Curvature effects in laminar and turbulent non-premixed combustion

在项目资助一期，对非预混燃烧中的曲率效应有了全面的了解。在10篇同行评议的论文中，表明曲率诱导效应显著影响层流和湍流扩散火焰的微观结构。在典型层流火焰模拟的基础上，对随时间变化的湍流射流火焰（H2-air、合成气-air）进行了直接数值模拟（DNS），为曲率效应分析建立了全面的参考数据库。分析总结在一个状态图，以评估曲率效应。第一阶段的一个方法论亮点是在DNS中对混合裂缝场的梯度轨迹（GTs）进行原位跟踪；可以跟踪和分析单个gt或小火焰的空间和时间演变。这些拉格朗日数据可以用来评估湍流火焰中小火焰概念的中心模型假设。研究表明，仅考虑小火焰方程中的曲率是不足以预测曲率效应的。在大曲率情况下，曲率诱导的切向扩散是沿混合断裂等值面的主导效应，这只能用扩展的Flamelet方程来充分描述。在第二个资助期，将研究曲率对非预混湍流火焰自燃的影响。与（统计上）静止的扩散火焰相反，这是之前的高度瞬态过程，其中未反应（或缓慢反应）的混合层在点火延迟时间后迅速发展成扩散火焰。文献中的实验和直接数值模拟表明，在低标量耗散率和负曲率的口袋中，点火核沿着首选混合分数（“最活跃的混合分数”）的等面形成。曲率效应因此可以显著影响点火，但以前的小火焰方法迄今几乎只关注标量耗散率的影响。将小火焰理论扩展到稳态条件下的曲率效应，是建立非预混火焰瞬态自燃模型的起点。除了对DNS数据的分析和小火焰的开发之外，在资助期2向LES的转移也是有针对性的。

In funding period 1 of the project, a comprehensive understanding of curvature effects in non-premixed combustion was achieved. In 10 peer-reviewed publications, it was shown that curvature-induced effects significantly influence the microstructure of laminar and turbulent diffusion flames. In addition to the simulation of canonical laminar flames, direct numerical simulations (DNS) of time-evolving turbulent jet flames (H2-air, syngas-air) were performed for this purpose, thus establishing a comprehensive reference database for the analysis of curvature effects. The analyses were summarized in a regime diagram for the assessment of curvature effects. A methodological highlight of funding period 1 is the in situ tracking of gradient trajectories (GTs) of the mixture fracture field in the DNS; individual GTs or flamelets can be tracked and analyzed in their spatial and temporal evolution. These Lagrangian data can be used to evaluate central model assumptions of the flamelet concept in turbulent flames. It has been shown that consideration of curvature in the flamelet equations alone is not sufficient for the prediction of curvature effects. Curvature-induced tangential diffusion along mixture fracture isosurfaces is the dominant effect in the case of large curvatures, which can only be fully described by the extended Flamelet equations. In the second funding period, the curvature influence on the auto-ignition of non-premixed turbulent flames will be investigated. In contrast to a (statistically) stationary diffusion flame, this is the preceding, highly transient process in which an unreacted (or slowly reacting) mixing layer quickly develops into a diffusion flame after an ignition delay time. Experiments and direct numerical simulations in the literature demonstrate that ignition kernels form along isosurfaces of a preferred mixture fraction, the "most reactive mixture fraction," in pockets of low scalar dissipation rate and negative curvature. Curvature effects can thus significantly influence ignition, but previous flamelet approaches have so far focused almost exclusively on the influence of scalar dissipation rate. The extension of flamelet theory to include curvature effects for steady-state conditions from funding period 1 is the starting point for modeling the transient auto-ignition of non-premixed flames. In addition to the analysis of the DNS data and the flamelet development, the transfer to the LES in funding period 2 is also targeted.