Pulsating instability and extinction in flames

Diffusional-thermal pulsating instability was numerically investigated for both laminar premixed and non-premixed flames characterized by Zeldovich numbers and Lewis numbers greater than unity, using detailed chemistry and transport. For premixed flames, these conditions occur near the rich and lean limits for hydrogen/air and heptane/air flames respectively, although the window of oscillation is very narrow for the latter. For adiabatic flames, four regimes of propagation were identified: steady propagation, pulsating with monochromatic oscillation, pulsating with period doubling, and "bursting," which is characterized by long periods of almost no chemical reaction separated by short bursts of intense reactivity and heat release. When optically-thin radiative heat loss is included in the model, the behavior is qualitatively unchanged except for the "bursting" regime where extinction occurs during the weakly reactive phase of pulsation. This unsteady behavior must therefore be taken into account in order to accurately evaluate the flammability limit for the appropriate mixture. Additionally, insights into the global chemical kinetics and transport in certain steady flames emerge through the present study.;The effect of flame stretch on the onset of instability and extinction was investigated both numerically and experimentally, using the premixed counterflow twin flame configuration. A CH4/O2/He mixture was chosen in order to safely explore large Lewis number flames in a laboratory setting. The presence of positive flame stretch was shown to promote the onset of instability compared to unstretched flames. As the stretch rate is further increased extinction occurs.;The possible occurrence of pulsations in non-premixed flames was also numerically through the burner-generated spherical flame. Oscillatory instability was observed near both the transport induced limit (low mass flow rate) and the radiative induced limit (high mass flow rate) of the isola response of flame extinction. Unlike premixed flames, no limit cycle was found; the oscillation grows in amplitude until it becomes large enough to extinguish the flame.