INVESTIGATING THE COMBUSTION BEHAVIOR OF Al/AP COMPOSITES WITH HIGH-SPEED VIDEOGRAPHY

In this study, we prepare 90 wt % loading composites of aluminum and ammonia perchlorate with equivalence ratios from 0.5 to 2 and study their combustion behavior with high-speed microscopic and macroscopic videography and pyrometry. For all equivalence ratios, microscopic videography reveals the formation of fractal-shaped agglomerates that transform into molten droplets eventually growing into larger droplets before departing the burning surface. Droplet size analysis suggests that droplets of similar sizes evolve from composites with equivalence ratios of 0.5 and 1, while dramatically larger droplets are observed for a composite with an equivalence ratio of 2. Temperature measurement from three-color pyrometry suggests that there is no difference in temperature among the agglomerates/droplets from different equivalence ratios. Burn rates obtained from the macroscopic imaging show that the burn rate of the composites increases as the equivalence ratio increases, although one expects that an equivalence ratio of 2 should have a lower burn rate due to its lower energy density. Analysis of the droplet residence time on the burning surface shows that while the droplets on the burning surface in the case of composites with equivalence ratios of 0.5 and 1 have similar residence times, droplets at an equivalence ratio of 2 have significantly longer residence times on the burning surface, which results in significantly more heat feedback to the unburnt propellant and leads to the unexpected higher burn rate than an equivalence ratio of 1.