Oxide Layer Effects in Metal Particle Combustion

Abstract

For a metal fuel in the form of a single particle, the existing models have been set up to describe its combustion in a steady-state gas-phase flame envelope by building on an analogy with the standard combustion model for a hydrocarbon fuel droplet. These models generally rest on the a-priori constraint that if gravity can be neglected, sperical symmetry will persist for the duration of the combustion process, thus excluding symmetry-breaking to occur. The talk is intended to show that contrary to this assumption, important aspects of combustion are associated with a changeover from the initial spherical symmetry to states of lesser symmetry so that the symmetry constraint of the earlier models needs to be removed. It is proposed that a number of important particle combustion modes exist where transport processes traversing the oxide surface layer exert a controlling influence on the overall reaction process. In this situation the metal surface corresponds to the site of the oxidation reaction which has to be supplied with oxygen from the ambient atmosphere while the intensity of the reaction depends critically on the oxide layer thickness h since oxygen transport across the surface layer in slow. In the standard models ignition is associated with a uniform thinning of the oxide layer, eventually leading to its disappearance. In the talk it isshown that uniform thinning results for much higher temperatures than rupturing and hole formation due to the combined action of capillary forces and the oscillatory Marangoni effect. The Marangoni effect arises from the temperature sensitivity of the surface tension in combination with a temperature gradient which is transverse to the layer interface. Critical conditions for ignition will therefore be associated with layer rupturing so that a spreading rupture is equivalent to a nonuniform transition from a thick layer to a thin one, which amounts to a transition from a regime of low to high reaction intensity, i. e. ignition. The converse effect, namely the critical conditions for drop or bulge formation in an oxide surface layer, determines particle extinction and the onset of cap formation. It is further proposed to investigate the influence of surface curvature on ignition and extinction phenomena, which causes combustion to be different for the case of a planar metal slab in comparison to the case of a spherical particle. It should be pointed out that the trend in applications is towards a decrease in particle sizes, so that there is a great interest in exploring the size effect, which may be expressed in terms of the surface curvature. Beyond criticality in asssociation with ignition and extinction it is further proposed to investigate the actuel spreading of a rupture or a drop in the oxide layer by way of a layer evolution equation.