Morphology pf instantaneous flame surfaces in laminar and turbulent lean H2-air flames

Paper in proceeding, 2025

Direct Numerical Simulation (DNS) data obtained from two lean (the equivalence ratio F=0.5 or 0.35) complex-chemistry hydrogen-air flames propagating in forced turbulence in a box are analysed. Karlovitz number Ka is varied from 1.2 to 6.4 or from 86 to 125 at F=0.5 or 0.35, respectively. The focus of consideration is placed on qualitative changes in morphology of instantaneous flame surfaces with increasing rms velocity  from zero to double laminar flame speed in the moderately lean (F=0.5) case. DNS data obtained from unstable laminar and weakly turbulent (Ka<1.5) flames show predominance of similar regular large-scale wrinkles of instantaneous flame surface. This observation indicates that diffusional-thermal instability dominates turbulence under such conditions. On the contrary, DNS data obtained from other explored turbulent flames show appearance of irregular small-scale wrinkles of instantaneous flame surface, but the lack of regular large-scale wrinkles associated with unstable laminar flames. This observation implies a decreasing role played by diffusional-thermal instability with increasing Ka. Since the analyzed DNS data do not show any solid sign of an important role played by the instability at Ka>3.5, the data are consistent with a recently introduced criterion of importance of laminar flame instabilities in turbulent flows.