From smoldering to flaming fire: Different modes of transition

Transition from smoldering to flaming fire in cotton is investigated experimentally for five different sample sizes. Two different modes of transition are identified: early and late transition. Early transition to flaming occurs when smoldering combustion forms a hot glowing core inside a sample. As the smoldering burns through to the outside of the sample, air moves more easily into the core, resulting in a change in smoldering direction from reverse to forward smoldering, causing increased heat production and flaming. Late transition to flaming occurs occasionally after most of a sample is consumed by smoldering, and where randomly appearing glowing spots ignite flammable vapor from smoldering processes. The current setup consists of cotton samples where one side of the sample is situated next to a lightweight concrete block forming a boundary. Cotton samples with density 80 kg/m³ with dimensions equal to or larger than 0.45 m · 0.45 m · 0.15 m high, consistently give early transition to flaming, while smaller samples have early, late or no transition. By identifying an experimental set-up with a sample size that assures transition from smoldering to flaming, the transition phenomenon can be studied in even more detail.