SOOT FORMATION IN PREMIXED HEPTANE-TOLUENE FLAMES

In this work soot formation was studied in a series of laminar premixed flames fuelled with binary heptane-toluene mixtures. The heptane/toluene mixtures have been extensively studied, due to their relevance as surrogates of the real fuels. The equivalence ratio (?=2), the cold gas velocity and the total carbon flow rate were kept constant changing accordingly the oxygen content and the diluent (N2) concentration. In this way it was possible to obtain a very similar temperature up to 80% of toluene concentration. The quite similar combustion conditions allow to have 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 generated with the different mixtures of heptane and toluene. As the toluene increases the aromaticity of the material increases. This means that soot particles formed burning mixtures having high toluene content present a minor propensity to be oxidized. The results from this study follow what already found for other aliphatic-aromatic binary mixtures. However, the heptane/toluene mixture seems to have some peculiarities different from other mixture. This is important for the future studies of the soot formation in surrogates and other synthetic fuels.