Influence of fuel load on the flammability of live Pinus Halepensis needles

Abstract

The fuel flammability, combustibility and fire spread properties depend on several parameters. Among these parameters the heat flux, moisture content, wind and the fuel load. Flammable ignition occurs when the emitted organic volatile components flow mixed with air reaches to a minimum rate corresponding to the lower flammability limit. The gas flow of these components depends on the fuel quantity (load) and its temperature. Therefore, ignition time depends on the fuel load. In this work the effect of the load on ignition time of live Pinus Halepensis needles is investigated using a cone calorimeter (providing three incident heat flux intensities). The average fuel moisture content is around 50% in wet basis. Ignition time exhibits an exponential trend for all the incident heat flux intensities considered with a correlation coefficient R^2>0.96. The exponent corresponds to a characteristic load m ̅_0 around 1 kg/m² slightly dependent on the incident flux within statistical errors. For very small loads compared to m ̅_0 ignition time is nearly independent of load, because the time required for water evaporation is neglected compared to that required for volatiles mixture with air to reach the lower flammability limit. For larger loads (in the range 0.38 kg/m² to 1.27 kg/m²), the time required for water evaporation introduces a shift in ignition time which increases linearly with load. Finally, for large loads compared to m ̅_0, the fuel bed thickness becomes larger than the optical length, and only its top layer is heated, transmitting thus heat to the internal layers by conduction. Therefore, the internal layers evaporate water and cool the upper one, leading to exponential of ignition time with the load. For dry fuels, ignition time appears independent of load.