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This experimental study is a contribution to the understanding of the phenomena of the wildfire. This work was realized on the dead twigs of Cistus monspeliensis exposed to the heat flux at 50 kW m2 using a cone calorimeter. The experiments were realized on the shrub particles with different thicknesses (range of leaves and twigs: 1–20 mm) in function of the stage of combustion (pre-ignition, flaming, and glowing phases). For this, heat release rate, smoke production rate, smoke extinction area, the aerosol yield ( Ya) via the light extinction measurements, and modified combustion efficiency were measured. The results show that there is a size limit for which the heat release rate had a different combustion behavior. The smoke production rate was also measured, and it showed that smoke was mainly emitted during the pre-heating phase. Aerosol yield ( Ya) was calculated depending on these combustion phases (pre-ignition and flaming stage) and for the range of thickness of the samples.

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The effect of the diameter of dead twigs of Cistus monspeliensis on their ignition was studied experimentally and theoretically. Autoignition experiments were carried out in a cone calorimeter. The ignition time, surface temperature before ignition, flame residence time, smoldering time and mass loss were measured. The particles were classified into two groups based on their ignitability. The first group contained the most flammable twigs, which had diameters smaller than or equal to 4 mm, along with leaves. The second one included twigs with diameters equal to or larger than 5 mm. For a radiant heat flux of 50 kW/m 2 , the 4-mm value appeared to be the upper limit for the size of the particles potentially involved in the spread dynamics of wildfires. However, bark detachment was observed on the thickest twigs, which greatly decreased their ignition time. Two ignition criteria were investigated: the ignition temperature and critical mass flux. The ignition temperature increased with the twig diameter, showing that this quantity should be carefully considered in ignition models. A thermal ignition model was proposed to determine the ignition time of twigs according to their diameter. The critical mass flux appeared to be fairly constant for any fuel diameter and could also be convenient for modeling the ignition of shrub fuels.

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