Experimental and theoretical study of diameter effect on the ignition of cistus twigs
Résumé
In this work, the effect of the diameter on the ignition of twigs of Cistus Monspeliensis was studied experimentally and theoretically. For this, piloted ignition experiments were carried out in a cone calorimeter. In the first part of the study, the location of ignition, the ignition time and the flame residence time were investigated according to the twig diameter. Different modes of ignition were observed. The ignition could be due to: glowing of embers; flaming near the solid; spark of the pilot. The ignition time and the flame residence time increase strongly with the diameter. For small diameters, ignition time can be considered as proportional to the diameter. For high diameters, the ignition time tends to stabilize around a constant value (about 80 s). The second part of the study was devoted to the modelling of the temperature evolution in a twig. A one-dimensional nonhomogeneous heat-conduction problem was considered in finite medium. An optimization was performed to determine the model parameters (ignition temperature, twig emissivity and total heat transfer coefficient). From the expression of temperature in the twig, the ignition time was calculated for the various diameters. The comparison between these values and the experimental data shows a good agreement. Finally, a sensitivity analysis was carried out highlighting the influence of the ignition temperature and of the total heat transfer coefficient on the results.