Soot formation in premixed heptane-toluene flames

In this work soot formation was studied in a series of laminar premixed flames fueled with binary heptane-toluene mixtures. The equivalence ratio (phi = 2), the cold gas velocity and the total carbon flow rate were kept constant changing accordingly the oxygen content and the diluent (N-2) concentration. A very similar temperature up to 80% of toluene concentration was obtained and, hence, a direct comparison between particle formation from such different fuels. In-situ optical techniques (laser induced incandescence - LII) and ex-situ particle size distribution (PSD) measured downstream of the flame front have furnished data on the total amount and size of combustion generated particles derived from heptane/toluene combustion. LII showed an increase of total volume fraction of soot as the toluene percentage increased. PSD profiles showed that particles with sizes less than 10 nm decrease as toluene percentage in the feed mixture increases, disappearing for toluene percentages above 60%. Conversely, large aggregates grow towards sizes larger than 100 nm when toluene concentration is increased. Bulk material was also sampled thermophoretically from the flames by means of fast insertion of quartz plate. UV-Visible and Raman spectroscopy was performed in order to obtain information on the aromaticity of the particles that was found to increase with toluene percentage. These results follow what already found for other aliphatic-aromatic binary mixtures. However, the heptane/toluene mixture peculiarities different from other mixtures will have to be taken into account for the future studies on surrogates and other synthetic fuels.