Firebrand Transport Research Articles

Investigating Fire–Atmosphere Interaction in a Forest Canopy Using Wavelets

Wildland fire–atmosphere interaction generates complex turbulence patterns, organized across multiple scales, which inform fire-spread behaviour, firebrand transport, and smoke dispersion. Here, we utilize wavelet-based techniques to explore the characteristic temporal scales associated with coherent patterns in the measured temperature and the turbulent fluxes during a prescribed wind-driven (heading) surface fire beneath a forest canopy. We use temperature and velocity measurements from tower-mounted sonic anemometers at multiple heights. Patterns in the wavelet-based energy density of the measured temperature plotted on a time–frequency plane indicate the presence of fire-modulated ramp–cliff structures in the low-to-mid-frequency band (0.01–0.33 Hz), with mean ramp durations approximately 20% shorter and ramp slopes that are an order of magnitude higher compared to no-fire conditions. We then investigate heat- and momentum-flux events near the canopy top through a cross-wavelet coherence analysis. Briefly before the fire-front arrives at the tower base, momentum-flux events are relatively suppressed and turbulent fluxes are chiefly thermally-driven near the canopy top, owing to the tilting of the flame in the direction of the wind. Fire-induced heat-flux events comprising warm updrafts and cool downdrafts are coherent down to periods of a second, whereas ambient heat-flux events operate mainly at higher periods (above 17 s). Later, when the strongest temperature fluctuations are recorded near the surface, fire-induced heat-flux events occur intermittently at shorter scales and cool sweeps start being seen for periods ranging from 8 to 35 s near the canopy top, suggesting a diminishing influence of the flame and increasing background atmospheric variability thereat. The improved understanding of the characteristic time scales associated with fire-induced turbulence features, as the fire-front evolves, will help develop more reliable fire behaviour and scalar transport models.

Firebrand burning under wind: an experimental study

Background Spot fires play a significant role in the rapid spread of wildland and wildland–urban interface fires. Aims This paper presents an experimental and modelling study on the flaming and smouldering burning of wood firebrands under forced convection. Methods The firebrand burning experiments were conducted with different wind speeds and firebrand sizes. Key results The burning rate of firebrands under forced convection is quantified by wood pyrolysis rate, char oxidation rate and a convective term. The firebrand projected area is correlated with firebrand diameter, char density, wind speed, and flaming or smouldering burning. A surface temperature model is derived in terms of condensed-phase energy conservation. We finally establish a simplified firebrand transport model based on the burning rate, projected area and surface temperature of firebrands. Conclusion The mass loss due to wood pyrolysis is much greater than that due to char oxidation in self-sustaining burning. The burning rate is proportional to U1/2, where U is wind speed. The projected area for flaming firebrands decreases more rapidly than that for smouldering ones. The firebrand surface temperature is mainly determined by radiation. Implications Knowledge about firebrand burning characteristics is essential for predicting the flight distance and trajectory in firebrand transport.

A fast, physically based model of firebrand transport by bushfire plumes

Spotfires are a major problem in bushfire management, greatly complicate suppression efforts, and contribute to fires breaking control lines. Firebrands are often implicated in structure loss, and in extreme circumstances have been observed to ignite new fires over 30km ahead of the parent fire. Existing prediction techniques do not accommodate the problem of such long-range spotting, and the meteorological and fire conditions that lead to such events are not well understood. We present a computationally inexpensive, physically based model of ember transport within bushfire plumes, with four components: an integral plume model, a model of turbulence within the plume, a probabilistic model of ember transport by the plume, and a model of transport beneath the plume. The predicted ember landing distributions from this model compare satisfactorily to the explicit ember-transport calculations of Thurston et al. (2017) and to observations (Cruz et al., 2012). We examine the sensitivity of the simple model to its input parameters. The 90th percentile of spotting distance increases with increasing fire power and decreases with increasing ember terminal fall velocity and fire radius. Where there is a meteorological inversion or stable layer and the plume updraft is sufficiently strong to carry embers to that height, the stable layer substantially limits the transport distance, with increased height favoring longer-range transport. The effect of wind speed is complex, due to the competing factors of faster horizontal transport but suppressed vertical plume development with stronger winds.

