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The ‘Balbi model’ is a simplified rate of fire spread model aimed at providing computationally fast and accurate simulations of fire propagation that can be used by fire managers under operational conditions. This model describes the steady-state spread rate of surface fires by accounting for both radiation and convection heat transfer processes. In the present work the original Balbi model developed for laboratory conditions is improved by addressing specificities of outdoor fires, such as fuel complexes with a mix of live and dead materials, a larger scale and an open environment. The model is calibrated against a small training dataset (n = 25) of shrubland fires conducted in Turkey. A sensitivity analysis of model output is presented and its predictive capacity against a larger independent dataset of experimental fires in shrubland fuels from different regions of the world (Europe, Australia, New Zealand and South Africa) is tested. A comparison with older versions of the model and a generic empirical model is also conducted with encouraging results. The improved model remains physics-based, faster than real time and fully predictive.

International audience

International audience; Nowadays, the needs for decision making tools useful for people involved in firefighting and/or in landscape management becomes more and more crucial, especially with the dramatic increase of the fire dangerousness and fire severity. These tools have to be accurate enough and faster than real time. Up to now, simulators and other tools are mainly based on empirical or semi-empirical models but the lack of physics in their formulation is a major flaw. The Balbi model is a simplified physical propagation model for surface fires which explicitly depends on the topography, the wind velocity and several fuel characteristics. It is a set of algebraic equations built from usual physical conservation laws (mass, momentum etc.) with some strong assumptions. This work aims at providing a new version of the Balbi model in which the resolution of the rate of spread (ROS) does not need any iterative method any more. This simplification is helpful in implementing the equations set into a fire propagation simulator or a coupled fire-atmosphere simulator. It needs a complete change in the structure of the model and the predicted ROS was tested at the field scale against 179 shrubland fires (burnt in Australia, South Africa, Turkey, Portugal, Spain, New Zealand) and 178 Australian grassland fires with a very good agreement with the observed ROS. Two statistical tools are used to check this agreement (Normalized Mean Square Error, NMSE and Mean Absolute Percentage Error, MAPE) and the Fractional Bias (FB) aims at understanding when the model over-predicts or under-predicts the ROS. The proposed model is accurate and its model parameters are calibrated against a small training dataset which makes it fully predictive whatever the environmental and topographic conditions and the fuel bed characteristics. Its more simple structure allows it to be a good candidate for the heart of a simulation or land management decision making tool.

International audience; The GOLIAT project is a consortium of academics and firefighting operators and land-use planning professionals of Corsica. One goal of GOLIAT project is to provide four operational decision support tools. To reach this goal, a survey of past fires occurred in Corsica since the twentieth century beginning is made. This inventory contributes to build up a database with a web display interface easy to use as fire patterns history. A fire behavior and impact simulator prototype for vegetation fires, a geolocation tool for hot spots using UAV images, and a guide of good practices of prescribed fires in the undergrowth are building. At the same time, experimental fires are carried out to improve knowledge about high intensity fire and the experimental results were compared to the predictions provided by a complete physical 3D model, namely FireStar3D.

International audience; Eruptive fires are one category of extreme fire behaviour. They are characterized by a sudden and unpredictable change in the fire behaviour which represents an extreme danger for people involved in firefighting. The major point is about the mechanism that turns a usual fire behaviour into an eruptive fire behaviour. Among the different explanations found in the literature, the pioneering interpretation consisting in a feedback effect caused by the convective flow induced by the fire under wind and/or slope conditions, has never been disproved with an example of fire accident. The main goal of this work lies in proposing a physical modelling of this fire induced wind. This modelling attempt is derived from the brand-new version of the Balbi model, which is a simplified physical model for surface fires at the field scale that explicitly depends on the triangle of fire (fuel bed, wind and slope). This work is a first step to the modelling of fire eruption. The model tries to represent accurately the acceleration of the fire rate of spread propagating on different sloped terrain under no-wind or weak wind conditions. It is tested against three sets of experiments carried out at the laboratory scale without external wind and against a high intensity experimental fire spreading on a steep sloped terrain and conducted under weak wind conditions in the north-western of Corsica. Some statistical tools are used to compare predicted and observed rate of spread (NMSE, Normalized Mean Square Error and MAPE, Mean Absolute Percentage Error) and to understand the model’s under-predictions or over-predictions trends (FB, Fractional Bias).

