Investigation of the Two-Dimensional Structure of a Methane–Air Flat-Burner Flame Using Laser-Induced Fluorescence

Abstract

We report a technique for the two-dimensional diagnostics of flames using laser-induced fluorescence in the saturated regime; the method involves measuring the absolute concentration of hydroxyl radicals (OH). A spatial resolution on the order of 50 μm is achieved, which is significantly smaller than the thermal flame thickness and the characteristic scales of heat-release zones. The technique is validated using a methane–air flame stabilized on a flat porous burner. It is shown that for rich mixtures, the combustion front structure is two-dimensional due to the formation of a secondary conical diffusion flame at the interface between the unreacted fuel and the surrounding air. It is found that the use of a nitrogen shielding stream co-flowing at the same linear velocity as the exiting fuel–air mixture from the burner completely suppresses the secondary flame and ensures a one-dimensional flame front structure in the central part of the burner, extending up to approximately 20 mm from its surface. It is demonstrated experimentally for the first time that, for pore diameters up to 400 μm, flow nonuniformities at the scale of the porous structure do not deform the combustion front. The developed technique enables the high-precision determination of laminar burning velocity based on critical flame blowoff conditions and can also be used to verify detailed kinetic mechanisms and validate numerical combustion models.