Flame structure analysis for categorization of lean premixed CH4/air and H2/air flames at high Karlovitz numbers: Direct numerical simulation studies

This paper presents direct numerical simulation studies of lean CH4/air and H2/air flames at high Karlovitz numbers utilizing detailed chemical kinetic mechanisms. Identical Karlovitz numbers are applied to the two flames; however, as the interaction of high turbulence intensity small scale structures with the reaction zones is considered, a significant difference in the effect of the turbulence structures on the inner reaction layer is observed. The heat release rate layer as well as layers of different species reaction rates is found to be more distributed for the H2/air flame compared with those in the CH4/air flame, owing to the difference in the Kolmogorov length scale and the differential diffusion effects. A modified, species-specific Karlovitz number is proposed, where the chemical time scale in the definition is a characteristic species-specific chemical time scale, which is obtained from stationary 1D simulations. The difference between the two flames for different intermediate species corresponds well to the difference in the obtained species-specific Karlovitz numbers. A validation study of the species-specific Karlovitz number is also conducted, where a turbulent effective chemical time scale is quantified for different intermediate species from instantaneous data sets. It is shown that in the preheat zone and reaction zone of the flame, the ratio between the laminar and turbulent chemical time scales of intermediate species corresponds well to the species-specific Karlovitz numbers for the two different flames.