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

Abstract

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 ∼ 100 in the propellant flame and allowed imaging of gradients in the gas-phase sodium distribution at the burning surface.

Liquid-to-gas transfer of sodium in a liquid cathode glow discharge

“…The laser system, operating at a 200 Hz repetition rate, was synchronized with the plasma discharge using a SRS DG535 delay generator, allowing time-resolved measurements of Na fluorescence during and after the discharge pulse. Based on the 2-level TaLIF model described by Zhu et al [18], a laser wavelength of 685.3 nm was used for 2-photon excitation from the 3 s ground state to 3d excited state. Fluorescence is observed at 3 wavelengths (818.3256 nm, 819.4791 nm, and 819.4824 nm [19]) corresponding to 3d-3p transitions.…”

Optical measurements in multiphase combustion

“…For the 3.5 wt.% case, the sodium temperature with microwave application increases by 1200 K. These time sequences also show unsteadiness in the microwave-enhanced cases, which may be attributed to the process of plasma kernel formation. Imaging of the Na distribution near the burning surface showed high Na concentrations at the burning surface-the result of NaNO 3 decomposition at the burning surface [155]. These regions of high Na density also correspond to regions of high electron concentration due to the low ionization threshold of the alkali metal additive.…”

“…shown the ability to color-shift and modulate pyrotechnics' light intensity (Barkley, 2020). The mechanisms of microwave energy deposition in alkali-doped solid propellant enhancement are highlighted in Figure 4.1, reproduced from Barkley et al [6].…”

“…Alternative methods have been explored in small molecules and atoms using multiphoton laser excitation and subsequent fluorescence [43,127,29]. For these species' systems, two-photon laserinduced fluorescence has the advantage of avoiding fluorescence trapping as compared to singlephoton resonant excitation [155]. Although two-photon excitation schemes have advantages, the loss process is significant, and accurate numerical modeling of the interaction is needed for correction of signals.…”

“…Moreover, for atomic single-photon LIF, the excitation and emission wavelength may be identical, and interferences are difficult to avoid. For species at high concentrations and in multiphase environments, single-photon LIF could also result in signal trapping, strong attenuation of the incoming laser beam, re-fluorescence, and interfering scattering from particles [155,144]. Signal trapping refers to the absorption of emitted photons before reaching the detector and generally leads to a decrease in effective signal yield, which requires correction for quantitative concentration To address these issues, multiphoton techniques, including two-photon LIF and resonanceenhanced multiphoton ionization (REMPI), have been explored for diagnostic measurements in combustion [151,144].…”