Soot formation in laminar acetylene/air diffusion flames at atmospheric pressure

The flame structure and soot-formation (soot nucleation and growth) propert ies of axisymmetric laminar coflowing jet diffusion flames were studied exp erimentally. Test conditions involved acetylene-nitrogen jets burning in co flowing air at atmospheric pressure. Measurements were limited to the axes of the flames and included soot concentrations, soot temperatures, soot str ucture, major gas species concentrations, radical species (H, OH, and O) co ncentrations, and gas velocities. The results show that as distance increas es along the axes of the flames detectable soot formation begins when signi ficant H concentrations are present, and ends when acetylene concentrations become small. Species potentially associated with soot oxidation-O-2 CO2, H2O, O, and OH-are present throughout the soot-formation region so that soo t formation and oxidation proceed at the same time. Strong rates of soot gr owth compared to soot nucleation early in the soot-formation process, combi ned dth increased rates of soot nucleation and oxidation as soot formation proceeds, causes primary soot particle diameters to reach a maximum relativ ely early in the soot-formation process. Aggregation of primary soot partic les proceeds, however, until the final stages of soot oxidation. Present me asurements of soot growth (corrected for soot oxidation) in laminar diffusi on flames were consistent with earlier measurements of soot growth in lamin ar premixed flames and exhibited encouraging agreement with existing hydrog en-abstraction/carbon-addition (HACA) soot growth mechanisms in the literat ure that were developed based on measurements within laminar premixed flame s. Measured primary soot particle nucleation rates in the present laminar d iffusion Games also were consistent with corresponding rates measured in la minar premixed dames and yielded a crude correlation in terms of acetylene and H concentrations and the temperature. (C) 2001 by The Combustion Instit ute.