Experimental study of Al agglomeration on solid propellant burning surface and condensed combustion products

Aluminum (Al) particles are commonly added to energetic materials including propellants, explosives and pyrotechnics to increase the overall energy density of the composite, but aluminum agglomeration on the combustion surface may lower the combustion efficiency of propellants, resulting in a loss in two-phase flow. Therefore, it is necessary to understand the agglomeration mechanism of aluminum particles on the combustion surface. In this paper, a high-pressure sealed combustion chamber is constructed, and high-speed camera is used to capture the whole process of aluminum accumulation, aggregation and agglomeration on the combustion surface, and the secondary agglomeration process near the combustion surface. The microscopic morphology and chemical composition of the condensed combustion products (CCPs) are then studied by using scanning electron microscopy coupled with energy dispersive (SEM-EDS) method. Results show that there are three main types of condensed combustion products: small smoke oxide particles oxidized by aluminum vapor, usually less than 1 μm; typical agglomerates formed by the combustion of aluminum agglomerates; carbonized agglomerates that are widely distributed, usually formed by irregular movements of aluminum agglomerates. The particle size of condensed combustion products is measured by laser particle size meter. As the pressure increases from 0.5 MPa to 1.0 MPa in nitrogen, the mass average particle size of aluminum agglomerates decreases by 49.7%. As the ambient gas is changed from 0.5 MPa nitrogen to 0.5 MPa air, the mass average particle size of aluminum agglomerates decreases by 67.3%. Results show that as the ambient pressure increases, the higher oxygen content can improve combustion efficiency and reduce the average agglomeration size of aluminum particles.