On Modeling the Combustion of a Single Micron-Sized Aluminum Particle with the Effect of Oxide Cap

The formation of oxide cap, which results from the condensation of gaseous aluminum oxide, makes a non-negligible impact on the combustion process of micron-sized aluminum particles, but its growth and effect are still unknown. Also, the transition of combustion modes during the combustion process, which affects the growth rate of the oxide cap, needs to be explored. Therefore, a detailed combustion model of a single micron-sized aluminum particle is developed to predict the transition of combustion modes and the effect of the oxide cap. This combustion model consists of a vapor-phase kinetic model and a particle model coupled by the Strang splitting algorithm. The predicted ignition delay and combustion times are compared with experimental data to validate the combustion model. Three combustion modes including vapor-phase, transitional, and surface combustions are considered in this combustion model. We find that the two modes coexist for particles between 100 and 200 μm when the ambient temperature and pressure are 2500 K and 1 atm, respectively. A higher ambient temperature extends the transition of combustion mode toward smaller sizes. An oxide cap model considering surface free energy is proposed to study the growth and effect of the oxide cap on the combustion process of micron-sized aluminum particles. We find that the formation of oxide cap limits the evaporation rate of aluminum directly due to the reduced active surface area. The oxide cap stabilizes the evolution of particle temperature and determines the burning time. The predicted burning time is reduced by a factor of 2 at least considering the growth of oxide cap.