Thermo-kinetic dynamics of near-limit cool diffusion flames

The dynamics of near-limit cool diffusion flames were studied experimentally and numerically by tracing transient flame evolution and instability. To determine the effects of chemistry-transport coupling on ignition and instability in a cool diffusion flame diffusion flames of n-heptane and pure oxygen were sensitized by ozone in a counterflow burner.The immediate addition of ozone to the oxidizer side of a frozen flow either initiated a cool diffusion flame or triggered a two-stage ignition into a hot flame. When the fuel mole fraction was instantaneously decreased for a stable cool diffusion flame near its extinction limit the measured flame chemiluminescence exhibited instability showing significant fluctuation of flame chemiluminescence just before extinction. The instability behavior of a cool flame was a thermo-kinetic instability which is triggered and controlled by the chemical kinetics associated with the OH radical population in the negative temperature coefficient chemical kinetic regime coupled with heat production and loss.