Transport of rod-shaped particles in a canopy flow with a buoyant plume*

Spot fire spread is a mode of wildfire spread in which a main wildfire launches smoldering firebrands, or embers, that ignite new "spot fires" upon landing at some distance downwind. Firebrands can be a wide array of shapes, and previous studies have shown that shape may impact their transport. To better understand the transport of these firebrands, and specifically the effect of firebrand shape, we experimentally investigated the transport of particles in a flow containing both canopy turbulence and a heated plume. We compared the transport of sphere- and rod-shaped particles with the same settling velocity to analyze the possible impacts of firebrand shape on spot fire spread. Rod behavior was characterized by examining orientation and spin rate. Cases with higher turbulence showed a greater distribution of rod orientations than cases with lower turbulence. Rod spin rates were similar in all cases except the short canopy, no plume case, where they were lower. These differences notwithstanding, we found that the spheres and rods landed with similar spatial distributions so long as either a plume or upstream canopy added turbulence to the system. If there was no plume and the upstream canopy was short, then the rods traveled farther and dispersed more than the spheres. These results suggest that it is not always necessary to account for firebrand shape to accurately predict spot fire spread, but they also reveal characteristics of how rod-shaped particles are transported in such a system.