Structural sensitivity, response and extinction of unsteady counterflow flames

Effects of sinusoidal velocity oscillation on counterflow premixed and diffusion flames were computationally investigated, with detailed descriptions of chemistry and transport, as functions of imposed frequency and amplitude. The phenomena of interest include (1) the response of oscillating counterflow flame structure to time variations of stretch rate; (2) the dynamic response of various nonequidiffusive premixed flames subjected to sinusoidal oscillation; (3) the transient sensitivity of flame response to the temporal variations of stretch rate; and (4) the extent of the extinction limits modified by flow oscillation for various nonequidiffusive premixed flames. Results on the structural response show that premixed flames exhibit weak sensitivity of the flame structure to oscillatory stretch rate variations, while the thermal and chemical structures of diffusion flames vary sensitively to the imposed oscillation. In addition, the disparate response of premixed flames due to effects of nonequidiffusion are demonstrated. It is also found that the transient burning rate characterized by the spatially-integrated reaction rate profile is a better physical representative than that based on the local mass flux. Furthermore, it is demonstrated that, since extinction is a transient process, for sufficiently rapid oscillations the premixed flame may not have enough time to extinguish before the flow condition again becomes favorable for burning, and as such with increasing frequency a flame can persist beyond the stretch rate regime in which steady-state flames do not exist. However, for mixtures with Le>l, a nonmonotoic transient extinction response is noticed because of the relatively larger sensitively of the flame temperature to stretch rate variations.