Analysis of the flame structure for lean methane–air combustion in porous inert media by resolving the hydroxyl radical

Abstract

The increasing need for more efficient and less emissive technologies has shifted the focus of combustion research towards technologies involving combustion in porous inert media. Although burners of this type have started to be examined and characterized over the past years, nonintrusive experimental methods are needed to describe the actual processes taking place inside the porous structure. In the present work, the technique of laser induced fluorescence (LIF) is employed to visualize the flame stabilization process inside porous media combustion, utilizing the excitation and subsequent detection of the hydroxyl radical (OH). In order to perform planar measurements inside the porous combustion zone, optical access along the porous structure had to be accomplished. Optical access along the porous structure allowed the laser beam to reach the probe volume and to detect sufficient fluorescence amount. This was realized by creating a thin gap of similar width as the porous cavities. The optical gap size and positioning were chosen so as to yield the most suitable configuration for a sufficient signal to noise ratio without significant disturbance of the combustion process in the porous reaction zone. The experiments were conducted for various thermal loads and excess air ratios for methane/air combustion. The main scope of this work is to demonstrate how the flame structure is influenced by the thermal load and the equivalence ratio over a wide operational range. The flame stabilization in the porous inert medium at fixed position, which is almost independent of the flow rate in some cases, is also observed. Moreover, an experimental characterization of the wide, stable operating conditions of the porous burner is given, along with the description of the flame zone inside the porous matrix.