Using a cone calorimeter for small-scale tests as a radiant heat source, the influence of compacting fuel beds on their flammability and combustibility for three different types of dry vegetation (Pinus halepensis and Pinus pinaster needles, and Wheat straw sticks) was experimentally investigated for a wide range of compactness (the degree of densification of the fuel bed) from <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\:3\)</EquationSource> </InlineEquation> to <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(\:50\:\%\)</EquationSource> </InlineEquation>. While the flaming duration increases monotonically with packing ratio (a quantitative measure of this compactness), ignition time shows at least one minimum value for the fuels studied. Using the mean free path of radiation as the controlling parameter instead of the packing ratio the range with minimum ignition time is significantly narrowed down to a few millimeters, which indicates the importance of radiation in the flammability process as expected for such a mainly radiative heating process. For Pinus halepensis needles, two minimum ignition times for values of packing ratio around 12.6% and 34% were identified both when the fuel bed thickness is varied or maintained constant. For low compactness, before ignition, the mass loss decreases linearly with time with a rate slightly affected by the packing ratio. After ignition, the flame spreads quickly to cover the entire fuel bed (fully developed flame) at the growth time. This marks the beginning of a sharp decrease in the mass corresponding to a sharp increase in the mass loss rate (MLR) towards a maximum value. For high compactness, the linear decrease of the mass still holds before ignition, followed by a long slightly non-linear trend corresponding to the slow flame spreading to cover the whole surface of the bed at the growth time (fully developed flame). This also marks the begining of a sharp mass decrease (MLR increase). The duration between ignition time and growth time is then very short for low compactness, and becomes very large for high compactness and corresponds to a plateau of MLR. The peak MLR is followed by a slow decrease until extinction (decay phase). To analyze the compactness induced combustion, the MLR curve was divided into two widths: the upper width is the Full Width at Half Maximum (FWHM), and the lower width is the duration (<InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(\:{x}_{t}\)</EquationSource> </InlineEquation>) between extinction time and growth time. The peak MLR, and the FWHM were found insensitive to the compactness (constant behavior within statistical errors for all compactness) whereas the <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\:{x}_{t}\)</EquationSource> </InlineEquation> was found to decrease for high compactness.

Influence of Compactness on Flammability and Combustibility of Wildland Fuels

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2026年2月1日

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H. Boutchiche,
N. Zekri,
D. X. Viegas,
A. Sahila,
O. Mosbah

摘要

Using a cone calorimeter for small-scale tests as a radiant heat source, the influence of compacting fuel beds on their flammability and combustibility for three different types of dry vegetation (Pinus halepensis and Pinus pinaster needles, and Wheat straw sticks) was experimentally investigated for a wide range of compactness (the degree of densification of the fuel bed) from \(\:3\) to \(\:50\:\%\) . While the flaming duration increases monotonically with packing ratio (a quantitative measure of this compactness), ignition time shows at least one minimum value for the fuels studied. Using the mean free path of radiation as the controlling parameter instead of the packing ratio the range with minimum ignition time is significantly narrowed down to a few millimeters, which indicates the importance of radiation in the flammability process as expected for such a mainly radiative heating process. For Pinus halepensis needles, two minimum ignition times for values of packing ratio around 12.6% and 34% were identified both when the fuel bed thickness is varied or maintained constant. For low compactness, before ignition, the mass loss decreases linearly with time with a rate slightly affected by the packing ratio. After ignition, the flame spreads quickly to cover the entire fuel bed (fully developed flame) at the growth time. This marks the beginning of a sharp decrease in the mass corresponding to a sharp increase in the mass loss rate (MLR) towards a maximum value. For high compactness, the linear decrease of the mass still holds before ignition, followed by a long slightly non-linear trend corresponding to the slow flame spreading to cover the whole surface of the bed at the growth time (fully developed flame). This also marks the begining of a sharp mass decrease (MLR increase). The duration between ignition time and growth time is then very short for low compactness, and becomes very large for high compactness and corresponds to a plateau of MLR. The peak MLR is followed by a slow decrease until extinction (decay phase). To analyze the compactness induced combustion, the MLR curve was divided into two widths: the upper width is the Full Width at Half Maximum (FWHM), and the lower width is the duration ( \(\:{x}_{t}\) ) between extinction time and growth time. The peak MLR, and the FWHM were found insensitive to the compactness (constant behavior within statistical errors for all compactness) whereas the \(\:{x}_{t}\) was found to decrease for high compactness.