Radiant Auto-Ignition of Wood

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Although piloted-ignition is well studied, auto-ignition is not, and under some conditions the latter may initiate the former. For that reason, the dynamics and mechanisms in the auto-ignition of redwood by radiant heating were experimentally studied. An open-cone radiant source heated an insulated cubical vertical sample 4 cm on a side. Infrared (IR) and normal video cameras were used to view the surface of the wood. Surface temperature (by thermocouple and IR) and mass loss were continuously recorded. The wood grain orientation was aligned either perpendicular or parallel to the incident heat flux. The time for flaming ignition is measured up to 70 kW/m2 and compared to piloted ignition results of Spearpoint and Quintiere [28,29] with little difference above 40 kW/m2. With no visible flame, the surface temperature of the wood can achieve 700 C, while the corresponding inert fibrous insulation achieves only 500 C. This is indicative of surface oxidation, which likely plays a role in the naming ignition or redwood. For example, at 40 kW/m2, heating perpendicular to the grain, flaming ignition occurs in 1000 s. The initiation of flaming ignition is seen in the gas phase above the sample consistent with the observations of Simms [25]. The purpose of this study is to examine experimentally and theoretically the auto-ignition of wood. The important parameters for ignition: ignition time, ignition temperature, mass loss rate, critical heat flux, and thermal inertia are examined and compared with the piloted ignition. The ignition contributed by glowing ignition, and flaming ignition is also discussed.