Structural Sensitivity, Response, and Extinction of Diffusion and Premixed Flames in Oscillating Counterflow

Authors: C. J. Sung and C. K. Law

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Effects of sinusoidal velocity oscillation on counterflowdiffusion and premixedflames were computationally investigated as functions of imposed frequency and amplitude and with detailed descriptions of chemistry and transport. The phenomena of interest investigated are: (1) the structuralsensitivity of premixed versus diffusionflames; (2) the effects of nonequidiffusion on the dynamic response of premixedflames; (3) the transient sensitivity of the premixedflame burning rate; and (4) the modification of the extinction limits of premixed and diffusionflames. Results show that premixed and diffusionflames, respectively, exhibit weak and strong structuralresponses, that the dynamic response of the premixedflames to mixture nonequidiffusion can be qualitatively disparate for mixtures with Lewis numbers (Le) greater and less than unity, and that the transient burning rate characterized by the spatially integrated reaction rate profile is a better physical representation than that based on the local mass flux. It is further demonstrated that for sufficiently rapid oscillations the 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 range of the stretch rate in which steady-state flames do not exist, and that for premixedflames with Le > 1, the transient extinctionresponse can be nonmonotonic because of the freely standing nature of the flame and the relatively stronger sensitivity of the flame temperature to stretch rate variations.

Citation: C. J. Sung and C. K. Law, “Structural Sensitivity, Response, and Extinction of Diffusion and Premixed Flames in Oscillating Counterflow,” Combustion and Flame 123, 375-388 (2000).