Science

The ‘Blue Whirl’ is a New Kind of Flame That Always Burns Cleanly

Researchers have already proposed using such a whirl to clear oil slicks, all the while keeping emissions in check.

A blue whirl over water in a swirl generator. Credit: University of Maryland

A blue whirl over water in a swirl generator. Credit: University of Maryland

Swirling tornadoes, tall columns of spinning air, are destructive – but fairly well documented and studied. Now, imagine a giant whirling column of fire, spewing burning debris and wrecking havoc to the surrounding environment. It sounds like a cool visual effects shot from a summer disaster movie but fire tornadoes, or fire whirls, are unfortunately real even if they are rare. Although they generally spell disaster, a group of engineers from the University of Maryland (UMD) have discovered a new kind of fire whirl that can potentially be useful in research and industry.

Fire whirls are created naturally when high-speed winds interact with a firestorm in a favourable environment. In the lab, however scientists study whirls at a much smaller scale and in situations they can stay in control of. Previous experiments have established that fire whirls originate from small fires seeded by a pool of fuel, called pool fires. These pool fires can then lead to the creation of whirls, which critically burn with greater efficiency. What if, instead of irresponsibly setting an oil-spill layer on fire, we can have a more controlled burn? Can we tame a fire whirl to clear oil slick, all the while keeping emissions in check?

Starting out with a pool fire, the UMD group intended to carefully study the formation and evolution of a fire whirl. Much to their surprise, however, they found a new type of fire whirl: one that was stable, remarkably clean and efficient. Christened as a ‘blue whirl’ in a paper published in the journal Proceedings of National Academy of Sciences in August 2016, this previously unobserved type of fire whirl might just be the remedial solution scientists have been searching for for the fuel-spill cleanup problem.

The study is significant because of a a unique setup. Unlike previous experiments, where pool fires were set on fuel beds on a solid ground, this study simulates a real world oil spill scenario by creating a pool fire on a layer of fuel, on the surface of a water bed. The towering fire whirl that initially develops after ignition is classical, but after sometime gives way to a radically different structure: a top-shaped flame with a clean blue glow and spinning. The transition is drastic. What makes the blue whirl so significant, aside from the fact that it hasn’t been reported before?

While the formation of a traditional fire whirl is marked by turbulence, the blue whirl is quiet and stable. A normal fire whirl burns with high efficiency, even in an incomplete combustion process, but a blue whirl is characterised by a nearly soot-less flame – indicating a complete combustion. This is important because a complete combustion event leads to far lower emissions as all the soot and ignitable matter is also burnt in the swirl of incoming oxygen and fuel.

Specific features of a blue whirl allow it to burn with such high efficiency. First: it spins on water, not on ground. This generates flow on the water and allows for a vaporised fuel layer to exist at the water-fuel interface continuously feeding the fire. Second: this rotational motion leads to a pressure drop at the centre of the whirl, keeping the fuel around it from spreading, as it would’ve in a randomly-lit pool fire.

The findings of the study should be cause for excitement because it a blue whirl might provide a clean and effective way of cleaning oil spills. While the exact cause of the blue whirl is still an open question, the scientists posit that the stage occurs when the column of a standard fire whirl undergoes what is known as a vortex breakdown transition – a change of the whirl from a spiral columnar structure (spiral mode) to the more exotic structure of a blue whirl (bubble mode).

A preliminary study of this kind about a newly discovered kind of flame will attract more research. It remains to be seen if scientists are able to go beyond the controlled experiments in a lab and create a blue whirl in the outdoors; they’d have to be able to keep the flame stable at all times, to predict how disturbances on the water and in the winds affect the whirl. But if they’re successful in creating a blue whirl in the wild, it wouldn’t be amiss to say its use will be a hot new industry.