Abstract
Using a cone calorimeter as a heat source, the influence of compactness on flaming ignition/residence times was explored experimentally for three different dry vegetation types (Pinus halepensis, Pinus pinaster, and wheat straw). While the residence time increases monotonically with the packing ratio, the ignition time exhibits a trend with at least one minimum for the fuels studied across a wide range of compactnesses from 3 to 50%. The range of the minimum ignition time is considerably narrowed to a few millimeters when the mean free path is employed in place of the packing ratio, demonstrating the importance of incident radiation in the heat transfer mechanism. For the Pinus halepensis needles, two minimum ignition times of approximately 12.5% and 34% were identified. A heat transfer analysis developed for this porous fuel using a theoretical model based on energy conservation showed that conduction and internal convection heat transfer enhance the heat flux received by the surface. The contribution of anomalous heating dynamics to ignition time behavior is further discussed. The model does not predict this anomalous dynamic because it does not account for the mass loss caused by heating.