Autoignition of wood under combined convective and radiative heating

Many wildland fire models assume radiation heat transfer controls fuel particle ignition. However, evidence suggests that radiation is insufficient to ignite the predominantly small, thin fuel particles in wildlands and that convective heating by flame contact is a critical component. Here, convective ignition was studied using an apparatus containing two 6.5 kW electrical heaters to heat air from 600 degrees C to 800 degrees C. Steel screens straightened the flow and provided background radiant heat. This apparatus produced autoignition of dry red oak cylinders and disks. Cylinder diameter was varied from 0.64 to 1.91 cm and length from 2.5 to 7.5 cm. Disk diameters varied from 2.54 to 5.08 cm and 1 to 4 mm thick. The airflow rate varied slightly from 1.55 to 1.71 m/s due to the density difference. A simplified analytical model was developed that predicted the measured times reasonably well for air temperatures of 700 degrees C and above. (C) 2016 by The Combustion Institute. Published by Elsevier Inc.