Abstract
Scientists have long studied fire in an effort to both understand the world around them and to prevent the destruction and devastation that uncontrolled fires can cause. Despite many advances in the understanding of fire phenomena, society offers continued challenges that require new approaches for the prevention and mitigation of unwanted fires. In this chapter, fire research is presented through a series of photographs that scale from small, buoyant flames in the laboratory up to large, uncontrolled wildfires and even fire whirls.
Introduction
Scientists have long studied fire in an effort to both understand the world around them and to prevent the destruction and devastation that uncontrolled fires can cause. Despite many advances in the understanding of fire phenomena, society offers continued challenges that require new approaches for the prevention and mitigation of unwanted fires. In this chapter, fire research is presented through a series of photographs that scale from small, buoyant flames in the laboratory up to large, uncontrolled wildfires and even fire whirls.
Our journey begins with a study of the most canonical problem in fire science, the pool fire, where liquid fuel in a pan is burned in a quiescent atmosphere. Interesting effects such as puffing and flickering are explored, as well as the influence of different fuels and configurations. Simulations of a pool fire show the complex instabilities that govern its behavior. Similar behavior is shown in very large fires, here up to 120 m in diameter. Another canonical problem in fire, upward flame spread, is presented in comparison to other drivers of the flame-spread process: orientation, ambient wind, and configuration. Together, the heat-release rate from a fire and flame-spread rate form the primary descriptors of material flammability, signifying the safety and appropriateness of materials for fire-safe design.
When design fails and accidents occur, active suppression of fires, often with water, is all important. Fire suppression over a range of scales, from a single droplet to large sprinklers, is explored. The extinction process is shown using a unique line burner that slowly transitions to extinction as nitrogen is added to the surrounding flow. A dramatic intensification of combustion, fire whirls, is also presented, occurring when appropriate levels of swirl are added to a pool fire. They result in a dramatic increase in flame length, heat release, and, in nature, unpredictable fire behavior. The efficiency of this process and its potential application for remediation and energy production are presented through the “blue whirl,” a soot-free regime of the fire whirl.
Larger uncontrolled fires, namely wildland fires, are also presented. In the laboratory, experiments using laser-cut fuelbeds have been studied in an effort to understand the processes controlling wildland fire spread. Also shown are firebrands, small burning embers that loft off burning material and cause much of the home destruction observed in wildfires. Large fires have also occurred in cities, and a recreation of the Great Fire of London of 1666 that occurred on the Thames in 2016 is a fantastic illustration of the destruction that has occurred in urban areas around the world.
Finally, the slow oxidation of carbonaceous material known as smoldering is presented. While it does not have a flame, the combustion process is critically important for biomass burning and the production of emissions from many different landscape-scale fires, impacting public health and the global climate.
As Michael Faraday once said, “There is no better, there is no more open door by which you can enter into the study of natural philosophy, than by considering the physical phenomena of a candle.” This journey through fire presents only a brief introduction to many captivating features encountered in the study of fire phenomena.
6.1 Pool Fires
6.1.1 Puffing Pool Fires
Buoyant diffusion flames, known as pool fires, form when a pool of liquid fuel is ignited. One interesting feature of pool fires is that they are known to “puff” at characteristic frequencies, on the order of 1 Hz, dependent on the diameter of the pool. In these photos two cycles of “puffing” are illustrated, where parcels of hot gas are formed at the base and transported upward, before the fire necks back in and the cycle begins again.
6.1.2 Synchronizing Twin Flickering Flames
Diffusion flames at a certain scale under a gravimetric field produce periodic motions caused by a buoyancy-induced instability: this is what we call flickering. We examine what happens when twin flickering flames are placed at a prescribed distance from one another. Interestingly, both in-phase and out-phase harmonic oscillation of twin methane–air jet flames (10.6 mm inner diameter of burners, issuing methane at 500 cm3/min) are found when the distance is modified. These images were taken with a high-speed camera (CASIO EX-F1 at 100 fps) at a distance of 25 mm, at which the out-phase mode with 13 Hz of flickering frequency is dominant. Left and right flames are elongated and their tips are separated in turns, making it look like the flames take turns jabbing at one another.
When the separation distance is set to smaller than 25 mm, twin flames merge together and show an in-phase oscillation. When it is larger than 25 mm, on the other hand, the oscillation mode suddenly switched to an out-of-phase one as shown in this figure. Interestingly, what is seen at 25 mm distance is that twin flames temporarily cease to suppress the buoyancy-induced instability together. In this way, we can study what causes this buoyancy-induced instability and how to suppress it if necessary.
Acknowledgements
This research was supported by Toyota Physical and Chemical Research Institute for fiscal year of 2015.
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6.1.3 Emulation of Pool Fires with a Gas Burner
Studying the combustion of condensed fuels can be difficult, so researchers often attempt to “emulate” these flames. In each pair of images shown, the flames of condensed and gaseous fuels have the same heats of combustion, heats of gasification, surface temperatures, and laminar smoke points. When these four quantities are matched, the resulting diffusion flames have similar appearances. The burner used here has been designed to eventually study flames in microgravity.
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6.1.4 Methanol Pool Fire
Pool fires have a complex structure, originating from the base upward into the plume. When burning methanol, little to no soot is produced, so the flame appears blue and structures can be clearly visualized. In this sequence of images, a pool of methanol ignited. The flame first is observed to propagate rapidly over the liquid fuel surface. Once the entire surface is ignited, a series of structures can be observed around the base of the pool fire, which play a complicated role in its structure and behavior.
