Detailed Analysis and Modeling of Stratified Combustion for Large-Eddy Simulation - DAMOSCLES

低排放燃烧器依靠湍流预混火焰作为波纹界面通过燃料和氧化剂的混合物传播。预混燃烧存在于燃气轮机、火花点燃的内燃机或加热器中。空间限制、可控性和安全问题限制了预混管道的体积和混合物的均匀性。在不均匀性会增加污染物排放的地方，它也可以帮助稳定火焰和减轻热声振荡。在内燃机中，分层可以在不完全燃烧的情况下降低整体等效比，因为分层可以提高整体火焰速度。这种增加可以用火焰结构的影响和更大的火焰表面来解释。前一种“反向支撑”效应发生在从化学计量到稀薄混合物的火焰燃烧中，因此热量和自由基可以扩散到通常“稀薄到不能燃烧”的反应物中。后一种效应是由不同的火焰速度引起的：火焰首先沿着“化学计量公路”传播，然后慢慢燃烧（反向支撑）到剩余的稀薄混合物中。这种行为产生额外的火焰起皱和火焰表面，从而产生消耗率。分层燃烧在许多实验和模拟中进行了研究，在直接数值模拟（DNS）中达到峰值，Chen, Dreizler， Hochgreb及其各自的同事进行了实验。在DNS尚未通过实验验证或用于开发更好的模型的地方，已经对这两种实验进行了许多大涡模拟（LES）。然而，尽管在达姆施塔特、剑桥和桑迪亚实验室测量得到了复杂的速度和物种数据，但LES集中在分层带上游的火焰上，在很大程度上忽略了分层的影响。本提案旨在缩小DNS， LES和验证实验之间的差距，以实现对物理的深入，验证的见解并开发验证的模型。这将通过剑桥火焰的DNS来实现，通过实验验证，挖掘DNS数据并从中开发模型，这些模型将在相同情况下的LES中进行测试。这种将DNS、LES和实验结合起来，用于开发经过验证的燃烧模型的新组合，需要现在刚刚可用的计算能力——它的可行性在Proch、Domingo、Vervisch和Kempf的两篇论文中得到了证明，这两篇论文刚刚在“燃烧与火焰”（combustion and Flame）中发布（在线）。提出的工作旨在对分层燃烧的物理学产生更深入的见解，在状态图中表示它们，并为分层燃烧提供更好的模拟模型-最终改进燃烧器，以实现化石燃料和生物燃料的更高效率和更低排放。

Low emission combustors rely on turbulent premixed flames that propagate as a corrugated interface through a mixture of fuel and oxidizer. Premixed combustion is found in gas turbines, spark ignited internal combustion engines or heaters. Space constraints, controllability and safety concerns limit the volume of premixing ducts and the achieved homogeneity of the mixture. Where inhomogeneity can yield increased pollutant emissions, it can also help stabilize the flame and mitigate thermo-acoustic oscillations. In internal combustion engines, stratification enables lower global equivalence ratios without incomplete combustion, as stratification can increase the overall flame speed. This increase is explained by an effect on the flame structure and a greater flame surface. The former effect of "back-support" occurs for a flame burning from stoichiometry into a leaner mixture, so that heat and radicals can diffuse into reactants that are normally "too lean to burn". The latter effect results from the varying flame speeds: the flame will first propagate along "stoichiometric highways" before slowly burning (back-supported) into the remaining lean mixture. This behavior generates additional flame wrinkling and flame sur- face, and hence consumption rate.Stratified combustion was investigated in many experiments and simulations, peaking in a direct numerical simulation (DNS) by Chen, experiments by Dreizler, Hochgreb and their respective coworkers. Where the DNS was not yet validated against experiments or used for developing better models, many large-eddy simulations (LES) have been carried out for both experiments. However, the LES focused on the flame upstream of the stratification zone, largely neglecting the effects of stratification - in spite of the availability of sophisticated velocity and species data from measurements in Darmstadt, Cambridge and the Sandia laboratories.The present proposal aims at closing the gap between the DNS, the LES and the validation experiments to achieve deep, validated insights into the physics and to develop validated models. This will be achieved through a DNS of the Cambridge flame, validating it against experiments, mining the DNS data and developing models from it, which will be tested in an LES of the same case. This new combination of DNS, LES and experiment for the development of validated combustion models requires computational power that is just available now - its feasibility was demonstrated in two papers by Proch, Domingo, Vervisch and Kempf that just became available (online) in "Combustion and Flame".The proposed work aims at generating deeper insights into the physics of stratified combustion, representing them in a regime diagram, and supplying better simulation models for stratified combustion - to eventually improve combustors to achieve higher efficiency and lower emis- sions from fossil- and bio-fuels.