Interaction between head fire and backfire in grasslands (original) (raw)
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Numerical Study of the Interaction Between a Head Fire and a Backfire Propagating in Grassland
Fire Safety Science, 2011
One of the objectives of this paper was to simulate numerically the interaction between two line fires ignited in a grassland, on a flat terrain, perpendicularly to the wind direction, in such a way that the two fire fronts (a head fire and a backfire) propagated in opposite directions parallel to the wind. The numerical simulations were conducted in 3-D using the new fuel element module recently implemented in WFDS. We chose grassland for the fuel layer, because it represents certainly one of the most studied ecosystem , in the frame of experimental campaigns to study the behavior of surface fires at large scale. The aim of this numerical study is to understand what are the physical phenomena and the favorable conditions of ignition of a counter fire, during a fire suppression operation. The preliminary results highlighted that the two fire fronts interacted only at a relative short distance (10-20 m), following this scenario: The thermal plume above the head fire (representing the main fire front) formed a sort of shelter, protecting the backfire to the direct effect of the wind flow, Before the merging between the two fire fronts, an in-draft flow was observed ahead of the head fire, promoting and accelerating the propagation of the backfire. During the last step of the merging of the two fire fronts, a sudden increase of the heat release rate was observed, indicating a significant interaction between the two fires, which can potentially represent a safety problem for people in charge of this kind of operation.
Numerical simulation of the interaction between two fire fronts in grassland and shrubland
The objective of this paper was to evaluate the potential for fully physical fire models to simulate the interactions between two converging fire fronts (a head fire and a back fire), in conditions similar to those encountered during suppression fire operations. The simulations were carried out using two fully physical models: FIRESTAR, in two dimensions, and Wildland Fire Dynamics Simulator, in three dimensions. Each modelling approach numerically solves a set of balance equations (mass, momentum, energy, etc.) governing the behaviour of the coupled system formed by the vegetation and the surrounding atmosphere. Two fuel profiles were tested: homogeneous grassland similar to landscapes in Australia and a shrubland representative of Mediterranean landscape (garrigue). Results from the two-dimensional and three-dimensional simulations were used to investigate how the two fire fronts interact together and mutually modify, or not, their own behaviour before merging. The results of these simulations showed that the merging of two fire fronts can result in a quick increase in fire-line intensity or in flame height. We concluded that physics-based simulations do reproduce reasonable and expected head-and back-fire interactions, but more work is needed to further understand the accuracy of such predictions.
A physics-based approach to modelling grassland fires
International Journal of Wildland …, 2007
Physics-based coupled fire-atmosphere models are based on approximations to the governing equations of fluid dynamics, combustion, and the thermal degradation of solid fuel. They require significantly more computational resources than the most commonly used fire spread models, which are semi-empirical or empirical. However, there are a number of fire behaviour problems, of increasing relevance, that are outside the scope of empirical and semi-empirical models. Examples are wildland-urban interface fires, assessing how well fuel treatments work to reduce the intensity of wildland fires, and investigating the mechanisms and conditions underlying blow-up fires and fire spread through heterogeneous fuels. These problems are not amenable to repeatable full-scale field studies. Suitably validated coupled atmosphere-fire models are one way to address these problems. This paper describes the development of a three-dimensional, fully transient, physics-based computer simulation approach for modelling fire spread through surface fuels. Grassland fires were simulated and compared to findings from Australian experiments. Predictions of the head fire spread rate for a range of ambient wind speeds and ignition line-fire lengths compared favourably to experiments. In addition, two specific experimental cases were simulated in order to evaluate how well the model predicts the development of the entire fire perimeter.
