Effect of moisture content on the spotting ignition of live wildland fuels

Fuel moisture content (FMC) is one of the most important fuel properties affecting fire spread in wildland fires. Knowing how it affects the ignition of live and dead fuels is of great importance as a necessary input for risk assessment mapping of future wildland fires. The present study aims to improve the current understanding of the spotting ignition behavior of live wildland fuels with different FMC under controlled boundary conditions using an idealized firebrand. A fuel bed of Pinus radiata needles is systematically studied in the Idealized Firebrand Ignition Test (I-FIT) apparatus, in which the firebrand is idealized by a controlled electric heater providing energy only by radiation while maintaining a cylindrical configuration. The experiments have demonstrated high repeatability under controlled conditions. In addition, this will provide necessary measurements for theoretical modeling of the ignition of live biomass. Moisture content was varied between 115% (completely wet) and 0% dry mass, controlled by drying live pine in an oven. Moisture contents above 60% showed inconsistent ignition under the conditions studied. Therefore, experiments were conducted with 0%, 30%, and 60% FMC. The results show that the critical incident heat flux increases with FMC, which also causes an increase in ignition time and temperature for a fixed incident radiant flux. These results were compared to a theoretical thermal model that considers the penetration of radiation into the medium, including the heat capacity of the introduced FMC. Experimental and theoretical results are generally in good agreement, with some discrepancies at high incident fluxes, where the results predict a deviation from a linear 1/t_ig−q̇_inc^′′ relationship, an effect that is less pronounced at higher FMC. A useful unified correlation is provided that relates normalized ignition time to normalized incident radiant flux. This provides useful information for fire hazard assessment and practitioners for fire risk assessment.