Transition from smouldering to flaming combustion of pine needle fuel beds under natural convection

Abstract

Smouldering transition to flaming combustion (StF) of forest fuels can rekindle wildfires, possibly causing the suppression effort to be useless or the firefighters to be stuck in danger. However, the StF mechanism of forest fuels remains to be elucidated. In this work, the StF of pine needle fuel beds with different bulk densities was investigated experimentally. Two modes of transition are identified: Middle- and Trailing-StF, which occur at the middle and trailing edges of the char region in the fuel bed, respectively. The conditions for the occurrence of both modes include (1) the length of the char region exceeding a critical value and (2) a hot glowing area (strong secondary char oxidation area) appearing before StF. An overhang structure is also necessary to induce Trailing-StF. These conditions are interpreted in terms of the permeability of fuel beds, which affects the diffusion and convection supply of oxygen for smouldering. Middle-StF occurs due to sufficient oxygen diffusion and convection supply in the middle of the char region with large pores. In Trailing-StF, the overhang structure forms a region with high permeability, which significantly enhances the convection mass flow rate of oxygen and provides a favorable space for the accumulation of mixed combustible gases. The oxygen mass flow rate leading to both StF modes is in the order of 10−2 g/s.