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Dynamic fire behaviour involves rapid changes in fire behaviour without significant changes in ambient conditions, and can compromise firefighter and community safety. Dynamic fire behaviour cannot be captured using spatial implem...
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Dynamic fire behaviour involves rapid changes in fire behaviour without significant changes in ambient conditions, and can compromise firefighter and community safety. Dynamic fire behaviour cannot be captured using spatial implementations of empirical fire-spread models predicated on the assumption of an equilibrium, or quasi-steady, rate of spread. In this study, a coupled atmosphere-fire model is used to model the dynamic propagation of junction fires, i.e. when two firelines merge at an oblique angle. This involves very rapid initial rates of spread, even with no ambient wind. The simulations are in good qualitative agreement with a previous experimental study, and indicate that pyro-convective interaction between the fire and the atmosphere is the key mechanism driving the dynamic fire propagation. An examination of the vertical vorticity in the simulations, and its relationship to the fireline geometry, gives insight into this mechanism. Junction fires have been modelled previously using curvature-dependent rates of spread. In this study, however, although fireline geometry clearly influences rate of spread, no relationship is found between local fireline curvature and the simulated instantaneous local rate of spread. It is possible that such a relationship may be found at larger scales.
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Junction fires, which involve the merging of two linear fire fronts intersecting at a small angle, are associated with very intense fire behaviour. The dynamic displacement of the intersection point of the two lines and the flow a...
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Junction fires, which involve the merging of two linear fire fronts intersecting at a small angle, are associated with very intense fire behaviour. The dynamic displacement of the intersection point of the two lines and the flow along the symmetry plane of the fire are analysed for symmetric boundary conditions. It is observed that the velocity of displacement of this point increases very rapidly owing to strong convective effects created by the fire that are similar to those of an eruptive fire. The change of fire geometry and of its associated flow gradually blocks the rate of spread increase and creates a strong deceleration of the fire, which ends up behaving like a linear fire front. Results from laboratory and field-scale experiments, using various fuel beds and slope angles and from a large-scale fire show that the processes are similar at a wide range of scales with little dependence on the initial boundary conditions. Numerical simulation of the heat flux from two flame surfaces to an element of the fuel bed show that radiation can be considered as the main mechanism of fire spread only during the deceleration phase of the fire.
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Background: In Pedróg?o Grande on 17 June 2017, two fire fronts merged and the propagation of the fire was influenced by the interaction of these non-symmetric fire fronts. Aims: This wildfire motivated us to study a junction fir...
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Background: In Pedróg?o Grande on 17 June 2017, two fire fronts merged and the propagation of the fire was influenced by the interaction of these non-symmetric fire fronts. Aims: This wildfire motivated us to study a junction fire with two non-symmetrical fire fronts. The analysis of the movement of the intersection point and the angle (γ) between the bisector of the fire lines and the maximum rate of spread (ROS) direction is of particular relevance. Methods: The study was carried out at Forest Fire Laboratory of the University of Coimbra in Lous? (Portugal) with laboratory experiments. Key results: We found that, for small rotation angles (δ), the non-dimensional ROS of the intersection point depends on the slope angle (α) and the initial angle between fire fronts. Conclusions: For high α, the non-dimensional ROS was highly influenced by the convection process and γ where the maximum ROS occurred, increased when δ increased. However, the radiation process was more relevant for lower α and influenced the non-dimensional ROS. For these cases, the maximum spread direction was close to that of the fire line bisector. Implications: The present work aimed to explain fire behaviour during the Pedróg?o Grande wildfire.
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A critical review of the mechanisms that are described in the literature to explain the onset and development of eruption or blow up in forest fires is presented, given their great relevance for fire safety, particularly in canyon...
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A critical review of the mechanisms that are described in the literature to explain the onset and development of eruption or blow up in forest fires is presented, given their great relevance for fire safety, particularly in canyons. The various processes described in the literature that are considered as potential causes of fire eruption are discussed. Some of them seem more likely to cause the phenomenon and the others seem to have a complementary role in some conditions. The current review highlights that more research is required to create a classification of Fire Eruption types and to allow the development of specific Fire Safety procedures for fire fighters to minimize accidents.
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In this paper, we present an experimental study at laboratory scale of a wind driven fire entering a lateral canyon perpendicular to the main fire spread direction. This study is inspired on a multiple fatalities accident occurred...
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In this paper, we present an experimental study at laboratory scale of a wind driven fire entering a lateral canyon perpendicular to the main fire spread direction. This study is inspired on a multiple fatalities accident occurred in Armamar (Portugal), in 1985. The laboratory experiments show an eruptive fire behaviour inside the canyon. A rapid increase in the rate of spread and intensity of the fire is noticed when the fire enters the canyon, even though the wind is blowing perpendicular to the canyon axis. This increase is due to the presence of three factors: flow circulation inside the canyon, junction, and eruptive fire behaviours. The findings of this study contribute to better understand fire behaviour in complex terrain and therefore to improve personnel safety.
