Time-resolved laser-induced incandescence for soot particle sizing during acetylene pyrolysis behind shock waves

Abstract

Time-resolved laser-induced incandescence (LII) and continuous wave (CW) laser extinction were applied for the sizing of soot particles originated from acetylene pyrolysis behind reflected shock waves. The LII technique is based on a fast heating of particles by a short laser pulse and on the time-resolved observation of the particle thermal emission. The time history of this emission is a measure for the particle cooling, which provides information about the particle size. Additionally, the soot formation was measured by a cw-laser extinction technique, which provides information about induction times and total soot concentration. Three group of experiments with mixtures of 5% C 2 H 2 diluted in Ar at average temperatures of around 1840 K. 1900 K, and 2020 K at pressures of about 2.8 bar were performed. For the individual experiments within each group, the delay time between shock-induced heating of the C 2 H 2 /Ar mixture and the LII laser pulse was varied between 200 μs and 1000 μs in steps of 100μs. The analysis of the LII data results in particle mean radii between 1.8 nm and 10 nm of a log-normal size distribution. Higher temperatures and longer reaction times yield bigger particles. In combination with the extinction data, the soot yield was found to be between 5% and 16% at the maximum reaction time of 1000 μs. The induction times for the soot-particle formation were found to be within the range of previous studies.