Dimensioning a fuelbreak remains always a challenging problem. For a long time, this problem was tackled using an empirical approach from the experience of operational users such as the fire fighters and the foresters. During the last decades, new approaches coming from fire safety engineering have completed the set of tools adapted to study this problem. These tools are all based on physical considerations, more or- less sophisticated. The simplest ones, consist in assimilating the flame as a radiant panel, calculating the distribution of radiant heat flux as a function of the distance separating the flame to a potential target and defining at what distance this heat flux reached a critical threshold level susceptible to produce damages on this target (pain for people or ignition for materials). The most complex ones, consist in solving the conservation equations (mass, momentum, energy ...) governing the behaviour of complex coupled problem formed by the vegetation, the flame front and the surrounding atmosphere. This new generation of engineering tool, based on CFD approach allows to directly predict the behaviour of a fire front propagating toward a fuelbreak, in order to evaluate its efficiency as a function of the amount of surface fuel (grass, shrubs) removed to reduce locally the fuel load and therefore the intensity of an incoming fire. These two approaches are fully complementary, only the first one has the potentiality to be spread operationally on the field, whereas the second one can contribute to improve the first one and to study with more detail some very sensitive situations such as those encountered in the wildland urban interface (WUI). The main part of this study concerns numerical simulations of the propagation of a fire front through a homogeneous vegetation layer (a grassland) in the vicinity of a fuelbreak represented by a band more or less wide inside which all the fuel was removed. The simulations were performed using a fully physical wildfire model (FIRESTAR3D), three variable parameters were considered in this study: the 1m open wind speed (U1 ranged between 3 and 10 m/s), the fuel height (HFuel ranged between 0.25 and 1m) and the fuelbreak width (LFB). With these conditions, the simulations covered a large range of values of the Byram’s convective number NC (0.3 < NC < 60) in order to explore wind as well driven fires (NC < 2) and plume dominated fires (NC > 10). The 72 simulations carried out in this study have been classified in three categories: 1/ Propagation (if the fire has crossed the fuelbreak with a propagation after); 2/ Overshooting or Marginal (if the fire has crossed the fuelbreak without a propagation after); 3/ No-propagation (if the fuelbreak has stopped the fire). The main objective of this study was to determine the optimal fuelbreak width LFBx separating between the Propagation and the No-propagation regimes, in order to generalize the conclusion, the results have been presented in dimensionless form (similitude theory) in representing as an example the ratio LFBx/HFuel versus the Byram’s convective number NC.

The 3D structure of a fire front propagating through a homogeneous porous solid-fuel layer was studied numerically at laboratory and field scales. At laboratory scale, wind-tunnel fires propagating through laser-cut cardboard fuel were numerically reproduced, while at field scale, simulations of grassland fires with quasi-infinite fire front were carried out for different wind speeds. These simulations were performed using FIRESTAR3D, based on a multiphase formulation that includes the main physical phenomena governing fire behavior. An unsteady RANS approach and a Large Eddy Simulation (LES) approach were used to simulate the reactive turbulent flow, whereas turbulent combustion was modeled using Eddy Dissipation Concept (EDC). Unlike other 3D wildfire tools available in the community, such as FIRETEC and WFDS, the model is based on an implicit, low-Mach number resolution of the governing equations, and makes no empirical assumptions in the resolution of the radiative transfer equation. The comparison with the experimental data concerned mainly the Rate of Spread (ROS) of fire, the fireline intensity, the flame-zone depth, and the wavelength characterizing the crest-and-trough structure of the fire front along the transverse direction. Particular attention was drawn to the similitude in the fire front dynamics between small and large scales. In order to highlight the physical mechanisms responsible for this dynamics, a dimensional analysis was carried out by introducing Byram's convection number NC based on the fireline intensity and Froude's numbers Fr based on the characteristic wavelength of the fire-front structure. The analysis shows that all the results (wind-tunnel fires and grassland fires, experimental and numerical) collapsed on a single scaling law in the form F r = N C − 2 / 3 .

International audience; The effectiveness of a fuelbreak, created in a homogeneous grassland on a flat terrain, was studied numerically. The analysis relies on 3D numerical simulations that were performed using a detailed physical-fire-model (FIRESTAR3D) based on a multiphase formulation. To avoid border effects, calculations were carried out by imposing periodic boundary conditions along the two lateral sides of the computational domain, reproducing that way a quasi-infinitely long fire front. A total of 72 simulations were carried out for various wind speeds, fuel heights, and fuelbreak widths, which allowed to cover a large spectrum of fire behaviour, ranging from plume-dominated fires to wind-driven fires. The results were classified in three main categories: 1- “Propagation” if fire crossed the fuelbreak with a continuous fire front, 2- “Overshooting” and “Marginal” if fire marginally crosses the fuelbreak with the formation of burning pockets, and 3- “No propagation” if fire does not cross at all the fuelbreak. The ratio of fuelbreak width to fuel height, marking the “Propagation”/“No propagation” transition, was found to be scaled with Byram's convection number Nc as 75.07 × Nc−0.46. The numerical results were also compared to an operational wildfire engineering tool (DIMZAL) dedicated to fuelbreaks dimensioning.

