Multi-Species Characterization of Aluminized Composite Propellant Flames via Optical Emission and Laser Absorption Spectroscopy

This work demonstrates a new absorption-referenced emission spectroscopy (ARES) diagnostic for quantitative measurements of multiple gaseous species, gas temperature, and particle temperature in aluminized composite-propellant flames. This ARES diagnostic combines broadband (300-900 nm) optical emission spectroscopy (OES) with ultrafast laser absorption spectroscopy (ULAS) near 395 nm. Together, this enabled quantitative column mole fraction measurements of Al, AlO, Ca, CaCl, and Na in aluminized AP-HTPB propellant flames burning in air or argon at 1 atm.The temperature of AlO was consistently near 3300 to 3500 K, depending on bath gas, which is much higher than that of the particles which varied from approximately 2500 to 2700 K. This is likely due to the AlO molecules being isolated to extremely hot diffusion flames surrounding the aluminum particles. The column mole fraction of aluminum was 14.33 ± 1.09 ppm-cm in air and 10.10 ± 0.83 ppm-cm in argon. In contrast, the inferred column mole fraction of AlO was approximately 5 to 10 times larger, varying from 120.68 ± 104.08 ppm-cm in air to 50.74 ± 11.15 ppm-cm in argon. Notably the inferred mole fraction of aluminum is several orders of magnitude below thermochemical equilibrium predictions which points to incomplete vaporization and combustion of the aluminum at the locations measured. These results demonstrate the potential of this technique for advancing our understanding of how the extent of aluminum combustion varies within aluminized propellant flames.