The impact of soot on laser-induced breakdown spectroscopy and the implementation of the breakdown delay thermometry method

Abstract

Multi-parameter laser-induced breakdown spectroscopy (LIBS) has the potential to provide significant diagnostic utility under a wide range of combustion environments and where composition, temperature, pressure and/or density is necessary for understanding and characterizing the chemical, physical and flow processes at hand. This Thesis details the development and implementation of a novel shot-to-shot based multi-parameter LIBS thermometry approach that operates on the temporal delay of the plasma development as a function of the breakdown threshold along the intensity gradient of the laser pulse. This configuration has the ability to simultaneously monitor both composition and temperature in a flame with only a single optical aperture when analyzing the plasma emission. The investigation of two (high and low temperature) flame cases, clean burning methane-air (φ = 0.85) and very rich pyrolizing propane-air (φ = 233) pre-mixtures, were made for heights above the burner (HAB) from a McKenna flat flame. The results showed a good match between measurements taken by the breakdown probability approach and a technical comparison is made with measurements from the continuum emission intensity and acoustic pressure wave. Although the uncertainties measured were significant (±0.22 – 0.25 ns), owing to the probabilistic nature of the breakdown event and the low sampling rate of the oscilloscope and photodiode (0.1 ns), precise determination of the delay is possible with prolonged sampling (300 – 1000 samples). Moreover, the extent that the breakdown threshold changes towards higher temperatures is observed to decrease significantly and perhaps crucially. The cause of this is thought to be due to the insensitivity of multi-photon ionization towards higher temperature, however it is also speculated that contributing to the increase in the threshold are molecular breakdown and electron diffusion effects. It’s suggested to simultaneously implement one of the more sensitive and environmentally dependent intensity or acoustic pressure wave methods.

Supplementing this study is the characterization of clean burning methane-air and propane-air flames with respect to composition, laser energy, HAB and position along the plasma, and an investigation on the impact of soot on the breakdown threshold and plasma emission. It is determined that the soot produced by the propane flame had a negligible impact on both parameters meaning that the nano-sized particles from the developing soot was insufficient enough to meaningfully influence the dynamics of the plasma initiation and development in a high temperature flame. However, incandescence was observable in the later stage of plume formation and strong thermal emission in the time-integrated spectra. These findings, nevertheless, support the conclusion of LIBS as a reliable diagnostic tool for harsh, sooting and turbulent environments.