Combustion of particles, agglomerates, and suspensions – A basic thermophysical analysis

Abstract

Ignition and combustion of single particles, particle agglomerates, and suspensions are analyzed using a simple thermophysical approach that considers ideal, non-volatile fuel particles undergoing heterogeneous reaction controlled by a combination of diffusion and kinetic rates.This approximation is useful to describe the combustion behavior in suspensions of refractory metal-fuel particles, where the lack of significant premixing of fuel vapor with oxidizer can lead to combustion in a diffusion micro-flame enveloping the particle.The transition from kinetic heterogeneous-fuel oxidation to diffusion-controlled combustion occurs via thermal runaway, customarily called particle ignition. There is, however, a critical particle size below which an individual particle cannot ignite at any temperature, and the combustion will be controlled by heterogeneous kinetics at a temperature close to that of the bulk gas. While individual particles may not be able to ignite, the collective effect, which results from the self-heating of particle suspensions to thermal runaway, can enable fast reaction in suspensions. Micron- and sub-micron-sized fuel particles are used for their high reactivity, but such particles often agglomerate before combustion. The ignition and combustion behavior of an agglomerate can be drastically different from its isolated constituent particles due to a large internal surface area inside of the agglomerate that is partially accessible to oxidizer, while only the external surface area of the agglomerate is available for heat loss. It is shown that the interplay between the thermal regimes of reaction, the collective effects of the suspension, and the collective effects of agglomeration can lead to a wide range of observed ignition and combustion phenomena that are independent of the material-specific fuel and oxide properties.