Flame–turbulence interaction near and during blow-off of lean premixed flames

Flameholding has been studied for decades but premixed flame-flow interaction near the point of blow-off, in practical combustion devices, still raises interesting questions. While theoretical ideas about blow-off generally consider the disruption of a reaction–diffusion balance (e.g., strain-based extinction) or a convection–reaction balance (well-stirred reactor), these balances have not been quantified previously during an actual blow-off event. In this work, strain-flame alignment and enstrophy budgets are investigated far from, near, and during total blow-off for a swirl injector. To quantify these, a two-flame, periodic model of interacting methane–air flames is constructed, fully resolving the flame structure using adaptive mesh refinement and detailed chemical kinetics. The flow is resolved down to twice the estimated Kolmogorov length scale in the reactant stream. Near blow-off, the Reynolds (Re), Damköhler (Da) and Karlovitz (Ka) numbers are estimated to be 24400, 0.1 and 250 respectively, which places the blow-off condition in the distributed/broken-reaction zones regime. Results indicate that the flame is passive with respect to alignment to the fluid-dynamic strain rate field but not with respect to vorticity dynamics and baroclinic torque may compete with or even exceed vortex-stretching as a source or sink of enstrophy. Species transport budgets, extracted during the final moments of total blow-off, indicate reaction–diffusion balances persist to the end. Scalar gradients are not observed to thicken to the point of distributing but exhibit complex dependencies on stretch effects.