Rebound characteristics of burning aluminum droplets impacting surfaces

An experimental system is established to study the behavior of burning aluminum droplets impacting a wall in a solid-rocket-motor environment. First, the rebound process of droplets impinging on the wall is observed using shadow shooting; the size range of the burning aluminum droplets is 150–500 μm, and their range of impact speed is 1–5 m/s. Second, a formula is developed for predicting the droplet–wall contact time, and combined with energy conservation, a model for predicting the behavior of aluminum droplets impacting the wall is established. Furthermore, analyzing the droplet velocity reveals that the total restitution coefficient decreases gradually with increasing Weber number, whereupon a model for calculating the restitution coefficient is proposed. Compared with experimental results, the errors of the prediction model are within 15%. Finally, a new phenomenon involving the oscillatory motion of burning aluminum droplets is discovered; analyzing this oscillatory behavior gives an equation for the attenuation of the oscillation of droplets moving in a gas flow, and the results of this equation agree well with experimental data. The present experimental system and measurement method are suitable for fine measurements of the rebound process of burning aluminum droplets, and the present results lay a foundation for (i) studying the law governing the motion of aluminum droplets in a solid rocket motor and (ii) revealing the mechanism for interaction between them and the inner surface of the combustion chamber.