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... For some parameters the maximum value was not well defined; in these cases, the average of five higher values in the test was considered as the maximum value. The maximum values of the parameters analysed here are higher than those obtained in similar size tests performed with liquid fuels, as for example, in our study heat power released by these fire whirls is higher than 1 MW (see Table 2 of ( Pinto et al., 2017)) in comparison with 600kW from fire whirls produced with liquid fuels by Lei et al. (2011) for containers with 500 mm diameter in both studies. ...
... Following the previous work ( Pinto et al., 2017), in the present study the potential of the FWG was explored to extend the range of parameters (burning rate, heat release rate and flame height) tested through the use of other sensors, Pitot tubes and thermocouples, to analyse the flow and temperature, respectively. As the repetition of tests in other conditions yielded similar results we will present the results of only one test. ...
Conference Paper
During a forest fire the formation of fire whirls can be observed but their characteristics and evolution are still difficult to predict. Due to the high thermal energy released, fire whirls can become a danger to firefighters and are, by their complexity and dangerousness, a way of extreme fire behavior that is poorly understood. This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale that analyses the influence of the variation of the following parameters: fuel load, density, fuel configuration, imposition of forced flow with different velocities through the use of fans. Comprehensive measurements were performed in order to establish correlations of the burning rate, heat release rate, flame height, temperature and velocity of fire whirls, and to clarify the similarities and differences between a fire whirl and a ‘normal’ fire, i.e., one in which vortex formation is not observed. Fire whirl experiments were performed in a vertical channel with a quadrangular section of 1×1 m2 with a height of 7.8 m, the fire whirl generator, using dried shrubs, which is a fuel mainly composed by heather (Erica australis) and gorse (Pterospartum tridentatum) quite common in forested areas in Central Portugal and in several Mediterranean climate regions. Given the transient nature of the experiments relationships between time correlated parameters or with average or extreme values were established. Comparison with similar experiments showed that the properties of the flames generated with the present configuration of the fire whirl generator are similar to fire whirls produced in other laboratory studies and with full scale fire whirls. The maximum heat release power was of the order of 1 MW which is higher than the reported values for similar laboratory tests. The results show that forced flow increases dramatically the burning rate and reduces the time needed to achieve a high rate of energy release. On the other hand, from a certain value of forced flow it is observed that the values of the burning rate and flame height decrease, thus existing a critical wind velocity for optimal fire whirl development. Comparison with results of other sources show that the flames that are generated in the present fire whirl generator are in a transition from fire whirl to pool fire regime and that it is possible to scale up some flow and thermal properties of field scale fire whirls and to derive predictive models on the basis of laboratory scale experiments.
... Laboratory experiments of fire whirls are mainly based on the generation of stable and controllable generating eddy through manually imposed rotating screen [217][218][219][220] or shear flow [221][222][223] , naturally induced entrained air due to combustion [80 , 214 , 224-231] , or their combination [232] . Rotating screen and fixed-frame facilities are most frequently used. ...
... Various flame patterns of fire whirls: (a-r) evolution of flame patterns of a propane fire with increasing imposed circulation ( ˙ Q = 7.5 kW)[220] , (s) a stationary fire whirl during field experiments[232] , (t) a moving fire whirl generated along a spreading fireline under a 30 °inclined fuel bed[97] , (u) a moving fire whirl formed over an L-shaped line fire[222] , and (v) a moving fire whirl formed over a line fire with an attack angle of 25 °[233] . ...
Article
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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.
... The experimental study of the combustion characteristics, like the burning rate, flame height and the temperature field of fire whirls has been carried out in the last decades in order to understand their mechanisms of formation and development [1][2][3][4][5][6]. In spite of this, few investigations have addressed the study of the flow field related to the development of fire whirls in laboratory experiments [1][2][3][4][5][6][7][8][9][10][11][12][13]. There are several techniques/methods and instrumentations available to measure the flow velocity of fire whirls, such as: particle image velocimetry (PIV) [8][9][10], tracer [11], stereo particle image velocimetry (SPIV) [1,12] and S-type Pitot tubes [2][3][4][5][6]13]. ...
... The fuel used in the present study is a solid porous fuel composed of natural forest type vegetation. In Ref. [7], the role of different factors, such as fuel type and size, the porosity of the fuel bed, the presence of the side walls and the characteristics of the forced flow, on various parameters used to characterize fire whirls, such as, the mass loss rate of the fuel, the heat release rate, the flame length and diameter, was analysed. Besides the former variables there are others, such as, the flow velocity and temperature fields, of great interest to better analyse and understand the physical mechanisms associated to the formation and development of the complex dynamics of fire whirls. ...
Article
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In this paper we present a pressure measurement instrument based on the S-type Pitot tube modified to measure the two velocity components of a high temperature flow assuming that the flow is locally two-dimensional. The development of this new device, which we designate as the Double S-type Pitot Tube, is related to the difficulty and the lack of precision of measurements with a standard S-type Pitot tube in flows with unknown directions like the case of fire whirls in laboratory experiments. The design construction and calibration method of the device are described. The pitch angle of the flow and the velocity coefficients of the probe are analysed experimentally in a wind tunnel calibration as well as the associated errors. The use of this sensor in a fire whirl laboratory test is shown and the results are compared with those of simple S-type Pitot tubes in the same test. The obtained results show the applicability and better performance of the novel device.
... Fire whirls, as compared to general fires, are characterized by significant enhancement in burning rates, flame temperature and flame heights, in addition to a strong swirling motion of the flame itself (for example, [7][8][9][10]). These peculiarities of them allow to intensify the processes of heat and mass transfer in different engineering devices. ...
... Sudden changes of fire behavior consist of unpredictable variations of fire intensity, erratic ROS and direction, spotting, and occurrence of fire-caused winds. Sudden changes of fire behavior are a significant safety danger to firefighters [120,121] and to population and assets, and can overwhelm suppression efforts. ...
... Fire whirls are "visually observable because of the presence of flame, smoke, ash, and/or other debris" [6]. Numerous investigations of fire whirls with an inner core of flame have been carried out with laboratory experiments [8][9][10][11][12][13][14][15], numerical analysis [9,[16][17][18][19], and theoretical analysis [20][21][22][23], primarily by combustion scientists. However, fire whirls without an inner core of flame have been studied by relatively few researchers. ...
