Firebrand Processes in Wildland Fires and Wildland-Urban Interface (WUI) Fires
Firebrands signify any hot object in flight that are capable to ignite other fuel types. Firebrands are produced or generated from the combustion of vegetative and structural fuels. Firebrand processes include generation, transport, deposition, and ignition of various fuel types, leading to fire spread processes at distances far removed from the original fire source. While the term ember has been used to sometimes indicate the same connotation as firebrand, these terms are in fact slightly different. Spotting, a term most often used with wildland fires, refers to fire spread processes ahead of the main fire front due to firebrands. The term spotting has also been used to indicate structure ignition by firebrands in WUI fires.
Wildland fires that spread into urban areas, termed wildland-urban interface (WUI) fires, are becoming more and more common across multiple locations of the world (Manzello et al. 2018). The 2018 WUI fires in the US state of California demonstrated the shear destruction that WUI fires are capable of by destroying more than 18,800 structures and resulting in multiple fatalities. An important component in rapid spread of wildland fires and WUI fires is the production or generation of new, far smaller combustible fragments from the original fire source referred to as firebrands. For WUI fires, the production of firebrands occurs from the combustion dynamics of vegetative and man-made fuel elements, such as homes. For wildland fires, the dominant process is from the combustion of vegetation, such as chaparral, conifers, and grasses.
Firebrand Generation from Structural and Vegetative Fuel Sources
During the combustion process of vegetative fuels, pyrolysis of the fuel elements is an important mechanism. Pyrolysis refers to the breakdown of solid-phase materials by heat; the interested reader may see the contribution in this encyclopedia focused on pyrolysis. For conifer tree combustion, fuel elements consist of needles, bark, and branches. During the combustion of vegetative fuel elements, these pyrolysis reactions result in the generation of gases and vapors and also act to weaken the structural integrity of the original fuel source itself as a result of the mass loss processes. As these combustion processes occur during wildland fires and WUI fires, the application of aerodynamic forces from the interaction of wind forces imposed by the atmospheric boundary layer to vegetative fuel elements results in the breakage of small elements, that once lofted, become firebrands. An interesting approach to model the generation of firebrands from vegetative fuels considered the use of fractal geometry to attempt to describe the various vegetative types (Barr and Ezekoye 2013). The interested reader is directed to the contribution on vegetative firebrand attack in this encyclopedia, as details of the such models are provided.
Firebrand generation from structure combustion is also an important mechanism to generate firebrands in WUI fires. Experimental work was first initiated on this topic in the 1960s, since in the USA there was concern about the nature of potential mass fire occurrence in the aftermath of global-scale thermonuclear war. Firebrand generation from structure combustion remains unsolved, and no rigorous modeling attempts have been made, as compared to vegetative studies. Many factors influence the firebrands generated from structure combustion. For more information, the interested reader is directed to the contribution in this encyclopedia on structure firebrand attack.
Firebrand Transport Processes
The most often studied aspect of firebrand processes is the transport of firebrands. Some of the earliest known experimental work on this problem was conducted by Tarifa and co-workers (1965). In their work fundamental combustion and drag properties were determined for different wood geometries, and these data were used to calculate how far firebrands might fly based on their terminal settling velocities. After the firebrands are generated, they may be initially lofted by the buoyant fire plume and transported in the atmospheric boundary layer. Most of more recent work has conducted analysis that has considered the case where the firebrands are simply transported from a given height rather than lofted from ground level by the buoyant fire plume. While these studies are very interesting, the numerical predictions cannot be validated on real scales in a simple fashion, so the overall robustness is quite limited, and the usefulness of the models is unknown.
To begin to remedy the lack of experimental validation for firebrand transport studies, an interesting approach is the model developed by Kortas et al. (2009) to describe firebrand transport. The model solves the conservation equations of firebrand mass, momentum, and volume. The model has also been validated using full-scale experiments of firebrand transport using an experimental apparatus known as a firebrand generator; details of the most current iteration of the device are described below.
Firebrand Deposition and Ignition Processes
After the firebrands have been generated and transported, they must deposit on vegetative fuels or structural elements in order to be a danger to initiate ignition processes. The most important hazard is if the deposited firebrands have sufficient energy to ignite these adjacent fuel sources. An important physical requirement is that sufficient energy must be added to the fuel bed where the firebrands have landed so that a combustion reaction may begin. Firebrands may initiate either a smoldering combustion reaction or a flaming combustion reaction. To sustain ignition, these combustion reactions must produce more heat than is lost to the surrounding environment. In most cases, the firebrands themselves are in a state of glowing combustion since it is difficult to sustain flaming combustion as they are transported through the atmospheric boundary layer (Tarifa et al. 1965). Once firebrands land on fuel beds, the initial ignition mechanism is that of smoldering combustion for structural and vegetative fuels considered here. If a glowing firebrand is able to provide enough energy, self-sustained smoldering combustion may occur in the fuel bed, and under the influence of an applied wind field, the smoldering combustion reaction may transition to a flaming combustion reaction. For the fundamentals of ignition processes not specific to firebrands, the reader is referred to the contribution on ignition in this encyclopedia.
The NIST Dragon technology has also been constructed at the Insurance Institute for Business and Home Safety (IBHS) wind facility (http://www.disastersafety.org). Here, an entire house may be placed inside the test section, and due to the large size of the facility, an array of firebrand generators is used. The IBHS facility, useful for full-scale demonstrations of building vulnerabilities to firebrands, complements ongoing research conducted by NIST in partnership with BRI and NRIFD that allows far more intricate and parametric studies due to much simpler, lower cost wind facilities. For further details, the reader is directed elsewhere (Manzello 2014).
An important component in rapid spread of wildland fires and WUI fires are the production or generation of new, far smaller combustible fragments from the original fire source referred to as firebrands. In this contribution, a terse overview of firebrand processes leading to fire spread processes at distances far removed from the original fire source was discussed.
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