Analysis of laser-induced incandescence and novel soot measurement approaches

Abstract

We used a time-dependent, numerical model of the soot particle heating and cooling processes to calculate signals from laser-induced incandescence (LII). Various laser sources were studied in order to compare their suitability for measurements of soot volume fraction and primary particle size. For modeling of LII imaging, each laser was assumed to produce a laser sheet, and each was assumed to have Gaussian spatial and temporal intensity profiles. The solutions show that substantial errors in volume fraction measurements resulting from changes in primary soot particle diameter and local gas temperature can be reduced with proper choice of detection wavelength, gate, laser source, and laser intensity. In addition, the ratios of LH images from two detection wavelengths, e.g., red (700 nm) versus blue (400 nm), can be used to determine primary particle diameter. This method of measuring soot size is valid even if the local gas temperature is unknown, because of the low temperature dependence of this ratio. While ratios from two tune gates, each with a different delay with respect to the laser pulse, provide a more sensitive measure of primary particle diameter, they are also much more dependent on the local gas temperature.