Spot Fires and Ignition of Fuels by Hot Particles
Spot fire formation is the primary vector for the spread of wildland and wildland-urban interface (WUI) fires under dry, hot, and windy conditions that produce the most devastating fires. Fire spotting leads to more rapid fire spread than flame front propagation because embers generated by burning vegetation or structures are lofted by fire plumes and transported downwind to ignite spot fires or structures remote from the flame front. Similarly, most structures destroyed during WUI fires are not ignited by direct flame impingement, but rather by embers or firebrands penetrating vents/eaves or direct ignition of roof construction and other "soft" targets.
Following the devastating 1994 Sydney (Australia) wildland fires, it was determined that 75% of houses were ignited by firebrands. In addition to causing staggering loss of property, rapidly spotting fires can also be deadly. Civilians and firefighters alike can become trapped between spot fires with no escape route. In addition to established fires spreading by spotting, many wildland, WUI, and structure fires are allegedly initiated by hot particles or embers. Common sources of these particles are hot work/welding, failed metal halide and halogen/tungsten lamps, powerline interactions (conductor clashing), overheated catalytic converters, failure of electrical heating filaments, seized train brakes, carelessly discarded cigarettes, and mechanical processes such as grinding that can generate incandescent particles. High winds can cause power lines to interact with one another and arc (conductor clashing), producing a shower of molten copper or burning aluminum particles that may initiate spot fires. Glowing or flaming woody embers can also be generated when powerlines contact trees or buildings. Depending on the characteristics of these particles and the target fuel bed on which they land, they may or may not represent a competent ignition source for initiating spot fires.
If it is suspected that a fire may have been ignited by embers or hot particles (e.g., from welding, grinding, or sparks generated by overhead electrical utilities) it is critical to establish whether the proposed source of embers or hot particles is capable of igniting the target fuel bed. This complex problem is best tackled through a combined theoretical, modeling, and experimental approach.
Reax Engineering Inc. has developed in-house computer software, based on peer-reviewed journal articles, for analyzing the trajectories and temperature histories of ejected particles or material fragments. This software allows us to determine how far a particle or fragment could travel in high winds and whether it represents a competent ignition source upon landing, i.e. whether it could ignite a particular material (polymer, cardboard, wood, etc.) or cause spot fires. The plot below shows the trajectories followed by burning aluminum particles generated by conductor clashing in high winds as a function of ejection angle.