Article
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Laboratory experiments were conducted to determine whether stationary fire whirls just downwind of a meter-scale turbulent flame are the lowest part of the counter-rotating vortex pair (CVP) of the plume from the flame. Plumes from a turbulent pool fire and air flow around the fire were visualized. There are two types of stationary fire whirls: those that contain flames (FWflame) and those that do not (FWair). We determined that FWairis most likely the lowest part of the CVP and that FWflame is most likely generated by flames entering the lowest part of the CVP, swirling and increasing in length. FWairs and FWflames form alternately at the same location just downwind of the fuel pools. During the period when stationary fire whirls occur, the plume tilt angle from the vertical direction is smaller, and the variation in the plume tilt angle and flame trailing length is greater compared to all other periods. Flow visualization also showed that the characteristic semi-circular space (SCS) partly surrounded by the flame trailing downwind from both edges of the fuel pools is generated by the swirling wind of FWairs inside the SCS.
... Forest fires are disturbances associated with significant social, economic and environmental impacts (Nunes et al., 2016;Pinto et al., 2017;Hong et al., 2018). In fire-prone forest ecosystems of the Mediterranean Basin, many authors have identified an increase in fire regime parameters, such as recurrence and severity, in large forest fires (Pausas and Keely, 2009;Ferreira-Leite et al., 2011;Bento-Gonçalves et al., 2012;Quintano et al., 2015), mainly as a consequence of land use change (Pereg et al., 2018) and global warming (Doblas-Miranda et al., 2017). ...
Article
In the Mediterranean Basin, wildland fires are major drivers of forest ecosystem dynamics. In the current context of global change, these fires are becoming more severe and recurrent because of climatic conditions, land use changes and invasive species. In areas affected by mega-fires (burned area > 10,000 ha), the patterns of regeneration may be heterogeneous due to local variations in fire regime, community composition and environmental features. The goal of this study was to analyze the post-fire structure of both Pinus pinaster Aiton. seedlings population and understory community in a Mediterranean fire-prone ecosystem at short-term by means of high spatial resolution satellite imagery within the perimeter of a full stand replacing mega-fire that burned around 12,000 ha of a Pinus pinaster forest in NW Spain. We established 234 field plots of 2 × 2 meters to cover four recurrence-severity scenarios. In each plot, we sampled 15 vegetation structural variables at both pine seedlings population and understory community levels. From the WorldView-2 satellite imagery, we obtained three sets of spectral variables (reflectance, spectral indices and image textures) that were used as predictors of vegetation recovery in generalized linear models. At population level, the number and cover of pine seedlings were successfully modeled with spectral indices and textural information (normalized root mean square error of 16% and 17%, respectively). At understory community level, woody species cover was correlated with first order textures (normalized root mean square error of 9%). Other understory structure variables (height and richness of woody species, percentage of bare soil, necromass and leaves) were predicted with an error lower than 20%. The predictive capacity of the models was similar for all recurrence-severity scenarios. Our results highlight the usefulness of spectral indices and textural data at high spatial resolution in the analysis of post-fire recovery in large and heterogeneous burnt areas. Given the accuracy and predictive capacity of the models obtained in this study, high spatial resolution satellite imagery together with field data provide useful information in post-fire decision making in fire prone ecosystems.
... Present day research has identified several manifestations of extreme fire behaviour events (e.g. Viegas and Simeoni, 2011;Werth et al., 2011;Viegas et al., 2012;Werth et al., 2016;Raposo et al., 2017;Pinto et al., 2017), but none of them is incorporated in current simulators. The more detailed are the demands on fire simulation outputs, the more detailed are the demands on input data. ...
Article
The incident wind velocity in a forest fire is one of the main factors affecting fire spread. Although buoyancy due to the fire heat release modifies the wind field, the standard procedure for fire simulation takes the undisturbed wind field, computed in the absence of the fire thermal effects. The present work addresses this problem, by using a method to take into account the mutual interaction (two-way coupling) between fire and wind. This study is based on the FireStation system, a software package that combines a semi-empirical fire spread model with a Navier-Stokes solver for wind calculation, using a dynamic interchange between the fire spread model and the wind model. The present work describes the application of this model to a real wildfire case, the two-way coupling approach proving to be a better option, with predictions of fire size and shape in closer agreement with the observed ones.
... Moreover, two spikes in heat flux were observed. As shown in Figure 12, a fire whirl (Pinto et al., 2017) formed behind the vehicle at about 70 min 45 s and moved toward the heat flux gauge H 3 . Although the fire whirl dissipated on the way, it still affected the data curves. ...
Article
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Fire accidents constitute a significant safety concern for automotive lithium-ion battery packs and have impeded the development of electric vehicles (EVs). While fire safety concerns have been raised about EVs, their fire performance remains unknown, especially for plug-in hybrid electric vehicles (PHEVs). Hence, this paper conducted full-scale fire experiments of PHEVs to explore their fire behavior and characteristics. Two brand new PHEVs were employed, and their power battery packs were ignited as the origin of the fire to simulate the representative fire scenario. Results showed that visible flames appeared around the chassis after about 60 min of the experimental procedure. Around the fire emerged, the battery packs intermittently released plenty of white smoke, which induced gas-phase explosions. The main component of the smoke was combustible gases. The SUV-type PHEV test took 9 min 11 s for the chassis flames to evolve into a passenger compartment fire. Due to the slow propagation of the fire in sedan-type PHEV, it required 9 min 56 s for flames to engulf the rear part of the sedan. The maximum temperature of PHEV fires was 843.6°C, while the maximum height of the fire reached around 3 m. At a distance of 1 m, the radiative heat emitted from burning PHEVs peaked at 1.151 kW/m2. Moreover, some secondary hazards of PHEV fires were illustrated. These results stimulate future experiments seeking novel flame retardant materials for PHEVs and provide helpful guidance on screening reliable PHEV fire prediction and protection strategies.
