Study of firebrand trajectories in a turbulent swirling natural convection plume

doi:10.1016/0010-2180(69)90072-8

Combustion and Flame

Volume 13, Issue 6, December 1969, Pages 645-655

https://doi.org/10.1016/0010-2180(69)90072-8 Get rights and content

Abstract

A combined theoretical and experimental study has been performed of the behaviour of firebrands in a turbulent, swirling natural convection plume. For the theoretical treatment, firebrands were idealized as spheres of constant density, the burning rate a constant for each type of combustible material. Concentration of firebrands was assumed low enough so that the fluid phase was considered undisturbed by the particulate phase. Results of the study showed that the trajectory of the firebrand was dependent upon a balance of forces on the firebrand. As the particle swirled about the axis of the plume, a balance was established between a radially inward drag force on the particle and the centrifugal force, which caused the particle to move either towards or away from the axis. The magnitude of the velocity field was dependent upon the altitude of the particle in the plume. Trajectories were terminated either by burnout of the particle or by grounding.

Experimentally, test particles were injected into a plume, and the trajectories observed. The important parameters that determined the particles' trajectory were the burning rate, the initial size of the particle, and the density of the particle. Initial placement in the plume also affected the particles' trajectory. Experimentally it was determined that flat particles were more stable in the plume than spherical particles.

