Measurements of the growth and coagulation of soot particles in a high-pressure shock tube

Abstract

The advantage of well-defined experimental conditions in shock tubes has been used to investigate the growth and coagulation of soot particles at high pressures. The measurements have been made for fuel-rich oxidation and pyrolysis of different hydrocarbons behind the reflected shock at pressures between 10 and 60 bar and temperatures between 1500 and 2300 K. In addition to soot volume fraction, time-resolved scattering measurements yielded particle diameters and number densities; all these give insight into both surface growth and coagulation at enhanced pressures. The temperature behind the reflected shock was monitored by two-color pyrometry. Soot growth was characterized by induction periods and soot growth rates. At low final soot yields, the growth rate of soot depends on the square of the carbon concentration. At high soot yields, reduced growth rates of soot volume fraction were observed and can be attributed to a lack of growth species. At constant carbon concentration no pressure dependence of soot volume fraction could be found. Particle diameters between 15 and 40 nm were measured. The number density of particles was found to increase strongly with soot volume fraction. Calculated and measured particle number densities agree well during early soot growth. However, at longer times the experiments reveal coagulation rates which are significantly smaller than predicted. This behavior indicates that collisions of deactivated soot particles are characterized by sticking probabilities lower than unity. A correlation for the sticking probability has been established to match both the experimental results and calculations.