Quantitative measurement of mixture fraction in counterflow diffusion flames by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) has been widely employed in various flames to determine local element compositions. Compared with other laser-based measurement techniques, LIBS is a robust and fast-to-implement method that allows for real-time monitoring and remote analyses in various industrial applications. However, quantitative LIBS measurements of elemental ratios in the gas phase are challenging as the atomic emissions also depend on laser intensities, gas densities, and gas compositions. In this work, we propose a new spectral correction method to eliminate these influences. In detail, the plasma temperature is obtained by fitting spectral lines of one single element based on the local thermodynamic equilibrium (LTE) assumption of the laser-induced plasma. The plasma temperature is then utilized to correct the intensity ratios of spectral lines from different elements. The proposed method is first applied to pure air flow to correct the ratios of the O line at 716 nm and the N line at 819 nm. The corrected O/N ratios are independent of laser intensities and gate delays in the investigated range. Shot-to-shot fluctuations are also decreased after correction. This new method is then used to perform measurements in a counterflow diffusion flame. The elemental ratios of C/N, O/N, and H/N at different positions are obtained to derive the mass fractions of C, H, O, N as well as the mixture fraction. Agreement between experimental and simulation results indicates the feasibility of this method for quantitative measurement of mixture fractions in flames.