further clarify the ignition characteristics of a nonpremixed H2/air mixture within a vortex, an additional parametric study was performed in a 2-D domain with different Ta, Umax, andτv.
• The flame structure, edge flame stabilization, and extinction characteristics of coun- terflow nonpremixed flames of CH4/He versus air at low strain rates were experi- mentally and numerically investigated. By adopting a novel experimental method- ology of He curtain flow in the counterflow burner, we could locate the flames in the middle of the burner, and hence, measure the critical He mole fraction,XHe,cr, for flame extinction even at very-low strain rates. The comparison between XHe,cr of the experiments and 2-D numerical simulations in normal and zero gravity re- vealed that the He curtain flow can effectively reduce the buoyancy effect on the flame characteristics such that the extinction dynamics of counterflow nonpremixed flames at low strain rates can be reasonably investigated even in normal gravity.
The dynamics of the outer edge flame for flame stabilization was numerically in- vestigated by performing 2-D transient numerical simulations of counterflow non- premixed flames of CH4/He versus air at low strain rates of 10 and 30 s−1, from which the transition of the edge flame from a bibrachial edge flame with positiveSe to a monobrachial edge flame with negativeSewas observed. The species transport budget analysis was performed to better understand the flame stabilization and extinction mechanism. The following results are obtained in the present study.
quire to be modeled. Rapid advances in LES modeling accompanied by simultaneous progress in computer science and hardware have enabled LES of realistic combustion ge- ometries [144–146]. Because of these, the present study could be extended to the following research directions.
• So far, we investigated the ignition and liftoff characteristics of hydrocarbon fuels such as methane, DME, andn−heptane. As the decarbonization becomes the most stringent issue in the world, carbon-free fuels such as hydrogen and ammonia have been recently highlighted. Thus, the flame structure and ignition characteristics of hydrogen and/or hydrogen/ammonia blends will be studied by performing 2-D laminar flame simulations and 3-D DNSs.
• Extending the fundamental numerical study of the combustion characteristics of carbon-free fuels using high-fidelity numerical simulations, a series of LES of igni- tion and stabilization mechanisms of future carbon-free fuels will be investigated.
OpenFOAM is utilized for the simulations.
• A novel turbulent combustion modeling framework is needed to reduce the compu- tational cost of turbulent combustion problems. The performance of the framework will then be evaluated against the 3-D DNSs of turbulent jet flames that we have obtained.
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