Simultaneous Single-Shot Two-Dimensional Imaging of Nanoparticles and Radicals in Turbulent Reactive Flows

Abstract

Recently, a phase-selective laser-induced breakdown spectroscopy (PS-LIBS) technique has been proposed and developed to diagnose nanoparticles. However, only a few kinds of nanoparticles can generate sufficient strong PS-LIBS signals for single-shot two-dimensional measurement. In this work, we report on the development and validation of a resonance-enhanced PS-LIBS for instantaneous two-dimensional imaging of nanoparticle formation and transport in a turbulent reactive flow. By matching the laser pulse with the absorption line of titanium atoms in the laser-induced nanoplasma, we find that the resonance-enhanced titanium atomic emissions are 1 order of magnitude stronger than the nonresonant spectra ranging from 498 to 502 nm. Based on the spectral characterization, a quantitative relation between the spectra and the particle volume fraction is theoretically established and validated by experiments. Combining the resonance-enhanced PS-LIBS and planar laser-induced fluorescence (PLIF) techniques, instantaneous two-dimensional distributions of the nanoparticle and the hydroxyl radical (OH radical) can be measured in situ in a turbulent reactive flow. The simultaneous measurement demonstrates a strong correlation between the nanoparticle formation and the reactive flow pattern in the turbulent region. The correlation further reveals that the precursor consumption is dominated by the turbulence effect, indicating the precursor chemistry-turbulence coupling.