Firebrand Generation From Thermally-Degraded Cylindrical Wooden Dowels

Abstract

During wildland fires, firebrands form once they break off of burning vegetation or structures. Many are then lofted into the fire plume where they are transported long distances ahead of the fire front, igniting new "spot" fires as they land. To date, very few studies have been conducted on the breakage mechanism of thermally-degraded vegetative elements. Knowledge of these mechanisms is needed to feed mathematical models of firebrand transport from traditional wildfires as well as those that spread into communities. First, a framework to understand the behavior of thermally-degraded wooden elements under simultaneous external loading is presented. A set of experiments were designed such that cylindrical wooden dowels of different species are exposed to different heating conditions similar to wildland fires, in order to model the breakage mechanisms of these elements in the absence of wind. The thermally-degraded elements are subjected to the three-point bending test to obtain the mechanical response of the materials after combustion. Assuming Hookean Orthotropic behavior for combusted dowels, dimensional analysis of the results reveals that the ultimate strength of the dowels is affected by the recoverable elastic strain during loading, which is found to occur under two distinct regimes. These results are not only important for better understanding of the breakage mechanisms but also are advantageous for developing a failure theory of thermally degrading wooden elements under simultaneous wind loading conditions.

Determination of Firebrand Characteristics Using Thermal Videos

“…Pine twig particles were 4-6 mm in diameter and had a length of 40 ± 2 and 60 ± 2 mm. The diameter of pine wood pellets was 6-8 ± 1 mm and the length of granules The size of the particles was selected in accordance with the data of field experiments [17][18][19]. Bark particles were made of pine bark (Pinus sibirica) with sides 10 ± 2, 15 ± 2, 20 ± 2, 25 ± 2, 30 ± 2 mm, and 4-5 mm in thickness.…”

“…The developed GUI (Figure 4a) is used to navigate through video frames (forward, backward, choice of frame with a required number), enlarge the window with frames by a given or arbitrary number of times using the mouse, as well as to select and adjust the parameters of the particle tracking algorithm, run the selected tracker with the given parameters, and finally, to generate the video in AVI format using the tracker. The size of the particles was selected in accordance with the data of field experiments [17][18][19]. Bark particles were made of pine bark (Pinus sibirica) with sides 10 ± 2, 15 ± 2, 20 ± 2, 25 ± 2, 30 ± 2 mm, and 4-5 mm in thickness.…”

“…Recently, several studies have been conducted in an attempt to solve this problem [16][17][18][19][20][21][22][23]. In order to conduct this research, a series of prescribed fire experiments within pine forests were performed to study the generation of firebrands and their properties as a function of fire behavior [16][17][18].…”

“…Hedayati et al [20] conducted experiments that were designed to generate firebrands from building materials in a wind tunnel and, upon doing so, proposed a probabilistic model for the estimation of firebrand mass/projected area/flying distance. The formation of firebrands from thermally degraded vegetative elements was also investigated in the study [19]. [23] used 3D Particle Tracking Velocimetry and 3D Particle Shape Reconstruction to reconstruct 3D models of individual particles and their trajectories.…”

The initiation of smouldering peat fire by a glowing firebrand

“…Spotting ignition by burning firebrands is a significant fire spread pathway both in wildlands and at the wildland-urban interface, contributing to significant loss of life and property (Manzello et al 2008;Caton et al 2017;Wang et al 2017). Due to its critical hazard, many research efforts have been taken to understand the mechanisms of firebrands, including generation (Manzello et al 2007a;Suzuki and Manzello 2016;Thomas et al 2017;Caton-Kerr et al 2019;Ju et al 2023), lofting (Tohidi and Kaye 2017), burning (Manzello et al 2007b;Song et al 2017), transport (Wadhwani et al 2022), deposition (De Beer et al 2023, and ignition of other fuels (Yin et al 2014;Manzello et al 2020;Tao et al 2021;Zhu and Urban 2023). For example, Urban et al (2019) investigated the ignition of a moist fuel bed (sawdust) by glowing firebrands of different sizes and found a minimum firebrand size of 3.17 mm in diameter and maximum fuel moisture content of 40% (wet-mass base) to achieve smouldering ignition.…”