Minimum oxygen supply rate for smouldering propagation: Effect of fuel bulk density and particle size

Smouldering is a low-temperature, flameless, and persistent combustion process driven by heterogeneous oxidations. Oxygen supply is a key parameter of smouldering and is sensitive to fuel density and particle size, but our understanding is still limited. Herein, we explore the oxygen threshold for smouldering propagation under upward internal airflow velocities up to 5 mm/s. Pine needles with different bulk densities (55-120 kg/m3) and wood samples with different particle sizes (1-50 mm) are tested. We found that the minimum airflow velocity for sustaining smouldering propagation increases with the decrease of the bulk density or the increase of the particle size. By increasing the airflow velocity, the smouldering front first propagates unidirectionally (opposed) and then bidirectionally (opposed + forward). Nevertheless, when the pore size is large (the fuel particle size is large or the fuel bulk density is small), bidirectional propagation always occurs, because the oxygen can leak through the opposed smouldering front. A simplified thermochemical analysis is proposed to reveal the influence of interparticle heat transfer on the minimum oxygen supply rate of smouldering propagation. This work advances the fundamental understanding of smouldering on solid fuel particles and their smouldering fire risks and helps optimize the efficiency and safety of smouldering processes.