Imaging the combustion characteristics of Al, B, and Ti composites

Abstract

In this study, we prepare 90 wt% loading composites of Al, B, and Ti with KClO4 by 3D printing and study their combustion characteristics with high-speed videography and pyrometry. Combustion characteristics are found to be strongly dependent on the fuel type. For Al, Al droplets with Al2O3 caps form and coalesce before departing the burning surface, while for B and Ti, fractal-shaped agglomerates form. Temperatures of the burning particles (droplet/agglomerates) are determined with color imaging-pyrometry. The combustion characteristics are attributed to the physical properties, e.g. melting and boiling points, of these fuels and their corresponding oxides. We observe particles residing and burning on the propellant surface for times on the order of ∼1–5 ms. This is significantly lower than the theoretical particle burn time, suggesting particles undergo incomplete combustion on the burning surface, consistent with the experimental observation that particles continue to burn after departing the surface. The estimated particle downstream burning distance is significantly larger than the observed luminous zone, implying it does not represent the complete flame zone. Since Al undergoes vapor phase combustion while B and Ti combust in the condense phase, burn rate of Al should be drastically higher than B and Ti, However, the differences are not as significant as expected. This is attributed to the formation of much larger droplets for Al that results in dramatically longer particle burn times.