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The onset of extreme fire behaviour in a mine drift with longitudinal ventilation was analysed. A fire in a mine drift with continuous fuel load, involving several separate fires may lead to flames tilted horizontally and filling ...
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The onset of extreme fire behaviour in a mine drift with longitudinal ventilation was analysed. A fire in a mine drift with continuous fuel load, involving several separate fires may lead to flames tilted horizontally and filling up the entire cross section. This will lead to earlier ignition, higher fire growth rate, higher fire spread rate and a severe fire behaviour. The focus has been on what changes take place at the onset and signs of the impending phenomenon. It was found that the fire gas temperature at the ceiling level provided a poor indicator. At the downstream far-field region of the fire, the sudden temperature increase at the lowest levels of the cross section and the sudden increase in flow velocities would provide signs of extreme fire behaviour. The corresponding fullscale heat release rates of the experiments at the onset of extreme fire behaviour were found to be very high for mining applications but not necessarily for tunnel fires. The heat release rate threshold for a mine drift with smaller cross-sectional dimensions would decrease considerably, increasing the likelihood of occurrence. The distance between the fuel items will play an important role during the initiation of horizontal flames.
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This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale. A particularity of the study is the use of typical forest fuels instead of fossil fuels as seen in some of previous st...
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This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale. A particularity of the study is the use of typical forest fuels instead of fossil fuels as seen in some of previous studies on this topic. The forest fuels tested in the experiments were dead needles of Pinus pinaster, straw of Avena saliva, dead leaves of Eucalyptus globulus and a mix of shrubs mainly composed by heather (Erica australis) and gorse (Pterospartum tridentatum). The experimental results of the tests with and without forced flow inside a fire whirl generator were compared with tests in similar conditions out of the generator. It was possible to evaluate the effects of fuel bed size, bulk density and external vorticity on several parameters like flame height and diameter, mass decay and heat release rate. The results show that forced flow increases dramatically the burning rate and reduces the time needed to achieve a high rate of energy release. 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.
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Background. Previous work by the author and others has examined weather associated with growth of exceptionally large fires ('Fires of Unusual Size', or FOUS), looking at three of four factors associated with critical fire weather...
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Background. Previous work by the author and others has examined weather associated with growth of exceptionally large fires ('Fires of Unusual Size', or FOUS), looking at three of four factors associated with critical fire weather patterns: antecedent drying, high wind and low humidity. However, the authors did not examine atmospheric stability, the fourth factor.Aims. This study examined the relationships of mid-tropospheric stability and dryness used in the Haines Index, and changes in surface wind speed or direction, to growth of FOUS.Methods. Weather measures were paired with daily growth measures for FOUS, and for merely 'large' fires paired with each FOUS. Distributions of weather and growth were compared between the two fire sets graphically and statistically to determine which, if any, weather properties correspond to greater growth on FOUS than on large fires.Key results. None of the factors showed a robust difference in fire growth response between FOUS and large fires.Conclusions. The examined measures, chosen for their anecdotal or assumed association with increased fire growth, showed no indication of that association.Implications. Focus on wind changes and mid-tropospheric properties may be counter-productive or distracting when one is concerned about major growth events on very large fires.
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Background Two fire ignitions in Pedrogao Grande on 7 June 2017 had very fast due to unusual physical processes associated with the interaction between an overhead thunderstorm and the fire and the subsequent merging of the fires ...
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Background Two fire ignitions in Pedrogao Grande on 7 June 2017 had very fast due to unusual physical processes associated with the interaction between an overhead thunderstorm and the fire and the subsequent merging of the fires as a junction fire, killing 66 persons in 2 h.Aims Using a laboratory simulation of the merging process, we explain the fire spread conditions and verify that the junction of the two fires was responsible for the very intense fire development.Methods The real fire spread was reconstructed from an extensive field survey and physical modelling tests were performed in the Fire Research Laboratory combustion tunnel using various fuels and scale modelling laws.Key results The spread and merging of the two fires in the tests agree very well with field observations, namely the periods of rate of spread (ROS) increase and decrease, peak values of ROS and area growth process using scaling laws.Conclusions Analysis of the Pedrogao Grande fire evolution and its physical simulation at laboratory scale showed the importance of the mechanisms of two fires merging in producing very important convective processes.Implications Our study showed the validity of performing the experimental analysis of complex fire spread situations provided that the similarity conditions are fulfilled.
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