On the trajectories of embers initially elevated or lofted by small scale ground fire plumes in high winds

Millions of acres and hundreds of structures are destroyed annually by wildfires. With many of these fires extending long distances due to spotting, detailed knowledge of ember transport by external and flame-generated winds is critical for fundamental understanding and prediction of the inception and evolution of such fires. This work presents a model that treats the burning and wind carrying of embers, and numerically compares their trajectories for spherical, cylindrical, and disk geometries. The embers may be launched at predetermined heights or lofted by a fire buoyant plume. Various terrain conditions and variable wind properties are considered. Results show that for embers of equal initial mass, disks propagate the farthest and have the highest remaining mass fraction upon impacting the ground. Spheres are carried the shortest distance, and cylinders have the smallest mass fraction upon impact. For disks in the range of diameters examined, initial diameter has no effect on the distance carried. Charring and extinction criteria are investigated for cylinders and spheres. Higher surface burning temperatures are found to lead to shorter propagation distances.