... A grande intensidade do fogo promove a turbulência do ambiente vizinho e, em circunstâncias especiais pode levar à formação de estruturas de fogo com vorticidade, vulgarmente designadas por "turbilhões de fogo" (Figura 100), com um tempo de duração que pode chegar a vários minutos e a comprimentos que podem chegar às dezenas de metros. O combate a estas estruturas é extremamente difícil, não apenas porque emitem muito calor, ignificando a área envolvente por radiação, mas também porque têm movimento associado, pondo em risco a segurança de quem estiver na sua proximidade (Pinto, et al. 2016;Werth, et al., 2016). Para além disso, turbilhões de fogo tem a capacidade de sugar a matéria combustível que se encontra no solo, muitas vezes arrancando-a literalmente do solo. ...
... It depends on the flux ratio of the angular momentum which generates the rotation by the flux of axial momentum generated by the buoyancy forces. According to previous studies, several researchers have concentrated to study the behavior of a rotating flame by forced and natural ventilation (Pinto et al. (2017) Thus, we can cite the work of Lei et al.2013Lei et al. , 2015b, who established experimentally a correlation linking the heating power, the flame height and temperature. Moreover, in a rotating fire cases they indicated that the rotating flame is characterized by a significant instability by noting that the entire flame is inclined and rotates about the symmetry axis with the increase of the angular velocity. ...
... The fuel burning rate, flame height, temperature, and velocity as a function of imposed circulation were measured in rotating screen facilities [5][6][7] . On the other hand, many researchers used fixed-frame facilities of various scales to perform measurements on the thermal or/and velocity fields by using invasive measurement methods and particle image velocimetry (PIV) [8][9][10][11][12][13][14][15][16][17] . These works mainly focused on the quasi-steady combustion characteristics of fire whirls and aimed at developing the burning rate, flame height, and radiation models. ...
Article
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This paper presents an experimental study on the flame and flow characteristics of the lifted flame in a fire whirl. A small-scale rotating screen facility was used for experiments with finely controlled imposed circulation (Γ) and heat release rate (Q˙=2.0−8.0kW), by using methane as the fuel. The instantaneous three-dimensional velocity fields in the vertical plane passing through the flame centerline were measured by stereo particle image velocimetry (SPIV) technique. It is found that the flame of fire whirl can be lifted off the burner under relatively weak Γ, for which, the lifted flame is mainly blue partially premixed flame under a certain lower heat release rate (Q˙=2.0kW in this work), while when the heat release rate is beyond a certain value (Q˙≥4.0kW in this work), the lifted flame is structured by blue partially premixed flame at the bottom and yellow diffusion flame downstream. For these two different lifted flames, the flame length and the flame base diameter have significantly different dependence on Γ. Experimental data indicate that under a specified Q˙, the flame liftoff height firstly increases slowly and then decreases quickly with increasing Γ after reaching the maximum value. The maximum liftoff height decreases with Q˙ slightly and Γ dominates the variation of liftoff height. It is also found that the instantaneous axial velocity in the lifted fire whirl increases quickly in the bottom inflow layer, then decreases with height upstream of the flame base and increases again in the hot flame region. The flow field diverges as the instantaneous flame base is approached. The mean axial velocity conditioned on the instantaneous lifted flame base generally increases with Q˙ and is less than two times the maximum laminar flame speed. The lifted fire whirl and the triple flame share similar characteristics of the flame front and velocity fields.
... Wang et al. [79] (2017) ...
Article
Full-text available
Fire whirls are a particular case of flame behaviour characterized by a rotating column of fire driven by intense convective heating of air close to the ground. They typically result in a substantial increase in burning rate, temperature, and flame height. Fire whirls can occur in any intense flame environment, including urban areas, particularly within combustible structures, and in wildland or forest fires. Recently, investigations on the creation of fire whirls have attracted much attention. However, most analyses are focused on fire whirl structure, formation, and controlling their unique state. In effect, revisiting the available experimental techniques and numerical simulations used in analyzing fire whirls has received less attention. In this paper, experimental arrangements including empirical set ups and employed fuels are presented in detail. Subsequently, major research progress focused on experimental studies and their laboratory setup is fully discussed, followed by the available numerical simulations, including combustion and turbulence models. Applied methodologies and chosen software in the recent numerical studies are also reviewed exclusively. Finally, the latest findings are featured, and prospective pathways are advised.
... Sudden changes of fire behavior consist of unpredictable variations of fire intensity, erratic ROS and direction, spotting, and occurrence of fire-caused winds. Sudden changes of fire behavior are a significant safety danger to firefighters [120,121] and to population and assets, and can overwhelm suppression efforts. ...
Article
Full-text available
Every year worldwide some extraordinary wildfires occur, overwhelming suppression capabilities, causing substantial damages, and often resulting in fatalities. Given their increasing frequency, there is a debate about how to address these wildfires with significant social impacts, but there is no agreement upon terminology to describe them. The concept of extreme wildfire event (EWE) has emerged to bring some coherence on this kind of events. It is increasingly used, often as a synonym of other terms related to wildfires of high intensity and size, but its definition remains elusive. The goal of this paper is to go beyond drawing on distinct disciplinary perspectives to develop a holistic view of EWE as a social-ecological phenomenon. Based on literature review and using a transdisciplinary approach, this paper proposes a definition of EWE as a process and an outcome. Considering the lack of a consistent "scale of gravity" to leverage extreme wildfire events such as in natural hazards (e.g., tornados, hurricanes and earthquakes) we present a proposal of wildfire classification with seven categories based on measurable fire spread and behavior parameters and suppression difficulty. The categories 5 to 7 are labeled as EWE.
... Forest fires are disturbances associated with significant social, economic and environmental impacts (Nunes et al., 2016;Pinto et al., 2017;Hong et al., 2018). In fire-prone forest ecosystems of the Mediterranean Basin, many authors have identified an increase in fire regime parameters, such as recurrence and severity, in large forest fires (Pausas and Keely, 2009;Ferreira-Leite et al., 2011;Bento-Gonçalves et al., 2012;Quintano et al., 2015), mainly as a consequence of land use change (Pereg et al., 2018) and global warming (Doblas-Miranda et al., 2017). ...
