Fast moving smoldering fronts in thin liquid fuel films dispersed on inert porous media

An experimental study of smoldering in thin layers of pyrolysis oil on an inert substrate of calcium silicate boards has been conducted. The smoldering fronts propagated with velocities up to 0.4mm/s, which is significantly faster than other types of smoldering without forced air flow. The smoldering propagation velocity increased both with thinner layer of pyrolysis oil and higher initial fuel temperature. Our experimental observations are in close correspondence with a theoretical model based on energy balance, where the availability of oxygen at the substrate surface is the main limiting mechanism. Both the front peak temperature and the front width are correlated to the initial fuel temperature. We also report observations of instability phenomena, including double fronts propagating in close succession. The temperature dynamics were observed by high resolution infrared thermography. The findings contribute to the understanding of the governing mechanisms of smoldering combustion of liquids on inert media, and in particular the fast smoldering fronts that have been observed along the surface of porous insulating materials in experiments with smoldering cotton.