Effects of soot volume fraction on local gas heating and particle sizing using laser induced incandescence

Several decades of developing laser induced incandescence (LII) as a combustion diagnostic have led to great advances in understanding soot formation. The bulk of diagnostic development has focused on time-resolved LII (TiRe-LII) where heating of soot particles using a laser, and the subsequent particle cooling, can be measured and matched to a model to determine soot primary particle size. These models vary significantly in both complexity and accuracy. This work focuses on the effect of local gas heating during the soot cooling process. Multi-color pyrometry is performed 5 μs following laser heating on flames ranging in soot volume fraction (SVF) of 10 ppm up to 43 ppm with laser fluences of 0.09 J/cm2, 0.07 J/cm2, and 0.05 J/cm2. Both laser fluence and SVF had significant effects on the local gas heating as measured by the soot temperature. Simulations were performed implementing local gas heating and showed prediction errors depend on pressure and soot volume fraction. As SVF increases, the errors in primary particle size predictions show a nearly linear rate of increase that can become significant at higher SVF values.