The amazing world of fireballs, donuts and horseshoes

On the left a fireball in the shape of a donut. On the right a fireball in the shape of a horseshoe. Credit: Eindhoven University of Technology

Flameballs are soft, fragile spherical flames that, until recently, could only exist under conditions of near zero gravity. TU / e researchers have successfully observed balls of flame under normal earth-bound conditions and thus uncovered new information on how lean fuel blends work. Lean hydrogen mixtures are considered the fuel of the future because they do not emit CO2 and only low concentrations of nitrogen oxides. Join our researchers on their exciting journey to understand the enigmatic fireball.

You don’t have to be a combustion specialist to understand that when an air-fuel mixture is ignited, flames begin to spread. The oxygen reacts with the fuel in the flame, heat is released and ignites the mixture next to the flame, and this process continues. It happens in your kitchen’s gas stove, in a cylinder in your car’s engine, or in a gas turbine at a power plant.

But even combustion scientists are bewildered when they see a flaming ball for the first time. “A flameball is a tiny, luminous spherical flame, which maintains the same size and shape for a virtually unlimited time,” explains Philip de Goey, Group Manager Combustion Technology at TU / e. “It looks like something impossible. It doesn’t expand, so there is a lot of fresh mixture around, and it doesn’t go out, even though there is no fuel in it. interior. “

The secret of the flame ball is that it is a so-called diffusion flame. Its combustion is supported by a continuous supply of oxygen and fuel which diffuses towards this spherical flame from the surrounding mixture. The heat released is also transmitted to the surrounding mixture by diffusion, and a fraction of it is carried away by radiation. Due to this heat loss, the flame ball is unable to ignite the neighboring mixture and expand. This makes it stable.

Soft and fragile

Predicted by Drozdov and Zeldovich in 1943, the flameballs have long been considered a theoretical curiosity as no one has ever observed them for nearly half a century after this prediction. The reason is that most combustion labs are built on Earth and, therefore, like everything on Earth, are subject to gravity.

In theory, a combustible mixture must be stationary for a fireball to exist. However, terrestrial gravity flames tend to generate upward convection flows due to buoyancy forces acting on the hot combustion product, such as in candles. While this natural convection helps candles burn, a ball of flame is too soft and too brittle to survive.

It was not until 1990 when the flaming balls were discovered experimentally by Paul Ronney, when gravity-free combustion experiments became possible. Such experiments were carried out inside free-falling chambers, dropped from high towers or on board planes flying in parabolic trajectories – a kind of flying roller coaster, where one can also feel weightless, but for a shorter time.

Experimenting with so-called lean-limit mixtures, which contain very small amounts of fuel and can barely sustain combustion, Paul Ronny observed that several 5-10mm balls of flame formed and burned in a mixture of hydrogen. and air.

Credit: Eindhoven University of Technology

Why fireballs matter

Shortly after the discovery, researchers recognized the potential importance of studying fireballs. First, these flames have much lower temperatures than those found in other flames. They are also extremely sensitive to small changes in the conditions in which they burn. This makes a flameball an excellent object to validate theoretical combustion models. Such validation becomes particularly important as modern combustion technologies evolve towards blends with low fuel concentrations. These so-called lean mixtures tend to generate cooler flames which produce less nitrogen oxides (NOX). And the flameballs are the leanest flames possible

Second, flame balls can exist in the leaner mixtures which can still burn – if less fuel is present in the air, no combustion is possible. The ultimate limits to which flames can exist are important for the development of safety standards and for the design of combustion devices.

Finally, the study of flameball phenomena can help us better understand the combustion mechanisms of lean hydrogen mixtures. Hydrogen is one of the main contenders to become the “green” fuel of the future, and lean combustion is seen as the future of combustion technologies.

Bring the flameballs back to Earth

It is therefore not surprising that the discovery of the flameballs triggered further intensive theoretical and experimental research. Experiments have even been carried out at the International Space Station, where “micro-gravity” conditions are optimal and permanent. However, extensive measurements under such conditions are not possible due to the very high cost and the limited possibilities of experimental diagnostics.

This changed, however, when the flaming balls were put down by researcher TU / e Yuriy Shoshin, working in the Combustion Technology group of Philip de Goey. As happened in the case of microgravity flameballs, Shoshin accidentally discovered “normal” gravity flameballs.

“When we filled a vertical glass tube with a mixture containing hydrogen and ignited from the lower end, we observed almost perfect balls of light that slowly rose to the upper end of the tube,” explains Shoshin. It turned out that the buoyancy forces induced by the flame create a small vortex in which the flame ball resides. So instead of destroying the ball of flame, as was the case in previous experiments, gravity-induced convection under appropriate conditions helps preserve it.

The amazing world of fireballs, donuts and horseshoes

Left, cross section of a ball of flame in a mixture of hydrogen, methane and air at high pressure; right: Simulation of a ball of flame residing inside a vortex. Credit: Eindhoven University of Technology

Living cells

Further intensive experimental and numerical studies have led to a lot of new information about how lean hydrogen flames work, explains Shoshin. which behave amazingly like living cells, dramatically “fighting for life”.

“Balls compete for fuel like food, constantly changing direction whenever new fuel becomes available. If a fireball is lucky enough to find a location with a lot of fuel, it splits in half, just like a living cell. Cells that are surrounded by better performing competitors are less fortunate and decompose. They can no longer resist the downward gas flow by self-induced buoyancy. These unfortunate bullets are removed from the fuel source by the gas flow and end up “starving”. ”

Donuts and horseshoes

The fact that flameballs exist in a vortex has given rise to the idea that flames with similar combustion mechanisms could possibly form under other conditions, where vortices are present. “And, indeed, in other experiences we have found other types of flames that burn the same way, shaped like donuts and horseshoes. “

Such flames form around what are called vortex filaments, lines around which gas revolves. In practical devices, combustion almost always occurs in turbulent mixtures, and such filaments are known to be present in turbulent gas. “This gives us hope that studying such flames can help understand turbulent lean hydrogen flames,” says Shoshin.

Flameball combustion mechanisms may also be relevant for flame stabilization. “The flames have to be stable to be usable in home boilers or gas-fired power plants, and the most common way to stabilize the flames is to create a vortex behind an obstacle placed in a flow of combustible mixture. “

Beyond theory

De Goey emphasizes the importance of finding flameballs in non-microgravity conditions. “While weightless flameballs remain the most basic and simplest example of a flameball, flameballs and their relatives studied in our group can exist under different conditions. It makes their physique much more interesting and also much more relevant. for other areas of combustion science.

“It is interesting to note that although our studies were to a large extent inspired by Paul Ronney’s microgravity experiments, for some of the members of the ‘flameball family’ discovered in our laboratories, the effects of gravity did. turned out to be unimportant at all. . “

The next step in the research of De Goey and his team is to incorporate the phenomenon of the flame ball into previous theories of normal flames. However, their interest in the enigmatic fireball goes far beyond mere scientific curiosity. “Ultimately, a full understanding of how they work will help us develop lean fuels that will pave the way for a sustainable energy future,” he said.

Reveal the structure of the mysterious swirling blue flame

Provided by Eindhoven University of Technology

Quote: The Amazing World of Flame Balls, Donuts and Horseshoes (2021, June 16) retrieved June 16, 2021 from donuts.html

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