An examination of the shrinking-core model of sub-micron aluminum combustion

Abstract

We revisit the shrinking-core model of sub-micron aluminum combustion with particular attention to the mass flux balance at the reaction front which necessarily leads to a displacement velocity of the alumina shell surrounding the liquid aluminum. For the planar problem this displacement simply leads to an equal displacement of the entire alumina layer, and therefore a straightforward mathematical framework can be constructed. In this way we are able to construct a single curve which defines the burn time for arbitrary values of the diffusion coefficient of O atoms, the reaction rate, the characteristic length of the combustion field, and the O atom mass concentration within the alumina provided that it is much smaller than the aluminum density. This demonstrates a transition between a ‘d  ²–t’ law for fast chemistry and a ‘d–t’ law for slow chemistry. For the spherical geometry, the one of physical interest, the outward displacement velocity creates not a simple displacement, but a stress field which, when examined within the framework of linear elasticity, strongly suggests the creation of internal cracking. We note that if the molten aluminum is pushed into these cracks by the high internal pressure characteristic of the stress field, its surface, where reaction occurs, could be fractal in nature and affect the fundamental nature of the burning law. Indeed, if this ingredient is added to the planar model, a single curve for the burn time can again be derived, and this describes a transition from a ‘d  ²–t’ law to a ‘d  ^ν–t’ law, where 0<ν<1.

Keywords:

• aluminum
• combustion
• submicron
• shrinking-core model
• fractal

Acknowledgements

This work was supported through IllinoiRocstar LLC under grant FA9300-33-M-2004 under a Phase II SBIR program with the Air Force, program manager S. Alexander Schumaker. Distribution A: Approved for public release; distribution unlimited.

We are grateful to Professor E. Dreizin for helpful email exchanges at a time when we were trying to learn something about the problem of sub-micron aluminum combustion. He is not responsible for any errors in this work, of course.