Laser-induced breakdown spectroscopy in inhomogeneous fuel sprays: A novel calibration methodology for local stoichiometry

Abstract

This paper presents a novel calibration for estimating the local stoichiometry of inhomogeneous sprays when using Laser-Induced Breakdown Spectroscopy (LIBS). The LIBS spectrum in inhomogeneous sprays appears to consist of coexisting spectra. This renders the traditional curve-fit calibration method inapplicable and impedes the determination of local equivalence ratios. A hypothesis is proposed that the spray LIBS spectra can be regarded as a discrete superposition of regions in the detection area with distinct compositions. A novel calibration approach is then proposed to test the simplest version of this hypothesis by identifying the best correlation between the spray LIBS spectra and a binary superposition of the calibration spectra. A local equivalent stoichiometry can then be estimated for the best correlated match. This approach generates spectra with high correlation to the test spray spectra for the majority of sparks, and hence provides a novel insight into LIBS application for inhomogeneous fuel/air mixtures. Test cases where the leaner region already contains fuel (ϕlean > 0) have high probability of successful ignition.