Article
Forest managers require reliable tools to evaluate post-fire recovery across different geographic/climatic contexts and define management actions at the landscape scale, which might be highly resource-consuming in terms of data collection. In this sense, remote sensing techniques allow for gathering environmental data over large areas with low collection effort. We aim to assess the applicability of remote sensing tools in post-fire management within and across three mega-fires that occurred in pine fire-prone ecosystems located along an Atlantic-Transition-Mediterranean climatic gradient. Four years after the wildfires, we established 120 2x2m plots in each mega-fire site, where we evaluated: (1) density of pine seedlings, (2) percentage of woody species cover and (3) percentage of dead plant material cover. These variables were modeled following a Bayesian Model Averaging approach on the basis of spectral indices and texture features derived from WorldView-2 satellite imagery at 2 m spatial resolution. We assessed model interpolation and transferability within each mega-fire, as well as model extrapolation between mega-fires along the climatic gradient. Texture features were the predictors that contributed most in all cases. The woody species cover model had the best performance regarding spatial interpolation and transferability within the three study sites, with predictive errors lower than 25% for the two approaches. Model extrapolation between the Transition and Mediterranean sites had low levels of error (from 6% to 19%) for the three field variables, because the landscape in these areas is similar in structure and function and, therefore, in spectral characteristics. However, model extrapolation from the Atlantic site achieved the weakest results (error higher than 30%), due to the large ecological differences between this particular site and the others. This study demonstrates the potential of fine-grained satellite imagery for land managers to conduct post-fire recovery studies with a high degree of generality across different geographic/climatic contexts.
Article
A method for mathematically solving the oscillatory infinite reaction rate diffusion flame is extended to the case where the oscillating convective coflow is either inside of, or adjacent to, a viscous vortex. The neglect of streamwise diffusion coupled with the restriction to infinite rate chemistry produces a Burke-Schumann boundary layer flame. The mathematical transformation, which does not require a priori restriction to small coflow oscillations, renders the transient oscillatory problem equivalent to a steady-state problem that can be solved mathematically and numerically evaluated to a high degree of accuracy. Flow fluctuations that are large fractions of the initial flow field are described exactly. Features of the flame response are examined without recourse to detailed time-dependent numerical simulations. There is no need to perform any small-perturbation analyses. The method is applied to examine the influences of the bulk inflow speed, the bulk inflow oscillation rate, and the interaction of the inflow and its oscillation rate with the viscous vortex.
Article
Fire whirl is frequently observed in wildland fires, and may cause serious difficulty in firefighting owing to its significant turbulent flow. In this paper, the radiation of fire whirl is investigated through experiments using a fire whirl facility made up of an air curtain apparatus, with five different sizes of n-heptane pools (25, 30, 35, 40 and 45 cm). The flame contour was extracted by image processing. By using infrared methods, the flame emissivity of fire whirl at different heights for different pool diameters was measured, and thereby a correlation was developed between the flame emissivity and the flame diameter. The soot volume fraction in the luminous flame is estimated to range within 2.5 × 10⁻⁶ to 4.0 × 10⁻⁶, much higher than that of general heptane pool fires, which provides an explanation of the higher flame emissivity of fire whirl. The emissive power profile v. normalised height is deduced from flame emissivity and flame temperature data. A multizone flame model (in which each zone is assumed as a grey body) is used, based on the measured data of flame emissivity, to predict the radiation of fire whirl. Comparison between the predicted and measured data of radiative flux shows good agreement.
Article
Wildland fires are frequent events worldwide, particularly in the European-Mediterranean region, USA, and Australia. These fires have been more frequent and intense in recent years due to climate changes and may cause significant damage, especially when reaching the Wildland-Urban Interface (WUI) areas. The presence of liquefied petroleum gas (LPG) cylinders may cause severe events in WUI areas, as occurred in Portugal during the large wildfires of 2017, which could have been avoided if the cylinders were protected. Devices for protecting the parts of houses under WUI fire were previously presented, but a protective device for cylinders was not. In this work, a protective device for LPG cylinders made with a thin fabric with an aluminum coating on the external face was tested in laboratory and field conditions. The cylinder and the fabric were equipped with thermocouples and heat flux sensors attached to their surfaces. The tests showed that the device gave effective protection to the cylinder, decreasing the radiative heat flux that reaches it and keeping it in a safe condition when exposed to a fire; consequently preventing extreme behavior such as an explosion.
Article
This work presents a numerical study of laboratory fire whirls. The Reynolds Stress Transport turbulence model RST is used together with Eddy Break-Up combustion and radiation models. The unsteady 3D fire whirl prediction assume the same swirl generator used in the laboratory consisting on enclosure walls to constrict airflow to enter tangentially in the facility. A low momentum fuel jet is located at the bottom centre of two halves of an offset hollow cylinder. A detailed comparison between available experimental data and numerical predictions, ranging from 2 to 300 kW is presented. The results are in good agreement with the available experimental data. The flame height dependence on circulation was deduced using scaling laws and validated with experimental data. For each heat release considered, a critical circulation was found that results in a maximum flame height. Turbulence fields predictions are discussed in light of the role of turbulent suppression and Richardson parameters.
Article
The dynamics of wood crib fires were investigated under fire whirl (FW) and free burning (FB) conditions in a small-scale apparatus. For open-packed cribs, the burning rates and fire spread rates of the FB and FW cribs were almost identical. However, as the number of sticks per layer (n) increased from 3 to 7, the burning rate for FW experiments increased 1.0–1.3 times that of the FB cribs and the spread rate for FW experiments was 0.92–1.66 times that of the FB cribs for the same geometric configuration. Likewise, for open-packed cribs the average flame height of the FB and FW cribs were very similar, but as n increased the average flame height for FW tests increased from 1.0 to 1.69 times that of FB cribs for the same configuration, which was almost the same increase as that for the fire spread rates. Temperature measurements along the centreline above the crib showed distinctly different profiles for FB and FW crib fires. The ratio of the absolute temperature in the continuous flame region of a FW to that of a FB crib of the same configuration increased from 0.99 to 1.2 as n increased.
