Spotting ignition by lofted firebrands is a significant mechanism of fire spread, as observed in many large-scale fires. The role of firebrands in fire propagation and the important parameters involved in spot fire development are studied. Historical large-scale fires, including wind-driven urban and wildland conflagrations and post-earthquake fires are given as examples. In addition, research on firebrand behaviour is reviewed. The phenomenon of spotting fires comprises three sequential mechanisms: generation, transport and ignition of recipient fuel. In order to understand these mechanisms, many experiments have been performed, such as measuring drag on firebrands, analysing the flow fields of flame and plume structures, collecting firebrands from burning materials, houses and wildfires, and observing firebrand burning characteristics in wind tunnels under the terminal velocity condition and ignition characteristics of fuel beds. The knowledge obtained from the experiments was used to develop firebrand models. Since Tarifa developed a firebrand model based on the terminal velocity approximation, many firebrand transport models have been developed to predict maximum spot fire distance. Combustion models of a firebrand were developed empirically and the maximum spot fire distance was found at the burnout limit. Recommendations for future research and development are provided.

This paper is one of a series on brand lofting and propagation. Here, spherical brand propagation in a constant ambient wind is addressed. Maximum propagation distances are calculated for wooden brands with diameters up to 0.18 m, which are lofted above axisymmetric pool fires with heat release rates, Qo, between 1 MW and 3 GW. Winds of 1.8 m/s ≤ Uw ≤ 92 m/s are considered. A maximum propagation distance equation is developed as a function of Qo, Uw and wood type, or β. Cedar brands (β = 1), lofted by fires with Qo = 1 MW, 50 MW and 1 GW, travel a maximum of 49 m, 290 m and 1100 m, respectively, in 10 m/s winds before landing at burn out. Brands between a "collapse" diameter, dcol = 0.49 Q0 0.269 β0.782, and a maximum loftable diameter, do,max = 0.454 β Q0 0.04, propagate the same maximum distance, since the larger brands move slower and therefore have more time to combust. Hence, only brands with 0 ≤ d ≤ d col need be studied for given Qo, Uw and β.

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