Thermal characterization of firebrand piles

• Published in: Fire safety journal 2019-03, Vol.104, p.34-42
• Main Authors: Hakes, Raquel S.P., Salehizadeh, Hamed, Weston-Dawkes, Matthew J., Gollner, Michael J.
• Format: Article
• Language: English
• Subjects: Burning; Burning embers; Critical mass; Firebrands; Fluctuations; Gauges; Heat flux; Heat transfer; Heating; Ignition; Measurement methods; Piles; Skin; Smoldering; Thermodynamic properties; Wildfire; Wildland-urban interface; Wind effects; Wind speed
• ISSN: 0379-7112; 1873-7226
• DOI: 10.1016/j.firesaf.2018.10.002

The cause of the majority of structure losses in wildland-urban interface fires is ignition via firebrands, small pieces of burning material generated from burning vegetation and structures. To understand the mechanism of these losses, small-scale experiments designed to capture heating from firebrand piles and to describe the process of ignition were conducted using laboratory-fabricated cylindrical wooden firebrands. Two heat flux measurement methods were compared, and the influences of firebrand diameter, pile mass, and wind on heating from firebrand piles were explored. Diameter had little effect on heating, pile mass a moderate effect, and wind a large effect. Peak heat fluxes showed distinct differences between heat fluxes produced by firebrand piles as opposed to individual firebrands, which have been studied exclusively on the small-scale in the past. Above a critical mass, piles did not produce higher heat fluxes; however, they heated fuels for an increasingly longer duration and over a larger area. Water-cooled heat flux gauges provided reliable heat flux measurements for large firebrand piles and an array of thin-skin calorimeters indicated significant spatial variation in heat flux. A recipient fuel transitioned from smoldering to flaming under an adequate wind speed soon after a firebrand pile was deposited on its surface. •Firebrand piles caused smoldering ignition of fuels which transition to flaming.•Two heat flux measurement methods compared for firebrand pile application.•Piled firebrands produce distinctly different heating from individual firebrands.•Heat transfer in firebrand piles is dominated by re-radiation and reheating.•Wind has a dramatic effect on firebrand pile heating and is required for ignition.