Chapter
The chapter presents the results of experimental-analytical modeling of the surface forest fire dynamics and the process of forest fuel ignition when exposed to thermal radiation from the fire line. The regularities are established for the occurrence and spread of fires in natural ecosystems of the temperate climatic zone. Analytical solutions have been obtained that make it possible to predict the level of heat load on the soil cover of coniferous stands. The special computer program has been developed to calculate the heat load during fires. The methods of field and laboratory modeling revealed patterns of forest fuel heating and ignition depending on moisture content. A practice-oriented method is proposed to calculate the width of fire barriers that limit the spread of forest fires. The methods for creating fire barriers are proposed.
Article
The paper first reviews the mode of generation of fire whirls, their properties, and operational regimes, under well-controlled experimental conditions. The situation is different with wildfires. These are uncontrolled and less well understood. A modified analytical approach is described for these conditions. This is based on global energy levels, per unit ground area, for different fuels, and their associated rates of fire spread, under different conditions. These enable regimes of possible fire whirl generation in wildfires to be defined. The associated accuracy is not that of a laboratory flame, but it is probably practically sufficient. Only in two instances were atmospheric fires experimentally controlled. In the remainder, the rate of fire spread was a more meaningful parameter than a burning velocity, and the critical lateral velocity was seldom known accurately. Nevertheless, it was possible to relate it to the heat release rate, with both in dimensionless form.
Article
Strong winds generated by fire-induced whirls without flames cause destructive damage. Laboratory experiments were conducted to clarify how the strength and size of this type of fire whirl are influenced by the wind speed, heat release rate (HRR), and fire source direction. This study uses a long narrow fire source placed parallel to the wind (parallel arrangement); the results were compared with those of previous experiments where the fire source was placed perpendicular to the wind (perpendicular arrangement). It was found that the direction of the fire source significantly impacts the circulation and radius of the fire whirl. The absolute value of the circulation (|Γ|) for the parallel arrangement is smaller than that for the perpendicular arrangement. The radius has the same tendency. |Γ| for the parallel arrangement tends to increase with increasing wind speed, which is the opposite of the tendency for the perpendicular arrangement. When the HRR increases, |Γ| for the parallel arrangement changes little, although |Γ| for the perpendicular arrangement increases. The results of both arrangements show that fire-induced whirls with large |Γ| and large radii, that is, large intense whirls, occur under low wind speeds and large HRRs in perpendicular arrangements downwind of the fire.
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This paper is aimed to demonstrate an opportunity of the generation of nonstationary wall-free fire whirls under laboratory conditions without using mechanical swirling devices and to estimate their integral parameters. A simple experimental facility, making possible the generation of concentrated fire vortex structures by means of combustion of solid fuel (urotropine) arranged symmetrically on a metallic underlying surface, is described. With the use of photography, some novel data on the probability of generation of fire whirls depending on the experimental mode have been obtained.
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In this paper, the effect of flow circulation on the combustion dynamics of fire whirl is systematically investigated by experiments. New correlations for the burning rate, flame height, radial temperature and mass flow rate are established for fire whirl. It is clarified that flow circulation helps increase both the fuel-flame contact area and the actual fuel surface area, which in turn increases both the heat feedback to the fuel surface and the radial velocity in the ground boundary layer, leading to increase of burning rate. A novel idea for correlation of fire whirl flame height is proposed by assuming that the ratio of the fire whirl flame height to the flame height without circulation solely characterizes the effect of circulation. This idea is fully verified, thereby a new formulation for flame height is established, which successfully decouples the burning rate and the circulation. It is indicated that the fuel-rich core in the flame body of fire whirl significantly affects the radial temperature distribution in the continuous flame region, and the flame body can be described by the combination of a cylinder and a cone. The flow circulation significantly suppresses fire plume radius and thus decreases its increasing rate with vertical distance. It is also demonstrated that the fire whirl flame involves laminarized regions in its lower section, coexisting with turbulent regions in the upper portion. The flow circulation enhances the air entrainment in the ground layer by altering the radial velocity profile and increasing the radial velocity. In the low section of flaming region, the significant decrease of mixture between the combustion products and surrounding air dominates the pure aerodynamic effect of flow circulation on the flame height. Finally, it is clarified that fire whirls maintain higher centerline excess temperature than general pool fires due to the effect of less air entrainment.
Article
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Whirling fire plumes are known to increase the danger of naturally occurring or post-disaster fires. In order for a fire whirl to exist, there must be an organized source of angular momentum to produce the large swirl velocities as air is entrained into the fire plume. These vorticity-driven fires occur over a large range of length and velocity scales, and significantly alter the entrainment and combustion dynamics. A new model is derived for a buoyant plume that incorporates angular rotation and neglects dissipation; the result is a form of the steady state Euler equations. Included is a general solution for large density and temperature variations. Results are presented that identify the mechanisms and their effects toward creating a fire whirl.
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A numerical investigation of swirling fire plumes is pursued to understand how swirl alters the plume dynamics and combustion. One example is the 'fire whirl' which is known to arise naturally during forest fires. This buoyancy-driven fire plume entrains ambient fluid as heated gases rise. Vorticity associated with a mechanism such as wind shear can be concentrated by the fire, creating a vortex core along the axis of the plume. The result is a whirling fire. The current approach considers the relationship between buoyancy and swirl using a configuration based on fixing the heat release rate of the fire and imposing circulation. Large-eddy methodologies are used in the numerical analyses. Results indicate that the structure of the fire plume is significantly altered when angular momentum is imparted to the ambient fluid. The vertical acceleration induced by buoyancy generates strain fields which stretch out the flames as they wrap around the nominal plume centreline. The whirling fire constricts radially and stretches the plume vertically.
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Vortices are almost always present in the wildland fire environment and can sometimes interact with the fire in unpredictable ways, causing extreme fire behavior and safety concerns. In this paper, the current state of knowledge of the interaction of wildland fire and vortices is examined and reviewed. A basic introduction to vorticity is given, and the two common vortex forms in wildland fire are analyzed: fire whirls and horizontal roll vortices. Attention is given to mechanisms of formation and growth and how this information can be used by firefighters.