International audience; This paper reported a high intensity experimental fire conducted during a field-scale experiment on a steep sloped terrain (28°) as part of a winter prescribed burns campaign managed by the local firefighter service in the northwestern region of Corsica. The rate of spread (ROS) of fire, measured using UAV cameras (thermal and visible), was evaluated at 0.45 m/s. The experiment was numerically reproduced using a completely physical 2D model, namely FireStar2D, and the comparison with the experimental measurements mainly concerned the fire ROS and the heat fluxes received by three distant targets placed at the end of the plot. The results analysis shows that the considered fire has a wind-driven regime of propagation with a fire intensity higher than 7 MW/m. The numerical results are in fairly good agreement with the experimental measurements, within 11% difference for the ROS and 5% for the heat fluxes, validating consequently the relevance of the numerical approach to tackle such high-intensity wildfires. Despite the unfavorable wind and humidity conditions for fire propagation (U = 1.67 m/s and RH = 82%), this experiment confirms that such fire can exhibit a dangerous behavior due to the steep slope of the terrain.

International audience; The safety during prescribed burnings could be achieved by conducting these operations under marginal conditions of fire propagation. This type of fire can or cannot propagate on account of small deviations of the burning conditions, mainly the wind speed, the fuel load, and the fuel moisture-content. In this context, numerical simulations of grassland fires were conducted under marginal conditions in order to relate the moisturecontent threshold of propagation success to the wind speed and the fuel load. The simulations were conducted using FireStar2D, a complete physical 2D fire simulator based on a multiphase modelling approach. The 10 mopen wind speed ranged from 0 to 10 m/s and the fuel load varied from 0.1 kg/m 2 to 0.7 kg/m 2. The effects of wind speed and fuel moisture-content on the fire behaviour and on the flame parameters are discussed. The results show that the moisture threshold increases with the fuel load until it reaches a value beyond which there is no dependence. A similar dependence of the moisture threshold on the wind speed is also observed. Finally, empirical formulae were constructed to relate the fuel moisture content threshold to the wind speed and the fuel loading implicitly through Byram's convective number.

International audience; The 'Balbi model' is a simplified steady-state physical propagation model for surface fires that considers radiative heat transfer from the surface area of burning fuel particles as well as from the flame body. In this work, a completely new version of this propagation model for wildand fires is proposed. Even if, in the present work, this model is confined to laboratory experiments, its purpose is to be used at a larger scale in the field under operational conditions. This model was constructed from a radiative propagation model with the addition of a convective heat transfer term resulting from the impingement of packets of hot reacting gases on unburnt fuel elements located at the base of the flame. The flame inside the fuel bed is seen as the 'fingers of fire' described in the literature. The proposed model is physics-based, faster than real time and fully predictive, which means that model parameters do not change from one experiment to another. The predicted rate of spread is applied to a large set of laboratory experiments (through homogeneous pine needles and excelsior fuel beds) and is compared with the predictions of both a very simple empirical model (Catchpole) and a detailed physical model (FireStar2D).

International audience; 12 This study reports 3D numerical simulations of the ignition and the propagation of 13 grassland fires. The mathematical model is based on a multiphase formulation and on a 14 homogenization approach that consists in averaging the conservation equations (mass, 15 momentum, energy …) governing the evolution of variables representing the state of the 16 vegetation/atmosphere system, inside a control volume containing both the solid-17 vegetation phase and the surrounding gaseous phase. This preliminary operation results 18 in the introduction of source/sink additional terms representing the interaction between 19 the gaseous phase and the solid-fuel particles. This study was conducted at large scale in 20 grassland because it represents the scale at which the behavior of the fire front presents 21 most similarities with full scale wildfires and also because of the existence of a large 22 number of relatively well controlled experiments performed in Australia and in the 23 United States. The simulations were performed for a tall grass, on a flat terrain, and for 24 six values of the 10-m open wind speed ranged between 1 and 12 m/s. The results are in 25 fairly good agreement with experimental data, with the predictions of operational 26 empirical and semi-empirical models, such as the McArthur model (MK5) in Australia and 27 the Rothermel model (BEHAVE) in USA, as well as with the predictions of other fully 3D 28 physical fire models (FIRETEC and WFDS). The comparison with the literature was 29 mainly based on the estimation of the rate of fire spread (ROS) and of the fire intensity, 30 as well as on the analysis of the fire-front shape. 31 32

International audience; A 3D multi-physical model referred to as “FireStar3D” has been developed in order to predict the behavior of wildfires at a local scale (