Journal of Geophysical Research, 2005
Numerical simulations using a fire model, FIRETEC, coupled to an atmospheric dynamics model, HIGRAD, are examined to investigate several fundamental aspects of fire behavior in grasslands, and specifically the dependence of this behavior on the ambient atmospheric winds and on the initial length of the fire line. The FIRETEC model is based on a multi-phase transport approach, and incorporates representations of the physical processes that govern wildfires, such as combustion and radiative and convective heat exchange. Results from the coupled model show that the forward spread of the simulated fires increases with increasing ambient wind speed, and the spread rates are consistent with those observed in field experiments of grass fires; however, the forward spread also depends significantly on the initial length of the fire line, and for a given ambient wind speed the spread rate for long (100 m) lines is greater than that for short (16 m) lines. The spread of the simulated fires in the lateral direction also depends on the ambient wind speed and the length of the fire line, and a possible explanation for this effect is given. For weak ambient winds, the shape of the fire perimeter is dramatically different from that seen with higher wind speeds. The shape of the fire perimeter is also shown to depend on the initial length of the fire line. These differences in fire behavior are attributed to the differences in the nature of the coupled atmosphere-fire interactions among these cases, and are described in terms of the complex interplay between radiative and convective heat transfer.
2009
This paper deals with the study of physical phenomenon observed between two converging fire fronts (a head fire and a back fire), in conditions similar to those encountered during suppression fire operations. The simulations were carried out using two fully physical models (FIRESTAR, Wildland Urban Interface Fire Dynamics Simulator), including the resolution of the set of balance equations (mass, momentum, energy) governing the behaviour of the coupled system formed by the vegetation and the surrounding atmosphere. Two fuel profiles were tested: homogeneous grassland similar to landscapes in Australia and a shrubland representative of Mediterranean landscape (garrigue). The numerical results showed clearly how the two fire fronts interact together and mutually modify their own behaviour, before merging.
Physics-Based Simulations of Flow and Fire Development Downstream of a Canopy
Atmosphere
The behavior of a grassland fire propagating downstream of a forest canopy has been simulated numerically using the fully physics-based wildfire model FIRESTAR3D. This configuration reproduces quite accurately the situation encountered when a wildfire spreads from a forest to an open grassland, as can be the case in a fuel break or a clearing, or during a prescribed burning operation. One of the objectives of this study was to evaluate the impact of the presence of a canopy upstream of a grassfire, especially the modifications of the local wind conditions before and inside a clearing or a fuel break. The knowledge of this kind of information constitutes a major element in improving the safety conditions of forest managers and firefighters in charge of firefighting or prescribed burning operations in such configurations. Another objective was to study the behavior of the fire under realistic turbulent flow conditions, i.e., flow resulting from the interaction between an atmospheric b...
International Journal of Wildland Fire
Grid-independent rate of spread results from a physics-based simulation are presented. Previously, such a numerical benchmark has been elusive owing to computational restrictions. The grid-converged results are used to systematically construct correlations between the rate of spread (RoS) and both wind speed and grass height, separately. The RoS obtained from the physics-based model is found to be linear with wind speed in the parameter range considered. When wind speed is varied, the physics-based model predicts faster RoS than the Mk III and V (McArthur) models (Noble et al. 1980) but slower than the CSIRO model (Cheney et al. 1998). When the grass height is varied keeping the bulk density constant, the fire front changes from a boundary layer flame mode to plume flame mode as the grass height increases. Once the fires are in plume mode, a higher grass height results in a larger heat release rate of the fire but a slower RoS.
A 3D physical model to study the behavior of vegetation fires at laboratory scale
Fire Safety Journal, 2018
A 3D multi-physical model referred to as "FireStar3D" has been developed in order to predict the behavior of wildfires at a local scale (< 500m). In the continuity of a previous work limited to 2D configurations, this model consists of solving the conservation equations of the coupled system composed of the vegetation and the surrounding gaseous medium. In particular, the model is able to account explicitly for all the mechanisms of degradation of the vegetation (by drying, pyrolysis, and heterogeneous combustion) and the various interactions between the gas mixture (ambient air + pyrolysis and combustion products) and the vegetation cover such as drag force, heat transfer by convection and radiation, and mass transfer. Compared to previous work, some new features were introduced in the modelling of the surface combustion of charcoal, the calculation of the heat transfer coefficient between the solid fuel particles and the surrounding atmosphere, and many improvements were brought to the numerical method to enable affordable 3D simulations. The partial validation of the model was based on some comparisons with experimental data collected at small scale fires carried out in the Missoula Fire Sciences Lab's wind tunnel, through various solid-fuel layers and in well controlled conditions. A relative good agreement was obtained for most of the simulations that were conducted. A parametric study of the dependence of the rate of spread on the wind speed and on the fuelbed characteristics is presented.