Oscillatory stretch effects on the structure and extinction of counterflow diffusion flames

The effects of oscillatory stretch on atmospheric laminar counterflow diffusion flames are investigated both numerically and experimentally. Measurements indicate that, at high excitation frequencies, the peak extinction strain rates of oscillating CH₄+N₂ versus air flames can be extended well beyond steady-state extinction limits. Hydroxyl radical concentrations are measured in a CH₄+N₂ versus air flame excited at moderate frequencies and are compared to numerical simulations including complex chemistry and detailed transport. Measurements and simulations of OH concentration oscillations show similar phase delays. However, the measurements indicate a larger variation of OH concentration. AT moderate frequencies, the time-dependent OH variation is quasi-steady where the time-dependent flame can be described by a series of steady-state flames. However, the time-dependent flame does not quite recover to its steady-state structure at the low-strain-rate extreme.