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Fire interactions between multiple 1 m tall, 0.7 m diameter chamise shrubs was studied utilizing the WildlandUrban Interface Fire Dynamics Simulator (WFDS, Mell et al., 2009). Two shrub arrangements were investigated. First, nine ...
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Fire interactions between multiple 1 m tall, 0.7 m diameter chamise shrubs was studied utilizing the WildlandUrban Interface Fire Dynamics Simulator (WFDS, Mell et al., 2009). Two shrub arrangements were investigated. First, nine shrubs were placed in a 3 x 3 horizontal region. The shrub separation distance and wind speed were varied. Two competing interaction mechanisms were identified: heat feedback enhancement, primarily due to thermal radiative heating, and air entrainment restriction. Shrub burning characteristics were examined and a global average burning rate was analyzed. For the no wind condition, the peak mass loss rate of the center shrub is 23% higher than the others, indicating that heat feedback enhancement is dominant. Air entrainment causes the surrounding shrubs to burn less intensely. At an imposed wind speed, air entrainment effects are dominant. Shrubs that are best shielded from the wind burn most intensely. Second, the vertical separation between two shrubs was varied under different wind conditions. With no ambient wind, nearly no interaction between the two shrubs was observed. At a wind speed of 1 m/s, significant interaction between shrubs occurred due to flame tilting. The downwind shrubs burned the most vigorously for vertical separation distances between 0.2 and 0.8 m.
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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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With the global move towards performance based fire design, fire safety assessment in and around buildings becomes increasingly important. However, key knowledge gaps still exist concerning the behavior of fire swirling, which may...
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With the global move towards performance based fire design, fire safety assessment in and around buildings becomes increasingly important. However, key knowledge gaps still exist concerning the behavior of fire swirling, which may be generated if one or more accidental fires are in the passage of the vortices behind an adjacent tall building. The present study is focused on the experimental investigations of the burning behavior of two pool fires behind 1/50 scaled tall buildings with heights varying from 0.565 to 1.165 m in a cross-wind. The objective is to gain insight of the effect of the distance between the two fires (D2), the distance between the fires and the building (DJ), wind speed (V), and the height of the scaled building (H) on the burning behavior. Important conclusions have been drawn about the influence of DJ and D2 on the fuel mass loss rate, the influence of DJ on fire swirling, the influence of D2 on the possible merging of the two fires and the effect of wind speed on the mass loss rate. The results suggested the existence of a critical velocity for the cross-wind on the initiation of fire swirling and an approximate value was identified for the conditions in the tests. The investigations also covered the effect of height of the scaled building on the fuel mass loss rate and the occurrence of fire swirling. This relationship was found to be also dependent on the wind speed. Analysis of the results has led to some important recommendations to enhance the fire protection of tall buildings.
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Junction fires, a type of dynamic fire behaviour, involve the merging of two active fire fronts. They have been known to result in extreme increases in rates of fire spread and intensity over short periods of time. When these phen...
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Junction fires, a type of dynamic fire behaviour, involve the merging of two active fire fronts. They have been known to result in extreme increases in rates of fire spread and intensity over short periods of time. When these phenomena occur during wildfires, they pose significant risks to emergency management and communities. Few studies have investigated rates of spread and fireline intensity of junction fires using field scale experiments. This study captured data from 16 junction fires in mallee-heathlands utilising a drone-mounted thermal camera. Mean rates of spread and fireline intensity were found to increase toward the 75 % stage of merging before decreasing, similar to those found in other studies. Although, normalised temperature areas increased continuously for the whole duration of merging. Additionally, a custom-built radiative heat flux device was used to measure temperature and radiant heat at the fireline within the junction fires. Peak radiative heat flux was also found to be highest at the 75 % stage of merging, following a gradual decrease as propagation progressed. Heat flux measurements were found to be similar to those of previous studies conducted in similar vegetation. Due to the many complex interactions between weather, fuel and fire behaviour, further studies need to be conducted to appropriately determine the effects of merging on rates of spread and fire intensity.
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In this study, the interaction of two parallel line fires with a length-width ratio of greater than 50 was investigated and compared to a single line fire. Considering different length–width ratios and spacings between the fire s...
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In this study, the interaction of two parallel line fires with a length-width ratio of greater than 50 was investigated and compared to a single line fire. Considering different length–width ratios and spacings between the fire sources, experiments were carried out to analyze the fire characteristics, such as the burning rate, the flame-merging state, the flame height, the flame tilt angle, and the flame length of the line fires. Its regularity was revealed by combining two mechanisms, namely, heat feedback enhancement and air entrainment restriction. The results revealed that the burning rate under different length–width ratios shows a uniform law, which increases first and then decreases with a greater spacing between the fire sources. There is a special relationship between the flame-merging probability Pm and the dimensionless characteristic parameters (S/ZC)/(L/d)0.27. Based on this relationship, a critical criterion of flame merging can be obtained as (S/ZC)/(L/d)0.27 = 2.38. In addition, the height and the length of the flame were studied to better describe the flame shape when the flame is tilted. Since the flame is bent, the flame length has an abrupt change at a specific position, and the inclination angle also has the same phenomenon. Finally, it was found that the influence of the length–width ratio on the line fires is relatively limited, which is further weakened under a greater length–width ratio.
