Self-extinction of wood plate: Effect of fuel thickness and flame spread orientation

Flame spread behaviour is a key element in evaluating material fire hazards, and studying the flame spread on wood helps understand the fire safety of timber structures. This work tests the flame spread on wood plates with thicknesses ranging from 1.0 mm to 30.0 mm under different irradiations and both upward and downward spread modes. Various flame spread phenomena are observed, including (1) flame spread without fuel burnout, (2) flame spread with fuel burnout, and (3) self-extinction. The flame tends to self-extinguish on wood, as the sample thickness increases or the flame spread changes from upward to downward. Minimum flame spread rate is found to be ∼0.03 cm/s for the upward flame spread, below which self-extinction occurs. Without external radiation, the self-extinction limit in terms of the maximum wood sample thickness for upward flame spread on one surface is quantified to be about 7.5 mm. As the irradiation increases, self-extinction becomes more difficult, because of the enhanced heating in both preheating and burning regions. For irradiation larger than 15 kW/m², the surface flame spread significantly increases and gradually transitions from the surface ignition process to the gas-phase flame propagation, so self-extinction no longer occurs. This work quantifies the self-extinction limits of wood flame spread as a function of sample thickness, irradiation, and spread orientation, which helps to improve fire resilience for future timber buildings.