Extinction and Reignition in Counterflow Spray Diffusion Flames Interacting with Laminar Vortices

The interaction of laminar vortices with a methanol spray counterflow diffusion flame was studied experimentally with vortices generated from either the fuel side or the oxidizer side. The overall stoichiometry was such that the flame resided on the fuel side of the gas-stagnation plane. Local extinction and subsequent reignition were investigated as the circulation of the vortex was varied. It was found that extinction required vortices of larger circulation if generated from the oxidizer side. The effect was attributed to stretching, and possibly, partial dissipation of the vortex, as it approached the stagnation plane before interacting with the flame. The robustness of the spray flame to vortex-induced extinction was compared with that of a similar gaseous flame. The spray flame was found to be comparatively weaker. Of the two potential culprits for such a difference, namely, the energetic handicap of spray flames due to the latent heat of vaporization of the liquid fuel and droplet inertia, the former was found to be the dominant factor. After extinction occurred, a hole was created in the diffusion flame, confining the combustion process to an annular region. The flame was then able to propagate back toward the centerline, re-establishing itself as a flat diffusion flame. The time interval for this reignition process was investigated as a function of the vortex circulation. It was found that, if the vortex approached the flame from the fuel side, the reignition time was much shorter than when the vortex was injected from the oxidizer side, and decreased for increasing values of the vortex circulation. In contrast, the reignition time increased with the circulation, if the vortex approached the flame from the oxidizer side. Only in the latter case, droplet inertia played a role in the reignition process.