Hassan, Ahmad (2022) Physics-based Modelling of Junction Fires at Laboratory Scale: Sensitivity, Validation and Parametric Studies. Research Master thesis, Victoria University.

Abstract

The process of modelling and replicating extreme fire behaviour like junction fires is essential for understanding the phenomena associated with extreme fires. Junction fires – an extreme wildfire phenomenon – occur when two fire lines merge in a wildfire. The junction point’s (apex) rate of spread and the fire intensity can quickly increase; this effect can be exacerbated by slopes and driving wind speed. The aim of this study is to look into junction fire behaviour using a physics-based model. In particular, the study is aimed at examining the key factors that influence junction fire spread, namely slope angle, junction angle and driving wind speed. A three-dimensional full physics-based model FIRESTAR3D, jointly developed by Aix-Marseille University, France, the Lebanese University, Lebanon, and Toulon University, France, was used in this study. For model validation, numerical simulations of laboratory-scale experiments of junction fire (conducted at the University of Coimbra, Portugal), replicating experiments with no imposed wind, were performed for a shrub fuel bed with slopes ranging from 0° to 30°. The simulations of junction fires were conducted for two junction angles, 30° and 45°. For each validation scenario, the sensitivities of the rate of spread (ROS) to various numerical, atmospheric and physical parameters were investigated. The behaviours of intersecting fire lines were explored in a parametric study for three crucial parameters (mentioned above) using the validated model. In the validation study, the experimental trends of the compared quantities were well reproduced by the simulations. Accelerating and decelerating propagation phases were observed in all simulations, with a dependence on the slope angle, while the maximum ROS depended critically on the junction angle. As was the case for wildfires in other studies simulated by FIRESTAR3D, it was found that this physics-based model is capable of simulating junction fire propagation. Validation tests performed using FIRESTAR3D for laboratory-scale experiments confirmed the potential of the model and provided a framework to extend the analysis to more general conditions, namely to explain the behaviour of real fires. The results suggested that junction fire spread appears to be sensitive to conditions change; with a slight reduction in junction angle, the ROS can increase significantly. For junction angles lower than 30°, accelerative and decelerative behaviour is observed, while the junction angle 45° was found to be the threshold angle at which propagation becomes steady. The heat release rate (HRR) followed the opposite trend and it was found that the peak value over time rose with the increase in junction angle. This may be due to the slow ROS and longer residence time. With no slope, radiation is the dominant method of heat transfer, but convection dominates on sloped terrain. In the case of wind-driven simulations of junction fire, strong interaction between fire lines was observed in response to wind, resulting in higher ROS. In the case of the 30° junction angle, junction point propagation and ROS were highly affected by both the slope and driving wind speed. However, for the cases with wider and narrower junction angles, the behaviour was not much affected by the driving wind speed. Considering the modes of heat transfer, higher wind velocity was accompanied by higher convection and lower radiation for the cases with wide junctions angles and non-sloped terrain; however, no significant changes were observed in the cases with higher slope. Overall, the physics-based modelling conducted in this study provided some important insights into junction fire behaviour. The modelling process gave insights into many crucial parameters over wide ranges of slope angle, junction angle and wind speed, and allowed the development of a significant initial understanding of such phenomena at the laboratory scale. By better understanding junction fires, operational predictions and firefighter safety can be improved.

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