Effect of particle size on combustion of aluminum particle dust in air

The combustion of aluminum particle dust in a laminar air flow is theoretically studied under fuel-lean conditions. A wide range of particle sizes at nano and micron scales is explored. The flame speed and temperature distribution are obtained by numerically solving the energy equation in the flame zone, with the particle burning rate modeled as a function of particle diameter and ambient temperature. The model allows for investigation into the effects of particle size, equivalence ratio, and chemical kinetics on the burning characteristics and flame structures of aluminum-particle/air mixtures. In addition, the flame behavior with ultra-fine particles in the sub-nanometer range is examined by asymptotically treating particles as large molecules. Calculated flame speeds show reasonable agreement with experimental data. As the particle diameter decreases from the micron to the nano range, the flame speed increases and the combustion transits from a diffusion-controlled to a kinetically controlled mode. For micron-sized and larger particles, the flame speed can be correlated with the particle size according to a d^{- m} relationship, with m being 0.92. For nano-particles, a d^-0.52 or d^-0.13 dependence is obtained, depending on whether the d^1.0- or d^0.3-law of particle burning time is implemented in the flame model, respectively. No universal law of flame speed exists for the entire range of particle sizes.