6.1.5 Simulation of Pool Fire Dynamics
This graphic depicts the turbulent instability dynamics of a 1 m-diameter methane–air fire plume. Instability dynamics are responsible for the unsteady heat transfer in fire environments, which have been observed experimentally. The mesh superimposed on the bottom of the plume is the underlying computational grid utilized to carry out the calculation. The puff cycle of the plume may be broken down into four distinct stages: formation of a base instability near the edge of the plume; growth of the instability due to a misalignment of the vertical pressure and radial density gradients generating a localized torque; formation of a large toroidal vortex that self-propagates and entrains a large quantity of surrounding air; and destruction of the toroidal vortex due to formation of secondary instabilities that grow causing a nonlinear breakdown of the toroidal vortex. The baroclinic torque instability dynamics is an important ingredient in characterizing the fire dynamics of large pool fires as it dictates the level of air entrainment and flame surface area available for combustion. An improved understanding of instability dynamics will therefore result in more accurate predictions of fire intensity and growth.
Acknowledgements
This research was supported by the National Science Foundation under grant CTS-0348110 and the Office of Naval Research under Grant No. N00014-03-1-0369. Computer resources were provided by the Center from Computational Research (CCR) at the University at Buffalo, the State University of New York.
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6.1.6 Extremely Large-Scale LNG Pool Fires and Indoor Fire Whirls
Figure 6.15 Largest experimental LNG pool fire on water (56 m-diameter, 146 m height) at Sandia National Laboratories, Albuquerque, NM.
Figure 6.16 Largest indoor experimentally created fire whirl (3 m-diameter pool) at Sandia National Laboratories, Albuquerque, NM.
While many experiments are conducted in the laboratory, it is important to verify and explore fire dynamics on a larger scale, where fire presents its greatest hazards. A large-scale liquefied natural gas fuel fire experiment simulating a spill of fuel on water was conducted at the 120 m-diameter water pool at Sandia National Laboratory, Albuquerque, New Mexico, on December 10, 2018. Other photos show an experimentally generated fire whirl formed from a 3-meter diameter JP-8 test conducted in the FLAME facility at Sandia National Laboratory.
6.2 Flame Spread and Fire Growth
6.2.1 Surface Inclination Effects on Upward Flame Spread
Flame spread, the process of fires moving over a surface, occurs due to ignition of virgin material as a result of heating. There are many mechanisms of flame spread, which vary depending on the materials, geometry, conditions, and scale under investigation. Changes in the rate of flame spread, and the nature of the flame, are easily visualized by changing the orientation of the fuel. Starting from the left “ceiling fire,” as the inclination angle or tilt of a burning surface is increased, underside flames transition from blue, well-mixed laminar flames to increasingly turbulent yellow flames on the topside that “lift” from the surface, dramatically increasing the flame thickness. These images were taken perpendicular to the surface of a thick sample of Polymethyl Methacrylate mounted flush into insulation board as flames spread upward. These tests have helped in finding critical inclinations with maximum flame spread rates, burning rates, and heat fluxes from the flame.
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6.2.2 Wind-Driven Fires
Changes in ambient wind, similar to slope, have a drastic affect on the spread rate of a fire. This series of images show the effect of wind (from 0.87 to 2.45 m/s) on a stationary flame, simulated using a 9.5 kW gas burner. As the wind, blowing from right to left, is increased, the fire transitions from more of a plume-dominated mode to an attached, boundary-layer flame. As flames become attached to the surface, heating and thus their rates of spread increase.
Reference
6.2.3 Asymmetric Buoyancy Induced Blow-Off
A 5 cm-wide × 20 cm-long composite fabric sheet woven with thread made of 75% cotton and 25% fiberglass is hung vertically and is electrically ignited at the bottom surface to cause a flame to spread upward. When burned, the cotton is pyrolyzed into fuel vapor leaving behind a woven fabric made only of fiberglass. This remaining fiberglass maintains the integrity of the fabric and acts as a flame barrier that does not allow ignition to penetrate it.
The image views the hanging fabric from the narrow edge. As the flame grows in size after ignition, the incoming air velocity induced by buoyancy becomes too high to remain stable. Image (a) shows the maximum flame length of about 20 cm after ignition and initial flame growth. (b) The flame base on the right-hand side is beginning to blow off due to the high velocity of the entrained air. (c) The flame base on the right-hand side continues to blow off, while the flame base on the left-hand side is maintained. Because the fuel is thermally thin, the left-hand side flame continues to vaporize fuel on the right-hand side, seen as white smoke. (d) The right-hand side flame has been blown too far downstream. The material surface is still too cold in this region for the flame base to be stable in this position. (e) The right-hand flame has completely blown off. A large amount of fuel continues to vaporize from the surface. (f) Due to the reduced fire power and heat feedback to the fuel, the left-hand flame shrinks in length. (g) The left-hand flame reaches its final stable length. As the cotton fuel burns out, this flame slowly propagates upward across the entire surface toward new fuel, but at this reduced length of about 10 cm. The remaining flame will not blow out due to the reduced entrained velocity. The fuel vapor mixing itself with air on the right-hand side is still flammable but is not reignited by the left-hand flame because of the flame barrier properties of the remaining fiberglass fabric.
Acknowledgements
This research was supported by NASA and Underwriters Laboratories.