Transient flow field effects on soot volume fraction in diffusion flames

Quantitative measurements of soot concentration made in an oscillating propane-air counterflow diffusion flame are presented. The non-intrusive laser induced incandescence (LII) technique was used to make spatially and temporally resolved measurements of soot volume fraction in these transient flames as a function of initial steady strain rate, forcing frequency, and forcing amplitude of the strain rate fluctuation. The results of this study show that the soot formation process becomes insensitive to fluctuations in strain rate at high initial strain rates. At low initial strain rates, however, the maximum soot concentration is drastically reduced with high frequency, high amplitude fluctuations compared to the corresponding steady strain soot volume fraction. Low frequency oscillations are found to always increase the maximum soot concentration, by up to a factor of six for some conditions. These measurements provide important insight into the response of the chemistry controlling the soot formation process in flamelets subject to unsteady rates of strain.