Extinction behavior of a partially premixed flame and a nonpremixed flame in turbulent counterflow

The present study experimentally investigates non-flamelet behavior leading to extinction in a NonPremixed Flame (NPF) and a Partially Premixed Flame (PPF) in a turbulent counterflow configuration. Both flames at Re-t similar to 900 are indistinguishable in terms of the turbulence properties that are imposed at the cold boundaries and persist up to the mixing layer. For each extinction event, the perturbation that leads to the first breach of the OH layer is tracked back in time in a Lagrangian manner using high-speed stereoscopic PIV and OH-PLIF imaging techniques, allowing the reconstruction of the time sequence of strain rate and vorticity that leads to flame extinction. The NPF is found to be more prone to extinction than the PPF, which is at odds with the computed laminar extinction strain rate, which is 23 % larger in the NPF than in the PPF, implying a greater resistance to strain for the NPF. Extinction appears to be caused by a combination of relatively intense strain rate and/or vorticity pockets interacting with the flame and causing a tear of the OH layer. The maximum strain rate norm exceeds the computed extinction limit in >85 % of the cases for the PPF, whereas it does so in approximately 75 % of the cases for the NPF, revealing a more pronounced strain rate effect on the extinction process for the PPF. Vorticity plays multiple roles in extinction. It manifests itself as: i) a pair of counterrotating vortices approaching the flame from either or both sides, creating a region of high strain rate; ii) a single vortex interacting with the flame by diluting the reactants with inert, thereby, weakening the flame; and iii) a vortex penetrating the oxidizer layer, making it thicker and, possibly, disrupting the laminar flame structure. The last two scenarios may explain extinction without a history of remarkably high strain rates. A 35 % reduction in the overall strain rate of the flames resulted in nearly complete suppression of extinction events in both PPF and NPF.