Numerical investigation of extinction in a counterflow nonpremixed flame perturbed by a vortex

The features of extinction in a CH 4/N 2–air strained counterflow nonpremixed flame perturbed by a vortex were investigated numerically. First, the extinction behaviors using two augmented reduced mechanisms (ARM) and their original full reaction mechanisms (the Miller and Bowman mechanism and GRI-Mech 3.0) were investigated with the numerical results for a steady counterflow flame and unsteady flamelet equations. The modified ARM, based on Miller and Bowman's mechanism (MB-ARM), and adjusted to predict an extinction limit reasonably, was the most suitable mechanism for the unsteady simulation, taking into consideration computational cost, stiffness during the ignition process, and prediction performance for an unsteady flame with sinusoidal transient disturbances. The unsteady 2D computations with the modified MB-ARM showed that fuel- and air-side vortices caused the unsteady effect, and a flame interacting with a vortex was extinguished at a much higher scalar dissipation rate than a steady flame. Moreover, an air-side vortex extinguished the flame more rapidly than a fuel-side vortex, since the air-side vortex was much stronger than the fuel-side vortex, given the same vortex jet velocity conditions. In addition, the degree of the unsteady effect experienced by a flame could be clearly understood by introducing characteristic time scales for the flame, vortex, and convective–diffusive layer.