Metal-particle ignition and oxide-layer instability

Abstract

The use of metals as high-energy fuel additives is generally compromised by the appearance of a strongly protective oxide layer that covers the fuel surface. The previous work concentrated on the elimination of the oxide layer by a global, symmetry-conserving attack, using an admixture of aggressive chemical constituents in the ambient atmosphere, a strong flux of radiation, or strongly shearing gas flows designed to intensely strain the surface layer. This paper shows that symmetry breaking leads to a different approach to ignition. For a liquid oxide layer, destabilization can be obtained via the Marangoni effect associated with longitudinal surface stress, thus breaking the translational symmetry in longitudinal directions. The thermodynamic state of the layer is described in a thin-film model, which leads to a creeping-flow approximation. Ignition by way of the Marangoni effect is then shown to result from spreading of punctures and ruptures in the oxide layer, which is a prerequisite for layer thinning or even complete layer removal. Boron-particle ignition is selected to illustrate the theory, because the well-known difficulties with boron ignitability have greatly impaired the use of boron fuel in propulsion devices. It is shown that, for ambient temperatures below 1634 K, the oxide surface layer can be destabilized by way of punctures and ruptures, owing to the peculiar property of the boron oxide, namely, positive Marangoni numbers. Previous models of boron-particle ignition insisted on conservation of symmetry and expressly excluded the appearance of punctures and ruptures. Because of this constraint, a critical ambient temperature of 1900 K for boron ignition was obtained, so that the new value of 1634 K opens up a novel approach to ignition.