Quenching of smoldering: Effect of wall cooling on extinction

Smoldering is the slow, low-temperature, and flameless combustion phenomenon in porous fuels. Smoldering is different from flaming regarding the chemical and transport processes, despite sharing many similarities in ignition and fire spread. In this work, we explore the applicability of quenching and quenching diameter in smoldering combustion. The smoldering of dry organic soil is initiated in the 25-cm long tubular reactor with different diameters from 4 cm to 15 cm. The thermal boundary and oxygen supply of the smoldering reactor are varied by using different wall materials and opening configurations, respectively. The quenching of smoldering is observed as the diameter of the reactor decreased, which is the same as the quenching of the premixed flame. The minimum smoldering temperature (~250 °C) and propagation rate (~0.5 cm/h or 0.1 mm/min) are found before quenching. The measured quenching diameter of smoldering is about 10 cm (much larger than the flame) and comparable to the thickness of reaction front (similar to the flame). The quenching diameter of smoldering increases as the wall cooling increases and the oxygen supply decreases. The influence of oxygen supply is unique to the smoldering quenching phenomenon because it affects the mode of smoldering propagation. This work helps understand the persistence and extinction limit of smoldering and supports the prevention and suppression strategies for smoldering fire.