Flame propagation in bimodal nano/micro-sized aluminum particles/air mixtures

The combustion of bimodal aluminum particles with air is studied theoretically in a well-characterized laminar particle laden flow. The flames are assumed to consist of several different regimes, including preheat, flame, and post flame zones, for fuel-lean mixtures. The flame speed and temperature distribution are obtained by solving the energy equation in each regime and matching the temperature and heat flux at the interfacial boundaries. The analysis allows the investigation of the effects of particle size, particle composition, and equivalence ratio on the burning characteristics of aluminum-particle/air mixtures. Reasonable agreement between theoretical results and experimental data was obtained in terms of flame speed. For a mono-dispersed particle laden flow, the flame speed increases with increasing particle concentration under fuel-lean conditions, but with decreasing particle size. A companion numerical model, which treats very Fine aluminum particles as large molecules, is developed to obtain the flame speed at the molecular limit. For a bimodal particle laden flow, the flame structure may display either an overlapping or a separated configuration, depending on the combustion properties of aluminum particles at different scales. At low percentages of nano particles in the fuel formulation, the flame exhibits a separated spatial structure with a wider flame regime. At a higher loading of nano particles, an overlapping flame configuration is observed.