Quantitive limitations of laser-induced incandescence in practical combustion environments

Abstract

Soot mass concentrations were measured with laser-induced incandescence in a nonreacting flow. The behavior of the LII signal with respect to soot concentration, particle size, and temperature was isolated with the use of a controlled soot generating device. This device was also intended to simulate an environment similar to a jet engine exhaust. Reduction of inteference signals and high detection sensitivity was achieved with the use of a Nd:YAG laser at its fundamental wavelength and broadband detection from 570 to 850 run. The technique yielded LII signals nearly proportional to soot concentration over 4 orders of magnitude, with promise of detecting soot concentration to 1 part per trillion. The detection setup was designed, according to a model of the LII process, to reduce dependence on local gas temperature and soot particle size. Experimental results agreed with the model predictions in terms of particle size dependence. Also, LII signals were observed for gas temperatures from 70 to 300 °C, the latter approaching temperatures typical of an engine exhaust. No dependence on temperature was found over this range.