Laboratory study of the effect of porous material on flame structure

It is well established that combustion in porous inert media holds important advantages like low pollutant emissions, high power density and the potential of operating in lean and ultra lean combustion regimes. Although burners of this type have started to be examined and characterized over the past years, non intrusive 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 zone, utilizing the excitation of the Hydroxyl radical - OH. A pumped Nd:YAG, frequency doubled dye laser was utilized for this purpose. Hydroxyl radical is the most widely examined species with the LIF technique on the combustion field, because it gives a good understanding of the flame zone and hence it was used in the present study. In order to perform LIF measurements inside the porous combustion zone, optical access along the porous structure for the laser beam to reach the probe volume and furthermore, to allow a sufficient amount of fluorescence to reach the detector was realized by a thin gap of a similar size as the porous cavity size. A parametric study for the optical gap size and positioning was made, yielding the most suitable configuration for a sufficient signal to noise ratio without significantdisturbance of the combustion process in the porous reaction zone. The experiments were conducted over an area of various thermal loads and excess air ratios for methane/air combustion. The main scope of this work is to demonstrate how an appropriate optical access in a porous combustion zone can be achieved for combustion diagnostics, hence demonstrating the capabilities of the methodology. Besides this, 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.

Keywords:

porous material , flame zone , utilizing the excitation , …