Extinction of Wood Fire: Modeling Smoldering and Near-Limit Flame Under Irradiation

Timber, as renewable and carbon–neutral construction material, has gained a new renaissance for tall buildings to meet the initiatives of sustainable construction, but their fire safety is still a major concern. Previously, we identified a unique combustion mode showing a near-limit flame that is weak, blue, discrete and tends to attach to the hot smoldering wood residue surface. Such a flame is an intermediate combustion mode between the typical yellow wood flame and pure smoldering and occurs when the irradiation is above 40 kW/m². This work proposes two numerical models based on the open-source code Gpyro and FDS to reproduce the solid-phase smoldering and the gas-phase near-limit flame, respectively. The solid-phase model demonstrates that the gaseous fuels for the near-limit flame mainly come from the pyrolysis of lignin that is maintained by the heat evolved from the internal char oxidation and external heating. The gas-phase model demonstrates the necessity of a hot surface and a small critical mass flux to maintain a near-limit flame that has a limited buoyancy effect. Finally, different flame regimes are obtained by the numerical simulations and summarized as a function of the fuel surface temperature and gaseous fuel mass flux. This is the first time that comprehensive models have been used to reveal the underlying mechanisms for smoldering-assisted flame, so it provides a better understanding of fire dynamics and helps evaluate the fire risk of timber materials under real fire scenarios.