Glowing and flaming autoignition of wood

Abstract

A study of the autoignition of wood by a radiant cone heater was conducted. Insulated redwood samples were exposed vertically to incident heat flux ranging from 10 to 70 kW/m2. IR thermography and normal video recording were used to view the sample surface. The surface temperature and mass loss were continuously recorded. Glowing and flaming autoignition were defined and examined. The times to glowing and flaming autoignition were measured and compared with the times to flaming piloted ignition. The study found that for incident heat fluxes less than 40 kW/m2, in some cases, the sample surface started to glow (glowing ignition) before a visible flame (flaming ignition) was eventually seen. However, for incident heat fluxes greater than 40 kW/m2, flaming ignition occurred very quickly (within 30 s). The measured ignition time, ignition temperature, and surface temperature history were compared with theoretical values. The mass flux (pyrolysis rate) was assumed to follow an Arrhenius reaction rate. The activation energy and the pre-exponential factor were determined from a best curve fit of the experimental data.