Experimental Study of Spatiotemporal Variations in Lab‐Scale Fire Whirls

Experimental investigation on flame patterns of buoyant diffusion flame in a large range of imposed circulations

Fire whirls can occur during urban fires, especially in intense fires in combustible building structures, and more often in forest or wildland fires. They are a special swirling diffusion flame characterized by significant enhancement in burning rates, flame heights and flame temperatures, along with a strong whirling motion of the flame. This whirling motion can pick up large firebrands and scatter them afar leading to spot fires. Many researchers have published experimental work on small- and medium-scale pool fire whirls and gaseous fuel fire whirls using split cylinders and various fixed-frame apparatus to investigate axial and tangential velocity profiles, axial and radial temperature distribution, burning rates, and flame heights. Likewise, several researchers have attempted to predict the experimental results of fire whirls using different modelling approaches and simulation software.In this paper, experiments were undertaken to study the dynamics of propane gas fire whirls in a small-scale, square-based, fixed-frame apparatus. Measurements of flame height and temperature profiles (both axial centerline and radial) were made for a low initial momentum burner of 76.2 mm internal diameter. The burner was operated at a volumetric flow rate of 6 dm3/min, which gave a heat release rate of 9.12 kW. Simulations using Fire Dynamics Simulator (FDS 6.6.0) and ANSYS Fluent 17.1 were performed to compare with the experimental measurements. Four separate mesh refinements were employed and four different sub-grid-scale (SGS) turbulence models were tested with FDS. The Deardorff, Wall-Adapting Local Eddy-viscosity (WALE), and dynamic Smagorinsky models, formed stable fire whirls for the two largest mesh refinements.The temperature profiles were overpredicted at the core of the flame with FDS and underpredicted with Fluent. The FDS simulation prematurely predicts the peak temperature for the axial centreline profile, whereas with Fluent the axial temperature profile matches the general trend of the experimental measurements.The visible flame height, determined through image processing, was approximately 0.88 ± 0.06 m, which corresponds to a measured temperature of ∼500°C. The 500°C temperature contour was used as a rough approximation of the flame height in the numerical simulations. It was found that with Fluent the 500°C contour grew until the fire whirl stabilized and reached the top of the hood at 1.6 m, clearly overpredicting the flame height. The height estimates based on the predicted 500°C contours show a strong dependence on the mesh resolution. This is primarily due to increased instability resulting in more mixing and spreading of the temperature for the coarser mesh size. However, the simulated flame heights show less dependence on the SGS turbulence models.

A fire whirl, a unique fire behavior, occurs when a vertical vortex of flames skyrockets due to specific surrounding temperatures and thermal gradient conditions during a fire. Compared with conventional fire plumes, fire whirls exhibit a higher air entrainment rate, tangential velocity, and axial velocity, thus presenting greater risks and destructive capabilities. Thus, studying the combustion characteristics of fire whirls becomes necessary. This experiment employed a small-scale, fixed-frame fire whirl generator. We investigated how varying air-inlet widths and fuel pan diameters influence the fire whirl’s combustion characteristics. Experimental images indicated a negative correlation between the fire whirl’s flame height and the air-inlet width, and a positive correlation with the fuel pan diameter. Our findings showed that the burning rate of the fire whirl during the quasi-steady-state combustion phase initially increased and then decreased as the air-inlet width expanded, peaking at a width of 7 cm. The data demonstrated a corresponding power-law relationship between the fire whirl’s dimensionless flame height and excess temperature. Ultimately, our results indicated a positive correlation between the 2/5 power of the fire whirl’s dimensionless heat release rate and the dimensionless flame height. The ratios of maximum to mean flame height and mean to continuous flame height are 1.35 and 1.5, respectively. Significantly, these ratios remain unaffected by the air-inlet width, fuel pan diameter, environmental temperature, and heat release rate.

Crosswind-induced fire whirl generation and development are investigated in this study, with particular emphasis on the effects of crosswind velocity, wind direction, and fuel mass loss rate on flame height and heat release behavior. Time-resolved flame heights and fire-whirl unsteadiness were quantified using frame-by-frame image analysis, supported by numerical simulations under both steady and time-varying mass loss conditions. The experiments show that crosswind-generated fire whirls differ qualitatively from normal stable whirls. A critical crosswind velocity of approximately 1 m/s was identified, above which a transition occurs from a buoyancy-controlled regime—in which flame height increases with heat release rate—to a wind-controlled regime, where enhanced air entrainment suppresses vertical flame growth despite higher heat release. Wind direction had a strong impact on vortex formation: wall-blocked flows produced tall, turbulent flames, while direct winds generated shorter, more stable flames with reduced angular momentum and limited whirl formation. Increased mass loss rates intensified flames under low-wind conditions, but in the dynamic regime, their influence lagged the flow structures induced by the wind. Computational modeling reproduced the overall experimental trends but consistently underpredicted flame heights due to the idealized representation of wind profiles. These findings underscore the nonlinear, regime-dependent relationship between flame height and heat release in crosswind fire whirls, providing insights essential for fire modeling, urban fire safety, and wind-informed architectural design.

