Structural sensitivity, response, and extinction of diffusion and premixedflames in oscillating counterflow

Effects of sinusoidal velocity oscillation on counterflow diffusion and pre mixed flames were computationally investigated as functions of imposed freq uency and amplitude and with detailed descriptions of chemistry and transpo rt. The phenomena of interest investigated are: (1) the structural sensitiv ity of premixed versus diffusion names; (2) the effects of nonequidiffusion on the dynamic response of premixed names; (3) the transient sensitivity o f the premixed name burning rate; and (4) the modification of the extinctio n limits of premixed and diffusion flames. Results show that premixed and d iffusion names, respectively, exhibit weak and strong structural responses, that the dynamic response of the premixed dames to mixture nonequidiffusio n can be qualitatively disparate For mixtures with Lewis numbers (Le) great er and less than unity, and that the transient burning rate characterized b y the spatially integrated reaction rate profile is a better physical repre sentation than that based on the local mass flux. It is further demonstrate d that for sufficiently rapid oscillations the flame may not have enough ti me to extinguish before the how condition again becomes favorable for burni ng, and as such with increasing frequency a flame can persist beyond the ra nge of the stretch rate in which steady-state flames do not exist, and that for premixed flames with Le > 1, the transient extinction response can be nonmonotonic because of the freely standing nature of the flame and the rel atively stronger sensitivity of the name temperature to stretch rate variat ions. (C) 2000 by The Combustion Institute.