Extinction of counterflow diffusion flames under velocity oscillations

The effects of sinusoidal velocity oscillation on counterflow diffusion flames of diluted methane against air were experimentally and computationally investigated. Experimentally, the unsteady flow characteristics as well as the local extinction strain rates were measured over extensive ranges in frequency and amplitude of the flow oscillation by using laser Doppler velocimetry (LDV). Computationally, the phenomena of interest were simulated with detailed descriptions of chemistry and transport. Results show that the hydrodynamic flow field does not respond instantaneously upon imposition of the velocity perturbation at the nozzle exits of the counterflow burners in that, with increasing frequency or amplitude, a noticeable lag develops between the imposed perturbation and the response of the flow further downstream. It is also demonstrated that extinction basically behaves quasi-steadily either for low-frequency oscillations, or for high-frequency oscillations imposed on weakly burning flames, in that the (maximum) extinction strain rate is largely independent of the mean strain rate, being only slightly larger than the steady-state extinction value. However, for strongly burning flames subjected to high-frequency oscillations, increasingly larger amplitudes are needed to effect extinction as the frequency increases. The present results therefore further substantiate and quantify the concept that since extinction is a transient process, for sufficiently rapid oscillation 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 strain rate regime in which steady-state flames do not exist.