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Structure of vegetation significantly influences its flammability and resulting fire spread. Despite considerable amount of laboratory studies, experimental works carried out with full plant specimens, representative of field conditions, are still limited. Present study aims to collect meaningful experimental data on structure and flammability of s...
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... the numerical shrub was modeled as the superposition of six fuel layers, each composed of one class of particles identified in the previous section. Finally, the bulk density of each particle class (Table 1) was determined from the mass measurements of each family at the height z and the associated frustum volume (): ...
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Structure of vegetation significantly influences its flammability and resulting fire spread. Despite considerable amount of laboratory studies, experimental works carried out with full plant specimens, representative of field conditions, are still limited. Present study aims to collect meaningful experimental data on structure and flammability of s...
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... En se basant sur les travaux de Tramoni et coll. [206], afin de caractériser spatialement la répartition des particules identifiées précédemment (section 2.1.3) dans les haies reconstituées, la structure de chaque haie a été divisée en 6 étages de la façon suivante (de bas en haut) : étage La base des haies a été prise égale à 0,5 m × 0,5 m. ...
Les “interfaces forêt/habitat”, zones où l’urbanisation et les espaces naturels se rencontrent, posent de sérieux problèmes dans la gestion du risque incendie en raison d'une augmentation des sources d'inflammation et de la vulnérabilité des infrastructures. Ces travaux de thèse s’inscrivent dans ce contexte et ont pour objectif d’étudier la vulnérabilité des constructions afin de définir des préconisations d’aménagement aux abords des habitations. Dans un premier temps, la vulnérabilité au feu de deux types de dalles de terrasse face à une source radiative et aux brandons a été étudiée. Face au rayonnement, deux dispositifs expérimentaux ont été utilisés et les critères de réaction au feu ont été analysés. L’échelle produit a montré que la forme des dalles influence l’inflammabilité. Les dalles thermoplastiques se sont avérées plus combustibles et émettrices de fumées que les dalles en bois. Pour caractériser l’inflammabilité des dalles face aux brandons, nous avons utilisé des copeaux de bois de différentes tailles et formes. Les brandons, enflammés ou incandescents, ont été disposés au contact des dalles à différentes positions. Notre étude a montré que les brandons incandescents ne permettaient pas d’allumer les dalles de terrasse étudiées. En revanche, avec les brandons enflammés, l’allumage des dalles intervient suivant plusieurs positions pour une masse minimale de brandons de 0,31 g pour le bois et 0,28 g pour le thermoplastique. Dans un second temps, nous nous sommes intéressés aux sollicitations thermiques engendrées par la combustion d’une haie et à son impact sur la dégradation des matériaux de construction. Pour cela une étude multi-échelle a été réalisée. A l’échelle du laboratoire, nous avons reconstituée une haie à l’aide de branches de ciste de Montpellier. La puissance dégagée, la perte de masse et la densité de flux de chaleur mesurée à 1,15 m de la haie ont été examinées. Les résultats obtenus lors du brûlage de la haie ont démontré qu’avec une forte vitesse de croissance au feu (FIGRA) et une puissance de feu (HRR) importante, les haies pouvaient significativement participer au développement d’un incendie aux interfaces forêt/habitat. Afin de se rapprocher de conditions réelles, la combustion de haies de ciste de surface (6 × 1 m²) a été étudiée à l’échelle du terrain en considérant deux hauteurs (1 et 2 m). Des fluxmètres radiatifs et totaux ont été positionnés à 3 m de la haie. Il a été mis en évidence que la densité de flux de chaleur mesurée pour la haie de 2 m est environ 1,6 fois plus importante que celle obtenue pour la haie de 1 m. Nous avons également reproduit une configuration d’interface forêt/habitat. Une haie (6 × 1 × 1 m³) a été placée au bord d’une terrasse faite de dalles en bois et à 3 m de différents types de menuiserie (PVC et aluminium). Les résultats ont montré que la densité de flux de chaleur générée par la haie était suffisante pour endommager les ouvrants et les dalles de terrasse en bois. Par la suite, nous avons utilisé le code de calcul WFDS (code CFD 3D) pour modéliser les expériences de combustion des haies de ciste à l’échelle du laboratoire. Les prédictions ont été confrontées aux résultats expérimentaux, montrant un très bon accord pour le HRR, la perte de masse et la géométrie du front de flamme.
