Vortex-induced extinction behavior in methanol gaseous flames: A comparison with quasi-steady extinction

Using a combination of HCHO planar laser-induced fluorescence and laser Doppler velocimetry measurements, the extinction behavior of methanol counterflow diffusion flames was examined experimentally under conditions in which the extinction was brought about by a vortex generated on the oxidizer side. Comparisons were made with quasi-steady extinction results for the same flames. It was found that the flames can withstand instantaneous strain rates as much as two-and-a-half times larger than the quasisteady ones. The finding was rationalized phenomenologically by comparing the characteristic times of the problem, that is, the mechanical time, the chemical time, and the vortex turnover time. Specifically, estimates of these times yielded the following ordering: τ_ch < τ_vort < τ_m. As a result, the vortex introduced an unsteady effect in the outer diffusive-convective layer of the flame, while the inner reactive-diffusive layer behaved in a quasi-steady manner. Consequently, the flame was subject to a damped strain rate through the outer layer. Results from a simple analytical model showed that the difference between vortex-induced extinction and quasi-steady extinction was much more modest in terms of instantaneous scalar dissipation rate or Damköhler number. Furthermore, the temporal history of the strain rate was found to be necessary to determine the effective strain rate felt by the flame. Implications of these findings for turbulent diffusion flame modeling by the flamelet approach are discussed.