International audience; An experimental and numerical study was carried out to assess the performance of the different sub-models and parameters used to describe the burning dynamics of wildfires. A multiphase formulation was used and compared to static fires of dried pitch pine needles of different bulk densities. The samples were exposed to an external heat flux of 50 kW/m 2 in the FM Global Fire Propagation Apparatus and subjected to different airflows, providing a controlled environment and repeatable conditions. Sub-models for convective heat transfer, drag forces, and char combustion were investigated to provide mass loss rate, flaming duration, and gas emissions. Good agreement of predicted mass loss rates and heat release rates was achieved, where all these submodels were selected to suit the tested conditions. Simulated flaming times for different flow conditions and different fuel bulk densities compared favorably against experimental measurements. The calculation of the drag forces and the heat transfer coefficient was demonstrated to influence greatly the heating/cooling rate, the degradation rate, and the flaming time. The simulated CO and CO 2 values compared well with experimental data, especially for reproducing the transition between flaming and smoldering. This study complements a previous study made with no flow to propose a systematic approach that can be used to assess the performance of the submodels and to better understand how specific physical phenomena contribute to the wildfire dynamics. Furthermore, this study underlined the importance of selecting relevant submodels and the necessity of introducing relevant subgrid-scale modelling for larger scale simulations.

International audience; In a recent communication, Cruz et al. (2017) called attention to several recurring statements (mantras) in the wildland fire literature regarding empirical and physical fire behaviour models. Motivated by concern that these mantras have not been fully vetted and are repeated blindly, Cruz et al. (2017) sought to verify five mantras they identify. This is a worthy goal and here we seek to extend the discussion and provide clarification to several confusing aspects of the Cruz et al. (2017) communication. In particular, their treatment of what they call physical models is inconsistent, neglects to reference current research activity focussed on combined experimentation and model development, and misses an opportunity to discuss the potential use of physical models to fire behaviour outside the scope of empirical approaches.

International audience; Several studies in the literature explore the connection between rate of spread (ROS) and wind in wildland fires. These studies show very different positions about the role of radiation and convection as heat transfer mechanisms. In the case when the fuel bed is well-ordered and vertically-oriented, there seems to be a consensus leading to suggest that convective heating is the dominant heat transfer mode in that case. The purpose of this work is to propose a convective semi-physical model for the behaviour of the rate of spread in wind, when the fuel bed is vertically-oriented. Due to a specific fuel bed arrangement, flame radiation -i.e. radiation from the part of the flame above the vegetal stratum is neglected. Only horizontal radiation from the fuel burning particles area and convective heating are taken into account. Convective heat transfer is assumed to be the primary heat transfer mechanism. The proposed model is confronted to 172 laboratory fires with a wide range of fuel characteristics. The predicted results are also compared with two simplified models from the literature. Statistical tools are used to check the agreement between the predicted ROS and the observed one where a strong agreement is generally observed, irrespective of fuel bed characteristics.

International audience; This paper was devoted to clarify and evaluate how fuel moisture content (FMC) characterising a homogeneous vegetation layer (grass or shrubs), can affect the behaviour of surface fire. The approach used in this study was based on numerical simulations performed using a detailed fire physical model. The numerical results were analysed in terms of fire residence time, fire front depth, mass loss rate and rate of spread (ROS). Two windy conditions (calm and weak) were studied to evaluate the decay of the rate of spread (ROS) resulting from an increase of the fuel moisture content. The effect of wind velocity upon marginal burning conditions was also analysed. The numerical results were compared with empirical data of the literature.

From August to November 2014 the Rosetta orbiter has performed an extensive observation campaign aimed at the characterization of 67P/CG nucleus properties and to the selection of the Philae landing site. The campaign led to the production of a global map of the illuminated portion of 67P/CG nucleus. During this prelanding phase the comet's heliocentric distance decreased from 3.62 to 2.93 AU while Rosetta was orbiting around the nucleus at distances between 100 to 10 km. VIRTIS-M, the Visible and InfraRed Thermal Imaging Spectrometer - Mapping channel (Coradini et al. 2007) onboard the orbiter, has acquired 0.25-5.1 micron hyperspectral data of the entire illuminated surface, e.g. the north hemisphere and the equatorial regions, with spatial resolution between 2.5 and 25 m/pixel. I/F spectra have been corrected for thermal emission removal in the 3.5-5.1 micron range and for surface's photometric response. The resulting reflectance spectra have been used to compute several Cometary Spectral Indicators (CSI): single scattering albedo at 0.55 micron, 0.5-0.8 micron and 1.0-2.5 micron spectral slopes, 3.2 micron organic material and 2.0 micron water ice band parameters (center, depth) with the aim to map their spatial distribution on the surface and to study their temporal variability as the nucleus moved towards the Sun. Indeed, throughout the investigated period, the nucleus surface shows a significant increase of the single scattering albedo along with a decrease of the 0.5-0.8 and 1.0-2.5 micron spectral slopes, indicating a flattening of the reflectance. We attribute the origin of this effect to the partial removal of the dust layer caused by the increased contribution of water sublimation to the gaseous activity as comet crossed the frost-line., 19 Figures, 5 Tables. Accepted for publication in Icarus journal on 29 February 2016

