Light extinction and scattering to determine nanoparticle formation rates during droplet jetting in aluminum dust flames

Abstract

The combustion of aluminum powder enables a CO₂-free generation of heat and electricity. To understand the asymmetric combustion of burning aluminum particles, we imaged light extinction and scattering with high speed and magnification. In the flame, particles melt, ignite, and build up a spherical flame sheet of condensed-phase alumina around the droplet. Shortly thereafter, the asymmetric stage occurs where the flame locally extinguishes, the droplet accelerates, i.e., droplet jetting, and leaves behind a condensation trail of alumina particles. Two alternately pulsed LEDs were used to image light extinction and scattering in the condensation trail at 200000 frames per second. This yields quasi-simultaneous images of transmission and scattered light. The geometry of the light-scattering experiment and Mie theory yield the collection efficiency of scattered light as a function of the particle size. An iterative calculation of the collection efficiency and the single-scattering albedo, the ratio of scattering and extinction, converges and yields the particle diameter in the Rayleigh regime. The correction for total scattered-light in the extinction yields the absorbance from which the nanoparticle volume is derived. Nanoparticles appear at the onset of the trails near the droplet and grow along the trail from around 40 nm to 110 nm until they are outside the Rayleigh regime. The nanoparticle formation rate is 50 % of the total alumina formation rate during the symmetric phase. The large occurrence frequency of droplet jetting makes it an important contribution to nanoparticle formation and to the total heat release in aluminum combustion.