Optical characterization of rich premixed CH4/O2 flamesacross the soot formation threshold

Light scattering, fluorescence, and C2 emission induced by an ultraviolet laser source and uv-visibleabsorption have been used to study the phenomenological aspects of formation, destruction, and chemical transformation of high molecular mass structures formed in rich premixed methane flames at atmospheric pressure. High molecular mass structures are formed in CH4/O2 flames with C/O ratios down to 0.35. This value, corresponding to their formation threshold, is noticeably lower than the soot threshold limit (C/O=0.45). Different flames have been analyzed across the soot threshold limit, varying the C/O ratio and the maximum flame temperature, and no dramatic changes in the properties of the high molecular mass structures have been found in the passage from nonsooting to sooting conditions. Their role as soot precursors seems to be connected to a kinetically controlled passage from aromatic functionalities with one or two rings to higher polycondensed aromatic functionalities more than to a carbonization process of these structures. High molecular mass structures are easily photofragmented by an ultraviolet laser source with high-energypulses, and the consequent C2 emission has been attributed to these structures more than to soot particles. From a practical point of view, the spatial distribution of these potential pollutants, whose toxic properties are largely unknown, can be easily followed in practical diffusion flames by monitoring the 2D fluorescence due to C2 photofragments employing an ultraviolet laser source