Jean-Luc Dupuy

Modeling of fire spread through a forest fuel bed using a multiphase formulation

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.

Fires from a cylindrical forest fuel burner: combustion dynamics and flame properties

Abstract 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.

Numerical study of a crown fire spreading toward a fuel break using a multiphase physical model

The propagation of a wildfire through a Mediterranean pine stand was simulated using a multiphase physical model of fire behaviour. The heterogeneous character of the vegetation was taken into account using families of solid particles, i.e. the solid phases (foliage, twigs, grass). The thermal decomposition of the solid fuel by drying and pyrolysis, and the combustion of chars were considered, as well as the radiative and convective heat transfer between the gas and the vegetation. In the gaseous phase, turbulence was modelled using a two transport equations model (RNG k-e) and the rate of combustion, which was assumed to be controlled by the turbulent mixing of fuel and oxygen, was calculated using an eddy dissipation concept. The radiation transfer equation, which includes absorption and emission of both the gas-soot mixture and the vegetation, was solved to calculate the contribution of radiation to the energy balance equations. Numerical solutions were calculated in a two-dimensional domain (vertical plane). Results showed the ability of this approach to simulate the propagation of a crown fire and to test the efficiency of a fuel break with success. The effects of the terrain slope were also tested. Some effects on fire behaviour of vortices resulting from the interaction of the wind flow with the canopy layer are shown.

Validation of FIRETEC wind-flows over a canopy and a fuel-break.

The wildfire model FIRETEC simulates the large coherent eddies of the wind-flows induced by the canopy. It has been qualitatively validated in its ability to simulate fire behavior, but there is still a need to validate physical submodels separately. In the present study, the dynamics and turbulence of the flow simulated by FIRETEC are validated in a manner similar to other air-flow models without fire, through comparison with measurements associated with flows within continuous and discontinuous forests captured through in situ and wind-tunnel experiments with neutral thermal stratification. The model is shown to be able to reproduce accurately all essential features of turbulent flow over both forests. Moreover, a short sensitivity study shows that the model is not very sensitive to uncertain parameters such as vegetation drag coefficient. Finally, it is shown in the discontinuous forest case that wind gusts on fuel-breaks can be very strong and significantly higher than in surrounding canopies, even if their directions are more stable. These results and others briefly reviewed in the present paper allow better understanding of wind-flow perturbations induced by fuel-breaks. This new validation added to previous ones confirms the ability of FIRETEC for investigating effects of fuel-break design on fire propagation.

Impacts of tree canopy structure on wind flows and fire propagation simulated with FIRETEC

Introduction Forest fuel management in the context of fire prevention generally induces heterogeneous spatial patterns of vegetation. However, the impact of the canopy structure on both wind flows and fire behavior is not well understood.Material and methods Here, a coupled atmosphere wildfire behavior model, HIGRAD/FIRETEC, was used to investigate the effects of canopy treatment on wind field and fire behavior in a typical Mediterranean pine ecosystem.Discussion First, the treatment-induced winds were simulated with the model. We observed that with decreasing cover fraction the wind velocity increased within the treated zone. The wind spatial variability increased when the vegetation was aggregated into larger clumps. Fire simulations indicated that a decrease of fire intensity occurred after several meters of propagation in the treated zone. This intensity decrease was significant with a cover fraction below 25%, but negligible with a cover fraction greater than 50%. The treatment also induced a more significant inclination of the plume away from vertical. The size of the tree clumps did not show significant effects on fire behavior.Conclusion This study was a preliminary investigation of wind/fire interaction over various canopy treatments, by using a physically based model. It gives some practical considerations for discerning the appropriate cover fraction and open perspectives for further investigations.

The effects of slope and fuel bed width on laboratory fire behaviour

A set of 109 laboratory fires in Pinus halepensis fuel beds (1 kg m–2) was used to test the effects of slope (0°, 10°, 20°, 30°) and fuel bed width (1, 2, 3 m) on fire behaviour variables such as rate of spread, fuel consumption, flame residence time, temperatures and flame geometry. The qualitative behaviour of the fires is also reported. The 20° and 30° upslope fires are pointed in shape and fire whirls moving along the fire flanks in the direction of the fire head are systematically observed in 30° upslope fires. Flame residence time increases with increasing slope angle, and both slope angle and fuel bed width affect rate of spread. The slope effects observed in 10° and 20° slope angles and in the narrowest fuel beds (1 and 2 m) are similar to those predicted by operational models. However, the observed slope effect at the 30° slope angle is underestimated by these models, in particular in 3 m-wide fuel beds. Flame temperatures correlate closely with dimensionless height and flame lengths correlate closely with fire line intensity. Mechanisms that could explain the different effects observed are suggested and discussed.

