Glowing to flaming transition in Douglas fir with varied moisture content

In wildfires, vegetation can ignite through convective and radiative heating, influenced by factors like wind and fire size. Understanding ignition from these modes is crucial for computational fire models. This study investigates the ignition behavior of Douglas fir branches under convective heating using a custom apparatus, thermogravimetric analysis (TGA), and computational simulations in Fire Dynamics Simulator (FDS). Ignition delay times were determined from color camera recordings. In all experiments resulting in flaming ignition, glowing combustion preceded it. Fuel moisture content (FMC) significantly impacted ignition, with higher FMC delaying ignition and reducing ignition probability, while lower FMC increased the likelihood of ignition. In tests with flaming ignition, gas-phase ignition occurred near the fuel sample in most cases, though in some, it occurred farther away. High-speed imaging was used to track ignition location and flame propagation when gas-phase ignition occurred at a distance from the sample. TGA revealed how thermal decomposition influenced ignition dynamics. Computational modeling aligned with experimental findings and clarified the role of heat transfer, fuel properties, and FMC on combustion. These results enhance the understanding of vegetation ignition and fire dynamics under convective heating conditions.