A physiochemical model for the combustion of aluminum nano-agglomerates in high-speed flows

A theoretical model of aluminum nanoparticle (ANP) and agglomerates has been developed to highlight the effect of flow-particle interactions and heat conduction within particles. To understand the atomic physicochemical mechanism, a series of reactive molecular dynamics (MD) simulations have been performed on single ANP and agglomerates combustion in high-speed flows. The results show that both single ANP and agglomerates experience the transition from diffusive oxidation mode to anisotropic and explosive oxidation modes as flow velocities increase. All the fundemental mechanisms in the model development are derived from fully atomic insights, including thermal and convective heat transfer between flow and particle, core-shell reaction, heat conduction, particle dispersion and drag force. The model predictions are compared with MD simulations, and the results show that our model can capture the ignition behaviors of ANP and agglomerates in a wide range of flow rates. The combustion of a serial of agglomerates (3–180 primary particles) is simulated to reveal the effects of agglomerates at different flow conditions and agglomerate sizes. At low flow rate conditions, the core-shell reactions contribute about two-thirds of the heat triggering the ignition of single ANP. At high flow rate conditions, the gas-particle convective heat transfer and heat conduction between particles become the major heat source rather than the core-shell reactions. In addition, the ignition delay of agglomerates is much shorter than that of single particle with the same equivalent radius, and the ignition becomes almost independent with the agglomerate size due to the heat accumulation inside at the flow-particle interface. Our study reveals the atomic mechanism of the ignition of ANP agglomerates and proposes a theoretic model of aluminum nanoparticles and agglomerates in high-speed flows, which is applicable to the numerical simulations of aluminum propellants and explosives.