Extinction of wood fire : a near-limit blue flame above hot smoldering surface

Abstract

Abstract: Timber is cost-effective and environmentally-friendly, which is a potential material for sustainable buildings, but its fire safety is still a significant concern. In this work, we investigate the burning behaviors of different types of woods and their self-extinction mechanism under external radiation. A unique near-limit flame is observed when the irradiation is above a critical value of about 40 kW/m2. Such a near-limit flame is weak, blue, and discrete that tends to attach to the wood residue surface, different from the normal buoyancy-controlled sooty yellow flame. If the irradiation is low (< 40 kW/m2), the yellow flame extinguishes and transits directly to smoldering at the mass flux of about 4 g/m2 s. However, above the critical irradiation level, the yellow flame transits to the blue flame that does not extinguish until the mass flux of around 1 g/m2 s, extending the flame extinction limit of timber materials. The near-limit blue flame may appear only if the char surface temperature exceeds 700°C. Two critical conditions are hypothesized for this unique blue flame, (I) in-depth pyrolysis (mainly lignin) sustained by the internal smoldering combustion, and (II) the hot surface maintained by large external radiation to extend the flammability limit. This unique blue flame may play an essential role in the transition between flaming and smoldering and help evaluate the fire risk of timber materials under real fire scenarios.