Experiments and simulations of aluminum particle agglomeration based on the real structure of propellants

In the combustion procession of solid propellants, aluminum particles tend to coalesce and eventually produce spherical agglomerates with diameters ranging from tens to hundreds of micrometers on the burning surface. This study proposed the characterization of the agglomeration process and accurate prediction of the agglomerate size of aluminum particles in solid propellants through the combined application of experimental and modeling techniques. The true structures of three solid propellants were obtained using 3D X-ray imaging technology, based on which the three-dimensional topological structure of the solid propellant was established. Considering physical processes such as the regression of the burning surface of the solid propellant, aluminum particle exposure, turbulent pulsation, and the heating and melting of aluminum particles, the agglomeration process, the size distribution of aluminum agglomerates were eventually obtained through simulation. The agglomeration process and the size distribution of aluminum agglomerates during the combustion of three solid propellants at 7 MPa were obtained using an electric heating wire ignition combustion experimental system combined with a high-speed camera. In this study, the re-agglomeration process of agglomerates occurring after detachment from the burning surface (both near and far from the burning surface) was captured using experimental methods for the first time. The simulation results were compared with the experimental results, validating the accuracy of the model from three aspects: the movement process of the burning surface and the agglomerate structure, special agglomeration behavior (re-agglomeration at different distances from the burning surface), and the size distribution of agglomerates. The deviation in the equivalent mean particle size (D₅₀, D₉₀, and D_4,3) of the aluminum agglomerate size distribution between the experimental results and the simulation results were all <8.60 %. This indicates that the model established in this study can accurately predict the size distribution of aluminum agglomerates. Further predictions were made using this model to study the effects of changes in pressure (3–10 MPa), aluminum particle volume content (8 %-14 %), and ammonium perchlorate (AP) particle size (46–456 µm) on aluminum particle agglomeration. The results showed that increasing the pressure reduces the degree of aluminum particle agglomeration, while increasing the aluminum content or AP particle size exacerbates aluminum particle agglomeration.