Simultaneous O and Al diffusion in the combustion of sub-micron aluminium drops

Abstract

We examine the problem of sub-micron aluminium drop combustion when both O diffusion and Al diffusion occur. During the unsteady evolution of the combustion field the oxide that is generated has the form of a shell whose inner surface encloses an aluminium bath and whose outer surface is exposed to an air bath. The diffusing atoms can react at these surface baths and also, in principle, between themselves in the interior of the oxide. However, the mass concentrations of the atoms are small so that the interior collisions necessary for this reaction are negligible.With this important simplification we are able to define a model that leads to an analytical discussion. For small Damköhler numbers (of which there are two, one defined by the O atoms the other by the Al atoms) the relative values of the bath reaction rates determine the proportion of oxide accretion on the outer surface to accretion on the inner surface. For large Damköhler numbers the relative values of the diffusion coefficients play the same role.

Keywords:

• sub-micron
• nano
• aluminium
• combustion
• modelling

Disclosure statement

No potential conflict of interest was reported by the author.

Additional information

Funding

Easy phases of the work reported here were supported through IllinoisRocstar LLC under a Phase II program with the Air Force, program manager S. Alexander Schumaker [grant FA9300-12-C-2001].