Effect of variable-speed wind on the transition from smouldering to flaming combustion

• A 1-D model of the StF transition under external winds was developed. • Transition behaviors under the constant and variable wind were compared. • Transition time under variable wind is greater than that under constant wind. • StF transition is controlled by cooling at a low wind speed. • A higher initial wind speed reduces the transition time due to more heat generation. Smouldering-to-flaming (StF) transition under external winds could lead to serious fire hazards. However, the StF transition mechanism under a variable-speed wind (VSW) remains to be explained. In this work, a one-dimensional model for the StF transition of a biomass stick was developed, which was successfully validated against the experimental results in the literature. VSW was defined as a sinusoidal function of constant-speed wind (CSW) in this work. Then, the StF transition behaviors under a CSW and a VSW were compared. It was found that the transition time under VSW is, on average, 1.9 times greater than that under CSW due to its minor heat generation. Moreover, the transition behaviors under VSW with different periods and initial wind speeds were also investigated. For VSW with different periods, the StF transition at a low wind speed (1.5 ∼ 3.7 m s −1 ) is mainly controlled by cooling, while by oxygen supply at a high wind speed (3.7 ∼ 5 m s −1 ). The effect of the initial wind speed on heat loss is insignificant, but a higher initial wind speed gives a higher heat generation, resulting in a decrease in the transition time.