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International Journal of Energetic Materials and Chemical Propulsion
ESCI SJR: 0.28 SNIP: 0.421 CiteScore™: 0.9

ISSN Print: 2150-766X
ISSN Online: 2150-7678

International Journal of Energetic Materials and Chemical Propulsion

DOI: 10.1615/IntJEnergeticMaterialsChemProp.v2.i1-6.130
pages 232-247

INTERFACIAL INSTABILITY INDICATING BORON PARTICLE IGNITION

M. Konczalla
Fakultät für Physik, Universität Bielefeld Postfach 8640 D-4800 Bielefeld 1, F.R.G.

ABSTRACT

The ignition of boron particles is generally described as a two-stage process. In the first stage, which is described in the present model, a solid boron kernel is covered by a thin layer of liquid B2O3. This layer acts as a diffusional barrier for the oxida-tor transport from the gaseous environment to the BB2O3 − interface, where the reaction occurs. Two competing effects, oxide evaporation at the outer interface and oxide formation at the solid-liquid interface, influence the layer thickness and its temperature. In theoretical models for boron particle ignition, it is generally assumed that the spherical symmetry is not broken during the evolution towards ignition, which is indicated by a vanishing film thickness. In the present model, a nonlinear partial differential equation for the local thickness f of a thin viscous liquid film on a spherical particle in the limit of a vanishing ratio of mean-film thickness and particle diameter is derived. This equation incorporates oxide evaporation and formation depending on the local film thickness and temperature, Marangoni-flow due to a surface temperature gradient and a varying capillary pressure due to a nonuniform curvature. We restrict ourself to the axisymmetric case.
A linear stability analysis of the governing equation shows that a steady, spherical symmetric state becomes unstable with respect to axisymmetric perturbations for certain values of the control parameters. For slightly supercritical parameter values, an adiabatic elimination procedure is used to describe the evolution of the interface. It is shown that this may result in a locally vanishing film, which indicates ignition. Thus boron particle ignition due to interfacial instability is predicted at relatively low ambient temperatures.


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