Calculation of fireline intensity by oxygen consumption and comparison with Byram's formulation

International audienc

Influence of Prescribed Burning on a Pinus nigra subsp. Laricio Forest: Heat Transfer and Tree Vitality

Surface fuel removal is crucial to facilitate the mitigation of severe fires in forests. Prescribed burning is often used by forest managers, thanks to its low cost and high efficiency in hard-to-reach areas. The determination of heat transfer between fires and trees has rarely been carried out on living species and consequently, their long-term effects on tree physiology are still not fully understood. In this study, a multidisciplinary approach was conducted to evaluate the impact of a late spring (June) prescribed burning on a Mediterranean pine forest (Pinus nigra subsp. laricio). The surface fuels consisted of a 656 g m−² needle litter, mixed with a few scattered living herbaceous strata. During the fire spread, measurements of the inner and outer trunk temperatures were made at the base of 12 trees with an average bark thickness of 19.4 ± 7.0 mm. The fireline intensity and flame residence time were in the range of 110–160 kW m−1 and 220–468 s, respectively. Despite a maximum heating rate at the cambial area of 4.37 °C min−1, the temperature of these tissues remained below 60 °C, a critical threshold above which thermal damage will occur. In addition, prior- and post-fire physiological monitoring was performed over a long time period (2.5 years) on 24 trees, using sap flow, chlorophyll fluorescence and gas exchange measurements. All parameters remain highly correlated and indicate that the burned trees did not suffer physiological damage. Moreover, drought resistance strategies were not altered by the prescribed burning. The thermal insulation capability of the bark allowed the functional tissues to experience low heat stress that did not affect tree vitality

Feasibility of particle image velocimetry in vegetative fire spread experiments

International audienceThis study is part of an ongoing effort to improve the understanding of mechanisms that control the spread of fires with a focus on the turbulent flow modified by the flame front. A large-scale PIV system was used to measure the flow field inside and in the vicinity of a flame front spreading across a bed of fuel in an open environment. The vegetative fuel consisted of a 10-m-long and 5-m-wide bed of excelsior (1 kg/m² fuel load) leading to a nearly 1.5-m-high flame front. The velocity field was investigated in a measurement region about 1.5 m high and 2 m long. In such a configuration, a 450-mJ laser source was used to generate the light sheet, and the flow was seeded using zirconium oxide particles (ZrO2). The PIV measurements in the presence of flame were improved by the use of a liquid crystal shutter in front of the PIV camera, allowing very short exposure times and eliminating the flame trace in the tomographic pictures. Despite the variability of the external conditions, leading to a difficult seeding over the whole PIV area, the present study shows the feasibility of the optical method of fluid visualization in the field. The measurements of the velocity fields show some features of the dynamics of fire plumes. This preliminary study demonstrates the feasibility of the method in the open, but some strong efforts to improve the seeding of the flow must be made

Modélisation 3D d'écoulements thermo-convectifs en milieu fracturé

Dans les écoulements en milieu fracturé, des considérations dimensionnelles montrent que si le nombre de Reynolds construit sur l'ouverture de fissure est faible, les équations de Navier-Stokes et de continuité conduisent à l'équation de Reynolds connue en lubrification. Nous construisons une formulation par éléments finis de cette équation adaptée aux écoulements en fractures 3D. Nous validons cette formulation en confrontant les résultats qu'elle produit à ceux issus de la résolution directe des équations de Navier-Stokes. Nous utilisons enfin cette formulation implémentée dans Flux-Expert® pour simuler l'écoulement convectif dans un réseau de fractures complexe

Determination of fireline intensity by oxygen consumption calorimetry

International audienc

Experimental investigation on the burning of shrubs

International audienc

Steady and Unsteady Fireline Intensity of Spreading Fires at Laboratory Scale

International audienc

Effects of slope on fire spread observed through video images and multiple-point thermal measurements

International audienceA series of nine fire experiments was conducted on a large-scale bench with an inclinable plate to study the effects of a slope on fire spread. The temperature and total and radiant heat flux densities were measured at the top of the fuel bed during the upslope fire spread in still air. Measurements were recorded at 5 different locations ahead of the fire front. The effects of the slope on the flame topology, fire dynamics, and heat transfers ahead of the fire front were observed. The thermal measurements and image analyses showed a change in the fire spread regime and its related behavior with an increase in the slope. This was observed through an elongation of the flame and its contact surface over the upper part of the fuel bed. When the slope increased, the fire topology changed from a U-shape to a V-shape, and the radiation-dominated thermal environment ahead of the flame front was progressively turned into a mixed convective-radiative one, in which convection finally dominated for steep slope configurations. For fire spreading across a 30° slope, this increase in the curvature of the fire front (V-shape) is associated with the appearance of fire whirls rolling along the flanks of the fire front, which indicates significant changes in the fluid mechanics and heat transfers. The span of the heat transfer increased and a significant amount of energy impinged 1 or 2 m ahead of the flame front because of the flame tilt (radiation) and hot gas flow (convection). The measurements also indicated a greater amount of energy impinging in the center of the fuel bed than at the edge. This study shows the importance of investigating the fluid mechanics of fire in future wildfire researches

Experimental investigation of flammability and numerical study of combustion of shrub of rockrose under severe drought conditions

International audienc