On the role of heterogeneous reactions in aluminum combustion

This work addresses the role of heterogeneous reactions occurring during aluminum combustion. Direct numerical simulations are conducted on a single aluminum burning particle accounting for both detailed gas-phase and surface kinetic mechanism. A surface kinetic solver has been developed to take into account surface and gas/surface interactions. The model is validated against experimental measurements and appears to be predictive. Calculations are performed through different pressure regimes (1 and 10 bar), atmospheres (O2, CO2, CO and H2O) and droplet sizes (from 1 to 400 µm). Flames structures (temperature and AlO profile) computed by the model are consistent with experimental measurements and observations. The reaction zones for CO and H2O atmospheres are computed to be very close to the surface which is an evidence of a combustion mostly driven by chemical surface phenomena. In some cases surface reactions promote self-sustained combustion for smaller size particles at low pressure regimes. Calculated burning times are also consistent with experimental measurements. In O2/Ar, surface chemistry is more important when particles are small (under 20 µm) which guides the combustion to a more kinetically-limited regime than a diffusion-limited regime. The importance of surface reactivity is different regarding the particle size and the atmosphere of interest. Calculations of alumina residue size reveals that surface reactions could explain alumina residue formation. The model predicts a strong oxidizer effect on alumina residue size. According to the model, CO2 environment lead to large residue size while H2O environment lead to small residue size. This work definitely stresses the potential role of heterogeneous reactions in aluminum combustion.