Transient local extinction and reignition behavior of diffusion flames affected by flame curvature and preferential diffusion

Experimental and numerical studies are made of transient H₂/N₂-air counterflow diffusion flames unsteadily strained by an impinging micro jet. Two-dimensional temperature measurements by the laser Rayleigh scattering method and numerical computations taking into account detailed chemical kinetics are conducted, paying attention to transient local extinction and reginition in relation to the unsteadiness, flame curvature, and preferential diffusion effects. The results are as follows: (1) Transient local flame extinction is observed where the micro jet impinges. However, the transient flame can survive instantaneously in squite of quite high stretch rate where the steady flame cannot exist. (2) Reignition is observed after the local extinction due to the micro air jet impingement. the temperature after reignition becomes significantly higher than that of the original flame. This high temperature is induced by the concentration of H₂ species due to the preferential diffusion in relation to the concave curvature. The predicted behaviors of the local transient extinction and reignition are well confirmed by the experiments. (3) The reignition is induced after the formation of combustible premixed gas mixture and the consequent flame propagation. (4) The reignition is hardly observed after the extinction by micro fuel jet impingement. This is due to the dilution of H₂ species induced by the preferential diffusion in relation to the convex curvature. (5) The maximum flame temperature cannot be rationalized by the stretch rate but changes widely, depending on the unsteadiness and the flame curvature in relation with preferential diffusion.