Heating and ignition from firebrand piles

Abstract

Smoldering firebrands, also known as burning embers, cause the majority of structural ignitions in the wildland-urban interface (WUI). Firebrands can be lofted several kilometers from the main firefront and smolder for several hours before causing ignition of vegetation or structural components, such as decks or roofs. Previous studies on ignition by firebrands have focused primarily on either vegetative fuels, large scale ignition of solid fuels (e.g. wood materials), or ignition by individual firebrands. This study focuses on small-scale heating and ignition phenomena by measuring heat fluxes from piles of firebrands and connecting heating to ignition conditions. In order to capture the heating from glowing firebrand piles and understand the ignition process of WUI fuels, small-scale experiments were conducted using laboratory-fabricated cylindrical wooden firebrands. Heating by firebrand piles was de-coupled from the ignition process by initially studying heating from firebrands on an inert surface. The influences of firebrand diameter, firebrand pile mass, and wind were explored. Firebrand diameter was found to have little effect on heating, while firebrand pile mass had a moderate effect, and wind had a large effect on measured heat flux. Piles did not produce higher heat fluxes as pile mass increased. Instead, larger piles heated a larger area of the fuel for an increasingly longer duration as pile mass increased. Based on the results from heating experiments, two firebrand pile masses were studied under three wind conditions to understand the process governing the transition to ignition of a recipient fuel. Video recordings indicated a qualitative description of the transition from smoldering to flaming. Flaming ignition of the recipient fuel occurred soon after deposition of the firebrand pile for 16.2 g of smoldering firebrands, with a minimum critical wind speed of 1.0 m/s required to cause transition to flaming. No smoldering of the recipient fuel occurred when only 5 g of firebrands were deposited.