Mysterious Soot-Free Blue Flame Is Actually Three Flames In One

A blue whirl (left) and a simulation of the heat released (right) that better highlights the location of the three flames. Chung et al., Sci. Adv. 2020; 6: eaba0827

Four years ago, researchers discovered a peculiar new type of flame – a blue whirl able to burn fuel entirely without soot and with low emissions. A new study now shows that this is not a single flame. It is a three-in-one deal. The work is published in Science Advances.

Combustion is crucial for energy production and many industrial processes. Its major drawbacks are visible all around us in the unfolding climate crises. To work out how the blue whirl behaves, the team created a sophisticated computer simulation that combined equations describing the motion of the fluid with models that describe how fuels behave chemically.

The work revealed that three flames are occurring at the same time, giving the blue whirl such efficiency. The base of the whirl is called the premixed rich flame and has excess fuel. On the edge of the whirl, there's the lean flame – a wispy structure that is air rich and fuel poor. The top part is the diffusion flame. The three meet in the bright blue ring.

“Only if we understand its structure can we tame it, scale it, and create it at will,” lead author Joseph Chung and colleagues, from the University of Maryland, wrote in the study.

The phenomenon of the blue whirl was first discovered by scientists studying fire whirls over water for oil-spill cleanup. To form, it first undergoes a sooty fire whirl state, before being able to burn all the fuel it encounters. Since its discovery, people wondered if it could be mastered for efficient combustion. The researchers argue that this question implies many others, such as our ability to control it, manipulate its size, and confine it to a combustor or a propulsion device.

“The lure of being able to burn any liquid hydrocarbon efficiently and cleanly is extremely attractive,” the researchers explained. “None of these questions can be answered easily until we at least understand the structure and dynamics of the flame and have a tool through which we can easily explore some of these questions. This paper describes a first step: a tool that can be used to explore and test the phenomenon, and how it has been used to reveal the blue whirl structure.”

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