Laser-induced breakdown spectroscopy measurements in spray flames

Laser Induced Breakdown Spectroscopy (LIBS) is a laser based spectroscopic technique which has been used widely in various fields, including the determination of the fuel to air ratio (FAR) in combustion. However, the flames examined so far with LIBS were mostly involving gaseous fuels, lean to stoichiometric compositions, and the burners used were of simple geometry. In the present work, LIBS was employed in an enclosed bluff-body swirling n-heptane spray flame, a geometry relevant to gas turbine combustors, in an effort to develop a measurement system for the local time-averaged FAR in liquid-fuelled flames, a quantity that has not been determined with laser-based techniques so far. In order to create the necessary calibration curves for the FAR measurement, the plasma was initially induced in homogeneous dispersions of C7H16 in air, with the overall (i.e. liquid and vapour) equivalence ratio in the range 0-10. From the acquired emission spectra, the most intense features were the atomic lines of hydrogen (Ηα) at 656.3nm and of oxygen (O(I)) at 777.3nm together with the molecular bands of CN (Δυ=0, Β2Σ+-­‐‐Χ2Σ+) at 388.3nm and of C2 (Δυ=0, d3Πg-­‐‐a3Πu) at 516.5nm. The ratios Hα/O and C2/CN increased monotonically with equivalence ratio, suggesting that these ratios can be used as calibration curves for the determination of the local equivalence ratio in spray flames. Using the acquired calibration curves, the spatial distribution of the mean equivalence ratio in the swirling flame was determined and the results were in qualitative agreement with our expectations of the fuel distribution from Mie scattering and OH-PLIF images. The results extend the potential of the LIBS technique to spray flames in practically-relevant configurations.