Journal Article•
Fire Whirl due to Interaction between Line Fire and Cross Wind
TL;DR: In this paper, the authors presented an elementary study on a moving fire whirl by conducting line fire experiments with cross wind and found that a line fire near the ground, a reasonable attack angle between the line fire and the cross wind, and wind speed within a critical range are the three essential conditions for the formation of fire whir in a linefire.
Abstract: During the wildland fire of Brazil in 2010 (http://www.dailymail.co.uk/sciencetech/article-1306088/Braziltornado-Whirling-column-flames-sweeps-burning-fields.html), a special fire whirl occurred over a narrow but long fire front and moved due to the wind effect. This paper presents an elementary study on such a moving fire whirl by conducting line fire experiments with cross wind. Experimental analysis indicates that a line fire near the ground, a reasonable attack angle between the line fire and the cross wind, and wind speed within a critical range are the three essential conditions for the formation of fire whirl in a line fire. By examining the advection and bending of vorticity, it is also deduced that the concentrated vortex of fire whirl results from the coupling of the line fire plume and the horizontal vortex line near the ground surface. By assuming the solid-body rotation of fire whirl flame, a possible mechanism of moving fire whirl is proposed, which states that the flame moving is mainly controlled by the drag force, lift force and ground friction. Accurate experimental measurements are needed to testify or verify this mechanism in the future work.
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01 Jan 2021
TL;DR: In this paper, the authors present an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control, including the complicated transformation of fuel preheating mechanisms, varied large-size flame fronts and unique spread modes induced in specific fire environments.
Abstract: A fact often overlooked is that large-scale wildfires, although occurring infrequently, are responsible for the overwhelming majority of fire-related suppression costs, economic losses, and natural resources damages. Fortunately, the increasingly severe problems of large-scale wildfires worldwide have been receiving ever-growing academic attention. The high-intensity burning behaviors in wildfires stem from the significant interaction of combustion with heat transfer and atmospheric flow under complicated fuel, meteorology, and topography conditions. Therefore, mitigating measures against large-scale wildfire disasters have grown into a challenging research focus for combustion scientists. Research over the past century has resulted in incrementally enhanced insights into the mechanisms of combustion dynamics underlying the various erratic behaviors in large-scale wildfires, with theories and models of fire accelerations developed and validated. These advances are expected to improve the efficacy of large-scale wildfire predictions significantly. Nevertheless, the physical interpretation of the acceleration of large-scale wildfires is far from adequate and complete. This paper intends not to make a comprehensive review of the entire wildfire research field, but to depict an overall pattern of the essential factors that lead an initial small-scale spreading flame to a large-scale wildfire beyond control. It is outlined that the complicated transformation of fuel preheating mechanisms determines the growth of surface fire spread, while varied large-size flame fronts and unique spread modes induced in specific fire environments play an essential role in fire spread acceleration. Additionally, multiple fires burning and merging often act as crucial steps for accelerating surface fire spread, generating large-size flames, and triggering unique spread modes. These major potential factors strike the energy balance of a low-intensity wildfire and push it to a high-intensity state. Several issues regarding intensely burning behaviors in large-scale wildfires are selected for in-depth discussions, for which an overview of the progress and challenges in research is presented. It is concluded that the fundamental exploration targeted at developing application tools capable of dealing with large-scale wildfires remains at its early stages. Opportunities for innovation are abundant, yet systematic and long-term research programs are required.
34 citations
TL;DR: In this article, the authors present an experimental study on the fire whirls over a line fire with cross wind, focusing on the occurrence frequency of fire whirs, and propose a scaling law for the critical wind speed inducing fire whirs based on the experimental data in this work and literature.
Abstract: Fire whirls are often reported to occur in wildland and urban fires due to the effect of ambient wind. This paper presents an experimental study on the fire whirls over a line fire with cross wind, focusing on the occurrence frequency of fire whirls. The experimental observations indicated that the fire whirls induced by a line fire may spread beyond the line fire region with the effect of wind. For the effect of cross wind, it is indicated that the cross wind basically increases the occurrence frequency, while the velocity components parallel or perpendicular to the line fire have competitive effects. A scaling law is presented for the critical wind speed inducing fire whirls based on the experimental data in this work and literature. A method is proposed to estimate the magnitude of the fire whirl height under the critical wind speed.
16 citations
TL;DR: In this article, the effect of the crosswind speed and heat release rate (HRR) on the circulation, size, and velocity of a fire whirl was investigated at HHRs of 0.06-0.54 m/s.
4 citations
TL;DR: In this article, a detailed description of double burner fire whirls is given, where two burners are placed symmetrically about the center of a fixed frame, and the flame from each burner sweeps along the ground until it reaches the point midway between the burners.
4 citations
TL;DR: In this article, the authors present an experimental study of the thermally-driven vortex induced by the linear heat wire under cross wind, where the straight, C-and S-shape wires were designed to simulate the fire front of different shapes and a wide range of heat release rate per length and wind speed were used to test the critical conditions under which vortex flows occur.