Effects of urban boundary layer turbulence on firebrand transport

This study investigates the role of topography-induced turbulence, generated by an idealized urban region, in the transport of firebrands and risk of spotting. Flight dispersion, deposition, and smoldering state of tens of thousands of individual mass and size-changing firebrands were investigated in the atmospheric boundary layer turbulence, which was obtained using Large-eddy simulations. Firebrands were assumed to be smoldering spherical particles of Stokes numbers ranging from 30 to 175. Resultsindicate that the presence of urban topography significantly affects the firebrand flight behavior, landing distribution, and risk of spotting. Compared to a case with flat topography, horizontal dispersions of the smallest size firebrands were significantly enhanced when urban topography was presented, while the largest firebrands landed closer to each other and closer to the release point. Consequently, a notably different and more compact spotting risk map was achieved. Within the urban boundary layer turbulence, firebrands had a shorter flight and smoldering times in comparison with the flat case. As a result, firebrands landed with larger temperatures, which contributed to a higher risk of spotting in the presence of urban topography.

A numerical investigation exploring the potential role of porous fencing in reducing firebrand impingement on homes

Firebrand impingement is a leading cause of home ignitions from wildland fire. The use of porous fencing has recently been proposed as a potential method for mitigating firebrand impingement on homes. A porous fence can act as a windbreak to alter the near-surface flow and induce particle deposition, as demonstrated in other applications, such as the use of snow fences to protect roadways from drifting snow. Conservation advocates have proposed the use of fire-resistant vegetation to act as a fence upwind of homes or subdivisions. Porous fences could also be constructed from fire-resistant materials such as metal, rock, or composites. This numerical investigation of the effectiveness of porous fencing to reduce firebrand impingement on homes conducted a series of experiments to explore the effect of porous fencing on the near-surface flow field and firebrand transport downwind of the fence. We also evaluated the sensitivity of the results to various fence, flow, and firebrand properties, including fence height, fence porosity, wind speed, firebrand source location, and firebrand size. To our knowledge, this is the first study to investigate the concept of using a fence to induce firebrand deposition upwind of homes. Our results showed that porous fencing can reduce firebrand impingement on homes by up to 35% under certain conditions; however, fencing can also increase impingement on homes. The mitigation effectiveness depended on the proximity of the firebrand source, distance between the fence and home as a function of fence height, wind speed, and firebrand size. A series of key findings and recommendations are provided.

Hierarchical Bayesian approach to developing probabilistic models for generation and transport of firebrands in large outdoor fires under limited data availability

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.

<i>Corrigendum to</i>: Firebrand transport from a novel firebrand generator: numerical simulation of laboratory experiments

Firebrands (often called embers) increase the propagation rate of wildfires and often cause the ignition and destruction of houses. Predicting the motion of firebrands and the ignition of new fires is therefore of significant interest to fire authorities. Numerical models have the potential to accurately predict firebrand transport. The present study focuses on conducting a set of benchmark experiments using a novel firebrand generator, a device that produces controlled and repeatable sets of firebrands, and validating a numerical model for firebrand transport against this set of experiments. The validation is conducted for the transport of non-burning and burning cubiform firebrand particles at two flow speeds. Four generic drag sub-models used to estimate drag coefficients that are suited for a wide variety of firebrand shapes are verified for their applicability to firebrand transport modelling. The four sub-models are found to be good in various degrees at predicting the transport of firebrand particles.

Determining Firebrand Generation Rate Using Physics-Based Modelling from Experimental Studies through Inverse Analysis

