Measurement of flame properties using Laser-induced breakdown spectroscopy at elevated pressures

To the best of our knowledge, we employ laser-induced breakdown spectroscopy (LIBS) to simultaneously measure multiple properties (pressure, equivalence ratio, and H2 ratio) of product gas under turbulent flow and highenthalpy conditions for the first time. We also introduce an improved LIBS signal acquisition configuration that enhances signal-to-noise ratio (SNR) and shot-to-shot stability. Under varying operating conditions (pressure: 3 to 15 bar, equivalence ratio: 0.7 to 1.2 and hydrogen enriched ratio: up to 50% by volume), the laser is focused at downstream of the multi-nozzle burner where combustion product gas flows turbulently. Laser-induced plasma (LIP) is generated and plasma spectra are simultaneously measured in both right-angle and backward directions. The Proper Orthogonal Decomposition (POD) and Gaussian Process Regression (GPR) model are employed to estimate multiple properties of product gas that exhibit non-linear correlations. POD extracts the principal features from the spectra and GPR model quantifies these properties. The results demonstrate that the proposed model achieves the highest prediction accuracy for pressure, hydrogen concentration, and equivalence ratio, with the average relative errors of model prediction (REP) values of 0.93%, 8.3%, and 1.45%, respectively. Furthermore, measurement in the backward direction yields a more stable and stronger signal intensity compared to those in the right-angle direction measurement. Specifically, the former successfully captures plasma spectra at 15 bar, whereas the latter fails to do so. This study significantly contributes to the precise measurement of combustion product gas in harsh high-enthalpy environments. Our approach provides important information for the design and optimization of combustion-related facilities and indicates promising applicability of LIBS in various other environments.