International audience; This study reports 3D numerical simulations of the ignition and the propagation of grassland fires. The mathematical model consists in solving the conservation equations governing the behavior of the coupled system formed by the vegetation and the ambient atmosphere. Based on a finite volume discretization, the computation code is parallelized using OpenMP directives and had been the subject of numerous validations. the model was previously tested on small scale in case of homogeneous litter fires; in this study it is proposed to test it on larger scale in case of grassland fires. the results are in fairly agreement with experimental data, with predictions of operational empirical-models developed in Australia (MK5) and in the USA (BEHAVE), as well as with predictions of physical models (FIRETEC, WFDS, FireStar2D). The comparison with the literature is based on the estimation of the rate of fire spread (ROS) and the analysis of the fire-front shape.

Original Russian Text published in Vychislitel'naya Mekhanika Sploshnykh Sred, 2010, Vol. 3, No. 2, pp. 34-45.; International audience; Three-dimensional large eddy simulation is used to solve the problem for a homogeneous forest canopy. The development of the Kelvin-Helmholtz instability above the canopy leads to the formation of coherent structures in the atmosphere flow, which are reproduced in the calculations. The statistical characteristics of the flow obtained from the numerical modeling are compared with experimental data. The passive admixture transfer from the canopy to the clean atmosphere is studied for two cases, namely, for constant and variable coupled concentration of the impurity in the canopy.

International audience; This paper reports large eddy simulations of the interaction between an atmospheric boundary layer and a canopy (representing a forest cover). The problem is studied for a homogeneous configuration representing the situation encountered above a continuous forest cover, as well as for a heterogeneous configuration representing the situation similar to an edge or a clearing in a forest. The numerical results reproduces correctly all the main characteristics of this flow as reported in the literature: the formation of a first generation of coherent structures aligned transversally with the wind flow direction, the reorganization and the deformation of these vortex tubes into horse-shoe structures. The results obtained when introducing a discontinuity in the canopy (reproducing a clearing or a fuel break in a forest), are compared with the experimental data collected in a wind tunnel; here, the results confirm the existence of a strong turbulence activity inside the canopy at a distance equal to 8 times the height of the canopy, referenced in the literature as the Enhance Gust Zone (EGZ) characterized by a local peak of the skewness factor.

International audience; This paper is an overview of the physical mechanisms and length scales governing the propagation of wildfires. One of the objectives is to identify the physical and mathematical constraints in the modelling of wildfires when using a "fully" physical approach. The literature highlights two regimes in the propagation of surface fires, i.e. wind-driven fires and plume-dominated fires, which are governed by radiation and convective heat transfer, respectively. This division leads to the identification of two governing length scales: the extinction length characterising the absorption of radiation by vegetation, and the integral turbulent length scale characterising the interaction between wind and canopy. Some numerical results published during the last decade using a fully physical approach are presented and discussed with a focus on the models FIRESTAR, FIRELES, FIRETEC and WFDS. Numerical simulations were compared with experimental data obtained at various scales, from laboratory to field fires in grassland and in Mediterranean shrubland. Some perspectives are presented concerning the potential coupling between physical fire models with mesoscale atmospheric models to study the impacts of wildfires at larger scale. Some of the topics on wildfire physical modelling that need further research are identified in the conclusions.

International audience; Self-heating is commonly observed when organic materials such as biosolids, coal, food grains and compost are stockpiled. A convection–diffusion model is presented that accounts for the roles of advection and the transport of oxygen in the self-heating process, as well as the development of an empirical correlation between dimensionless Darcy number, Frank–Kamenetskii parameter and pile aspect ratio, to predict the critical permeability above which thermal runaway can be avoided. It is apparent that the permeability of the stockpile determines the likelihood of the thermal runaway. However, the solids that form a stockpile are poly-disperse and it is essential to determine an effective permeability. This has been achieved using experimental data on biosolids obtained from a wastewater treatment plant in Australia. With this method the model is used to demonstrate how the permeability of a stockpile might be adjusted to reduce the incidence of thermal runaway.

International audience; A method to accurately understand the processes controlling the burning behavior of porous wildland fuels is presented using numerical simulations and laboratory experiments. A multiphase approach has been implemented in OpenFOAM, which is based on the FireFOAM solver for large eddy simulations (LES). Conservation equations are averaged in a control volume containing a gas and a solid phase. Drying, pyrolysis, and char oxidation are described by interaction between the two phases. Numerical simulations are compared to laboratory experiments carried out with porous pine needle beds in the FM Global Fire Propagation Apparatus (FPA). These experiments are used to support the use and the development of submodels that represent heat transfer, pyrolysis, gas-phase combustion, and smoldering processes. The model is tested for different bulk densities, two distinct species and two different radiative heat fluxes used to heat up the samples. It has been possible to reproduce mass loss rates, heat release rates, and temperatures that agree with experimental observations, and to highlight the current limitations of the model.

