A unified scaling law for bushfire junctions

The study of extreme fire phenomena is limited by the experimental capabilities, especially in terms of geometric scale. Scaling tools provides a solution to extrapolate limited laboratory-scale results to large real-world-scale scenarios. In this paper, a new scaling law is proposed for merging bushfires (which propagates quasi-steadily) through a relationship between the normalised rate of spread of junction fire and Byram’s convective number. The proposed law accounts for wind and slope effects, highlighting the role played by the two forces governing the flame-front dynamics and the plume trajectory: buoyancy force and wind inertia. A large set of numerical simulations of the junction fire at a wide range of scales, slopes, wind speeds, junction angles and two types of fuel (grass and shrub) was carried out using fully physical modelling. Results show that the normalised rate of spread of a junction fire depends only on a modified expression of Byram’s convective number and on fuel type. Moreover, the proposed expression of Byram’s number yields a unified scaling law for both junction fires and single straight fire lines for quasi-steady fires. The research helps assess the effects of some topographical parameters in extreme fires, improving situational awareness, operational predictions and firefighter safety. • A modified Byram’s convective number is proposed for junction fires. • Normalised junction fire rate of spread is scaled using Byram’s convective number. • Unified scaling law is found for both junction fire and single straight fire line. • The scaling law depends on fuel type. • Fire scale, when considered in isolation, does not affect junction fire behaviour.