Experimental and numerical studies characterizing the burning dynamics of wildland fuels
Introduction
Many experimental studies on wildland fuel flammability are conducted at small bench scale or even at microscopic scale, such as Thermogravimetric Analysis (TGA) [1], [2] and Differential Scanning Calorimetry (DSC) [3], [4], [5] to understand the physical and chemical processes involved during the decomposition of the fuel when heated [2], [6]. While it is difficult to perform large scale experiments, to maintain repeatable and fully controlled environments [7], [8], [9], and to monitor all the dynamics involved, the use of numerical models becomes essential. Different types of models exist and they are used to predict wildfires. They are either based on empirical correlations [10], [11], [12], [13], simplified models [14], [15], or detailed computational fluid dynamics (CFD) models [16], [17], [18]. These CFD models are often used to study large fires [18], [19], [20], [21], in which many parameters and complex submodels are included in order to provide satisfactory results.
The first aim of this study is to perform experiments at an intermediate scale that is small enough to maintain a controlled environment, and large enough to be comparable to real fire conditions. Therefore, experiments are conducted in the FM-Global fire propagation apparatus (FPA) [22], [23], which allows repeatable conditions to be achieved, and to monitor temperature, heat release rate (HRR), gas production and mass loss rates (MLR) as in [24], [25], [26]. In tests performed for the present work, temperature was measured inside the fuel bed to make sure that the heat transfer (radiation and convection) is modeled correctly during the heating phase, before ignition, where there is no flame radiation and soot oxidation yet. Measurements of gas production allow determining the HRR, which indicates how the energy is released. Finally, mass loss indicates the degradation rate, including evaporation, pyrolysis and char oxidation. Mass loss is also linked to the burning rate, which, along with the surrounding conditions, will affect the HRR, flame height and the burning rate. Separately, spectral measurements have been done for dead pine needles under a wide spectrum (0.25–20 µm) to determine the effective absorptivity of the fuel under the FPA halogen lamps, used to heat up the samples.
Numerical simulations are then conducted to mimic the same experimental conditions to verify how well the model behaves and to understand its limitations. The numerical approach is based on the multiphase model [17], [27], [28], [29] that was implemented in OpenFOAM [30] and called ForestFireFOAM. The latter is built following the structure of FireFOAM, a LES code for fire modeling [31]. The multiphase formulation is used to include the process of degradation of the forest fuel by drying, pyrolysis and heterogeneous combustion, and to simulate it by assuming a volumetric reaction rate. This approach was not yet implemented neither in OpenFOAM, nor in FireFOAM. Consequently, part of this study has been dedicated to the implementation of this new model. The multiphase approach was introduced by Grishin [27], in which he presented an extensive review of the work conducted in USSR in the 1970s and 1980s on wildland fires. Grishin's model was the first to incorporate kinetics to describe pyrolysis, combustion, and hydrodynamics through a fuel bed using a multiphase approach. Thermal equilibrium was initially assumed between gas phase and solid phase and the equations were averaged over the height of the forest canopy to simplify the formulation. Later, Larini et al. [28] presented the bases of the multiphase formulation for a medium in which a gas phase and N solid phases in thermal non-equilibrium are treated individually along with some one-dimensional applications. A detailed review of these models are presented in [32].
The multiphase model includes the Navier–Stokes conservation equations [33] for radiative and reactive multiphase medium. The closure models, or submodels that are used for degradation, heat transfer, combustion, and radiation are typically applied to simulate large-scale wildfires in complex environments. However, they were obtained from micro- and small- scale laboratory experiments using very different conditions than the ones where they are often applied in the literature [20], [27], [28], [34], [35]. These submodels are described and examined hereafter at intermediate scale. The multiphase model is tested with experiments using fuel beds with different bulk densities that are representative of litter conditions. Two distinct North American species with different surface to volume ratios are burnt at two different heat fluxes imposed by the FPA heaters. Pine needle beds are used as a reference fuel because they are well characterized in the literature [36] and they allow obtaining repeatable fuel bed properties under laboratory settings. Moreover, pine needle beds often accumulate on forest floor and near structures in the Wildland Urban Interface (WUI), increasing the fire risk [37].
Section snippets
FPA experiments
Experiments were performed with the FM Global Fire Propagation Apparatus (FPA), which provides controlled and repeatable conditions [24], [26] (Fig. 1), such as the ability to produce a constant incident radiative heat flux. No forced flow was applied in this study, only natural convection was allowed through the porous samples. As the lower end of the FPA is closed, natural convection is limited at the backface of the sample. Dead needles were packed in cylindrical open baskets of 12.6 cm
Multiphase approach
The interactions between solid fuel constituting of a forest fuel layer and the gas phase are represented by adopting a multiphase formulation [27], [28], [33]. The general formulation of the multiphase approach is particularly relevant to forest fuels and our small scale fires of pine needle beds. The following assumptions are made for simplicity:
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The fuel bed is considered as a homogeneous distribution of solid particles whose dimensions and physical properties are evaluated from experimental
Results & discussion
During the heating process from the start of the test until ignition only heat transfer plays an essential role in the temperature evolution, since there is neither combustion nor flame involved yet. Since no forced flow was applied ( is small), and if we neglect change of properties (dehydration), we can assume that the gas and the fuel are in thermal equilibrium before ignition. Figures 5 and 6 show the numerical predictions and experimental results for the temperature evolution in
Conclusion
An experimental and numerical study on the burning dynamics of wildland fuels was presented. The FPA was used to obtain a controlled environment and repeatable conditions to burn litters of pine needles. Additional experiments were done in order to understand and to describe with mathematical formulations all the different aspects involved in these experiments, such as analyzing the spectral emissivity of dead pine needles to better describe the radiative heat transfer. The effective
Acknowledgments
The authors gratefully acknowledge Drs Y. Wang, K. Meredith, P. Chatterje, A. Gupta, and N. Ren for the numerous discussions and help provided in the development of ForestFireFOAM. The reviewers are also acknowledged for their useful analysis, which has significantly improved the article. This work was granted access to the HPC resources of Aix-Marseille Université financed by the project Equip@Meso (ANR-10-EQPX-29-01) of the program «Investissements d'Avenir» supervised by the Agence Nationale
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