A computational study on oscillatory extinction of spherical diffusion flames

The transient behavior of a spherical diffusion flame was studied in both the high velocity induced limit (low mass flow rate) and the radiative induced limit (high mass flow rate) of the isola response of flame extinction. Oscillatory instability was observed near both steady-state extinction limits. This oscillation typically grows in amplitude until it becomes large enough to extinguish the flame. The oscillatory behavior was numerically observed using detailed chemistry and transport for methane (50%CH₄/50%He into 21%O₂/79%He) and hydrogen (100% H₂ into 21%O₂/79%He) diffusion flames where the fuel was issued from a point source, and helium was used as an inert to increase the Lewis number, facilitating the onset of oscillation. In both methane and hydrogen diffusion flames, the oscillation always leads to extinction, and no limit cycle was found. For the oscillating methane flames, the amplitude of the temperature oscillation was as great as 50K just before the flame extinguishments with the frequency of oscillation about 0.4 Hz. Although the onset of instability leads to extinction, the growth rate is small enough to allow some flames to oscillate for over 450 seconds, suggesting that such oscillations can be observed experimentally. For the hydrogen flames, however, the -premixed.