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The concept of extreme forest fire behaviour is analyzed and defined as the set of forest fire spread characteristics and properties that preclude the possibility of controlling it safely using available present day technical resources and knowledge. Using some basic properties of fire behaviour, the following expressions or manifestations of extreme fire behaviour are characterized and described: (i) Conflagrations that are very large fires which spread in extreme weather conditions of low humidity and strong wind; (ii) Eruptive fires that occur usually in canyons or steep slopes and are characterized by a quick acceleration of the head fire rate of spread; (iii) Crown fires consisting on the transition and spread of surface to canopy fuel layer with very high rates of energy release; (iv) Spot fires produced by burning particles that are transported by the fire plume and wind flow and can produce new ignitions while landing at places that are difficult to predict; (v) Jump fires that are associated to the merging of fire fronts making a small angle between them producing very high rates of spread and with the potential to generate fire whirls and tornadoes and (vi) Vortex structures that can be generated by boundary conditions or discontinuities in the flow near the fire and can cause situations very difficult to control.
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A fire whirl, observed during a wildland fire in Brazil in 2010, occurred over a narrow but long line fire and moved along the line fire at nearly a constant speed. There appeared to be no mountains, tall buildings or trees near the scene, indicating that the fire whirl was generated merely by the interaction between the line fire and background wind. Scale-model experiments having different line fire configurations were designed and performed to reconstruct the above-mentioned Brazil fire whirl. Moving fire whirls were successfully reconstructed during the scale-model experiments, the mechanism and conditions of which are discussed herein.
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This paper presents the first experimental effort to explore the large scale 3-D flame instabilities of fire whirls, including the inclined flame revolution during the transition from a general pool fire to fire whirl, and the swirling flame precession in a quasi-steady fire whirl. The experimental medium-scale fire whirls were produced by a fixed-frame facility. Experimental observations indicate that flame revolution is an important flame instability during the formation of fire whirl, showing that the entire flame is inclined and revolves around the geometrical axis of symmetry with increasing angular velocity until the critical point, without the self-rotation of the flame. It is found that the inlet velocity fluctuates synchronously with the flame revolution. As soon as the fire whirl forms, the erect swirling flame starts to precess around the geometrical axis of symmetry. Analysis indicates that during flame precession the periodic fluctuations of inlet velocity disappear and a local annular external recirculation zone (ERZ) is produced outside the flame (vortex core), while the flow is upward inside. It is found that the inlet velocities are nearly constant within the continuous flame in order to maintain a stable generating eddy. A good linear correlation exists between the average inlet velocities and average ambient circulations for all fuel pan sizes. The precession frequency is relatively stable during one test. The frequencies of flame revolution and precession are both proportional to the average inlet velocity, and the corresponding Strouhal numbers are constants of 0.42 and 0.80, respectively. The flame revolves and precesses in the same direction as the self-rotation of the fire whirl flame in all tests. The flame revolution is related to the periodical fluctuations of inlet flow, while the flame precession is considered to be linked to the occurrence of ERZ in fire whirls.
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Among all the design fire scenarios studied for the purpose of satisfying the current-generation engineering performance-based fire codes, one important scenario for fire hazards in building compartments that has not received much attention deals with multiple fires occurring in a single compartment. Under certain commonly-encountered conditions, such multiple fires merge into an almost continuously rotating fire whirl close to the confining walls near the floor, which is highly destructive. In the present study, experiments and numerical simulations have been carried out for a rectangular walled open-ceiling enclosure with a single corner gap and four equi-distant fires in a 2 x 2 array on the enclosure floor, simulating a room-fire scenario with horizontal fire whirls. Results of the study are discussed in conjunction with the relevant engineering performance-based fire design scenarios.
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This paper presents the first comprehensive experimental study on the dynamical behaviors of small scale buoyant pool fires under imposed circulation before the formation of fire whirls. A rotating screen facility was used for experiments. It was found that as the screen rotated slowly, the original vertical buoyant pool fire inclined with a certain angle (α) and revolved around the facility central axis steadily. The deviation of the flow field from axial symmetry is proved to be induced by the additional Coriolis force in the rotating coordinates. A correlation is proposed to couple the tangent of inclination angle to the screen angular speed and fire characteristics. The correlation agrees well with the experimental data in this work. The flame revolution was entirely dominated by aerodynamics and its angular speed was strictly equal to that of the rotating screen. It was found that the inclined flame surface was steady at the base and the oscillation was originated at a higher position due to suppression of vorticity generation. The nominal averaged pulsation frequency of the inclined flame was always larger than that of the free buoyant fire. The flame inclination and the change of pulsation mode can affect the flame heights and lengths significantly. At a critical screen angular speed, the inclined flame became vertical again with self-swirling and the fire whirl formed finally.
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One of the formation mechanisms of fire whirls, which often occur in a cross flow downwind of a fire, was studied experimentally in a wind tunnel using a flow visualisation technique. We observed that vortices forming along and just downwind of a flame were occasionally shed downwind. Such vortices differed markedly from wake vortices, which occurred downwind of the flame and exhibited periodic shedding behaviour in the downwind direction. We demonstrated that these vortices were a counter-rotating vortex pair (CVP) of the plume of the flame. This result suggests that the CVP of a large fire is a possible origin of the fire whirls that occur downwind of a fire area and are shed downwind.
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A fire whirl from a liquid-fuel pool (acetone) is formed at the center of a rotating screen which imparts a controlled angular momentum to the ambient air. Measurements show that outside of the core the fluid motion is that of a free vortex. A hot-wire method of measurement of the radial temperature distribution is developed. The temperature distribution shows that the fire whirl consists of a rotating cylinder, fuel rich inside, lean outside. The turbulent plume theory is extended to include combustion and angular momentum. This theory checks the experimental results and provides: 1.The turbulent mixing coefficient decreases with increasing angular momentum, as is to be expected.2.The turbulent mixing coefficient increases with elevation above the ground. This effect was not expected. Its cause remains unknown, although it may be in some way related to the vertex-jump (vortex breakdown) phenomena which may be required if the whirl is to satisfy both ground-level and “high”-altitude boundary conditions.
