Ignition of different size Al particles in a powder bed by electro-static discharge (ESD) stimulation

Experiments were conducted to investigate metal particle heating and ignition by an electric spark. Al powder was chosen as a popular metal fuel additive for many energetic formulations, and as a metal, for which spark initiation typically results in ignition of individual particles rather than in a readily detectable aerosol flame (as is the case for Mg). Al powders with nominal particle sizes of 3-4.5 μm and 10-14 μm were used in experiments. SEM investigation showed that the finer powder was strongly agglomerated, even after it was processed to reduce the degree of agglomeration. Alternatively, effectively no agglomeration was observed for the coarser powder. Emission streaks produced by an empty steel sample holder struck by the spark and by heated and ignited Al particles were detected and differentiated. Emission traces of burning particles were acquired by a photodiode to determine burn times for the particles ignited by sparks with different energies. From burn times, particle diameters were estimated using a correlation reported in the literature. ESD current measurements were taken to determine circuit impedance and energy delivered to the powder by Joule heating. Burn times were clearly correlated with the Joule heat energy for the coarser (nom. 10-14 μm) powder, but not for the finer powder used in this work. The results are interpreted assuming that the particles are Joule heated and that heating is more efficient for finer particles, with greater surface to volume ratio. It is further suggested that strong agglomeration observed for the finer Al powder used in these experiments skewed the expected correlation between the Joule heating energy and the size of particles ignited by the ESD stimulation.