Unburned mixture fingers in premixed turbulent flames

Data obtained in 3D direct numerical simulations of statistically planar, 1D premixed turbulent flames indicate that the global burning velocity, flame surface area, and the mean flame brush thickness exhibit significant large-scale oscillations with time. Analysis of the data shows that the oscillations are caused by origin, growth, and subsequent disappearance of elongated channels filled by unburned gas. The growth of such an unburned mixture finger (UMF), which deeply intrudes into combustion products, is controlled by a physical mechanism of flame-flow interaction that has not yet been highlighted in the turbulent combustion literature, to the best of the present authors knowledge. More specifically, the fingers grow due to strong axial acceleration of unburned gas by local pressure gradient induced by heat release in surrounding flamelets. Under conditions of the present DNS, this physical mechanism plays an important role by producing at least as much flame surface area as turbulence does when the density ratio is equal to 7.5. Although, similarly to the Darrieus-Landau (DL) instability, the highlighted physical mechanism results from the interaction between a premixed flame and pressure field, it is argued that the UMF and the DL instability are different manifestations of the aforementioned interaction. Disappearance of an UMF is mainly controlled by the high-speed self-propagation of strongly inclined flame fronts (cusps) to the leading edge of the flame brush, but significant local increase in displacement speed due to large negative curvature of the front plays an important role also.