Thermo-kinetic dynamics of near-limit cool diffusion flames
Published on Jan 1, 2017
· DOI :10.1016/j.proci.2016.05.049
Abstract The dynamics of near-limit cool diffusion flames are investigated experimentally and numerically by studying transient flame evolution and instability. In order to observe the effects of chemistry-transport coupling on ignition and instability in a cool diffusion flame, diffusion flames of n-heptane and pure oxygen are sensitized by ozone in a counterflow burner. First, it is found from experimental observations that the immediate addition of ozone to the oxidizer side of a frozen flow either initiates a cool diffusion flame or triggers a two-stage ignition into a hot flame. Second, a cool flame near its extinction limit can be destabilized by a perturbation of the fuel mole fraction, eventually leading to flame extinction. The recorded flame chemiluminescence signals reveal repeated flame oscillations before extinction. Next, details of the transient dynamics of near-limit cool diffusion flames are explored through numerical calculations by adopting the same perturbation method. Experimental observations of unsteady flame initiation and instability are simulated, and it is found that the ignition process is highly sensitive to the perturbed ozone mole fraction and that the ignition delay times in a counterflow configuration are significantly affected by the diffusive transport of species with high concentration. The instability mechanism of a cool flame is found to be distinct from that of diffusive-thermal instability. The results show that the instability behavior of a cool flame is 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.