Ignition and combustion behavior of pressure treated wood and wood-plastic composite exposed to glowing firebrand piles: Impact of air flow velocity, firebrand coverage density and pile orientation

Pressure treated wood (PTW) and wood-plastic composite (Trex®) were exposed to glowing firebrand piles in a bench-scale wind tunnel. The air flow velocity was 0.9–2.7 m s⁻¹, the firebrand coverage densities were 0.06 and 0.16 g cm⁻², and the pile footprint was 5 × 10 cm² with either the 10-cm or 5-cm sides perpendicular to the incident air flow. Several types of flaming ignition events were observed including flames attached to the substrate surface in front of the pile (preleading zone ignition), and flames attached to the pile that sometimes spread onto the substrate downstream of the pile (downstream ignition). The most frequent and long-lasting flaming combustion occurred in experiments performed at 2.4–2.7 m s⁻¹ using 0.16 g cm⁻² firebrand coverage density piles with 10-cm sides perpendicular to the air flow. Trex® was less prone to preleading zone ignition but was more prone to downstream ignition. Unlike Trex®, PTW exhibited a propensity for sustained smoldering for a wide range of air flows.