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Special flame merging behaviour and complex combustion characteristics can be formed under the coupling effect of tangential flow and heat transfer of multiple pool fires (MPFs) in a shaft. In this study, quadruple pool fires (DPF...
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Special flame merging behaviour and complex combustion characteristics can be formed under the coupling effect of tangential flow and heat transfer of multiple pool fires (MPFs) in a shaft. In this study, quadruple pool fires (DPFs) evolutionary experiments in shaft were carried out, and the influence of pooled spacing and side slit width of the shaft on flame merging behaviour were analyzed. The primary influencing mechanism of merged flame evolution was expounded from heat transfer and thermal feedback between fire sources. The shape of conical merged fire whirls that can completely fill the gap between the oil pools with flame is confirmed, and its cone angle is determined by the position of outer edge of oil pools. In the formation of merged fire whirls, the critical conditions are that the distance between fire sources is less than three times the pool diameter and the width of side slits is 1/4 of the side length of the shaft, respectively. Compared with the single pool fire whirls with the same liquid surface area, the thermal flow field of DPFs in shaft is relatively unfavorable to the formation of fire whirls. The formation time of merged fire whirls is nearly five times that of the single pool fire whirls, and the flame height can be reduced by nearly half. The evolution process of the DPFs that can form merged fire whirls exhibits the multi-stage morphological characteristics over time and all of these stages possess different heat transfer mechanisms. Before forming the merged fire whirls, intermittent merging among pool fires is a necessary stage. And the duration of this stage is relatively less affected by the width of slit on the side of shaft. The transverse force of asymmetric tangential flow on the flame is difficult to be completely overcomed by merged fire whirls. Because of the inclination of merged flame, the overall vertical height is unsuitable as a direct characterization parameter of combustion intensity.
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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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This paper presents an experimental analysis on the flame characteristics of multiple fires burning by tests of square fire array consisting of even number of equidistant burners, using propane as the fuel. We propose to use the t...
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This paper presents an experimental analysis on the flame characteristics of multiple fires burning by tests of square fire array consisting of even number of equidistant burners, using propane as the fuel. We propose to use the temperature along the centerline of fire array, instead of visual observations, to identify flame merging and examine the characteristics of flame structure under different states of flame merging. It is revealed that with decreasing flame spacing, the flames undergo a physical transition in the centerline temperature of the fire array, induced by the formation of the fuel-rich region in the lower region above the burner surface. Analyses on fuel rich region and axial temperature, in a quantitative way, provide physical details of the changes of flame structure when the flames that burn on discrete fuel beds undergo physical transition to become one single flame with decreasing flame spacing. It is inferred from the temperature data that the air entrainment is more restricted with increasing merging level of flames. A new method in terms of the centerline temperature of fire array is proposed to determine the critical heat release rate for the onset of flame merging. It is found that the heat release rate Q(D)(*) using the fire array size D as the characteristic length scale, can be used to correlate the flame height of a merged fire array. It is further indicated that a general linear form L/D = 3.7Q(D)(*2/5) - c can be used as a general correlation of flame height for fire arrays in different merging levels, in which the parameter c characterizes the deviation from the Heskestad curve under different flame spacings. For non-merged fire array, the flame heights of separate flames mainly depend on the heat release rate, while the flame spacing and the number of flames have minor effects.
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In order to characterize fire merging, pool fires on hollow trays with varying side lengths were burned under quasi-quiescent condition and in a wind tunnel with the wind speed ranging from 0 m/s to 7.5 m/s. Burning rate and flame...
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In order to characterize fire merging, pool fires on hollow trays with varying side lengths were burned under quasi-quiescent condition and in a wind tunnel with the wind speed ranging from 0 m/s to 7.5 m/s. Burning rate and flame images were recorded in the whole combustion process. The results show that even though the pool surface area was kept identical for hollow trays of different sizes, the measured burning rates and fire evolutions were found to be significantly different. Besides the five stages identified by previous studies, an extra stage, fire merging, was observed. Fire merging appeared possibly at any of the first four stages and moreover resulted in 50-100% increases of the fire burning rates and heights in the present tests. The tests in wind tunnel suggested that, as the wind speed ranges from 0 m/s to 2 m/s, the burning rates decrease. However with further increase of the wind speed from 2 m/s to 7.5 m/s, the burning rate was found to increase for smaller hollow trays while it remains almost constant for larger hollow trays. Two empirical correlations are presented to predict critical burning rate of fire merging on the hollow tray. The predictions were found to be in reasonably good agreement with the measurements. (C) 2015 Elsevier B.V. All rights reserved.
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