Role of pocket formation in the extinction of methane flames subject to strong turbulence

Despite extensive studies of local and global extinction, it remains unclear how local flame dynamics contribute to the global evolution of a turbulent flame at large Karlovitz numbers. Furthermore, the extent to which canonical laminar flame properties, such as laminar flame speed and extinction strain rate, govern the behavior of highly turbulent flames is not yet well understood. Unique to these conditions is the formation of isolated Product-in-Fuel (PiF) pockets from an initially continuous flame surface. The present study proposes that local extinctions are coupled to the global ignition or extinction of a turbulent flame through the PiF pocket mechanism. Direct numerical simulations of a methane/air flame kernel subject to homogeneous isotropic turbulence at Karlovitz numbers larger than 100 are conducted. A 17-species reduced mechanism derived from FFCM-1 is employed and later perturbed to conduct the parametric studies. The topology and evolution of the PiF pockets are quantified. The distribution of thermochemical states in spatial coordinates relative to the flame surface demonstrates that the quenching pockets serve to deposit sensible enthalpy, combustion products, and radicals (particularly CH 2 O) into the fresh mixture ahead of the flame. To evaluate the role of canonical flame properties on the pocket formation and flame evolution, the baseline methane/air chemical kinetics model is perturbed using a Monte Carlo method to generate modified reaction mechanisms that individually alter the laminar flame speed and extinction strain rate. The laminar flame speed is found to be the most relevant parameter for the current configuration, counteracting the turbulent deformation of the flame and ensuring the necessary product support for flame propagation. • Quantification of the formation, dynamics, and impact of pinched-off product pockets. • The pinched-off pockets alter the thermochemical state in the fresh mixture. • The laminar flame speed is critical in controlling turbulent flame kernel extinction.