Fire tornadoes, despite their name, are more like dust devils than your typical tornado. In nature, they’ll often form in wildfires, but here the Slow Mo Guys simulate one for the high-speed cameras using a ring of box fans set up to provide rotational flow, or vorticity, around a kerosene fire. As the fire burns, the warm air over the flame moves upward due to buoyancy. This creates a low-pressure area around the fire that draws in the spinning air from further out. Like an ice skater who pulls her arms in when spinning, the rotating air spins faster as it moves in toward the fire, resulting in a swirling turbulent vortex of flame. Hopefully it goes without saying, but, seriously, don’t try this at home. (Video credit: Slow Mo Guys; submitted by Chris S.)
Tag: combustion
Ignition
Shown here are the first instants after a bubble full of methane gas is ignited via laser. Using the schlieren optical method and a high-speed camera, scientists recorded the deflagration at 10,000 frames per second. Because schlieren imaging is very sensitive to small changes in density, we see not only the expanding flame front as the methane ignites but also the subtle waviness of the methane expanding into the surrounding air as the bubble bursts. (For comparison, check out what bursting a water balloon looks like at high-speed.) Be sure to head over to ScienceTake for the full schlieren video, and also check out this award-winning video of a match lighting made by the same researcher. (Image credit: V. Miller et. al.; full video: The New York Times; submitted by Rebecca M.)
ETA: An earlier version of this post mistakenly said the demo used a balloon full of methane rather than a bubble. Thanks to jump-first-think-later for the correction.
Bullet-Time Inferno
Remember the bullet time effect from The Matrix? This spectacular video gives you a similar effect with the turbulent flames created by firebreathers. To capture this level of detail, Mitch Martinez uses an array of 50 cameras placed around the performers, allowing him to reconstruct the full, three-dimensional representation of the flames. Similarly, some scientists use arrays of high-speed video cameras to collect 3D, time-resolved data about phenomena like combustion. Because these flows are so complex in terms of their fluid dynamics and chemistry, capturing full 3D data is important to help understand and model the flow better. (Video credit: M. Martinez; via Rakesh R.)
Extinguishing Flames
Putting out fires can be a difficult, water-intensive task. In this video, scientists demonstrate how using a non-Newtonian fluid can make it easier to extinguish and suppress flames. Where water tends to splatter and scatter against an object, a yield-stress fluid can cling and coat to smother the flame. The fluid used here is water with a 0.1% polymer additive, which is enough to significantly change the fluid’s rheological properties. Pre-treating flammable objects with the fluid is also effective at suppressing combustion, raising additional possibilities for using such techniques in fighting the spread of wildfires. (Video credit: B. Blackwell et al.)
Water in Oil
Pouring water on an oil fire is a quick way to cause almost explosive results. Since water is denser than oil, it quickly sinks to the bottom of a container, heating up as it does. When the water reaches its boiling point, it evaporates and expands as steam. That phase change involves a huge change in volume, a fact made especially clear in the video below. The steam expands and rises, throwing droplets of oil upward and outward. These smaller atomized droplets are easier to combust, which, in the case of the video above, causes a veritable cloud of flames if a fire has already started.
(Video credits: The Slow Mo Guys and N. Moore)
Re-lighting a Candle
When you blow out a candle, you can re-light the wick using the smoke trail left behind. This is a topic we’ve discussed before, but I’m thrilled to finally see the process in true high-speed, thanks to the Slow Mo Guys. The plume that rises from the extinguished candle is an atomized mixture of fuel (wax) and air. When you bring a new combustion source–the match–close enough, that mixture ignites and the flame spreads downward back to the wick. (Video credit: The Slow Mo Guys)
Extinguishing Fires With Sound
Engineering students from George Mason University have built a fire extinguisher that uses sound to put out flames. Since sound waves are mechanical pressure waves, they can move the air surrounding a burning material. Through trial and error the students found the high-frequency sound had little effect, but at frequencies between 30-60 Hz the sound waves could jostle enough oxygen away from the flame to extinguish the fire. They’re hoping the solution is scalable and can be applied to larger fires. For other wild ideas for chemical-less fire extinguishers, check out how researchers put out fires with explosions. (Video credit: George Mason University; submitted by @isanaht)
Fire-Breathing
In this high-speed video, the Slow Mo Guys demonstrate fire-breathing. Rather than using a liquid fuel like kerosene, they utilize cornstarch, which is both easily flammable and non-volatile thanks to its powdered form. Blowing out the cornstarch creates a turbulent jet of cornstarch and air. Combine that with a combustion source, and the cornstarch quickly deflagrates, meaning that the flame propagates via heat transfer. When neighboring regions of cornstarch become hot enough, they ignite and the flame front expands. You can observe this in the flame growth shown in the video; just after ignition the cornstarch jet is much wider than the fire and it takes some time for the flames to catch up with the jet. Although a liquid-fueled fireball operates by the same principles, it can look rather different. For comparison, check out this high-speed video of a WD-40 fireball. And, hopefully it goes without saying, but don’t try this stuff at home. (Video credit: The Slow Mo Guys)
Propagating Flames
Like many flows, flames can be unstable and undergo a transition from orderly laminar flow to chaotic turbulent flow. The timelapse image above shows the propagation of a flame front travelling downward. Each blue line represents the forwardmost position of the flame at a specific time. The flame is essentially two-dimensional, held between two glass plates separated by a 5-mm gap. The V-like points in the flame front are called cusps, and if you look closely, you can see cusps forming and even merging as the flame moves downward. Also notice how the flame front is more uniform near the top of the image, but, by the bottom, it has split into many more cusps. This is one of the indications that the flame is unstable. Check out the full poster-version of the image in the Gallery of Fluid Motion. (Photo credit: C. Almarcha et al., original poster)
The Hidden Complexities of the Simple Match
Striking a match and blowing it out seems rather simple to the naked eye. But with high-speed video and schlieren photography, the act takes on new complexity. Schlieren photography is an optical technique that is incredibly sensitive to changes in density, which makes it a prime choice for visualizing flows with temperatures variations or shock waves. Here it shows the hot gases generated as the match is lit. Once the match ignites, the flow calms somewhat into a gently rising plume of exhaust and hot air. When someone enters the frame to blow out the match, the frame rate increases to capture what happens next. The flow field around the match becomes very complex as the air and flame interact. The range of length scales in the flow increases, from scales of several centimeters down to those less than a millimeter. This complexity and range of sizes is a hallmark of turbulence. (Video credit: V. Miller et al.)