Article
This paper presents semi-empirical investigations on the quasi-steady burning rates of laminar and turbulent fire whirls established over liquid fuel pools. The inflow boundary layer above the fuel surface consists of two regions: outer reactive region and inner non-reactive region. Based on the momentum boundary layer solutions with the applications of stagnant film model and Chilton–Colburn analogy, the burning rates are correlated with ambient circulation and pool size for laminar and turbulent fire whirls respectively. It is shown that in general pool fires the mass and heat transfers on the fuel surface are controlled by natural convection, while in fire whirls they are strongly enhanced by forced convection. Fuel evaporation rate in the outer region is relatively larger than that in the inner region. The large proportion of fuel evaporated from the outer region is mainly due to its larger area. The predictions agree well with the data from the present experiments and the literature. Furthermore, the flame height is confirmed to be proportional to the ambient circulation for both laminar and turbulent liquid fire whirls.
Article
Burning rates of Douglas fir wood were measured using crosspiled sticks 1/4, 1/2, 3/4, and 1 in. (0.64, 1.27, 1.90, and 2.54 cm) in cross-sectional dimensions. Burning was 1.4 to 4.2 times as fast with the whirlwind as without.
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This paper adds vortex flows around Burke–Schumann diffusion flames to predict the flame heights and the flame shapes of small fire whirls. The resulting model matches the measurements of methanol flames in a previous laboratory experiment and the results of numerical calculations in this paper. Burgers Vortex is assumed inside the vortex core radius, while ideal flow is assumed outside the vortex core radius. The ideal flow is corrected for the viscosity changes inside and outside the flame. If the two vortices are combined, they can be approximated as a Sullivan Vortex. Both the experiments and the numerical calculations show that vortex flows stabilize the flame shape, allowing the flame height as defined in a regular diffusion flame to increase. In fact, regular diffusion flames chop off unburned fuel to form separate plumes. With vortex flow, the flame stretches as if the diffusion rate had been reduced. We adjust Roper’s flame height equation to account for the vortex flow and find that the flame height depends on the volume fuel rate and the vortex core radius. If more flows than that required to stabilize the flame were supplied, the radial flows start reducing the flame diameter near the pan, which in turn is balanced by an increase in the volume fuel rate. In the experiment, a balance between the flame temperature, the volume fuel rate, and the flame shape explains why the flame height stops increasing with vortex flows after a fire whirl is generated. In the numerical calculations, we find that the temperature gradient above the port, which controls the fuel evaporation rate, increases with the vortex flows.
Article
The medium-scale fire whirl was extensively investigated by experimental means, in order to establish correlations of the burning rate, flame height and flame temperature of fire whirl, and to clarify the difference between fire whirls and general pool fires. Experimental observations and data confirmed that a free burning fire whirl is a highly stable burning phenomenon with large quasi-steady periods. Burning rates of fire whirls depend on pool diameter similarly to those of general pool fires; however the transition turbulent burning occurs sooner as the pool diameter increases. The lip height seems to have little effect on the burning rate of fire whirls. The correlation H∗=K·(Q˙∗·Γ∗2)m was proposed to couple the height of fire whirl to the fire release rate and ambient circulation. It correlates the data from both this work and the literature. Radial temperature profiles in the continuous region of the fire whirl were confirmed to be hump-type, implying the existence of fuel-rich inner core. The pool diameter and heat release rate do not significantly affect the radial temperature profiles in non-dimensional radial coordinates. It was found that the fire plume of fire whirl involves three distinct zones just like that of pool fire, but with different normalized ranges. Fire whirls maintain a higher ratio of continuous flame height to the overall flame height, and also higher maximum centerline excess temperature in continuous flame region, as compared to general pool fires. It was further demonstrated that the fire whirl plume at its origin behaves like a turbulent jet with moderate swirling, and then tends to become buoyancy dominated downstream, with slight swirling. With an increase in dimensionless height adjusted by the plume origin, the plume centerline excess temperature decays rapidly and approaches the theoretical value of −5/3 for free buoyancy plume.
Article
This work was motivated from the knowledge of a historical fire whirl accident in which a fire whirl was responsible for 38,000 deaths and serious damage to the environment, all of which happened within 15 minutes. To understand the general features of fire whirls, records of example fire whirls were collected and deduced to be of three different types. Scaling laws were derived using dimensional analysis; three types of scale-models, with scaling ratios of , , and , corresponding to three actual fire whirls (prototypes), were designed in the laboratory. Wind tunnel fire experiments were performed burning methanol as the fire source and by changing the wind velocity to understand conditions under which fire whirls can occur. The model fire whirls reconstructed in the laboratory were visualized with water vapor; and profiles of velocity and temperature were, respectively, measured by a hot wire anemometer and a fine thermocouple. Because the dynamic structure of prototype fire whirls is not known due to lack of direct measurements in actual flows, a large scale-model with scaling ratio of to the prototype was designed to be studied in an open field. This model consisted of 85 circular pans filled with 2.3 m3 of methanol that were distributed in an area of 400 m2. Air movement was visualized by colored smoke. Wind velocity and direction were measured at five different locations and air temperatures at nine different locations. It was found that the visually determined whirl diameter and whirl column height, and the maximum tangential velocity of the whirl can be correlated to the scale model by the proposed scaling laws.
Article
An analytical model was developed of the radially averaged fluid mechanical properties of the fire whirl core, including the coupling between the fire whirl core flow, the ground boundary layer, and the fuel heat transfer. A parameter related to the ground boundary layer radial and tangential velocity profiles, necessary for obtaining solutions from the analytical model, was obtained from correlation of the model with extensive measurements of the fluid mechanical properties of a subscale fire whirl. The fire whirls were created by burning either acetone or natural wood fuels at the center of the bottom of a large vortex generating device. The temperature, core radius, and tangential and vertical core velocity behavior predicted by the model were reasonably verified by the experiments. Extension of the model to full scale fire whirls was made. The effect of the change in size scale was accounted for by utilizing the mean core temperature as a correlation variable and including dependence on the vertical extent of the ambient circulation in the temperature correlation. Calculations for full scale fire whirls were made which show physically plausible results and trends for the fire whirl fluid mechanical properties with varying ambient circulation and fuel characteristics. The model depends on a minimum of inputs and is easy to apply.
