Ignition and combustion of low-exothermicity porous materials by a local hotspot

We present numerical investigations of a spherically symmetric model for the ignition and subsequent combustion of low-exothermicity porous materials exposed to a constant, continuous heat source. We account simultaneously for oxidant, the gas-dynamic processes, including a gaseous product of reaction, and a solid product that is allowed to assume different physical properties from the solid reactant. For external conditions that are typical of natural convection, the model exhibits striking novel behaviour, including the possibility of a potentially dangerous high-temperature ‘burnout’ at the external surface of the material, which triggers a reverse combustion wave propagating from the outer surface of the solid towards the heat source. This phenomenology is controlled largely by the diffusion of oxygen entering the system. We identify the effects that convection and product properties have on combustion of the solid, particularly on the formation of a reverse wave. Applications of the approach to specific problems are discussed and future work is outlined.