Use of water and mercury droplets to simulate Al2O3 collision/coalescence in rocket motors

Abstract

Combustion of aluminized solid propellants produces aluminum oxide droplets in a wide range of sizes. Droplets of different sizes accelerate at different rates and may collide, especially in nozzles of rocket motors. It has generally been assumed in the past that all collisions among these droplets result in coalescence. However, a model for coalescence efficiency previously developed and verified experimentally for free-falling water droplets indicates that permanent coalescence will not occur if the rotational energy of the spinning (temporarily coalesced) agglomerate exceeds the surface energy holding it together. Consequently, the extrapolation of this model to the high surface tension and impact energies of A12O3 droplets in nozzles has been studied experimentally using mercury droplets because 1) their liquidity at room temperature allows for simple and inexpensive testing, 2) they have a surface tension that is nearly that of A12O3, 3) their large diameters are easily visible, and 4) their high density generates high-impact energy at low-impact velocity. Single mercury droplets (bullets) of various diameters were rolled into a stationary mercury droplet (target) and the collision/coalescence process was recorded on videotape; bullet size, speed, and offset were obtained from a superposed grid. It was concluded that the water model accurately predicts the collision-induced coalescence of mercury and, therefore, A12O3 droplets because 1) the qualitative similarity between the collision/coalescence behavior of water droplets and liquid metal (mercury) droplets was excellent, 2) the model predicts well quantitatively whether a collision will result in coalesced mercury droplets, and 3) the model predicts well the degree of A12O3 coalescence apparently occurring in the nozzle of the Space Shuttle booster.