Abstract
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.
Highlights
Transition from smoldering to flaming fire is an interesting phe nomenon within the subject of fire, where weak ignition sources initiate smoldering fires, and transition to flaming fires may occur hours, days or weeks later
Hadden et al [12] investigated the effect of sample size on radiant ignition of PU-foam, and found that an increase in sample size reduced the critical radiation heat flux needed for smoldering and flaming
Smoldering and smoke were observed in all experiments, and transition to flaming occurred in 16 of 25 experiments
Summary
Transition from smoldering to flaming fire is an interesting phe nomenon within the subject of fire, where weak ignition sources initiate smoldering fires, and transition to flaming fires may occur hours, days or weeks later. Previous research shows that transition from surface reactions (smoldering) to gas phase burning (flaming) is affected by different external factors such as: increased airflow, narrow gaps in the samples or differences in densities. Ohlemiller [5] reported that increased airflow in voids between cellulose insulation and wood frames could cause glow ing and transition to flaming in some instances. Tse et al [6] found that increasing airflows would increase the smoldering rate, causing oxida tion of char (secondary char oxidation) left by smoldering of the initial material. Wang et al [8] found that increased airflow around samples resulted in increasing smoldering rates and transition to flaming. Alex opoulos and Drysdale [9] reported that transition to flaming in gaps occurred more rapidly in narrow gaps. Hadden et al [12] investigated the effect of sample size on radiant ignition of PU-foam, and found that an increase in sample size reduced the critical radiation heat flux needed for smoldering and flaming
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