Experimental and Numerical Investigation of Fire Whirl’s Impact on the Firebrand’s Lofting Mechanism

Wildfire, which is one of the most destructive phenomena on earth, is known as very difficult to suppress, especially in large-scale fires such as forest fires in dry season. Countless acres of land and properties are destroyed due to it are reported each year. One of the most important reasons for its difficulty to control is its spreading mechanism. Among all of the spreading mechanisms for wildland fire, the spotting phenomenon is the most remarkable. The spotting phenomenon is defined as the discontinuous spread of the wildland fire caused by a firebrand, which could be a burning particle generated from a burning tree. With the help of wind, a firebrand could travel miles away from the main fire zone and cause a secondary ignition far from the initial wildland fire. In order to study the spotting phenomenon, a huge amount of publications have focused on the spotting distance of the firebrand under different conditions. However, another topic about the firebrand’s takes off mechanism is quite limited. And the taking off mechanism for firebrand is called lofting. As the fact of the maximum spotting, the distance could be more than 10 miles away, the current understanding of the detailed physics of the spotting is not accurate.
In this thesis, the lofting of the firebrands is investigated. The investigation is being undertaken from both experimental and numerical sides. In the experimental study about the firebrand’s lofting, a setup for collecting data about the firebrand’s lofting mechanism in a fire whirl generator has been developed. In order to qualitatively understand the fire whirl that is being utilized in the experiment, the PIV test was also conducted on the fire whirl generator. Based on the experimental result, a hypothesis that aerodynamic lift might be one of the key points to the hyper-long spotting distance.
Then, in the numerical study about the firebrand’s lofting, a developed mathematic model for predicting the lofting trajectory of the firebrand was developed. The comparisons about including aerodynamic lift and other factors are also included. Meanwhile, a semi-theoretical approach to solving mathematical models has also been developed. After then, the modified approach is proven to be reliable after testing it with the traditional Runge-Kutta method.
Based on the modelling of the lofting mechanism, the impact of aerodynamic lift, tumbling effect, and mass loss effect have been studied. Among the three aspects, the mass loss effect is the less important one, and the aerodynamic lift is the most important one. This conclusion also fits the result of the experimental study.
This study helps build the inner connection between the fire whirl and hyper-long-distance spotting, which deepens the understanding of the detailed physics of the firebrand’s lofting mechanism, which could be helpful for firefighters in controlling wildland fire.