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, symmetryconserving 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.

Experiments and Numerical Calculations on Laser‐induced Ignition of Single Micron‐sized Aluminum Fuel Particle

“…In the past five decades, ignition of metal fuels has been investigated both experimentally and theoretically in detail [1][2][3][4][5][6], and of significant practical interest is aluminum since aluminum fuels possess the advantages of high oxidation enthalpy and combustion temperature, environmentally friendly by-products and relatively low cost [7][8][9][10]. Micronsized aluminum fuels have been intensively applied in various energetic materials, including pyrotechnics, explosives, and propellants [11][12][13].…”

Oxidation and ignition of aluminum nanomaterials

“…The whole oxidation process can be explained according to the changing properties of the external alumina layer. 38 Rai et al 39 found that the oxidation was due to the diffusion of species, which took place in two regimes separated by the melting temperature of the metal particles. The first regime was slow and occurred before the melting whereas the second regime was fast occurring after the melting.…”

Experimental and theoretical study of iron and mild steel combustion in oxygen flows

“…[3][4][5][6][7] Many important results have been obtained in these directions which are still subjects of ongoing intensive experimental and theoretical studies. [8][9][10][11][12][13][14][15] Yet, the large amount of energy released by a burning metal has undesirable consequences and represents a source of significant fire hazards, especially when metals are used in high-temperature and/or high-pressure oxidizing environments such as those prevailing in nuclear plants and oxygen supply systems. 16,17 The research conducted in metal-fire prevention has mostly consisted of carrying out standard tests that quantify the relative flammability of different metals, that is, their relative propensity to sustain combustion of metallic materials of standardized dimensions in oxygen atmospheres.…”