2D Direct Numerical Simulations of methane/air turbulent premixed flames at initial Karlovitz numbers ranging from 600 to 9500 are performed. Instantaneous results are then extracted and analyzed with a focus on the inner flame structure. Snapshots reveal that the distributed reaction zone regime, theoretically reached around Ka a parts per thousand aEuro parts per thousand 100, is not attained before Ka a parts per thousand aEuro parts per thousand 2000. A correction of the definition of Ka is proposed in order to account for gas expansion across the flame, and is found to be consistent with the previous observations. The fuel-consumption zone is shown to be highly affected by turbulence and the characteristics of flames developing at lower Ka cannot be seen: the reaction zone is indeed strongly stretched and curved by intense turbulence leading to the formation of large protruding structures. In addition, the heat release rate layer is found to be broader and more distributed than at lower Ka as small turbulent eddies are able to survive inside it. No local flame quenching is however noticed. A statistical analysis of the distributed flame highlighted three major features characterizing this regime: significant broadening of the whole flame results from the presence of small eddies inside the reaction zone, temperature evolves linearly with respect to the progress variable and minor species peak mass fractions are lower than in a laminar flame. These results have important consequences for turbulent combustion modelling of flames in the distributed combustion regime.
Subject classification (UKÄ)
- Fluid Mechanics and Acoustics
- Turbulent premixed flames
- High Karlovitz number
- Inner flame