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A review of firebrand studies on generation and transport
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Firebrands play a vital role in the propagation of fire by starting new fires called spotfires, ahead of the fire front during wildfire progression. Firebrands are a harbinger of damage to infrastructure; their effects particularly pose a threat to people living within the wildland-urban-interface, they can hamper the suppression of wildfire and block evacuation routes for communities and emergency services. Short-range firebrands which travel along with the wind, with little or no lofting, are particularly crucial in increasing fire front propagation and damaging structures situated close to the wildland-urban interface. In the Daylesford fire of 1962 in Australia, massive short-range spotting (the process of spot fire ignition and merging of spots caused by firebrands) occurred in the eucalyptus forest and increased the rate of fire spread by roughly three times more than that computed using an operational fire model. Similarly, long-range firebrands can be transported by the fire plume and ambient wind and can ignite new fire up to 30–40 km from the source of fire as observed in the 2009 Black Saturday fire in Australia.
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Citation Excerpt :
Regarding the generation of firebrands, their behavior has often been analyzed through investigations of actual or prescribed fires [9–16] or burn experiments on combustibles, including extracted and full-scale samples of vegetation and structures [17–33]. Regarding the transport of firebrands, most early studies solved the equation of motion of a rigid particle in a flow with a steady profile to analyze its behavior [34–44]. In recent studies, computational fluid dynamics (CFD) simulations have become a dominant approach for tracking the trajectories of firebrands in unsteady flow fields [45–52].
Spot ignition by wind-dispersed firebrands is an important factor for large outdoor fires. However, the large probabilistic variability in the generation, transport, and ignition processes makes it difficult to quantify the risk. Data collection under a wide range of conditions is essential for quantifying uncertainty. Generally, this can be addressed only with relatively small-scale experiments conducted repeatedly under the desired conditions, simultaneously involving ambiguity in the consistency of the full-scale behavior. However, data collection in full-scale experiments is complex, particularly for behaviors such as large outdoor fires, which makes it difficult to acquire sufficient data for statistical analysis. To fill this gap, we employed a hierarchical Bayesian approach to alleviate the estimation variance of the probabilistically varying characteristics of the spot-ignition processes involved in full-scale behavior. In the present analysis, the results of two previous experiments were used: a full-scale experiment that burnt a three-story wooden building, and a wind tunnel experiment that burnt wood cribs in an open-top combustion apparatus. Pooled models, using mixed data from all experiments without considering the relationship between data sources, were less successful in developing probabilistic distributions in some instances. By contrast, hierarchical Bayesian models (HBMs) are relatively robust, compensating for the scarcity of full-scale data with small-scale data. The obtained HBMs were applied to estimate the probabilistic distributions for the size and transport distance of firebrands under the hypothesized fire conditions. A comparison between the HBMs and existing probabilistic models demonstrated a significantly short dispersal range of firebrands by the latter, which may lead to an underestimation of hazards in practical applications. This study presents a methodology for constructing a reasonable probabilistic model for the processes of spot ignition behavior, which often encounter the issue of data limitations for statistical analysis.
Quantifying rare events in spotting: How far do wildfires spread?
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Citation Excerpt :
This assumption subsequently became a common approximation in firebrand research. Ever since the pioneering work of Tarifa et al. [7,22], many similar studies have been conducted which improve or build upon it [23–27]. In particular, Koo et al. [19] cast doubt on the terminal velocity assumption.
Spotting refers to the transport of burning pieces of firebrand by wind which, at the time of landing, may ignite new fires beyond the direct ignition zone of the main fire. Spot fires that occur far from the original burn unit are rare but have consequential ramifications since their prediction and control remains challenging. To facilitate their prediction, we examine three methods for quantifying the landing distribution of firebrands: crude Monte Carlo simulations, importance sampling, and large deviation theory (LDT). In particular, we propose an LDT method that accurately and parsimoniously quantifies the low probability events at the tail of the landing distribution. In contrast, Monte Carlo and importance sampling methods are most efficient in quantifying the high probability landing distances near the mode of the distribution. However, they become computationally intractable for quantifying the tail of the distribution due to the large sample size required. We also show that the most probable landing distance grows linearly with the mean characteristic velocity of the wind field. Furthermore, defining the relative landed mass as the proportion of mass landed at a given distance from the main fire, we derive an explicit formula which allows computing this quantity as a function of the landing distribution at a negligible computational cost. We numerically demonstrate our findings on two prescribed wind fields.
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Citation Excerpt :
However, they focused more on the thermo–mechanical properties of wooden cylindrical materials and less on the probabilistically varying shape and mass of firebrands intrinsic to the generation process [18,24]. Regarding the transport of firebrands, most early studies solved the equation of motion of a rigid particle in a flow of steady profile for analyzing its behavior [26–36]. In more recent studies, computational fluid dynamics (CFD) simulations have become a dominant approach for tracking the trajectories of firebrands in an unsteady flow field [37–44].
Spot ignition by wind-dispersed firebrands is an important factor in the spread of fire in large outdoor fires. However, the large probabilistic variability in its generation, transport, and ignition processes causes difficulty in quantifying its risk. In this study, a series of wind tunnel experiments was conducted to understand the probabilistically varying characteristics of two among three processes involved in spot ignition, i.e., the generation and transport of firebrands. Analysis samples were generated by combusting a wood crib in an apparatus that simulates the behavior of a fully developed structural fire. The experimental parameters were the ejection height of firebrands, $H$ , crosswind velocity, $U_{\infty}$ , and thickness of wood stick used for assembling the wood crib, $d$ . Utilizing the experimental results, the projected area of a firebrand and its transport distance downwind of a fire source were formulated based on simple physical considerations. However, because the obtained correlations represent deterministic relationships of the parameters, they were further incorporated into probability distributions (a truncated exponential distribution for the projected area and a truncated normal distribution for the transport distance) to enable the consideration of intrinsic variations. The obtained models reasonably reproduced the experimental results for both the projected area and transport distance. Although structural fire is the targeted type of fire source, the proposed models exhibit forms extensible to other types of fire sources.
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A fact often overlooked is that large-scale wildfires, although occurring infrequently, are responsible for the overwhelming majority of fire-related suppression costs, economic losses, and natural resources damages. Fortunately, the increasingly severe problems of large-scale wildfires worldwide have been receiving ever-growing academic attention. The high-intensity burning behaviors in wildfires stem from the significant interaction of combustion with heat transfer and atmospheric flow under complicated fuel, meteorology, and topography conditions. Therefore, mitigating measures against large-scale wildfire disasters have grown into a challenging research focus for combustion scientists. Research over the past century has resulted in incrementally enhanced insights into the mechanisms of combustion dynamics underlying the various erratic behaviors in large-scale wildfires, with theories and models of fire accelerations developed and validated. These advances are expected to improve the efficacy of large-scale wildfire predictions significantly. Nevertheless, the physical interpretation of the acceleration of large-scale wildfires is far from adequate and complete. This paper intends not to make a comprehensive review of the entire wildfire research field, but to depict an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control. It is outlined that the complicated transformation of fuel preheating mechanisms determines the growth of surface fire spread, while varied large-size flame fronts and unique spread modes induced in specific fire environments play an essential role in fire spread acceleration. Additionally, multiple fires burning and merging often act as crucial steps for accelerating surface fire spread, generating large-size flames, and triggering unique spread modes. These major potential factors strike the energy balance of a low-intensity wildfire and push it to a high-intensity state. Several issues regarding intensely burning behaviors in large-scale wildfires are selected for in-depth discussions, for which an overview of the progress and challenges in research is presented. It is concluded that the fundamental exploration targeted at developing application tools capable of dealing with large-scale wildfires remains at its early stages. Opportunities for innovation are abundant, yet systematic and long-term research programs are required.

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Study of firebrand trajectories in a turbulent swirling natural convection plume

Abstract

Combustion & Flame

Mass fires and fire behaviour

U.S. Forest Service Research Paper No. PSW-19