Analysis of the physical processes associated with junction fires at laboratory and field scales

Abstract

Junction fires, which involve the merging of two linear fire fronts intersecting at a small angle, are associated with very intense fire behaviour. The dynamic displacement of the intersection point of the two lines and the flow along the symmetry plane of the fire are analysed for symmetric boundary conditions. It is observed that the velocity of displacement of this point increases very rapidly owing to strong convective effects created by the fire that are similar to those of an eruptive fire. The change of fire geometry and of its associated flow gradually blocks the rate of spread increase and creates a strong deceleration of the fire, which ends up behaving like a linear fire front. Results from laboratory and field-scale experiments, using various fuel beds and slope angles and from a large-scale fire show that the processes are similar at a wide range of scales with little dependence on the initial boundary conditions. Numerical simulation of the heat flux from two flame surfaces to an element of the fuel bed show that radiation can be considered as the main mechanism of fire spread only during the deceleration phase of the fire.

Effect of Canyons on a Fire Propagating Laterally Over Slopes

“…The merging of two fire fronts that was studied in Viegas et al (2012), Sharples et al (2013), and Raposo et al (2015Raposo et al ( , 2018, also exhibits a behavior similar to a fire eruption leading to extremely high values of R' and are accompanied by very strong convective effects.…”

Modeling Vorticity-Driven Wildfire Behavior Using Near-Field Techniques

“…Given that several modes of dynamic fire propagation are now known to influence the development of a fire, the limitations of current operational fire spread simulators constitute a significant gap in operational capability. Documented examples of dynamic fire propagation include that exhibited by junction fires (Viegas et al, 2012;Thomas et al, 2017;Raposo et al, 2018), eruptive fires (Viegas and Pita, 2004;Viegas, 2006), and vorticity-driven lateral spread (VLS) (Sharples et al, 2012;Simpson et al, 2013Simpson et al, , 2014Simpson et al, , 2016. These modes of dynamic fire propagation are driven by complex interactions between the fire and the atmosphere, or between different parts of the fire itself.…”

Modelling of junction fires with analytical and numerical analysis of the phenomena

“…In (Raposo et al 2018) general empirical laws of fire behaviour, which can be used to predict the evolution of a junction fire episodes, namely the evolution of non-dimensional rate of spread as a function of the independent slope angle α and angle between fire fronts θo were presented. The same kind of analysis will be used for the tests with wind relating the different wind speeds with the evolution of the ROS.…”