On the extinction of igniting kernels in near-isotropic turbulence

Extinction dynamics of ignition kernels of rich H-2-air mixture (phi = 4, Le > 1) in near-isotropic turbulence, are studied using three sets of direct numerical simulations and the recently introduced flame particle tracking. Turbulence is found to extinguish a freshly ignited, initially spherical premixed flame kernel, which otherwise sustains in a quiescent flow field by propagating beyond the minimum radius. The mechanism of kernel extinction is investigated by tracking lifetime trajectories of flame particles on an O-2 mass fraction isosurface in the flame speed-curvature (S-d,S- kappa) space using the well-known concept of minimum radius from laminar flames. The classical S-curve in the temperature-Damkohler number (T, Da) space is also analyzed. Ensemble averaged S-d - kappa and S-curves display corresponding turning points which help to elucidate the intricate mechanisms involved in turbulent premixed flame extinction dynamics. Turbulence locally wrinkles the iso-surface into positively curved structures which lead to turning points in (S-d, kappa) space, such that the minimum radius is never reached, locally and as an ensemble. A budget analysis of the principal curvature evolution equation highlights the role of turbulence in bending the surface to form positively curved, pointed structures where heat loss is enhanced, further lowering Da towards extinction. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.