Thermal infrared emission–transmission measurements in flames from a cylindrical forest fuel burner

We describe emission–transmission measurements performed at different heights in a flame from a cylindrical forest fuel burner, using a camera operating in the thermal infrared (7.5–13 µm). The forest fuel burner was made of a cylindrical wire mesh basket filled with a forest fuel (Pinus pinaster needles), which was ignited at the base of the basket. Three diameters of basket were used (20, 28 and 40 cm). Heat release rates, as calculated from weighing of the basket and heat of combustion of the fuel, ranged between 50 and 170 kW and flame heights ranged between 1 and 2 m. The emission–transmission device allows the determination of the transmittance of the flame and of a radiometric temperature. We show that radiation was dominated by soot in the spectral range of the camera, but that radiation from gaseous products of the combustion was not negligible. Using the Mie theory in its Rayleigh limit, we deduced some average volume fractions of soot from the measurements, which peaked at 6.8 × 10−6 in the persistent region of the flame. Then the total extinction coefficient and the total emissivity of the flame due to soot were calculated according to a standard method. Measured transmittance, soot volume fraction, total extinction coefficient and total emissivity were found to scale with the normalised height of measurement Z, defined as the ratio of the height of measurement to the height of the flame (0.25 < Z < 1.6).

Slope effect on laboratory fire spread: contribution of radiation and convection to fuel bed preheating

Two series of 16 and 18 laboratory fire experiments were conducted to explore the respective roles of radiation and convection heat transfer in slope effect on fire spread. The first series attempts to measure fuel temperature and gas temperature simultaneously and at the same location using an infrared camera and thermocouples respectively. The second series measures the incident radiant heat flux as would be received by a small fuel bed volume ahead of the fire line. These measurements are used to compute a fuel bed heat balance for each slope angle (0°, 10°, 20° and 30°). Overall, radiative heating is found to be the heat transfer mechanism that dominates in the slope effect between 0° and 20°, but close to the fire line (<10 cm), the flux due to convective heating is also significant, reaching one-third of the net heat flux at a 20° slope angle. When the slope angle increases from 20° to 30°, the rate of spread rises by a factor of 2.5 due to a marked increase in convective heating, while radiative heating no longer increases. Far from the fire line, cooling by convection is found to be substantial except at the 30° slope angle.

Fuel bulk density and fuel moisture content effects on fire rate of spread: a comparison between FIRETEC model predictions and experimental results in shrub fuels

Fuel bulk density and fuel moisture content effects on fire rate of spread were assessed in shrub fuels, comparing experimental data observed in outdoor wind tunnel burns and predictions from the physically-based model FIRETEC. Statistical models for the combined effects of bulk density and fuel moisture content were fitted to both the experimental and the simulated rate of spread values using non-linear regression techniques. Results confirmed a significant decreasing effect of bulk density on rate of spread in a power law in both laboratory burns and simulations. However, experimental data showed a lesser effect than simulations, suggesting a difference in the effective drag. Fuel moisture content effect was highly consistent, showing a similar exponential relationship with rate of spread in laboratory and in simulations. FIRETEC simulations showed similar orders of magnitude with predictions of two field-based empirical models, finding a significant correlation between rate of spread values. The study confirms the efficacy of the combined approach through experimental data and simulations to study fire behaviour.

Exploring three-dimensional coupled fire–atmosphere interactions downwind of wind-driven surface fires and their influence on backfires using the HIGRAD-FIRETEC model

The obstruction of ambient winds and the possible existence of indrafts downwind of a wildfire are aspects of coupled fire–atmosphere interaction influencing the effectiveness of a backfiring operation. The fire-influenced winds behind a headfire as well as their influences on backfire spread are explored using the three-dimensional HIGRAD-FIRETEC model. Fires are simulated under weak to strong wind speeds and in shrubland and grassland fuel types. The importance of three-dimensionality in the simulation of such phenomena is demonstrated. Results suggest that when fire–atmosphere interaction is constrained to two-dimensions, the limitations of air moving through the head fire could lead to overestimation of downwind indrafts and effectiveness of backfiring. Three-dimensional simulations in surface fuels suggest that backfires benefit from the obstruction of ambient winds and potentially the existence of an indraft flow in only a limited range of environmental conditions. Simulations show that flows are most favourable when the wildfire is driven downslope by a weak wind and the backfire is ignited at bottom of the slope. Model simulations are compared with backfiring experiments conducted in a dense shrubland. Although this exercise encountered significant difficulties linked to the ambient winds data and their incorporation into the simulation, predictions and observations are in reasonable agreement.