Modeling differential molecular diffusion in turbulent non-premixed combustion remains challenging for flamelet models. Laminar flamelet is a key building block of flamelet models for turbulent combustion. One significant challenge that has not been adequately addressed is the representativity of laminar flamelet for the characteristics of differential molecular diffusion in turbulent combustion. Laminar flamelet is typically generated based on two conceptual burner configurations, the opposed jet burner and the Tsuji burner. The two burners are commonly viewed to be equivalent for the description of laminar flamelet structures. In this work, a major difference between them is revealed for the first time when they are used to represent differential molecular diffusion. The traditional opposed jet burner yields almost fixed equal diffusion locations defined as the points with equal values of mixture fractions based on different elements in the mixture fraction space. The Tsuji burner, with a slight extension, can produce a continuous variation of the equal diffusion locations in the mixture fraction space. This variation of the equal diffusion locations is shown to be an important feature of non-premixed combustion, as demonstrated in a laminar jet mixing layer, a turbulent jet mixing layer, and a turbulent jet non-premixed flame. Theoretical analysis is conducted based on an idealized opposed jet mixing layer to identify the major cause of the difference between the two burners. It is found that the inlet diffusion caused the major difference in differential molecular diffusion in the two burners. The curvature difference, another main difference between the two burners, however, is not found to cause any major difference in differential molecular diffusion in the two burners. Based on the finding, a modified opposed jet burner with disabled inlet diffusion is introduced and is shown to be able to reproduce the feature of differential molecular diffusion observed in the Tsuji burner. Both burners, after properly generalized with suitable boundary conditions and additional parameters (like the velocity ratio of fuel and oxidizer), are expected to be suitable and equivalent choices for the generation of representative laminar flamelets that can be used eventually in flamelet modeling of differential molecular diffusion in turbulent non-premixed combustion.

中文翻译：

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对置喷射燃烧器和 Tsuji 燃烧器，用于具有用于非预混燃烧建模的微分分子扩散的代表性层流火焰

对于小火焰模型来说，模拟湍流非预混燃烧中的微分分子扩散仍然具有挑战性。层流火焰是湍流燃烧火焰模型的关键构建模块。尚未得到充分解决的一项重大挑战是层流火焰对湍流燃烧中分子扩散差异特征的代表性。层流火焰通常基于两种概念燃烧器配置生成，即对置喷射燃烧器和辻燃烧器。对于层流火焰结构的描述，这两种燃烧器通常被认为是等效的。在这项工作中，当它们被用来表示微分分子扩散时，首次揭示了它们之间的主要区别。传统的对置喷射燃烧器产生几乎固定的相等扩散位置，定义为基于混合分数空间中的不同元素具有相等的混合分数值的点。 Tsuji 燃烧器具有轻微的延伸，可以在混合分数空间中产生相等扩散位置的连续变化。等扩散位置的这种变化被证明是非预混合燃烧的重要特征，如在层流射流混合层、湍流射流混合层和湍流射流非预混合火焰中所证明的那样。基于理想化的对射混合层进行理论分析，以确定两种燃烧器之间差异的主要原因。研究发现，入口扩散导致了两个燃烧器中分子扩散差异的主要差异。然而，曲率差异是两个燃烧器之间的另一个主要差异，但并未发现两个燃烧器中分子扩散差异有任何重大差异。基于这一发现，引入了一种改进的具有禁用入口扩散的对置喷射燃烧器，并且被证明能够再现在 Tsuji 燃烧器中观察到的微分分子扩散特征。在使用适当的边界条件和附加参数（如燃料和氧化剂的速度比）进行适当概括后，两种燃烧器预计将成为生成代表性层流火焰的合适且等效的选择，最终可用于微分分子的火焰建模湍流非预混燃烧中的扩散。