OPTICAL AND SPECTROSCOPIC CHARACTERIZATION OF RICH PREMIXED FLAMES ACROSS THE SOOT FORMATION THRESHOLD

Phenomenological aspects of formation, destruction, and coagulation of high molecular mass structures formed in the main oxidation zone of r ich premixed flames and in rich flames well below the soot threshold l imit have been examined. High molecular mass structures transparent to the visible radiation, previously detected in the preinception region of soot forming flames, are also present in flames below the soot for mation limit. The onset of ultra-violet fluorescence within the main o xidation zone implies that the formation of these species is a very fa st process and can be considered as a ''polymerization'' of small arom atic groups activated by the presence of oxidizing agents. The final c oncentration of this material falls down as the C/O ratio is decreased and below C/O = 0.35 it is not anymore present. It appears that rich premixed flames present two critical C/O ratios: a first one for soot formation and a lower second one for high molecular mass structure for mation. Ultra-violet scattering/extinction technique has indicated tha t the typical average size of these structures is around 4-6 nm and th eir overall coagulation rate is very low.