Fire whirls are powerful, spinning disasters for people and surroundings when they occur in large urban and wildland fires. Whereas fire whirls have been studied for fire-safety applications, previous research has yet to harness their potential burning efficiency for enhanced combustion. This article presents laboratory studies of fire whirls initiated as pool fires, but where the fuel sits on a water surface, suggesting the idea of exploiting the high efficiency of fire whirls for oil-spill remediation. We show the transition from a pool fire, to a fire whirl, and then to a previously unobserved state, a "blue whirl." A blue whirl is smaller, very stable, and burns completely blue as a hydrocarbon flame, indicating soot-free burning. The combination of fast mixing, intense swirl, and the water-surface boundary creates the conditions leading to nearly soot-free combustion. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this state points to possible new pathways for reduced-emission combustion and fuel-spill cleanup. Because current methods to generate a stable vortex are difficult, we also propose that the blue whirl may serve as a research platform for fundamental studies of vortices and vortex breakdown in fluid mechanics.

/pdf/from-fire-whirls-to-blue-whirls-and-combustion-with-reduced-3c2bmxn7qi.pdf

From fire whirls to blue whirls and combustion with reduced pollution

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.

Combustion dynamics of large-scale wildfires

In this work, a small-scale fire whirl with pool diameter of 5 cm was produced by a fixed-frame type facility with two split cylinders, and the tangential, axial and radial velocity profiles in the central vertical plane of the fire whirl at different gap sizes (2.54 cm, 3.81 cm, 5.08 cm, 6.35 cm, 7.62 cm) of the two split cylinders were measured using stereo particle image velocimetry. A correction method for the effect of flame wander was developed to obtain velocity profiles corresponding to the characteristics of the instantaneous whirl. The magnitudes of the corrected radial velocity are zero to within experimental error. The whirl consists of an inner vortex core and an outer free vortex and the tangential velocity fits well with the model of a Burgers vortex. The profile of axial velocity follows a Gaussian distribution and the maximum axial velocity appears at the flame center. Moreover, at the vortex core radius, the axial velocity is 0.3 times the maximum axial velocity in the same level.

Measurement of the Flow Field of Fire Whirl

Numerical study of the development and angular speed of a small-scale fire whirl

The blue whirl is a recently discovered regime of the fire whirl that burns without any visible soot, even while burning liquid fuels directly. This flame evolves naturally from a traditional fire whirl in a fixed-frame self-entraining fire whirl experimental setup. Here, detailed thermal measurements of the flame structure performed using thermocouples and thin-filament pyrometry are presented. Thermocouple measurements reveal a peak temperature of ∼2000 K, and 2-D temperature distributions from pyrometry measurements suggest that most of the combustion occurs in the relatively small, visibly bright, blue vortex ring. Different liquid hydrocarbon fuels such as heptane, iso-octane and cyclohexane consistently formed the blue whirl with similar thermal structures, indicating that blue whirl formation is independent of fuel type, and also that the transition from a fire whirl to a blue whirl may be influenced by vortex breakdown.

https://par.nsf.gov/servlets/purl/10089265

Thermal structure of the blue whirl

Scaling of a small scale burner fire whirl

The interaction of double burner fire whirls

http://absimage.aps.org/image/DFD13/MWS_DFD13-2013-000844.pdf

Experimental Investigation of Laboratory Fire Whirls

http://absimage.aps.org/image/DFD14/MWS_DFD14-2014-002242.pdf

Katherine Hartl

Papers

Scaling of a small scale burner fire whirl

Measurement of the Flow Field of Fire Whirl

The interaction of double burner fire whirls

Experimental Investigation of Laboratory Fire Whirls

Experimental investigation of the velocity field of a laboratory fire whirl