Firebrand spotting is a potential threat to people and infrastructure, which is difficult to predict and becomes more significant when the size of a fire and intensity increases. To conduct realistic physics-based modeling with firebrand transport, the firebrand generation data such as numbers, size, and shape of the firebrands are needed. Broadly, the firebrand generation depends on atmospheric conditions, wind velocity and vegetation species. However, there is no experimental study that has considered all these factors although they are available separately in some experimental studies. Moreover, the experimental studies have firebrand collection data, not generation data. In this study, we have conducted a series of physics-based simulations on a trial-and-error basis to reproduce the experimental collection data, which is called an inverse analysis. Once the generation data was determined from the simulation, we applied the interpolation technique to calibrate the effects of wind velocity, relative humidity, and vegetation species. First, we simulated Douglas-fir (Pseudotsuga menziesii) tree-burning and quantified firebrand generation against the tree burning experiment conducted at the National Institute of Standards and Technology (NIST). Then, we applied the same technique to a prescribed forest fire experiment conducted in the Pinelands National Reserve (PNR) of New Jersey, the USA. The simulations were conducted with the experimental data of fuel load, humidity, temperature, and wind velocity to ensure that the field conditions are replicated in the experiments. The firebrand generation rate was found to be 3.22 pcs/MW/s (pcs-number of firebrands pieces) from the single tree burning and 4.18 pcs/MW/s in the forest fire model. This finding was complemented with the effects of wind, vegetation type, and fuel moisture content to quantify the firebrand generation rate.

Firebrand transport from a novel firebrand generator: numerical simulation of laboratory experiments

Firebrands (often called embers) increase the propagation rate of wildfires and often cause the ignition and destruction of houses. Predicting the motion of firebrands and the ignition of new fires is therefore of significant interest to fire authorities. Numerical models have the potential to accurately predict firebrand transport. The present study focuses on conducting a set of benchmark experiments using a novel firebrand generator, a device that produces controlled and repeatable sets of firebrands, and validating a numerical model for firebrand transport against this set of experiments. The validation is conducted for the transport of non-burning and burning cubiform firebrand particles at two flow speeds. Four generic drag sub-models used to estimate drag coefficients that are suited for a wide variety of firebrand shapes are verified for their applicability to firebrand transport modelling. The four sub-models are found to be good in various degrees at predicting the transport of firebrand particles.

Coupled Assessment of Fire Behavior and Firebrand Dynamics

The hazards associated with firebrands have been well documented. However, there exist few studies that allow for the hazard from a given fire to be quantified. To develop predictive tools to evaluate this hazard, it is necessary to understand the conditions that govern firebrand generation and those that affect firebrand deposition. A method is presented that allows for time-resolved measurements of fire behavior to be related to the dynamics of firebrand deposition. Firebrand dynamics were recorded in three fires undertaken in two different ecosystems. Fire intensity is shown to drive firebrand generation and firebrand deposition—higher global fire intensities resulting in the deposition of more, larger firebrands at a given distance from the fire front. Local firebrand dynamics are also shown to dominate the temporal firebrand deposition with periods of high fire intensity within a fire resulting in firebrand shower at deposition sites at times commensurate with firebrand transport. For the range of conditions studied, firebrand deposition can be expected up to 200 m ahead of the fire line based on extrapolation from the measurements.

Generalization framework for varying characteristics of the firebrand generation and transport from structural fire source

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∞, 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.

A Methodology for Experimental Quantification of Firebrand Generation from WUI Fuels

Over the past few years, numerous large-scale disasters have occurred due to wildfires at the wildland-urban interface (WUI). In these fires, spread via the transport of firebrands (burning embers) plays a significant role. Several models have been developed to describe the transport of firebrands but few, if any, are available which can provide a quantitative means to generate firebrands at the source of a fire. In this regard, a new methodology is proposed here that uses a wind tunnel to experimentally quantify the generation of firebrands from WUI fuels under different ambient conditions. The setup allows for the collection of all generated solid firebrands and major downstream gaseous species concentrations. Unique firebrand yield correlations can then be generated for each tested fuel, while also accounting for the heat-release rate, providing unique validation targets for numerical simulations. Generation of firebrands from branches of two conifers at a fixed wind speed of 4 m/s are presented to demonstrate the capabilities of this new methodology. A carbon mass balance was utilized to analyze preliminary results and understand how much of the fuel mass transitions to firebrands vs. gases. These results provide a description of the mass burning process and ultimately tie firebrand production to a time-dependent heat-release rate for initialization of firebrand transport in numerical simulations. An average firebrand yield ranging from 3–4% of initial dry mass is ultimately presented for lodgepole pine and Douglas fir. Future work is required with larger fuel sizes pertaining to real wildfire scenarios; however, the presented methodology can provide valuable data needed to initialize numerical simulations of firebrand transport, necessary for reconstruction of WUI fires and to aid in the development of mitigation strategies for the prevention of future disasters.

