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

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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.