Anisotropic Combustion of Aluminum Nanoparticles in Carbon Dioxide and Water Flows

Shock-induced combustion of aluminum nanoparticles was examined in the CO₂ and H₂O flows up to 8 km/s using reactive molecular dynamics. The morphological evolutions and heat/mass transfer of ANPs were discussed to reveal the nature of anisotropic combustion. The breakage of triatomic gas molecule and the formation of key intermediates were identified to illustrate the reaction mechanisms at the atomic level. It was found that surface reactions prevail for cases in lower flow velocity (≤6 km/s), and gas-phase reactions govern the oxidation process under the intense impact (8 km/s). In particular, we converted the flow velocity to the initial kinetic energy of flow molecules to highlight the impact of oxidizing ability on the shock-induced combustion. In the regime of low initial kinetic energy (<122.2 kJ/mol), the oxidation follows the diffusion mechanism, and the ignition delay is mainly affected by the reaction rate and heat release of oxidizers. Further increasing the initial kinetic energy (<458.1 kJ/mol), the impact of oxidizers weakens and the heat transfer becomes dominant. In the extreme scenarios (>458.1 kJ/mol), the overall oxidation is governed by the microexplosion mechanism, and different oxidizers share almost the same ignition delay.