Diurnal Pine Bark Structure Dynamics Affect Properties Relevant to Firebrand Generation

Firebrands are an important agent of wildfire spread and structure fire ignitions at the wildland urban interface. Bark flake morphology has been highlighted as an important yet poorly characterized factor in firebrand generation, transport, deposition, and ignition of unburned material. Using pine species where bark flakes are the documented source of embers, we conducted experiments to investigate how bark structure changes in response to diurnal drying. Over a three-day period in a longleaf pine (Pinus palustris Mill.) stand in Florida, we recorded changes in temperature, moisture content, and structure of bark across different facing aspects of mature pine trees to examine the effects of varying solar exposure on bark moisture. We further compared results to bark drying in a pitch pine (Pinus rigida Mill.) plantation in New Jersey. Under all conditions, bark peeled and lifted away from the tree trunk over the study periods. Tree bole aspect and the time of day interacted to significantly affect bark peeling. General temperature increases and moisture content decreases were significantly different between east and west aspects in pitch pine, and with time of day and aspect in longleaf pine. These results illustrate that bark moisture and flakiness is highly dynamic on short time scales, driven largely by solar exposure. These diurnal changes likely influence the probability of firebrand production during fire events via controls on moisture (ignition) and peeling (lofting).

Fire Behavior, Fuel Consumption, and Turbulence and Energy Exchange during Prescribed Fires in Pitch Pine Forests

Prescribed fires are conducted extensively in pine-dominated forests throughout the Eastern USA to reduce the risk of wildfires and maintain fire-adapted ecosystems. We asked how fire behavior and fuel consumption during prescribed fires are associated with turbulence and energy fluxes, which affect the dispersion of smoke and transport of firebrands, potentially impacting local communities and transportation corridors. We estimated fuel consumption and measured above-canopy turbulence and energy fluxes using eddy covariance during eight prescribed fires ranging in behavior from low-intensity backing fires to high-intensity head fires in pine-dominated forests of the New Jersey Pinelands, USA. Consumption was greatest for fine litter, intermediate for understory vegetation, and least for 1 + 10 hour wood, and was significantly correlated with pre-burn loading for all fuel types. Crown torching and canopy fuel consumption occurred only during high-intensity fires. Above-canopy air temperature, vertical wind velocity, and turbulent kinetic energy (TKE) in buoyant plumes above fires were enhanced up to 20.0, 3.9 and 4.1 times, respectively, compared to values measured simultaneously on control towers in unburned areas. When all prescribed fires were considered together, differences between above-canopy measurements in burn and control areas (Δ values) for maximum Δ air temperatures were significantly correlated with maximum Δ vertical wind velocities at all (10 Hz to 1 minute) integration times, and with Δ TKE. Maximum 10 minute averaged sensible heat fluxes measured above canopy were lower during low-intensity backing fires than for high-intensity head fires, averaging 1.8 MJ m−2 vs. 10.6 MJ m−2, respectively. Summed Δ sensible heat values averaged 70 ± 17%, and 112 ± 42% of convective heat flux estimated from fuel consumption for low-intensity and high-intensity fires, respectively. Surprisingly, there were only weak relationships between the consumption of surface and understory fuels and Δ air temperature, Δ wind velocities, or Δ TKE values in buoyant plumes. Overall, low-intensity fires were effective at reducing fuels on the forest floor, but less effective at consuming understory vegetation and ladder fuels, while high-intensity head fires resulted in greater consumption of ladder and canopy fuels but were also associated with large increases in turbulence and heat flux above the canopy. Our research quantifies some of the tradeoffs involved between fire behavior and turbulent transfer of smoke and firebrands during effective fuel reduction treatments and can assist wildland fire managers when planning and conducting prescribed fires.

Firebrand Generation From Thermally-Degraded Cylindrical Wooden Dowels

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.

