Two dimensional flame structure of oscillating burner-stabilized methane-air flames

The highly transient relaxational diffusive-thermal oscillations of flat burner-stabilized flames can be very attractive to probe the performance of detailed reaction mechanisms in the regimes close to ignition/extinction. In such regimes, certain reaction zones can travel over distances of the order of 10 mm and this raises an important question if one dimensional numerical models can be accurate in describing them. The question of quantitative comparison of modeling and experiments becomes crucial to study, to understand these regimes and to utilize them for validation. In this work, we experimentally investigate relaxational oscillations of methane-air flames on a flat porous burner with a surrounding nitrogen co-flow and perform fully resolved 2D numerical simulations of the same burner configuration, using a detailed reaction mechanism and molecular diffusion model, buoyancy and radiation, alongside corresponding experiments. The focus is on the effect of the nitrogen co-flow on the flame oscillations, which can only be studied numerically in 2D simulations due to the mutual effect of the complex flow field and flame dynamics. The results of both numerical and experimental approaches are found to be in quantitative agreement. They show that there is an optimal co-flow velocity that removes the secondary diffusion flame and extinguishes the edge flame settled in the stagnation flow region. This optimal regime makes the flame flatter and closer to a one-dimensional configuration and this is a most favorable condition for validation of kinetic mechanisms. The detailed data from the simulations will guide the design of the next generation of the burner configurations to study the kinetics and dynamics of complex fuels required for a sustainable energy transition. Novelty and Significance Statement The novelty of this research lies in the synergy of these modeling, computations with experimental measurements, allowing both parametric studies of the oscillation regime and deeper insights into the flame structure. These results are significant because they allow to develop more accurate burner configurations for studying flames near extinction and ignition conditions, which will be an important task for more complex fuels from renewable sources required for a sustainable energy transition. Ultimately, our understanding of the interplay between chemistry and diffusion controlled combustion regimes under transient conditions can be approved and validation data for e.g. chemical reaction mechanisms can be generated. The latter becomes extremely important since efficiency and pollutant mitigation issues require lean and chemistry controlled combustion processes used in the combustion facilities. Thus understanding, optimization and control of such regimes has become a crucial point for further development of the sustainable combustion.