Laser-Induced Breakdown Spectroscopy for Analysis of High Density Methane-Oxygen Mixtures

The applicability of laser-induced breakdown spectroscopy (LIBS) toward greater than atmospheric density combustion diagnostics is examined. Specifically, this involves ascertaining the feasibility of measuring chemical equivalence ratios directly from atomic emission spectra at high density. The need for such measurement arises from the desire to quantify real time, localized combustion performance in weakly mixed flows. Insufficiently mixed flows generally result in unwanted byproducts, possess the propensity for overall combustion instability, and are increasingly likely to experience localized flame extinction.

We simulate methane/oxygen combustion in ambient pressures ranging 1 to 4 atmospheres, demonstrating these results to be analogous to what would be obtained under combustion conditions at pressures roughly 10 times greater. Given that LIBS measurements are independent of the sample's initial state, we introduce a reaction quenching species (neon) at large mole fraction into the mixture, thereby eliminating any chance of ignition and thus many experimental difficulties associated with observing high density atomic spectra.