Soot formation from heavy hydrocarbons behind reflected shock waves

Soot formation from heavy hydrocarbons (n-hexadecane, toluene, n-heptylbenzene, and 1-methylnaphtalene) was studied behind reflected shock waves, using a light extinction technique. The highly diluted mixtures (99 to 99.8% of argon) were heated between 1300 and 2700 K. The pressure ranged from 200 to 1800 kPa. Soot induction delay times, growth rates, and yields were determined under pyrolysis and for two equivalence ratios (5 and 18). The effect of aromaticity, oxygen content, temperature, and pressure on these parameters were investigated. Small quantities of oxygen have little influence on soot induction delay times and soot growth rates but decrease the soot yield. The maximum soot yield was found to depend strongly on the initial number of aromatic rings of the fuel. The optimum temperature for which the soot yield is maximum is a function of the hydrocarbon nature (aromatic or linear). Samples of soot particles formed behind shock waves and collected after experiments were analyzed by transmission electron microscopy at a magnification of 5 x 10(4) in order to determine die size of elementary spheres. This parameter was studied in relation with the experimental conditions. The elementary sphere size decreases with increasing temperature, especially in the presence of oxygen. For relatively low equivalence ratios (phi = 5), the shape of the primary spheres deviates strongly from the spherical form.