Three-dimensional spatial distributions of agglomerated particles on and near the burning surface of aluminized solid propellant using morphological digital in-line holography

The particle size distribution of aluminum particles during combustion plays an important role in predicting the combustion performance of aluminized solid propellants. However, conventional models face difficulties in estimating the distributions of the agglomerated particles in propellants with novel or complex compositions. This study investigated the accuracies of the experimental techniques applied to propellant combustion to obtain the three-dimensional (3D) size distributions of agglomerated particles on the burning surface and particles in the plume. The 3D spatial distributions of aluminum particles on the burning surface and in the plume during solid aluminum composite propellant combustion were obtained using digital in-line holography (DIH) and morphological watershed image segmentation. Overall, 68,321 individual particles at distances of up to approximately 20 mm from the burning surface of the propellant were analyzed under 1 MPa pressure. The field of view results suggest that approximately 7% of the particles were detected on the burning surface, displaying a trimodal distribution in the particle size range of 30–1,200 μm, and almost all the particles (>96%) were agglomerated. The mean diameters and agglomeration fractions in the height range of 0–1 mm were larger than those in the overall combustion field; the mean diameters in this height range were similar to those of the particles on the burning surface. We found that the process of “second mergence” occurred at and up to 1 mm above the burning surface. As the height from the burning surface increased, the mean particle diameter and agglomeration fraction decreased rapidly; however, when the height exceeded 11 mm, the mean diameter did not change significantly, indicating that the particles had almost burned completely and formed condensed combustion products. Significant differences were found between the results obtained from the empirical agglomeration models and those obtained from the 3D DIH because these models were developed based on 2D data. The results presented herein further provided an insight into the agglomeration characteristics of aluminum particle combustion in composite propellants.