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

Journal article, 2025

Direct Numerical Simulation (DNS) data obtained from lean (the equivalence ratio = 0.5 or 0.35) complex-chemistry hydrogen-air flames propagating in forced turbulence in a box are analyzed. Karlovitz number is varied from 1.2 to 6.4 or from 86 to 125 at = 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 ( = 0.5) case. DNS data obtained from unstable laminar and weakly turbulent ( 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 . Since the analyzed DNS data do not show any clear sign of an important role played by the instability at 3.5, the data are consistent with a recently introduced criterion of importance of laminar flame instabilities in turbulent flows.