Snowy holidays and long, dark nights are a great time to sit by the fire or enjoy some candlelight. We’ve talked before about how buoyancy affects a flame’s shape, how atomization mixes liquid fuel and oxidizers, how flames propagate, how internal combustion works and how instabilities can end combustion. But in all that we haven’t addressed what fire actually is! Combustion is a chemical process–a reaction between a hydrocarbon fuel and oxygen, but the flame we’re accustomed to seeing is a combination of blue light produced by the complete reaction and incandescent red/orange/yellow light from glowing soot particles produced when there is insufficient oxygen for the reaction. If you have time after the Minute Physics version, this video from Ben Ames has a wonderful explanation of flames. Of course, if you just prefer your holiday fun with more explosive high-speed videos, you’re going to want to see this Christmas tree made from detonation cord (see 2:40 for the start of the best part). This wraps up our holiday-themed fluid dynamics series. Happy holidays from FYFD! (Video credit: Minute Physics)
Tag: flame
Fire in Microgravity
In the movie “Gravity” Sandra Bullock’s character battles a fire aboard the International Space Station. Combustion is a huge concern in space habitats. Microgravity fires are challenging to detect and fight because they behave very differently in the absence of buoyancy. On Earth, buoyancy makes hot air rise from a flame while cooler air is pulled in near the base. This feeds fresh oxygen to the teardrop-shaped flame. In space, there is no buoyancy and flames are spherical. They also burn at lower temperatures and lower oxygen concentrations–so low, in fact, that the oxygen depletion necessary to extinguish a fire is lower than what humans require to survive.
No buoyancy makes it harder for fires to spread, but it also makes them harder to detect since smoke doesn’t rise toward a detector on the ceiling. Instead, fire detectors aboard the Space Station are housed in the ventilation system that moves air through the modules constantly. In the event of a fire, astronauts use a three-step fire suppression system. First, they shut off the ventilation system to delay the fire’s spread. Then they shut off power to the affected unit, and, finally, they use fire extinguishers on the flames. The Russian module is equipped with a foam extinguisher and the others use CO2 units. (Image credit: Warner Brothers)
Flame Feedback
When a flame is enclosed in a combustion chamber, it can create violent oscillations in the pressure field. Flames have a natural unsteadiness in their heat release. These temperature fluctuations create pressure waves in the chamber. In the right enclosure, those pressure waves resonate and feed energy back into the initial perturbation. This creates a self-exciting oscillation, not dissimilar from aeroelastic flutter. This combustion instability is known as a thermoacoustic instability because of the coupling between temperature and pressure (acoustic) waves. The quick demo above lets you see and hear such an instability; here’s the same setup in high-speed, which makes the oscillating flame even clearer. The violence of this instability can be great enough to destroy engines. Famously, the F1 engine used in the Saturn V rocket had a history of instability issues before the fuel-injector was redesigned. For another great demo of this effect, check out this video from T. Poinsot. (Video credit: V. Anandan)
How Flames Expand
Combustion is a remarkably complicated phenomenon fluid dynamically. The schlieren images above illustrate a couple of the variables that affect flame propagation. The top image shows an idealized, essentially spherical flame expanding in a quiescent hydrogen-air mixture at atmospheric pressure. The middle flame is expanding in a high-pressure environment, similar to an internal combustion engine. The lowest image shows a flame in a highly turbulent environment, which is also typical of internal combustion engines in order to promote mixing of the air and fuel. (Photo credit: C.K. Law, S. Chaudhuri, and F. Wu)
Turbulent Flames
The flames surrounding a burning tree stump flicker and billow in this image from photographer Serdar Ozturk. The chaotic motion of the flames is indicative of turbulence, a state of fluid flow known for its many scales. Note the range of lengthscales and structures in the fire. In turbulent flows, kinetic energy cascades from large scales, like the width of the top of the plume, down to the small scales, which may be even smaller than the wisps of flame at the edges of the fire. At the largest scales, the structures and behaviors we observe are all flow- and geometry-dependent, but theory predicts that, at the smallest scales, all turbulent flows look the same. (Photo credit: trashhand/Serdar Ozturk)
Flame Thrower Physics
This high-speed video–which we do not recommend recreating yourself–features burning gasoline flying through the air. In addition to the sheer entertainment value, there are some neat physics. In the first segment, when they kick a tray of gasoline, one can see lovely fiery vortices forming around the backside of the tray as it’s launched. This is the start of the tray’s wake. In the latter half of the video, they launch the flaming gasoline from a bucket. Notice how the flames are in the wake while liquid gasoline streams out ahead without burning. This is because it is primarily gaseous petrol that is flammable. As the liquid fuel breaks up into droplets heated by the burning gasoline vapors nearby, the rest of the fuel changes to a vapor state and catches flame. (Video credit: The Slow Mo Guys; submitted by Will T)
Detonation in a Bubble
Accidental releases of combustible gases in unconfined spaces can be difficult to recreate in a laboratory environment. Here researchers simulate the conditions using detonation inside a soap film bubble. Combustible gases are pumped inside the soap film and then a spark creates ignition. The resulting flame propagation is visualized using high-speed schlieren photography, making the density gradients in the flame visible. When the mixture of hydrogen fuel to air is balanced, the flame is spherically symmetric with a high flame speed. In contrast, weaker mixtures of fuel/air produce slow flame speeds and mushroom-like flames that leave behind unreacted fuel. This is due to buoyant effects; the time scale associated with buoyancy is smaller than that of the flame speed and chemical reactions when the fuel/air mixture is lean. (Video credit: L. Leblanc et al.)
Relighting a Candle
When a candle is blown out, a buoyant plume of unburned fuel/air mixture continues to rise for several seconds. By bringing a combustion source close to the plume, the mixture can ignite and flames will propagate back down to the candle wick to reignite it. Watch the slow motion replay near the end of the video and you can actually see the flame front propagate downward. (Video credit: G. Casavan, University of Colorado)
Fireball in Slow Motion
The high-speed video above shows an atomized spray of flammable liquid being ignited using a lighter. It was filmed at 10,000 fps and is replayed at 30 fps. Although uncontained, this demonstration is similar to the combustion observed inside of many types of engines. Automobiles, jet engines, and rockets all break their liquid fuel into a spray of droplets to increase the efficiency of combustion. The turbulence of the flames dances and swirls, with small-scale motions close to the sprayed droplets and larger-scale motions around the vaporized fuel. This variation in size of the scales of motion is a hallmark feature of turbulence and can be used to characterize a flow.
Reader Question: Fire as a Fluid?
Reader David L asks:
I understand that fire is a form of energy rather than a fluid in the physical/tangible sense. However, is it possible for fire to exhibit fluid-like behaviours to a certain extent.
In other words, could the dynamic properties of fire be described with pseudo-variables analogical to variables that describe a physical fluid (i.e. viscosity, density, Re, etc.)?
Actually, combustion is a major topic of research among fluid dynamicists. Since the part of fire that we identify as visible flame is a reacting mixture of gas and some solid particles, it moves according to the same equations of motion as any other gas. However, when studying combustion thermodynamical equations and chemical reactions must also be tracked in addition to mass and momentum, which makes modeling fire very difficult. Combustion plays a major role in internal flows like those in car, jet, and rocket engines. (Photo credit: master.blitzy)
