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Theses

Etude expérimentale et numérique du comportement au feu de végétaux

Abstract : For several years, forest management are conducted to limit the spread of wildland fires and thus to reduce their impacts. The common technique consists in a reduction of the fuel load to reduce the fire intensity and its rate of spread. Theses fuel break can be used during firefighting activities. However, the management of vegetation areas is still done without a sound scientific knowledge of the combustion dynamics of vegetative fuel. The main objective of this thesis work is to improve our fundamental knowledge on the combustion dynamics of natural and isolated shrubs while providing a validation basis for physics-based models. Thus, four studies have been carried out. The rockrose (cistus monspeliensis) was first characterized according to its structure and the flammability criterion. The characterization data was then used as input for the calculation code WFDS. As a first step, this code was validated for the fire spread of litters composed of a single type of particles based on experimental results obtained from a previous study. As a second step, WFDS was tested for the combustion of rockroses, composed of particles with different sizes. In addition, a sensitivity analysis of the parameters related to the source term of mass was conducted. The data collected during the characterization highlight the small variation of the composition of rockroses with similar dimensions. This allowed us to develop a numerical model for a typical cistus of size similar to those studied. A significant variation in the structure of the cistus was also observed during the summer period. Most of the leaves have fallen to form a litter of dead fuel under the shrub. In addition, flowers containing seeds, that appeared earlier in the year, are dry. The rockrose flammability has been experimentally studied according to the four criteria defined in the literature (ignitability, sustainability, combustibility and consumability). The experiments allowed us a better understanding of the ignition position. Moisture content was observed to significantly influence the combustion processes. An ignition in the upper part of the crown with a low surrounding quantity of fuel leads to a long sustainability and weak or moderate combustibility and consumability (low and moderate flammability). Conversely, an ignition in the upper part of the shrub, with a large amount of fuel in its vicinity, causes high flammability. The reaction to fire is fast with a moderate intensity (high and fast flammability) if the moisture content of the particles is high. It is slow with a high intensity if it the moisture content is low, which leads to a high consumption of the thick twigs (high and consumer flammability). The moisture content of the leaves also has a significant effect on the ignition time (ignitability). The obvious change in the shrub properties, with the presence of flowers, observed for the rockrose in summer, led to a change in combustion dynamics. The weaker quantity of leaves in summer, compared to other seasons, leads to a low flammability. However, the flaming flowers, which fall during the combustion, not only ignite the leaves on their path but can also be a potential source of ignition for the litter under the shrub. Preliminary tests of WFDS were first performed for a fuel bed of pine needles, in order to assess its performance under more easy simulation case. The predicted intensity and steady rate of spread state agree with those observed experimentally but the model failed to represent accordingly the extinction phase that was too fast. Finally, simulations of the combustion of a typical cistus were performed. The dimension of the ignition zone was first characterized for a dry cistus. Then simulations of the combustion of cistus with fuel moisture content, were done. This revealed a good ability of WFDS to represent the parameters related to the flammability criteria.
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Jean-Baptiste Tramoni. Etude expérimentale et numérique du comportement au feu de végétaux. Sciences de la Terre. Université Pascal Paoli, 2018. Français. ⟨NNT : 2018CORT0002⟩. ⟨tel-03448838⟩

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