Four years ago, a team of aerospace and fire protection engineers from the University of Maryland, US, described a unique soot-free flame called a blue whirl, which consumes all fuel it encounters.
It was discovered serendipitously while performing experimental studies with fire whirls – whirlwinds naturally induced by fire – and its potential was clear.
“Understanding and control of the blue whirl and its predecessor, the fire whirl, will suggest new ways for fuel-spill remediation, reduced-pollution combustion and fluid mechanics research,” the researchers wrote in a paper.
Subsequent research produced temperature maps of blue whirls and informed how they can be stabilised, but their flame structure and dynamics – especially how they form from a fire whirl – have remained mysterious.
Now another Maryland team has taken a significant step forward by revealing, in a paper in the journal Science Advances, that the blue whirl actually consists of three different flame structures that swirl together into one.
Joseph Chung and colleagues (including Elaine Oran, a co-author of the original paper) analysed how they emerge by coupling 3D, time-dependent equations that describe the motion of viscous fluid substances to a model for fuel conversion and chemical energy release.
They started by simulating experimental conditions, then tweaked physical parameters such as fuel and air size and velocity in their calculations until a blue whirl materialised.
They found that the ethereal flame is composed of three different flames: a diffusion flame, in which the fuel and oxidizer are separated before burning, and premixed rich (with excess fuel) and lean (with excess air) flames.
It is, they write, “a curious phenomenon that has many intriguing aspects” and “very beautiful, both in its stable state, as a spinning blue top-like flame, and when it went slightly unstable, perhaps revealing some of its inner structure”.
Questions remain, however. Can it be useful in any way for efficient combustion with no soot formation? Might it scale to larger sizes? And can it can be made directly without going through the fire whirl state?
The research will continue. “Only if we understand its structure can we tame it, scale it and create it at will,” Chung and colleagues say.
Nick Carne is the editor of Cosmos Online and editorial manager for The Royal Institution of Australia.
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