Autoignition of Dead Shrub Twigs: Influence of Diameter on Ignition

Abstract

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.

Multi‐scale simulations of the thermal degradation of thin wooden plates: Evaluation of two kinetic mechanisms

“…These kinds of plates were chosen in order to obtain a characteristic thickness close to those of the thin particles (twigs, leaves, and needles) involved in the spread of wild land fires. [1][2][3]36 For all experiments, the plates were oven-dried at 60 C for 24 hours to remove moisture. This protocol was already used to remove totally the moisture content of small particles of biomass or wood 1,27 without damaging the material.…”

“…[1][2][3]36 For all experiments, the plates were oven-dried at 60 C for 24 hours to remove moisture. This protocol was already used to remove totally the moisture content of small particles of biomass or wood 1,27 without damaging the material. However, since the start of the experiments at matter and material scales is not immediate after the removing of the plates from the oven, a rehydration occurred and the moisture content of the samples was around 1%.…”

“…The propagation of wildland fires is mainly driven by thin fuel particles [1][2][3] (leaves, needles, grass, and so on). Most authors consider that particles smaller than 6 mm are responsible for fire growth and development.…”

“…First of all, this thickness is characteristic of that of the fine particles involved in the propagation of forest fires, such as needles or twigs. [1][2][3]36 Second, the low thickness makes it possible to avoid the superposition of the different phases of thermal degradation, that is, the gasification and the char oxidation. Finally, since the temperature gradients are small inside the wood thickness, the influence of transport phenomena are minimized which prevent the imprecisions caused by the uncertainty on the thermal properties of the plates.…”

Multi-scale modeling of the thermal degradation of woods

“…Pieces of 100 × 100 × 0.6 mm 3 were cut in the wooden sheets for samples. Then, they were placed on ceramic wool in a sample holder mesh basket of 10 ×10 cm 2 made of stainless steel (Tihay-Felicelli et al 2016). They were oven dried at 60°C during 24 hours before each experiment.…”