The propagation of wildfire through a Mediterranean shrub has been numerically simulated using a multiphase formulation. This approach was based on a complete description of the physical phenomena contributing to the propagation of a surface fire through a solid fuel layer. The heterogeneous character of the vegetation (nature, foliage, twigs, trunk, etc.) was taken into account using families of solid particles, which are characterized by specific physical properties such as the surface area/volume ratio, the moisture content, the density, and the volume fraction. Using this approach, the interaction between dead and living fuel, which plays a significant role in the propagation of a wildfire, was taken into account. The evolution of the state of each solid fuel family was computed by solving the mass and energy balances, including the effects of thermal degradation of the vegetation (drying, pyrolysis, and glowing combustion) and the exchanges of mass, momentum, and energy with the surrounding gas. The calculations were performed for a surface fire propagating through Mediterranean vegetation composed of shrubs (Quercus coccifera) and grasses (Brachypodium ramosum). The results show the effects of wind on heat transfer between the fire front and the vegetation. Two modes of fire propagation are identified: plume-dominated fires and wind-driven fires, respectively dominated by radiation and convection heat transfer.

International audience; With fuel moisture content and slope, wind velocity (U W) is one of the major physical parameters that most affects the behaviour of wildland fires. The aim of this short paper was to revisit the relationship between the rate of spread (ROS) and the wind velocity, through the role played by the two forces governing the trajectory of the flame front and the plume, i.e. the buoyancy of the plume and the inertia due to wind. A large set of experimental data (at field and laboratory scale) from the literature was analysed, by introducing the ratio between these two forces, namely Byram's convective number N C and considering the relationship between the fire ROS/wind speed ratio and Byram's number. This short note was also an opportunity to make a point on particular issues, such as the existence of two regimes of propagation of surface fires (wind-driven fire vs plume-dominated fire), the relative importance of the two modes of heat transfer (by convection and radiation) on the propagation of a fire front, and others scientific debates animating the wildland fire community.

International audience; This paper deals with the modelling of living fuel ignition, suggesting that an accurate description using a multiphase formulation requires consideration of a thermal disequilibrium within the vegetation particle, between the solid (wood) and the liquid (sap). A simple model at particle scale is studied to evaluate the flux distribution between phases in order to split the net flux on the particles into the two sub-phases. An analytical solution for the split function is obtained from this model and is implemented in ForestFireFOAM, a computational fluid dynamics (CFD) solver dedicated to vegetation fire simulations, based on FireFOAM. Using this multiphase formulation, simulations are run and compared with existing data on living fuel flammability. The following aspects were considered: fuel surface temperature, ignition, flaming combustion time, mean and peak heat release rate (HRR). Acceptable results were obtained, suggesting that the thermal equilibrium might not be an acceptable assumption to properly model ignition of living fuel.

International audience; The efficiency of a firebreak, built in a shrubland has been studied numerically using a multiphase physical model. The physical mechanisms governing the propagation of the surface fire and the consequences upon the temperature signal and the radiative heat flux received by a target located at 1 m above the ground level, have been firstly studied before positioning the firebreak. The role played by the flame and the recirculation of hot gases to the ignition of unburned fuel (especially the dry grass) ahead of the fire front have been clearly identified. Four values of the firebreak width LC (ranged between 5 and 20 m) and 3 values of wind velocities (ranged between 1 and 8 m/s) have been tested. The simulations show that above a threshold value of this parameter, even if a small amount of the fuel located on the opposite side of the firebreak was ignited, the released energy was not sufficient to sustain the propagation of the surface fire after crossing the firebreak.

We describe a multiphase formulation to study numerically the propagation of a line fire in a forest fuel bed. One of the objectives of these studies is the improvement of knowledge on the fundamental physical mechanisms that control the propagation of forest fires. In complement of the experimental approach, this simulation tool can also be used for the development of simplified operational models used for instance for the prediction of the rate of spread (ROS) of wildland fires. The decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented by adopting a multiphase formulation. This approach consists in solving the conservation equations (mass, momentum, energy) averaged in a control volume at a scale sufficient to contain several solid particles in the surrounding gas mixture. After a presentation of the equations and closure sub-models used in this approach, some numerical results obtained for the propagation of a line fire in a pine needles litter are presented and compared with experimental data obtained in laboratory. These results show that the rate of spread of fire in the fuel bed is primarily controlled by the radiative heat transfer. By increasing the fuel load (with a constant packing ratio), the results show the existence of two modes of propagation. A first area where the ROS varies linearly with the fuel load followed of a second where the ROS becomes independent of the load. By introducing the optical thickness characterizing the fuel bed, this difference in mode of propagation was interpreted like the demonstration of two modes of radiative transfer (optically thin and thick, respectively). The analysis of the distributions of the mass fractions of fuel and oxidant present in the gas mixture integrated through the depth of the fuel bed shows that the propagation velocity could also be limited by the lack of oxygen or fuel available in the ignited zone to maintain the pilot flame.

