Investigation of particle inception in sooting premixed hydrocarbon oxygen low pressure flames

Abstract

Particle size distributions of soot particles have been measured in flat, smooting low-pressure hydrocarbon oxygen flames by a sampling technique. Independent of the various fuels investigated the measured particle size distributions can be approximated very well by long-normal distributions. If distributions are considered, very good agreement results between previous light scattering and extinction measurements and measurements by the technique applied in this work. Furthermore the ratio fN of the second moment of the volume distribution μ2 to the square of the first moment μ12 shows only slight time dependence for the reaction times used in this study. This leads to the conclusion that the refractive index of soot particles is dependent on the age of the soot particles because the ratio (I/I0)/(kext·μ1), where I/I0 and kext are taken from corresponding optical measurements, containing as variables solely fN and the complex refractive index, varies with reaction time. The asymptotic value of fN equals the value which can be obtained from numerical solutions of the particle balance including particle inception and surface growth of particles. The measured values of fN compared with the numerical simulation elucidate that soot formation consists of two processes, particle inception and surface growth. The numerical simulations allow estimates of particle inception rates and times for particle inception and also of the portions of the soot being formed by particle formation and surface growth respectively.