Simultaneous temperature and velocity measurements of airborne firebrands

Firebrand spotting is an important wildfire spread mechanism in wildland and wildland-urban interface (WUI) fires. In this process, firebrands are transported ahead of the fire front where they may ignite receptive fuels beyond the flame front. This work aims to quantify the physical and thermal characteristics of the firebrands generated from burning Douglas fir branches with needles at wind speeds from 0.5-3 m/s in an intermediate-scale wind tunnel. The size, velocity and orientation distributions of airborne firebrands were obtained through an image-processing methodology applied to motion-blurred firebrand streak images taken with a commercial camera. The average glowing surface temperature of firebrands was measured with color ratio pyrometry and the projected velocity was measured with particle streak velocimetry. The results revealed that the time-averaged projected velocity of firebrands approached the applied wind speed. The majority of measured firebrands had a characteristic size smaller than 5 mm with glowing temperatures between 900 and 1100 ∘ C . The measured glowing temperatures were found to be not very sensitive to the wind speed within the range examined in this work. The methodology provides a new possibility for firebrand measurements and the data obtained in this study is important for improving understanding the subsequent deposition patterns and ignition potential of firebrand groups.