Extinction processes during a non-premixed flame-vortex interaction

Studies of flame-vortex interactions are quite valuable in the analysis of turbulent combustion. As turbulence may be viewed as a collection of vortices with different scales and intensities, the interaction of isolated vortical structures with flames defines the elementary process by which turbulence acts on flames. Experiments and interpretation are thus simplified because the unperturbed flame and the incoming vortex may be controlled with precision. We here investigate the influence of vortex velocity (directly related to its induced strain rate) and of global mixture ratio on the extinction limits. Three vortex types with different velocities interact with a non-premixed diluted hydrogen-air flame. The global mixture ratio of this flame has been varied between 0.5 and 1.2. Four different kinds of interaction are described, and the limits of the connected-flame regime, relevant for flamelet modeling, are identified. The growth of the flame surface during the interaction is also examined, showing very different effects depending on vortex velocity and global mixture ratio. The increase in flame surface area is maximum for slow vortices and intermediate values of the mixture ratio. The main features of the interaction and the relative importance of the increase in flame surface are then explained in the light of characteristic times and extinction strain rates obtained by asymptotic analysis. The extinction of the flame front is finally examined using direct numerical simulations of flame-vortex interactions, including complex chemistry, detailed thermodynamics, and multicomponent diffusion velocities. The relative importance of the strain rate acting on the flame front and of mixing effects is assessed, proving that unmixedness is not responsible for the extinction.