Correction of laser-induced incandescence signal trapping in soot measurement in a microgravity boundary layer laminar diffusion flame

This work presents a novel method to auto-correct the trapping effect of laser-induced incandescence (LII) signals inside the flame to obtain soot volume fraction distributions in laminar axisymmetric (2D) and three-dimensional (3D) flames using planar LII. The development of the proposed model is described with an estimation of the propagated uncertainties. The proposed model is firstly numerically validated in a canonical 2D coflow Santoro flame using synthetic LII signals and soot properties predicted by the CoFlame code. Secondly, experimental measurements of LII were carried out in the same 2D coflow Santoro flame and the LII signals were converted to soot volume fraction using the proposed model. Simultaneous LII and line-of-sight attenuation measurements were conducted to obtain the calibration factor. The proposed model automatically took into account the signal trapping effect in the conversion of the detected LII signal to soot volume fraction. Finally, the validated methodology is applied to a 3D laminar boundary layer diffusion flame established in microgravity. The significant differences in the measured soot volume fraction distributions with and without considering signal trapping, particularly in the 3D zone of the flame, demonstrate the importance of considering signal trapping to LII measurements of soot volume fraction in this flame. The model developed in this work can be readily applied in planar LII measurements of soot in any flame configuration as long as it is steady or statistically steady, to allow measurements to be performed at different positions inside a flame, such as for pool-fires or pulsating flames.