Modelling the Burning of an Ornamental Vegetation with WFDS: From Laboratory to Field Scale ^†

Among the vectors of fire propagation towards buildings in the WUI, ornamental hedges have been identified as one of the main elements [1]. In terms of regulations, there is no global consensus on the distances between ornamental plants and buildings. Consequently, it is necessary to scientifically study how the burning of ornamental plants and more particularly hedges impacts constructions according to their distance from the structure elements considered. Numerical simulations may be an appropriate approach for this type of study. However, it is necessary to ensure beforehand that the numerical model correctly reproduces the phenomena involved. Studies on the capacity of WFDS to reproduce the combustion of plants at the laboratory scale have already been carried out [2,3,4].

To go further in the test of WFDS, this work presents the results concerning the numerical prediction of the combustion of a hedge at the laboratory and field scales. The experimental data used for this study were obtained from dedicated experiments of burning reconstructed hedges of rockrose composed of dried branches. At both scales, the numerical model had a computational domain filled with Cartesian cells whose size depended on the scale studied. Vegetation is modelled using a Fuel Element approach [3] to consider the fire spread throughout the raised volume of the hedge. The three stages of the thermal degradation of the fuel (dehydration, pyrolysis, char oxidation) are described using Arrhenius laws [4].

The results show a good agreement between experiments and simulations for both scales. The phases of the fire growth and fully developed fire are well represented for both the heat release rate (at the laboratory scale) or for the heat flux levels (at the field scale). Fire extinction occurs faster in WFDS, especially because WFDS cannot model the collapse of particles during combustion.