A criterion for ignition of combustible solids exposed to firebrand piles

Firebrands are glowing particles, typically millimeters to centimeters in size, that represent a common ignition source for structures in wildland-urban interface fires. In this work, three distinct types of ignitions of combustible substates by firebrands differing in mechanism have been identified through a review of the bench-scale wind tunnel experiments performed in earlier studies. A model for the most frequently observed type of ignition, preleading zone ignition, was developed. This model simulates a substrate’s response to firebrand exposure using a combination of an empirical firebrand pile heat feedback sub-model and a detailed pyrolysis model of the combustible substrate. Using the ignition model, an analysis of the experimentally observed preleading zone ignitions of Western Red Cedar, pressure-treated wood, and a wood-plastic composite at 0.9 – 2.7 m s -1 air flow velocities and 0.06 and 0.16 g cm -2 firebrand coverage densities was performed. It was determined that the attainment of the 10-s-averaged heat release rate of 120 kW m -2 or higher, computed under the assumption of complete combustion of the gaseous pyrolyzate, was a reliable predictor of the ignition occurrence. It was also found that the same critical heat release rate value can be used to predict the preleading zone ignition time. • Three ignition types of combustible substrates by firebrands were identified • Detailed pyrolysis modeling was used to analyze most common type of ignition • Heat-release-rate-based criterion for the ignition was derived