Numerical simulation of laser ignition of an isolated carbon particle in quiescent environment

A one-dimensional, fully transient numerical model is used to simulate combustion experiments performed on isolated carbon particles (similar to 200 mu m in diameter) ignited by laser irradiation. The model incorporates a well established comprehensive gas-phase kinetic mechanism, a detailed transport model, and a five-step semiglobal heterogeneous reaction mechanism. When the ambient gas is at room temperature, the numerical results show that ignition by heating the particle is possible only in an oxygen-enriched environment. The ignition of the particle corresponds to the initiation of gas-phase CO oxidation in the boundary layer. The subsequent rapid burning is transient throughout the particle lifetime. Transiency is produced by the interactions between the gas-phase boundary layer (temperature and species) and the heterogeneous reactions at the particle surface. In contrast to typical effects of gas-phase accumulation observed in liquid droplet combustion problems, this chemically reactive boundary layer is contiguous to the particle surface and interacts with the heterogeneous system through transport of heat, and both stable and radical gas-phase species. This interaction changes the relative rates of the heterogeneous surface reactions in gasifying the particle and the overall gasification rate as well. The model also predicts a finite extinction diameter which increases with decreasing ambient oxygen mass fraction. The stimulated temporal variation of the surface temperature and the aforementioned trend of extinction diameter agree with experimental observations. Quantitative agreement with experimental results can be obtained by using a single multiplying factor on the particle surface area to account for (as a first approximation) the effects of porosity. Thus, this model can be used in engineering practice, for instance, to estimate the ignition criteria, and burning time of carbon particles for known ambient conditions.