like thresholds, iso-values or similar, and they can be applied to premixed as well as to diffusion flames. We determine flame fronts by ridge analysis of the heat release rate field in turbulent premixed combustion data sets obtained by 3D Direct Numerical Simulation (DNS). The suitability of other, iso-surface based flame identifiers is evaluated using the ridge-based, intrinsic flame front as a reference. Iso-surfaces of various scalars (several species and temperature) are compared to the ridges. The study shows that for a methane flame, iso-surfaces of several scalars can serve as good flame front markers, and suitable iso-values for these scalars can be derived from ridge-conditioned statistics. However, different fuel/air mixtures (we study two fuels, hydrogen and methane) exhibit different characteristics. For the hydrogen/air flame, none of the studied scalars was a suitable flame marker. This flame featured ‘holes’ (regions of vanishing chemical heat release). These are correctly detected as non-flame regions by the ridge method, but not by the iso-surfaces of classical flame marker scalars like temperature.

中文翻译：

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湍流预混燃烧火焰前沿标记的 DNS 研究

预混湍流燃烧中的火焰锋识别和分析通常依赖于一些标量等值面作为火焰标记，因此寻求合适的标量和适当的火焰识别等值就变得重要。为了对这一领域做出贡献，我们使用脊分析的概念研究了各种火焰锋识别方法的适用性。这用于标记给定标量场的中心区域。在燃烧中，一个基本的标量场是放热率 (HRR)。因此，应用于 HRR 场的脊线分析提供了一个燃烧的中心区域，我们将其与火焰前沿相结合。与等值面类似，脊可以定义片状物体，在给定的燃烧数据集中跟踪火焰前沿。脊具有作为通用的、固有的火焰标识符的优点，因为它们不依赖于用户定义的参数，如阈值、iso 值或类似参数，并且它们可以应用于预混火焰和扩散火焰。我们通过对由 3D 直接数值模拟 (DNS) 获得的湍流预混燃烧数据集中的热释放率场进行脊线分析来确定火焰锋。使用基于脊的固有火焰前沿作为参考来评估其他基于等值面的火焰标识符的适用性。将各种标量（几种物质和温度）的等值面与脊进行比较。研究表明，对于甲烷火焰，几个标量的等值面可以作为良好的火焰前沿标记，这些标量的合适等值可以从脊条件统计中推导出来。然而，不同的燃料/空气混合物（我们研究了两种燃料，氢和甲烷）表现出不同的特性。对于氢气/空气火焰，所研究的标量都不是合适的火焰标记。这种火焰具有“洞”（消失的化学热释放区域）。这些可以通过脊法正确检测为非火焰区域，但不能通过经典火焰标记标量（如温度）的等值面检测到。