Article
This paper discusses why the visibly-determined flame length of a weak fire whirl increases as compared with the corresponding pool fire without spin. Here, a fire whirl is called weak when the pure aerodynamic effect of flow circulation has a negligible influence on the flame length. Split cylinders were used to apply a flow circulation to a 3-cm-diameter methane burner flame and a 3-cm-diameter ethanol pool fire. After applying the flow circulation, the flame length of the ethanol pool fire increased about three times, while little change was observed in the flame length of the methane burner flame. The difference is explained by the fact that the burning rate of the methane burner flame was fixed constant, whereas that of the ethanol pool fire increased due to the increased heat input to the fuel surface caused by a change in flame shape pushed toward the fuel surface. The experimental observations thus demonstrate that the burning-rate effect can significantly increase the flame length even under a weak circulation condition. Computational fluid dynamics (CFD) simulations were conducted to understand the detailed flow structure of a fire whirl. An analytical model was then developed based on the experimental observations and CFD calculations; the predicted relationship between the flame height and the burning rate agreed with experimental data.
Article
Air entrainment, leading to strong fire whirls, is commonly thought to be caused by the buoyant rise of the hot combustion products under the influence of gravity. We have, however, created in the laboratory steady, axisymmetric strong fire whirls with axes inclined 30° from the vertical orientation, whirls which model an inclined fire whirl, about 30 m tall, observed in California wildland near the Cleveland National Forest. The results contradict the common notion of buoyancy being significant for the structure of the whirl, implying that strong fire whirls instead are dominated by rotation only, even if their axis is vertical. The new concept of rotation-controlled fire whirls is explained by a Rossby number displacing the Froude number (or the Richardson number) in describing the phenomenon.
Article
We conducted a series of laboratory-scale fire whirl experiments spinning 5-cm-diameter methanol pool fires and observed elongated flame height compared with the pool fire without spin. A simple scaling analysis was conducted to obtain dependency of the axial flame height on the momentum-controlled circulation and the effect of buoyancy. To obtain a specific functional relationship for the parameters obtained by the scaling analysis, we developed an analytical model consisting of coupled species and energy equations and Burgers vortex for circulation generated by a fire whirl. The solution of the coupling equations shows that the average rate of heat transfer from the flame to the fuel surface is a function of the vortex core radius; a smaller vortex core radius provides more heat to the fuel surface enhancing evaporation thereby producing the longer flame height. This new model predicts both flame height and flame shape. The flame height prediction compare favorably with results from the scaling analysis and experiment.
Article
Wildfire spread in living vegetation, such as chaparral in southern California, often causes significant damage to infrastructure and ecosystems. The effects of physical characteristics of fuels and fuel beds on live fuel burning and whether live fuels differ fundamentally from dead woody fuels in their burning characteristics are not well understood. Toward this end, three common chaparral fuels prevalent in southern California, chamise, manzanita, and ceanothus, were investigated by burning them in a cylindrical container. The observed fire behavior included mass loss rate, flame height, and temperature structure above the burning fuel bed. By using successive images of the temperature field, a recently developed thermal particle image velocity (TPIV) algorithm was applied to estimate flow velocities in the vicinity of the flame. A linear regression fit was used to explain the observed time difference between when maximum flame height and maximum mass loss rate occur, as a function of fuel moisture content. Two different methods were used to extract power laws for flame heights of live and dead fuels. It was observed that the parameters defined in the well-known two-fifths power law for flame height as a function of heat release rate were inadequate for live fuels. As the moisture content increases, the heat release rate in the power law needs to be calculated at the time when the maximum flame height is achieved, as opposed to the maximum mass loss rate. Dimensionless parameters were used to express local temperature and velocity structure of live and dead chaparral fuels in the form of a Gaussian profile over different regimes in a fire plume.
Article
A fire whirl in an open space can cause devastating damage as was experienced in Hifukusho-ato, Tokyo, after the Great Kanto Earthquake in 1921. To understand the generation mechanism of the open-space fire whirls, 1/1000th scale-model experiments were conducted in a large, low-speed open-loop wind tunnel. In analyzing the experiments, there was found to be a critical lateral wind velocity that generated intense fire whirls. A scaling law that predicts the critical wind velocity was developed and validated by various data including scale-model experiments by other researchers and real urban fire whirls. A dimensional analysis is conducted to understand the effect of flow circulation on the increase in flame height. The simple analysis was supported by the results of numerical simulations by other researchers.
Article
A new generation of models of a forest fire is currently being developed; they include more and more physical mechanisms. The main objective of the present study was to provide experimental measurements to test such models. An apparatus was designed to simulate and study in laboratory conditions the flame and the near-field plume stemming from the combustion of an isolated shrub. The burner was made of a cylindrical wire mesh basket filled with a forest fuel ignited at the lower circumference of the basket. Three diameters (20, 28, and 40 cm) for the basket and two kinds of fuel (Pinus pinaster needles and excelsior) were used. Temperatures were measured inside and above the burner using type K thermocouples of 50-μm diameter. Three pairs of these sensors were especially used to estimate the upward gas velocity, thanks to the expected cross-correlation of thermal fluctuations. We obtained varied and non-steady regimes of combustion as desired, in particular the maximum heat release rates ranged between 30 and 180 kW. The structure of the visible flame and its temperatures were analyzed for a fully developed flame. The height of a flame was found to scale with the heat release rate according to the usual two-fifths power law, which enables a normalized flame height to be defined. Vertical temperature profiles in the flame were found to scale with the normalized height and radial temperature profiles fitted Gaussian laws well. The determination of gas velocity was uncertain, but the usual scaling also applied to the measurements. Scaling rules, which apply well to either steady turbulent diffusion flames on a porous gas burner or pool fires, hold for the non-steady flame on our forest fuel burner, when observing a fully developed flame.
Causes and Behavior of a Tornadic Fire-Whirlwind
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