... Some data can be found in the literature on plant flammability obtained in large and intermediate scale tests, which can be more realistic. For example, the pHRR of a small rockrose shrub (with up 3-20% MC in its leaves) was 228 kW [59]. With a rapid decrease after the peak, the flameout appeared in less than a minute, leaving the sample to be only partly consumed. ...
Assessing the fire risk of vegetated roofs includes the determination of their possible contribution to fire. Green roof components such as plants and growing media are organic materials and present a fuel that can catch and support the spread of fire. The flammability characteristics of these components were analyzed and compared to a typical roof covering. Growing media with 15% of organic matter were tested using cone calorimeter apparatus. The fuel load and heat release rate of the growing media were measured in both moist (30%) and dry conditions. It was observed that growing media in a moist condition do not present a fire risk, reaching a maximum heat release rate of 33 kW/m2. For dry substrates, a peak heat release rate of 95 kW/m2 was recorded in the first minute, which then rapidly decreased to 29 kW/m2 in the second minute. Compared to a typical bitumen roof membrane, the green roof showed a better fire performance. The literature data report more severe results for plant behavior, reaching peak heat release rates (HRRs) of 397 kW/m2 for dried and 176 kW/m2 for a green material. However, a rapid decrease in HRR to much lower values occurs in less than 2 min. The results also show that extensive and intensive types of green roofs present 22% and 95% of the additional fire load density when installed on a modified bitumen membrane, 19.7 and 85.8 MJ/m2, respectively.
... A more in-depth overview is given by Perez-Ramirez et al. [28]. Tramoni et al. [29] validated WFDS for the burning of a rockrose shrub (Cistus monspeliensis) and found that WFDS was able to accurately predict relevant fire properties, such as heat release rate, flame duration, and consumption rate. The shrubs were modeled as a series of frustums of a porous medium composed of two elements, foliage and branches. ...
The current work focuses on better understanding of the phenomenon of fire interactions between multiple burning chamise shrubs situated in a square array. The Wildland-Urban Interface Fire Dynamics Simulator (WFDS , Mell et al. 2009) was utilized to study burning behavior of nine 1 m tall, 0.7 m maximum diameter chamise shrubs placed in a 3 × 3 horizontal array arrangement. All shrubs were simultaneously ignited from their bases by individual ignition zones located on the ground beneath the shrubs. Several simulations were performed by varying the shrub separation distance from zero to 0.875 meters and wind speed from zero to 1.0 m/s. Two competing interaction mechanisms were identified: heat feedback enhancement, primarily due to thermal radiative heating, and air entrainment restriction. The burning characteristics of the shrubs were examined as well as a global average burning rate. For the no wind condition, the peak mass loss rate of the center shrub is 23% higher than the rest of the shrubs, indicating that the heat feedback enhancement is dominant. However, air entrainment causes the surrounding shrubs to burn less intensely. At an imposed wind speed, air entrainment effects are dominant. The shrubs that are best shielded from the higher wind speeds are the shrubs that burn most intensely.
Fire interactions between multiple 1 m tall, 0.7 m diameter chamise shrubs was studied utilizing the Wildland-Urban Interface Fire Dynamics Simulator (WFDS, Mell et al., 2009). Two shrub arrangements were investigated. First, nine shrubs were placed in a 3×3 horizontal region. The shrub separation distance and wind speed were varied. Two competing interaction mechanisms were identified: heat feedback enhancement, primarily due to thermal radiative heating, and air entrainment restriction. Shrub burning characteristics were examined and a global average burning rate was analyzed. For the no wind condition, the peak mass loss rate of the center shrub is 23% higher than the others, indicating that heat feedback enhancement is dominant. Air entrainment causes the surrounding shrubs to burn less intensely. At an imposed wind speed, air entrainment effects are dominant. Shrubs that are best shielded from the wind burn most intensely. Second, the vertical separation between two shrubs was varied under different wind conditions. With no ambient wind, nearly no interaction between the two shrubs was observed. At a wind speed of 1 m/s, significant interaction between shrubs occurred due to flame-tilting. The downwind shrubs burned the most vigorously for vertical separation distances between 0.2 and 0.8 m.
