Analysis of the physical processes associated with junction fires at laboratory and field scales
J. R. Raposo A , D. X. Viegas A D , X. Xie B , M. Almeida A , A. R. Figueiredo A , L. Porto A and J. Sharples CA ADAI/Center of Studies of Forest Fires (CEIF), University of Coimbra, Rua Luís Reis dos Santos, Coimbra 3030-788, Portugal.
B State Key Laboratory of Fire Science, University of Science and Technology of China, Jinzhai Road 96, Hefei, Anhui 230026, P. R. China.
C University of New South Wales, UNSW Canberra, ACT 2610, Australia.
D Corresponding author. Email: xavier.viegas@dem.uc.pt
International Journal of Wildland Fire 27(1) 52-68 https://doi.org/10.1071/WF16173
Submitted: 19 January 2016 Accepted: 27 September 2017 Published: 23 January 2018
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.
Additional keywords: accelerating fires, convective effects, dynamic fire behaviour, extreme fire behaviour, fire behaviour, merging fires.
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