3 citations
References
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01 Jan 2007
TL;DR: In this article, an experimental and methodological investigation on the behaviors of square fire arrays which are composed of 3.5 to 7.7 n-heptane fires initiated from fuel pans of 5 cm in diameter and 2 cm in height is presented.
Abstract: This paper gives an experimental and methodological investigation on the behaviors of square fire arrays which are composed of 3 × 3 to 7 × 7 n-heptane fires initiated from fuel pans of 5 cm in diameter and 2 cm in height. It is intended to develop a burn-out time (BOT) data analysis method to analyze the interaction effects (which may induce fire merging and fire whirls) among the multiple fires. In 26 fire tests the fire point spacing D varied from 20 to 50 cm for each array size and in several cases shear flow was added from one side of the array. By considering the flame height L a reasonable critical condition for initiation of fire merging was implied to be D/L = 0.29 − 0.34, which is independent of the fire array size and fire point spacing. By burn-out time data the Interaction index I (m) and Interaction link index A (m, n) were defined to characterize the fire interactions. The assumptions essential to solve the equation system of I (m) = ∑n A (n, m) were examined in detail, whereby the equation system was solved. The analysis showed that the burn-out time data analysis realizes a quantitatively reasonable comparison of the fire interaction effects, thus indicating that it is reasonable to regard the burn-out time as a measure for the average burning rate for each specific fire point. An apparent criterion of BOT(m)/BOTR = 0.5 was summarized to identify whether any fire point m will be completely involved in fire merging (where BOTR is the burn-out time of the free burning reference fire point). It was implied that the interaction effect imposed on any fire is mainly ascribed to its adjacent four fires. The effects of shear flow to fire burning and occurrences of fire whirls were also discussed.
88 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Generally, the concentrated vortex of fire whirl can be generated in leeward slope [1], large fires [2] and L-shape urban fires [3, 4] under the effect of cross wind, as well as by multiple fires interaction [5]....
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TL;DR: In this paper, a two-dimensional numerical study on the laminar incompressible flow past a rotating circular cylinder in the Reynolds number range 60-Re⩽200 and at rotational rates 0-6 was carried out.
Abstract: To examine in detail the behavior of a new vortex shedding mode found in a previous investigation [Phys. Fluids 14, 3160 (2002)], a two-dimensional numerical study on the laminar incompressible flow past a rotating circular cylinder in the Reynolds number range 60⩽Re⩽200 and at rotational rates 0⩽α⩽6 was carried out. The results obtained clearly confirm the existence of the second shedding mode for the entire Reynolds number range investigated. A complete bifurcation diagram α(Re) was compiled defining both kind of shedding modes. The unsteady periodic flow in the second mode is characterized by a frequency much lower than that known for classical von Karman vortex shedding of the first mode. The corresponding Strouhal number shows a strong dependence on the rotational velocity of the cylinder, while only a weak dependence is observed for the Reynolds number. Furthermore, the amplitudes of the fluctuating lift and drag coefficients are much larger than those characterizing classical vortex shedding behind...
87 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Stojkovic et al [14] presented the profile of CD as a function of α ranging from 0 to 6 with Re being from 60 to 200....
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67 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Kumar et al [13] studied the vortex shedding at Re=200, 300 and 400 with the dimensionless rotary velocity varying from 0 to 5 by experiment, and observed that the vortex shedding activity only occurs at 0 α αΙ ≤ < or α α α ΙΙ ΙΙΙ < < , where 1....
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...Kumar et al [13] studied the vortex shedding at Re=200, 300 and 400 with the dimensionless rotary velocity varying from 0 to 5 by experiment, and observed that the vortex shedding activity only occurs at 0 α αΙ≤ or α α αΙΙ ΙΙΙ , where 1.95αΙ ≈ , 4.34αΙΙ ≈ and 4.70αΙΙΙ ≈ ....
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TL;DR: In this article, the fluid dynamical scaling laws of forest fires are derived by dimensional analysis and a scale model is constructed to investigate a forest fire which seriously injured several fire fighters.
Abstract: The fluid dynamical scaling laws of forest fires are derived by dimensional analysis and a scale model is constructed to investigate a forest fire which seriously injured several fire fighters. The result of the small-scale experiment indicate that a fire whirl could be generated in such a fire.
60 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Emori and Saito [1], Soma and Saito [3], Kuwana et al [4, 6])....
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...Generally, the concentrated vortex of fire whirl can be generated in leeward slope [1], large fires [2] and L-shape urban fires [3, 4] under the effect of cross wind, as well as by multiple fires interaction [5]....
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...A reasonable wind speed is essential for the formation of fire whirl, which is validated by scale-model experiments (e.g. Emori and Saito [1], Soma and Saito [3], Kuwana et al [4, 6])....
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58 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Byram and Martin [8] suggested three essential conditions for fire whirl formation, i.e. a generating eddy, a fluid sink located within the eddy and some friction or drag offered to the movement of air in the ground of the eddy....
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...Moreover, the angular velocity of fire whirl inside the vortex core decreased with increasing vertical distance, however, the angular momentum outside the vortex core kept constant independent on the vertical distance [8]....
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...Byram and Martin [8] suggested three essential conditions for fire whirl formation, i....
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