Ignition, stabilization and particle-particle collision in lifted aluminum particle cloud flames

Micron-sized aluminum (Al) particles have recently been proposed as promising carbon-free energy carriers. To facilitate the application of micron-sized Al particles as fuel in practical energy generation system, this study experimentally investigates the underlying mechanisms of the ignition, stabilization and particle-particle collision in lifted Al particle cloud flames. High-resolution shadowgraphy and luminosity measurements at a frame rate of up to 50 kHz are implemented to reveal the transient dynamics involved in these processes. It is shown that small particles, e.g., with diameters less than 10 µm, ignite and combust further upstream than larger ones. This leads to the formation of massive and wide spreading hot Al2O3 smokes, which contributes to the ignition of particles with larger diameters (e.g., > 30 µm) downstream. The combustion of these large particles, in turn, promotes the ignition of adjacent particles through the deposition of hot Al2O3 product particles on the particle surfaces. This process then sustains the group combustion of Al particle cloud. Additionally, the critical interparticle distance that triggers the ignition of the cloud flame is estimated to be around 6.5 times the mean diameter of the fresh particles. This again suggests that the individually burning Al particles can have a much broader influence on the surrounding non-burning particles due to the wide spreading hot Al2O3 smokes. Moreover, the collision and the consecutive coalescence of two burning micron-sized Al particles are firstly studied. Interesting features, e.g., the contact of the two flame envelopes, the collision and coalescence of the Al droplet cores, the variations in the droplet velocity and flame envelope radius, are analyzed and discussed. A schematic model accounting for this process is also proposed.