The behaviour of a turbulent diffusion flame subjected to cross wind conditions is studied numerically using a finite-volume procedure. A k-∈-gRNG turbulence model and a s-probability density function (pdf) are used for the description of the turbulent nonpremixed combustion process. The gas/soot radiation model used in the analysis is based on the PI-differential approximation method and coupled with a two-equation submodel for soot formation. This model is applied to methane/air flames for cross wind velocity ranging from 0.5 m/s to 2 m/s. For a moderate wind of 1 m/s, the transient behaviour of the flame associated with the development of buoyancy-driven large structures is examined. Under different windy conditions, numerical results show that the nature of instabilities which develop along the thermal plume, depends on the orientation of the density gradient with respect to the gravity. The wind effects upon the flame trajectories are analyzed from the angle of deflection between the hot gas stream a...

A one dimensional numerical model is proposed to study the counter-current fire spread in a pine needles fuel bed confined in a tube. The physical mechanisms which control the fire spread and the interactions between the gas flow and the solid fuel particles are described using a multiphase formulation. The set of partial differential equations which governs the physical behavior of the gas flow and of the solid fuel bed is solved using a finite volume method. To describe accurately the small area where the thermal degradation (drying, pyrolysis, char combustion) of the solid fuel occurs, the calculations are carried out using an adaptative grid algorithm consisting in refining locally the grid on both sides of the pyrolysis front. The computation results have not been confronted with experimental measurements, therefore the objective of this study is to highlight the behavior of this physical system according to the rate of air supply. While varying the inlet flow velocities ranging from 2 to 30 cm/s, th...

The propagation of wind-aided line fires through fuel beds is simulated by using a multiphase approach. In this approach, a gas phase flows through N solid phases which constitute an idealized reproduction of the heterogeneous combustible medium. A set of time-dependent equations is obtained for each phase and the coupling between the gas phase and the solid phases is rendered through exchange terms of mass, momentum, and energy. Turbulence is approached by using a RNG k−e statistical model constructed from the Favre averaging method. The radiative transfer equation extended to multiphase media is solved using the discrete ordinates method (DOM). Soot formation is taken into account for the evaluation of the absorption coefficient of the soot/fuel particles/combustion products mixture using the gray gas assumption. First-order kinetics is incorporated to describe water vaporization, pyrolysis, and char combustion processes. The solution is performed numerically by a finite-volume method including a high-order upwind convective scheme and a flux limiter strategy along with a projection method for the pressure–velocity coupling. This model has been applied to describe the unsteady behavior of wind-aided fires spreading through a litter of dead pine needles and the induced hydrodynamics. The numerical results obtained from our model are presented and compared to measurements and predictions from other laboratory-based models.

Reports numerical simulations of an unconfined methane‐air turbulent diffusion flame expanding from a porous burner. Turbulent combustion is simulated using the eddy dissipation concept (EDC) which supposes that the reaction rate is controlled by the turbulent structures which enhance the mixing of fuel and oxidant. Two statistical k‐ε turbulence models have been tested: a standard high Reynolds number (HRN) and a more recent model based on the renormalization group theory (RNG). Radiation heat transfer and soot formation have been taken into account using P1‐approximation and transport submodels which reproduce the main phenomena encountered during soot production (nucleation, coagulation, surface growth). The set of coupled transport equations is solved numerically using a high order finite‐volume method, the velocity‐pressure coupling is treated by a projection technique. The numerical results confirm that 20‐25 percent of the combustion heat released is radiated away from the flame. Unsteady and unsymmetrical flame behaviour is observed for small Froude numbers which results from the development of Rayleigh‐Taylor like instabilities outside the flame surface. For higher Froude numbers the steady‐state and symmetrical nature of the solution is recovered.

We describe a numerical model to study the propagation of a surface fire in a fuel bed. The objective, in the long term, of these studies is to improve the mode of evaluation of the rate of spread (ROS) of a wildfire in simulations and thereby help to prevent and reduce the risks related to forest fires. The decomposition of solid fuel constituting a forest fuel bed as well as the multiple interactions with the gas phase are represented by adopting a multiphase formulation. This approach consists of solving the conservation equations (mass, momentum, energy, etc.) averaged in a control volume at a scale sufficient, to contain several solid particles in the surrounding gas mixture. The calculations, which were carried out in pine needle litter on flat ground and without wind, are compared with experimental data obtained in the laboratory. These results show that the rate of spread of fire in the fuel bed is mainly controlled by radiative heat transfer. When the fuel-bed depth is increased near the extinction limit, the ROS increases quickly, reaches a linear mode, and tends to stay in a saturated mode for which the ROS becomes independent of the height of the litter. By introducing the optical thickness characterizing the fuel bed, this difference in mode of propagation is interpreted as demonstrated: two modes of radiative heat transfer corresponding, respectively, to an optically thin and thick medium.

