Two-photon laser-induced fluorescence of sodium in multiphase combustion

Alkali metals are prevalent in coal combustion, biomass thermal conversion to fuels, and energetic material combustion, and have applications as dopants for combustion diagnostics and plasma-based combustion control. Recently, the dynamic control of propellant burning rate and pyrotechnic luminosity have been demonstrated through combined microwave field radiation and incorporation of alkali dopants. Understanding of the alkali distribution within the flame is essential to predicting field-flame interaction. However, the multiphase propellant combustion environment exhibits significant particle scattering, broadband emission background, and high optical density, which complicate optical measurements. Here, multiple two-photon laser-induced fluorescence (LIF) schemes for atomic sodium were compared in gas-phase flames and sodium-doped solid propellant combustion. For the two-photon Na LIF scheme, a rate equation model incorporating fluorescence, amplified spontaneous emission, and ionization shows good agreement with experimental characterization in a gas-phase flame. The technique was then extended to sodium-doped solid propellant flames, a multiphase combustion environment characterized by strong particle scattering. The 3s-3d two-photon excitation LIF achieved a better signal-to-noise ratio of similar to 100 in the propellant flame and allowed imaging of gradients in the gas-phase sodium distribution at the burning surface. (C) 2020 Optical Society of America