Analysis of nano-aluminum particle dust cloud combustion in different oxidizer environments

The combustion of nano-sized aluminum particles with various oxidizers, including oxygen, air, and water, is studied in a well-characterized laminar particle laden flow by means of both numerical and theoretical approaches. In the numerical analysis, nano-sized aluminum particles are treated as large molecules, which can be regarded as a limiting case when the particle size approaches to zero. The particle laden-flow is then modeled as a one-dimensional, laminar, steady flow of a premixed gas mixture. Emphasis is focused on the detailed chemical kinetics and its ensuing effect on the flame structure. A companion theoretical model is also established. The flame is assumed to consist of three zones: preheat, flame, and post flame regimes. By solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries, an algebraic equation for the flame speed is obtained. The analysis allows for the investigation of the effect of particle size, ranging from nano to micro meters in diameter, on the burning characteristics of aluminum oxidizer mixtures. In the molecular limit, the numerical results indicate that the flame speeds of an aluminum mixture with air are significantly higher than that with H₂O, mainly due to the greater reaction rate of Al with O₂ versus H₂O. The kinetic bottleneck results from the Al₂O₃ formation, in which O atoms play an influential role, in the aluminum-steam system. For micro-sized particles, the theoretical analysis shows that the aluminum-steam flame speed is slightly larger than that of an aluminum-air mixture. The phenomenon may be attributed to the high diffusivities of oxidizer and the presence of H₂ and H in reaction products for a H₂O system.