During a forest fire the formation of fire whirls can be observed but their characteristics and evolution are still difficult to predict.Due to the high thermal energy released, fire whirls can become a danger to firefighters and are, by their complexity and dangerousness, a way of extreme fire behavior that is poorly understood.This work presents a study on the formation of fire whirls with vertical axis on wildfires at laboratory scale that analyses the influence of the variation of the following parameters: fuel load, density, fuel configuration, imposition of forced flow with different velocities through the use of fans.Comprehensive measurements were performed in order to establish correlations of the burning rate, heat release rate, flame height, temperature and velocity of fire whirls, and to clarify the similarities and differences between a fire whirl and a 'normal' fire, i.e., one in which vortex formation is not observed.Fire whirl experiments were performed in a vertical channel with a quadrangular section of 1×1 m2 with a height of 7.8 m, the fire whirl generator, using dried shrubs, which is a fuel mainly composed by heather (Erica australis) and gorse (Pterospartum tridentatum) quite common in forested areas in Central Portugal and in several Mediterranean climate regions.Given the transient nature of the experiments relationships between time correlated parameters or with average or extreme values were established.Comparison with similar experiments showed that the properties of the flames generated with the present configuration of the fire whirl generator are similar to fire whirls produced in other laboratory studies and with full scale fire whirls.The maximum heat release power was of the order of 1 MW which is higher than the reported values for similar laboratory tests.The results show that forced flow increases dramatically the burning rate and reduces the time needed to achieve a high rate of energy release.On the other hand, from a certain value of forced flow it is observed that the values of the burning rate and flame height decrease, thus existing a critical wind velocity for optimal fire whirl development.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.

Fire whirls occur occasionally in wild land fires and large scale urban fires. When a fire whirl occurs, the height of flame increases significantly and the fire damages could become more severe. In this study, small and middle scale experiments (up to 20 cm in diameter) were performed to understand the mechanisms of the flame height increase at fire whirls. In the experiments, fire whirls were established on a fuel pool using a fixed-frame-type fire whirl generator. At the small scale pools (\( d \le 5\,{\text{cm}} \)), the flame behaved as a laminar diffusion flame and the measured flame height almost agreed with our scale model based on Burke–Schumann theory. Meanwhile, at the middle scale pools (\( d \ge 10\,{\text{cm}} \)), the flame comes away from a laminar diffusion flame. The measured flame height was correlated to the dimensionless heat release rate at the conditions without swirling flow and also with swirling flow. The measured data at the middle scale pools almost laid on a line of slope 2/3 in the logarithmic diagram between the flame height and dimensionless heat release rate. Also the radiant emittance and shape of base part flame, which were important to understand the fire whirl phenomena, have been examined. It was found that the radiant emittance of flame increased considerably by the swirling flow at the middle scale pools. It was also confirmed experimentally that the viscous effect near the floor (Ekman layer) made the base part flame approach to the fuel pool.

Experimental research on combustion dynamics of medium-scale fire whirl

Effect of circulation on flame heights over liquid fuel pools

Fire whirls, also known as fire tornadoes, represent an extraordinarily severe fire phenomenon, exemplified by the catastrophic events shortly after the great Kanto earthquake in 1923. This research aims to propose an early-warning analysis employing image processing techniques for predicting before the fire-whirl formation. Scaled-down experiments were conducted using two speed-adjustable fans to control the wind movement and generate fire whirls. Through image processing, a set of input flame images is transformed into the evolution of flame height as output data, which serves as the basis of the signal processing. The difference between whether fire whirls occur or not can be detected from the flickering of the flame-height signal. Without delivering external winds, minor changes in noise components are observed at subsequent times, showing no signs of fire whirls. On the other hand, the noise component preceding the fire whirl occurrence highlights a significant increase in standard deviation and autocorrelation, attributing to a slower recovery rate from a perturbed state near a transition to a fire whirl. In this paper, a dynamical marker is constructed as a composite metric of smoothed flame height, standard deviation, and autocorrelation coefficient at lag 1 of the noise component, showing upward trends prior to fire whirl formation. The effectiveness of the dynamical marker as a warning sign for predicting fire-whirl occurrences is validated through experiments of three different wind speeds with the alarm threshold of +3σ to mitigate an unnecessary false detection.

