Predicting the piloted ignition of wood in the cone calorimeter using an integral model : effect of species, grain orientation and heat flux
This paper experimentally and theoretically examines the ignition of 50 mm thick samples of wood in the Cone Calorimeter. Four species of wood were exposed to a range of incident heat fluxes up to 75 kW/m² with their grain oriented either parallel or perpendicular to the incident heat flux. The time to ignition measurements obtained from the Cone Calorimeter were used to derive characteristic properties of the materials. These properties were used as input to a one-dimensional integral model that describes the transient pyrolysis of a semi-infinite charring solid subject to a constant radiant heat flux. The integral model predictions and experimental data compare well at incident heat fluxes above around 20 kW/m². At lower heat fluxes it was found that the ignition mechanism of wood is different from that at higher incident fluxes. This difference is believed to be due to char oxidation that precedes flaming ignition. The lowest radiant heat flux to cause ignition was found to be approximately 10 kW/m² depending on species, grain orientation or moisture content. Ignition at low heat fluxes could take up to 1½ hours.