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.
Abstract: Dust-devil-like fire whirls without an inner core of flame cause considerable damage due to the strong winds during wildfires and urban fires. The purpose of this study is to clarify the effect of the crosswind speed (U) and heat release rate (HRR) on the circulation, size, and velocity of this type of fire whirl. A U of 0.06–0.54 m/s was investigated at HHRs of 0.84 kW and 2.23 kW. The particle image velocimetry measurements clarified the velocity fields of the vortices and complicated flow structures downwind of a long narrow fire source. The absolute value of the circulation of the vortex increases when the wind speed decreases in the range of 0.28 m/s ≤ U ≤ 0.54 m/s. The radius and velocity of the vortex have the identical tendency. This change in absolute value of the circulation with the crosswind speed is determined by the vortex radius and not by the average vorticity in the vortex. The absolute value of the circulation of the vortex increases when the HRR increases. This relationship is determined by the increase in average vorticity in the vortex and not by the radius of the vortex. This relationship does not depend on the crosswind speed.
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.
Abstract: A detailed description is given of double burner fire whirls that are similar in structure to the type of combined whirls seen in nature. The whirls are generated using the fixed frame method, and two burners are placed symmetrically about the center of the fixed frame. The flame from each burner sweeps along the ground until it reaches the point midway between the burners, where the two flames wrap around each other in rotation as they stretch vertically. Stereo Particle Image Velocimetry is used to examine the structure and behavior of the whirls under different burner separation and burning rate conditions. The circulation, Froude number and whirl height for the double burner whirls are found to follow similar scaling relationships to those found for single centered and offset burner whirls, but the velocity profiles display significant differences, including the presence of lobed radial velocity profiles in the core of the combined whirl. The air entrainment contours suggest double whirls may be more effective at entraining, lifting, and ejecting flaming debris.
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.
Abstract: Fire whirl is a typical thermally-driven vortex which often occurs in large forest and urban fires. This paper presents an experimental study of the thermally-driven vortex induced by the linear heat wire under cross wind. The straight, C- and S-shape wires were designed to simulate the fire front of different shapes. A wide range of heat release rate per length and wind speed were used to test the critical conditions under which vortex flows occur. It is found that the vortex can appear over and away from the heat wire under certain conditions. As the wire shape became complex from the straight to C-shape and to S-shape, the clockwise vortex was firstly observed and then was replaced by the vortex pair that rotated oppositely. The critical conditions of strong vortex over the heat wire well follow the previous dimensionless correlation that couples the heat release rate per length and wind speed, but the characteristic length should be different for the three types of wires. A physical mechanism of vortex formation is proposed, which indicates that the boundary vorticity near the ground surface is the possible eddy source for the vortex formation under critical conditions. The conditions of similar vortex phenomena induced by S-shape wires of two different length scales follow the previous scaling law that was built for straight line fires, but the interaction of upward thermal plume over S-shape wire results in a minus intercept of linear fitting. This work can provide a positive reference for the numerical simulation of the fire spread under cross wind.
3 citations
References
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TL;DR: In this article, structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry.
Abstract: Structural features resulting from the interaction of a turbulent jet issuing transversely into a uniform stream are described with the help of flow visualization and hot-wire anemometry. Jet-to-crossflow velocity ratios from 2 to 10 were investigated at crossflow Reynolds numbers from 3800 to 11400. In particular, the origin and formation of the vortices in the wake are described and shown to be fundamentally different from the well-known phenomenon of vortex shedding from solid bluff bodies. The flow around a transverse jet does not separate from the jet and does not shed vorticity into the wake. Instead, the wake vortices have their origins in the laminar boundary layer of the wall from which the jet issues. It is argued that the closed flow around the jet imposes an adverse pressure gradient on the wall, on the downstream lateral sides of the jet, provoking 'separation events’ in the wall boundary layer on each side. These result in eruptions of boundary-layer fluid and formation of wake vortices that are convected downstream. The measured wake Strouhal frequencies, which depend on the jet-crossflow velocity ratio, match the measured frequencies of the separation events. The wake structure is most orderly and the corresponding wake Strouhal number (0.13) is most sharply defined for velocity ratios near the value 4. Measured wake profiles show deficits of both momentum and total pressure.
1,186 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...Wake vortex shedding can be observed in the backside of a cylinder in a cross stream, as well as that of a jet fire or a pool fire in a cross wind [15, 16]....
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TL;DR: In this paper, the authors numerically investigated two-dimensional laminar flow past a circular cylinder rotating with a constant angular velocity, for the purpose of controlling vortex shedding and understanding the underlying flow mechanism.
