Tuesday, November 01, 2016

The Wrinkling Nature of Flames

Many people are mesmerized by the dancing flames of a fire, watching them flicker and evolve through half-glazed eyes. Flames may be relaxing and comforting in a fireplace or campfire ring, but don’t forget that fire is also a powerful tool that can drive jet engines. The more we understand about how flames behave at a fundamental level, the better we can use them to our advantage.

In research soon to be published in the APS journal Physical Review E, a team of Italian researchers led by Francesco Creta from the University of Rome La Sapienza report key findings related to the shape and speed of flames. Their work explores the stability of flames traveling through a moderately turbulent environment. By understanding how turbulent flames behave, we can better predict what they will do in environments like the inside of a combustion chamber. Combustion chambers use turbulent flows to stabilize flames and extract as much heat as possible.

Bring together a fuel and an oxidizer, and the chemical combustion leads to a flame—a release of heat and light. The type of flame depends on how the two elements are mixed and how quickly the fire travels. For example, the flames in a wood fire are different than the flames in a gas stove. The flames studied by Creta and his colleagues are called premixed turbulent flames, which you can think of as the flame on a gas stove or the Bunsen burner you may have used in chemistry class traveling through a low-turbulence environment.

Under certain conditions, Bunsen flames start to wrinkle. The image below shows flames with decreasing ratios of flame thickness to burner diameter. When this value is less than a critical value, the flame starts to wrinkle as shown in (c), the edges become pointy and cusp-like shapes emerge. In (d), the ratio is lower than in (c) and so the wrinkling is even more pronounced.

Two-dimensional simulations of flames. The colors represent temperature.
Image Credit: Creta, et al, Physical Review E.
This isn’t just interesting to look at, wrinkling fundamentally changes the flame by increasing its surface area and therefore its burning rate. It is the result of an intrinsic instability in the flame.

The trouble is that wrinkling can also be caused by turbulence, and it can be difficult to untangle the effects of an intrinsic instability from the effects of turbulence. In this work, the scientists detangled the two by investigating questions like: How does the instability affect the shape of a flame and the way it travels? If you increase the turbulence, does this suppress the effects of the instability?

Through simulations carried out at La Sapienza by Pasquale Eduardo Lapenna and Rachele Lamioni, the team found that the curvature of a wrinkled flame is a good indication of whether it is unstable. An unstable flame has a kind of asymmetry in its curvature—it is skewed toward negative curvature. If the flame is wrinkled by turbulence only, this isn’t the case. This means that during experiments, researchers can differentiate between flames that have been wrinkled by turbulence only and those that have an intrinsic instability by measuring the curvature profile of the flames.

Simulation results also showed that unstable flames travel significantly faster than flames wrinkled by turbulence alone. However, as you increase the turbulence, this difference starts to fade. As you increase the turbulence the difference in curvature starts to fade too. This indicates that you may be able to control the effects of the instability by adjusting the turbulence.

In a series of follow-up experiments led by Guido Troiani at ENEA C.R., researchers created Bunsen flames with different ratios, like those shown in the image, and tested them to see whether the results matched those of the simulation. Indeed, they did. Although this research explores the fundamental behavior of flames, the results provide guidance on how to optimize practical combustion devices.

The next time you find yourself lost in thought while gazing into a crackling fire, take a moment to focus on a flame. The seemingly random flickering and shape-shifting is, in reality, a complicated interplay between the flame and its environment. It’s a dance that is predictable if you know enough about the situation. That’s one of the amazing things about physics.

Kendra Redmond

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