Morphology and structure of spherically propagating premixed turbulent hydrogen - air flames

Abstract

Three-dimensional direct numerical simulations of spherically propagating premixed turbulent stoichiometric hydrogen-air flames with detailed chemistry and detailed diffusion are employed to clarify the influence of turbulence-flame interactions with respect to flame structure and morphology. Four cases are considered within the corrugated flamelets and the thin reaction zone regimes. The most significant fuel consumption and heat release rates occur at the negatively-curved flamelets. Furthermore, the increments in burning velocity are lower than the increments in flame surface area, which is due to the reduction in the local burning intensity at the positively-curved flamelets. The morphology of intense reaction zones is quantified using Minkowski functionals and their shapes include ”tubes”, ”pancakes” and more complex shapes, which are compared to their counterparts in planar flames. As turbulence level increases, the number of locally defined intense reaction zones increases, and their boundaries expand to cover more extensive parts of the flame front. However, intense reaction zones’ geometrical dimensions do not significantly differ for each flame as it propagates. Local turbulence properties are obtained for each intense reaction zone. The conditional averages of local Taylor microscale and local Kolmogorov scale, conditioned based on the shapefinders, are investigated. The conditional averages of the local Taylor microscale scale correlate with the planarity and filamentarity of intense reaction zones. Therefore, turbulent motions at Taylor microscale size have a significant role in characterizing turbulence-flame interactions relevant to flame morphology and relevant to the local flame thickness and reaction layers of developing-flames. On the other hand, local Kolmogorov scales’ turbulent motions show weaker or no such correlations. There is a dissidence between global turbulence statistics and local ones representing the interactions at the flame front. Local turbulence-flame interactions of Taylor microscale sizes occur at specific length scales, depending on the flame size and irrelevant of eddies with other length scales.