An experimental study and modeling of aluminum particle asymmetric combustion

Abstract

The study focuses on phenomena accompanying asymmetric combustion of aluminum particles in gaseous media, including particle rotation, sinusoidal tracks, abrupt trajectory swerves, helical smoke tail formation, and the dependence of rotation frequency on droplet size, ambient temperature, and oxidizer concentration. An extensive literature review, combined with original high‑speed video observations, reveals two rotation modes with a transition diameter near 30 µm. For larger particles, the mean rotation frequency scales as d_p^–1.7, and for smaller ones as d_p^–0.15, where d_p denotes particle diameter. This disparity arises from the difference in combustion regimes of coarse and fine particles. Rotation frequency follows a characteristic "two‑humped" temporal pattern and increases with oxidizer concentration and temperature. Swerves occur without fragmentation, and smoke helix diameter scales linearly with Al/Al₂O₃ droplet size. The chemical interaction between molten alumina and aluminum can produce either discrete bubbles beneath the oxide cap (at high pressure above 5–20 atm) or a “quasi-cleft” (at lower pressure). The transition from bubbles to the quasi‑cleft resembles that from nucleate boiling to film boiling. The developed semi-analytical quasi-cleft model describes jet outflow beneath the oxide cap, linking rotation dynamics, swerve onset, and helical tail formation. The growth of the oxide cap on the aluminum droplet surface leads to the emergence of the “two-humped” frequency-time dependence, with a trajectory swerve occurring near the minimum rotation frequency. This can cause detrimental deposition of oxide residues on a combustion chamber wall. A number of additives in Al-based composite induce and enhance rotation. Assessments based on the literature support that the burning rate increases appreciably due to rotating convection, particularly under conditions of high ambient temperature and oxidizer concentration at pressures below ∼10 atm, typical for ramjet mode.