Micron-sized aluminum powder is added to solid propellants in order to increase their performance and to suppress combustion instabilities. During the combustion, aluminum particles accumulate and form agglomerates. Agglomeration may cause loss of potential chemical energy due to incomplete combustion, two-phase flow losses, and accumulation of slag in the motor. A well-known method to reduce agglomeration is the use of nano-aluminum powders (such as ALEX). The specific surface of nano-powders is larger than that of micron‑sized powders, therefore their reaction and heating rates are higher. The nano-aluminum particle ignites faster with shorter burning time, resulting in a higher burning rate.
In order to further improve the performance, different materials were investigated for coating nano-aluminum particles. Two of the powders considered are V-ALEX, a fluoropolymer (Viton) coated nano-aluminum; and L-ALEX, stearic acid coated nano-aluminum. A thin layer of coating may promote particle ignition, which decreases the agglomerate size and, thus, reduces the two-phase flow losses.
The purpose of the research is to examine the effect of ALEX, V-ALEX and L‑ALEX powders on the agglomeration phenomenon in solid AP/HTPB propellants, at different operating pressures. The agglomeration phenomenon was measured by high speed video photography in order to record the combustion process of propellant strands. The strands were burned inside a windowed pressure chamber, allowing measurements of the agglomerates size and number. The experimental data show that coated nano-powders exhibit reduced agglomeration, in terms of number of agglomerates and their volume, and that propellants containing V-ALEX powder experienced the largest decrease in agglomeration.