The Near Extinction Flame Behavior and Extinction Limit of Timbers

In this work, we investigate the near extinction flame behavior and the self-extinction mechanism for a range of wood samples under external radiant heat ux. A special surface flame is observed above a critical heat flux (35~45 kW/m² for different wood samples). Different from the normal buoyancy-controlled yellow flame, the surface flame is weak, blue, and discontinuously floating above the wood residue surface, which is almost free of buoyancy effect. When the mass flux decreases to about 4.0 g•m^-2•s^-1 for all wood samples, the buoyancy-controlled yellow flame transits directly to smoldering under the low heat flux, while to the blue surface flame under the high heat ux. The surface flame eventually extinguishes and transits to smoldering at the critical mass flux around 1 g•m^-2•s^-1, redefining the flame extinction limit of timber materials. The analysis shows that there are two necessary conditions for the occurrence of near-limit blue flame, 1) flammable gases emitted during the smoldering combustion of char, and 2) the charring surface temperature exceeding the minimum autoignition temperature elevated by large irradiation. This newly observed surface flame may play an important role in the transition between flaming and smoldering and help evaluate the fire risk of timber structure under real fire scenarios.