Experimental characterization and analysis on fire whirls using forest fuels

Abstract

During a forest fire the formation of fire whirls can be observed but their characteristics and evolution are still difficult to predict. Due to the high thermal energy released, fire whirls can become a danger to firefighters and are, by their complexity and dangerousness, a way of extreme fire behavior that is poorly understood. This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale that analyses the influence of the variation of the following parameters: fuel load, density, fuel configuration, imposition of forced flow with different velocities through the use of fans. Comprehensive measurements were performed in order to establish correlations of the burning rate, heat release rate, flame height, temperature and velocity of fire whirls, and to clarify the similarities and differences between a fire whirl and a ‘normal’ fire, i.e., one in which vortex formation is not observed. Fire whirl experiments were performed in a vertical channel with a quadrangular section of 1×1 m2 with a height of 7.8 m, the fire whirl generator, using dried shrubs, which is a fuel mainly composed by heather (Erica australis) and gorse (Pterospartum tridentatum) quite common in forested areas in Central Portugal and in several Mediterranean climate regions. Given the transient nature of the experiments relationships between time correlated parameters or with average or extreme values were established. Comparison with similar experiments showed that the properties of the flames generated with the present configuration of the fire whirl generator are similar to fire whirls produced in other laboratory studies and with full scale fire whirls. The maximum heat release power was of the order of 1 MW which is higher than the reported values for similar laboratory tests. The results show that forced flow increases dramatically the burning rate and reduces the time needed to achieve a high rate of energy release. On the other hand, from a certain value of forced flow it is observed that the values of the burning rate and flame height decrease, thus existing a critical wind velocity for optimal fire whirl development. Comparison with results of other sources show that the flames that are generated in the present fire whirl generator are in a transition from fire whirl to pool fire regime and that it is possible to scale up some flow and thermal properties of field scale fire whirls and to derive predictive models on the basis of laboratory scale experiments.