Structural Studies on Locally Strained Diffusion Flames. 3rd Report. Analysis on Local Flame Extinction and Reignition Phenomena of Unsteady Strained Flames.

Abstract

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 reduce the following results. (1) Transient local flame extinction is observed where the micro jet impinges. The transient flame can survive instantaneously in spite of the quite high stretch rate where the steady flame cannot exist. (2) Reignition is observed after the local extinction induced by an impinging micro air jet. The predicted behaviors of the local transient extinction and reignition are well confirmed by the experiment. (3) The reignition is hardly observed after the extinction by the micro fuel jet impingement. (4) These characteristic phenomena of (2) or (3) are dominated by the concentration or dilution of H2 due to the preferential diffusion with the concave or convex flame curvature, respectively. (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 to preferential diffusion.