Abstract: The present study numerically investigates two-dimensional laminar flow past a circular cylinder rotating with a constant angular velocity, for the purpose of controlling vortex shedding and understanding the underlying flow mechanism. Numerical simulations are performed for flows with Re=60, 100, and 160 in the range of 0⩽α⩽2.5, where α is the circumferential speed at the cylinder surface normalized by the free-stream velocity. Results show that the rotation of a cylinder can suppress vortex shedding effectively. Vortex shedding exists at low rotational speeds and completely disappears at α>αL, where αL is the critical rotational speed which shows a logarithmic dependence on Re. The Strouhal number remains nearly constant regardless of α while vortex shedding exists. With increasing α, the mean lift increases linearly and the mean drag decreases, which differ significantly from those predicted by the potential flow theory. On the other hand, the amplitude of lift fluctuation stays nearly constant with in...
279 citations
TL;DR: In this paper, the authors measured the flame length, velocity, temperature, and mass flux for line fires in a very similar manner to axisymmetric systems and found that the air entrainment coefficient for non-reacting, buoyant plume region in the Boussinesq approximation and assuming Gaussian distributions for horizontal velocity and temperature is 0.13.
Abstract: Flame length, velocity, temperature and mass flux measurements for some line fires are reported. As would be expected from theoretical considerations, laminar flame lengths are shown to scale as Q1 4 3 whilst turbulent flames scale as Q1 2 3 The velocity and temperature data in and above the turbulent flame system are shown to be reduced by a scaling related to flame length in a very similar manner to axisymmetric systems. An interesting adjustment occurs just above the flame tip as the constant velocity associated with the ‘intermittent’ flame region decays to a new, apparently different constant velocity associated with the non-reacting plume. The air entrainment coefficient deduced for the non-reacting, buoyant plume region in the Boussinesq approximation and assuming Gaussian distributions for horizontal velocity and temperature is 0.13.
113 citations
TL;DR: In this article, three types of scale-models, with scaling ratios of 1 235, 1 2500, and 1 4837, corresponding to three actual fire whirls (prototypes) were designed in the laboratory.
Abstract: This work was motivated from the knowledge of a historical fire whirl accident in which a fire whirl was responsible for 38,000 deaths and serious damage to the environment, all of which happened within 15 minutes. To understand the general features of fire whirls, records of example fire whirls were collected and deduced to be of three different types. Scaling laws were derived using dimensional analysis; three types of scale-models, with scaling ratios of 1 235 , 1 2500 , and 1 4837 , corresponding to three actual fire whirls (prototypes), were designed in the laboratory. Wind tunnel fire experiments were performed burning methanol as the fire source and by changing the wind velocity to understand conditions under which fire whirls can occur. The model fire whirls reconstructed in the laboratory were visualized with water vapor; and profiles of velocity and temperature were, respectively, measured by a hot wire anemometer and a fine thermocouple. Because the dynamic structure of prototype fire whirls is not known due to lack of direct measurements in actual flows, a large scale-model with scaling ratio of 1 100 to the prototype was designed to be studied in an open field. This model consisted of 85 circular pans filled with 2.3 m3 of methanol that were distributed in an area of 400 m2. Air movement was visualized by colored smoke. Wind velocity and direction were measured at five different locations and air temperatures at nine different locations. It was found that the visually determined whirl diameter and whirl column height, and the maximum tangential velocity of the whirl can be correlated to the 1 2500 scale model by the proposed scaling laws.
112 citations
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TL;DR: In this article, a scaling law that predicts the critical lateral wind velocity was developed and validated by various data including scale-model experiments by other researchers and real urban fire whirls, and a dimensional analysis was conducted to understand the effect of flow circulation on the increase in flame height.
Abstract: A fire whirl in an open space can cause devastating damage as was experienced in Hifukusho-ato, Tokyo, after the Great Kanto Earthquake in 1921. To understand the generation mechanism of the open-space fire whirls, 1/1000th scale-model experiments were conducted in a large, low-speed open-loop wind tunnel. In analyzing the experiments, there was found to be a critical lateral wind velocity that generated intense fire whirls. A scaling law that predicts the critical wind velocity was developed and validated by various data including scale-model experiments by other researchers and real urban fire whirls. A dimensional analysis is conducted to understand the effect of flow circulation on the increase in flame height. The simple analysis was supported by the results of numerical simulations by other researchers.
95 citations
"Fire Whirl due to Interaction betwe..." refers background in this paper
...The used wind system has no sidewalls, which is different from that of Kuwana et al [6]....
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...Emori and Saito [1], Soma and Saito [3], Kuwana et al [4, 6])....
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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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...Kuwana et al [6] have also studied the critical wind speed for small-scale line fires to generate the fire whirls by experiment, suggesting the semi-empirical correlation log(Uc (gL) 1 2 ) = 1 3 log( Q*)−0.5 where the characteristic length L corresponds to the line burner width for a line fire and the dimensionless heat release rate ranges from 10-4 to 10....
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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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