High-Speed Thermal Imaging of Disk-shaped Firebrands in a Wind Tunnel

Abstract

The transport and temperature-time history of disk-shaped firebrands was studied with high-speed thermal imaging in a wind tunnel. One millimeter diameter wooden disks were ignited using a novel firebrand generator. The firebrands dropped vertically into a wind tunnel crossflow. Optical images of the firebrands were recorded at 960 frames/second during their transport in the wind tunnel test section at three different wind speeds. The resulting high speed color videos were processed using a particle tracking algorithm and a two color pyrometry technique to determine temperature, position, and velocity time histories of individual firebrands. The firebrand temperatures were observed to oscillate at distinct frequencies that ranged between 50Hz and 480Hz (the Nyquist frequency of the imaging system). A positive correlation was observed between the temperature fluctuation frequency and the relative speed of the firebrand compared to the wind speed. The observed high speed temperature fluctuations suggest that rapid transitions between glowing and flaming combustion may be an important aspect in firebrand transport phenomena and should be considered within the context of model development.