RandomFront 2.3: a physical parameterisation of fire spotting for operational fire spread models – implementation in WRF-SFIRE and response analysis with LSFire+

Abstract. Fire spotting is often responsible for dangerous flare-ups in wildfires and causes secondary ignitions isolated from the primary fire zone, which lead to perilous situations. The main aim of the present research is to provide a versatile probabilistic model for fire spotting that is suitable for implementation as a post-processing scheme at each time step in any of the existing operational large-scale wildfire propagation models, without calling for any major changes in the original framework. In particular, a complete physical parameterisation of fire spotting is presented and the corresponding updated model RandomFront 2.3 is implemented in a coupled fire–atmosphere model: WRF-SFIRE. A test case is simulated and discussed. Moreover, the results from different simulations with a simple model based on the level set method, namely LSFire+, highlight the response of the parameterisation to varying fire intensities, wind conditions and different firebrand radii. The contribution of the firebrands to increasing the fire perimeter varies according to different concurrent conditions, and the simulations show results in agreement with the physical processes. Among the many rigorous approaches available in the literature to model firebrand transport and distribution, the approach presented here proves to be simple yet versatile for application to operational large-scale fire spread models.

On the Importance of Firebrand Processes in Large Outdoor Fires.

Firebrands are produced from combustion of both vegetative and structural fuels in large outdoor fires. It is well known that firebrand generation, transport, and ignition mechanisms result in rapid and potentially devastating fire spread processes in large outdoor fires. In this article, the basic mechanisms of firebrand generation, transport, and ignition are discussed with an emphasis on how fundamental combustion knowledge may play an important role in this complex problem.

Wind and Fire Coupled Modelling—Part I: Literature Review

Wind and fire phenomena can together be a devastating force, whether in the case of a building fire, release of smoke in an urban area or forest fire near an urban habitat. Most of the fire phenomena are influenced by the wind, usually for the worse. If we want to understand fires, we have to understand wind as well, and model it appropriately. This modelling is described by the discipline of Computational Wind Engineering, from which we are able to transfer invaluable knowledge to coupled wind-fire analyses. This two-part review is dedicated to such a transfer. In Part I, the authors describe the historical and most current instances of wind and fire coupled modelling, referred to as simple models, in situ measurements, wind tunnel experiments or numerical studies with CFD. The review is subdivided into six categories, namely flame behaviour in wind conditions, indoor flows, natural smoke ventilators, tunnel ventilation, wildfires and firebrand transport, and urban dispersion of smoke. Besides flame behaviour, all remaining topics are covered, to the best of the authors’ knowledge, with multiple references to valuable experimental and numerical studies. In Part II of the review, the authors describe the best practices of Computational Wind Engineering, that may be transferred to fire-oriented numerical studies. This part provides good practice guidelines, reference studies and a proposal for the optimisation of the simulation procedure for coupled wind and fire models.

Aerodynamic characterization of rod-like debris with application to firebrand transport

Aerodynamics of the firebrands' flight is fundamental to fire spotting; yet, this phenomenon is relatively poorly understood. Variants of Tachikawa's transport models have been developed for different shapes of debris but, only simplified versions are implemented in firebrand transport models. Failure to include lift and rotational forces reduces the problem to a two-dimensional one, in contrast to the 3D trajectories observed. As such, many studies may have under-/overestimated the flight distance and failed to capture the spotting distribution, accurately. Also, there is virtually no detailed quantitative experimental validation available for rod-like debris transport models. Hence, a set of free-fall experiments with non-combusting model firebrands were run in the absence of cross-wind. The results are employed for verification and performance evaluation of a 3D deterministic 6-Degrees-Of-Freedom (DOF) rod-like debris transport model. It is shown that a transport model must include the complete 6-DOF aerodynamics of the debris for estimating the flight characteristics, such that results are not statistically significantly different from the experimental data. The findings have applications beyond firebrand transport, as they represent the most comprehensive experimental data set and analysis of rod-like debris transport, currently in the literature.

Stochastic modeling of firebrand shower scenarios

Firebrand shower and its subsequent spot fires are responsible for more than half of the ignitions reported during wildfires, in particular at wildland urban interface (WUI) areas. The firebrand transport is a highly stochastic and nonlinear problem which directly influences the spotting distribution. Hence, a coupled stochastic model of firebrand showers, that is thoroughly and systematically validated against large scale wind tunnel experiments of lofting and downwind transport of model firebrands, is presented. It is shown that the developed model predicts the first and second order statistics of the flight accurately in relation to the experimental data. The sensitivity of the model to the initial conditions of the flight as well as the velocity field characteristics are examined.