On the transition from classical diffusion-limited combustion behavior for fine and ultrafine aluminum particles

The characteristics of aluminum particle combustion for fine particles burning below the diffusion limit are examined experimentally and theoretically. Many of the most common assumptions about aluminum combustion flame structure fail once the diffusion-limited structure begins to be lost, and new behavior is observed. Heterogeneous shock tube measurements at UIUC have been able to accurately characterize this behavior for micron and sub-micron particles. Among the observed changes are a reduced ignition temperature, a reduced peak combustion temperature, enhanced pressure dependence on the burning rate, a variation in the relative oxidizer efficiencies, and a change in the burning rate exponent. As particles approach the nanoscale, the behavior continues to change as particles approach the kinetic limit. Recent experimental results are reviewed, and a theoretical basis for these changes is proposed.