Effect of flow circulation on combustion dynamics of fire whirl

In this work, a fire whirl facility with no enclosure of solid walls was designed, for experimental simulation of fire whirls in open field. Air curtains were used to produce the generating eddy of fire whirl. Tests were conducted to evaluate the capability of the new facility. It was found that there was an optimal tilt angle for the air curtains to produce stable fire whirls. Experiments of fire whirls under different pool diameters showed that the mass loss rate $$ \dot{m} $$ depended on the circulation $$ \varGamma $$ by $$ \dot{m}\sim \varGamma^{1.18} $$ and the dimensionless flame height satisfied $$ H^{*} \sim \varGamma^{*0.71} $$ (where the asterisks denote dimensionless variables). It was also found that the ratio of the continuous flame height to the whole flame height for different pool diameters was 0.67. The centerline temperatures varied with the normalized height $$ Z $$ by $$ Z^{0.08} $$ , $$ Z^{ - 1.30} $$ and $$ Z^{ - 2.18} $$ , respectively in the regions of the continuous flame, intermittent flame and plume. The radiative fraction was calculated to be close to 44%. All these results fit well with literature data, thereby the capability of the new facility for producing stable fire whirls was fully validated. Some potential applications of the new facility were discussed in detail. Especially, instruments such as infrared camera can be used to investigate the flame radiation of fire whirls by the new facility.

In this paper, the flame wander of fire whirl is investigated by experimental means. Small-scale fire whirls were produced by two split cylinders, and the data of vertical velocity measured by stereo particle image velocimetry are analyzed to track the flame displacement along the horizontal direction. Medium-scale fire whirls were produced by a fixed wall facility, in which a video camera was used to monitor the flame position along the horizontal direction, thereby the flame displacements are determined by image analyses. It is found that during each test the flame displacements at different heights vary synchronously, suggesting that the flame is wandering as a whole. The flame displacements at different heights involve different variation ranges. By using the flame displacement data, the appearance probabilities of flame along the horizontal direction are calculated for small- and medium-scale fire whirls. The results show that the probabilities follow the Gaussian distribution, suggesting that a fire whirl almost always wanders at the very vicinity of the pool center. Finally, it is verified that the frequency of flame wander linearly depends on the circulation of fire whirl, while the correlation formulations differ between small-scale and medium-scale fire whirls.

Fire whirls, a combustion-intensifying phenomenon, offer potential for cleaner, more efficient burns with reduced emissions in environmental applications like oil spill remediation. While most knowledge of fire whirls stems from laboratory-scale experiments, and only a limited subset of these experiments has captured emissions, this study addresses a critical research gap by conducting one of the largest controlled fire whirl experiments to date, focusing on enhancing in-situ burning. Using a 1.5-meter diameter crude oil pool with 15 mm and 40 mm slick thicknesses on an open water surface, fire whirls were successfully generated within a 5-meter-tall three-wall structure under the influence of various wind conditions. Measurements of flame geometry, flow velocity, temperature profiles, heat flux, mass consumption, and emissions were compared to pool fires. Results revealed that fire whirls, with mean flame heights nearly double those of pool fires, increased burning rates by 40% and reduced soot emissions by 40%. The highest fuel consumption efficiency of 95% was achieved in a 15 mm slick thickness fire whirl experiment, while in other fire whirl experiments, especially those with 40 mm slick thickness, premature extinguishment was observed lowering ultimate fuel consumption efficiencies. Enhanced performance compared to traditional pool fires suggests that fire whirls might be more effective for in-situ burning, however this efficiency is currently only achieved under calm ambient conditions. This study discusses the potential interactive effects of ambient conditions, configuration sizing, and boilover phenomenon on fire whirl dynamics, highlighting the need of continued research across scales to optimize configurations and mitigate detrimental emissions, thereby enhancing the efficacy of in-situ burning in large-scale oil spill scenarios.

Background Fire whirl is an extreme fire behaviour in wildland fires, and an essential factor for its formation is the surrounding generating eddy. No systematic experimental study has been conducted on natural fire whirls with varying heights of the initial generating eddy. Aims The aim of this research was to provide a comprehensive experimental study on the effect of initial generating eddy height on fire whirl formation and flame characteristics. Methods The experiments were conducted in a fixed-frame facility with varying channel wall height (representing the initial generating eddy height). A 20-cm-diameter propane burner (10.0–100.0 kW in heat release rate) was used. Key results The critical channel wall height for fire whirl formation decreases with the heat release rate. The mean flame height grows remarkably with initial generating eddy height for large heat release rates, but it varies only slightly at relatively small heat release rates. Conclusions The formation of fire whirl depends on the initial generating eddy height, rotational strength, and heat release rate. A flame height correlation of the fire whirl is obtained by considering the initial generating eddy height. Implications This work provides a basis for improving the prediction accuracy of natural fire whirls in wildland fires.

Effect of imposed circulation on temperature and velocity in general fire whirl: An experimental investigation