The vertical Bridgman-Stockbarger technique is a directional solidification technique. The material is loaded into an ampoule, and solidified by varying the temperature field with a translation of the ampoule through a furnace. The hydrodynamic is governed by both solutal and thermal convective instabilities. The resolution of the problem uses an homogenised formulation of the conservation equations, the enthalpy approach, issued from classical mixture theories. The energy equation is formulated with the enthalpy variable. Solving the conservation equations in the whole domain allows not to track the interface, as its shape is not needed in the resolution. The numerical simulation is done using a finite-volume method. We present here studies on two type of Bridgman configurations. The first one is an inverted Bridgman configuration of a pure material. The results show the interaction between the interface and the hydrodynamic of the unsteady melt. The second is a solidification of a binary alloy, where the temperature gradient has a stabilising effect. The results show the influence of the convection on the distribution of dopant in the solidified crystal.

The Bridgman technique is a directional solidification technique. The material is loaded into an ampoule, and solidified by varying the temperature field with a translation of the ampoule through a furnace. The hydrodynamic is governed by both thermal and solutal convective instabilities. The resolution of the problem uses an homogenized formulation of the conservation equations, the enthalpy approach, issued from classical mixture theory. The numerical simulation is done using a finite volume method. We present here studies on two types of Bridgman configurations. The first one, corresponding to an inverse Bridgman configuration (heated from below), shows the interaction between the interface and the hydrodynamic of the unsteady melt. The second one shows the solidification of a binary alloy in a normal Bridgman configuration, and shows the influence of thermo-solutal convection on the crystal constitution.

The influence of latent heat and natural convection upon the melt–crystal interface in a vertical Bridgman–Stockbarger crystal growth system is studied by numerical simulation. The temperature and velocity field are computed using an average technique which consists of representing the solid–liquid mixture as a single continuum medium. The physical properties of the equivalent medium are evaluated from average values which characterize each phase present in the system. The numerical resolution is performed using a finite volume method including a high-order upwind scheme and PISO algorithm for the pressure–velocity coupling. The numerical results show the effects of latent heat and gravity magnitude upon the flow pattern in the melt. The consequences upon the melt–crystal interface are also analyzed.

Resume L'interaction de la convection thermique instationnaire d'un bain fondu avec un front de solidification est etudiee numeriquement. L'application concerne la croissance cristalline par une technique de type Bridgman verticale. Le domaine physique comporte une zone liquide, une zone solide et une zone intermediaire de transition de petite dimension, modelisee comme un milieu poreux. Une formulation homogene de type enthalpie-porosite est proposee avec une approximation numerique de type volumes finis. Le modele permet la localisation du front de solidification et la description de ses deformations. La methode est qualifiee par rapport a une approche de suivi de front a differentes valeurs du nombre de Rayleigh.

This paper reports numerical simulations of unconfined CH4/Air turbulent diffusion flame which expands from a porous burner. Turbulent combustion is modelled using an Eddy Dissipation Concept (EDC) coupled with a RNG κ − e statistical model for the turbulent flow. Radiation heat transfer which results from the formation of soot particles in the flame is represented using PI-approximation for the calculation of the irradiance field and transport submodels for the determination of soot volume fraction and soot number density. Appropriate source terms allow to reproduce the main phenomena (nucleation, coagulation, surface growth[tdot]) which appear during the expansion of soot field in the flaming zone. The set of coupled transport equations is solved numerically using a finite volume method, including a high order Ultra-Quick convective scheme to avoid numerical dissipation. The velocity-pressure coupling is treated with a projection technique. The numerical results obtained for pool fire regime show that t...

The effects of cross wind on the behaviour of a turbulent diffusion flame have been studied numerically. The multicomponent turbulent reacting flow is approached using a two-equation (κ - ϵ) statistical model constructed from the Favre averaging method. To improve the representation of turbulent fields in fully developed and weak turbulent regions, RNG (Renormalization Group) κ - ϵ turbulence modelling is adopted. Infinitely fast reaction is assumed and the combustion rate is fully determined by the turbulent mixing rate of fuel and oxygen using the Eddy Dissipation Concept (EDC). Energy lost by radiation is taken into account and soot formation is computed for the evaluation of the absorption coefficient of the soot-gas mixture using the gray gas assumption. Calculations have been performed both with and without wind. The numerical results show that the flow is characterized by oscillations which affect significantly the flame behaviour. The development of shear buoyancy-driven instabilities (Kelvin-Helm...