Four types of flames join forces to make this eerie ‘blue whirl’

The ghostly blue flame stunned researchers when it first showed up unexpectedly in a laboratory experiment. Now scientists have determined the structure of this “blue whirl.”

Four separate types of flames make up the blue whirl, researchers report August 12 in Science Advances. “It’s amazingly complex,” says engineer Elaine Oran of Texas A&M University in College Station.

Reported in 2016, the blue whirl appeared when scientists ignited liquid fuel floating on water, within an enclosure designed so that air sweeping in created a vortex (SN: 8/18/16). A tornado of fire blazed before settling into a spinning blue flame several centimeters tall. The hue indicates that it burns without soot, suggesting that such flames could be useful in cleaning up oil spills or for more environmentally friendly power generation.

Most flames fall into two general classes: premixed and diffusion. In diffusion flames, the fuel and the oxidizer — typically, oxygen — are initially separate, limiting how fast the fire can burn. In premixed flames, the two swirl together. Premixed flames come in three different varieties. They can have either an excess or dearth of fuel relative to oxidizer, called rich or lean premixed flames. Stoichiometric flames are the Goldilocks variety, with just the right amount of fuel for complete combustion.

Comparing computer simulations to experimental observations allowed Oran and colleagues to pin down the blue whirl’s structure. Its conical base is a rich premixed flame, topped by a diffusion flame. On the sides, a lean premixed flame appears as a faint wisp. Where those three flames meet, a stoichiometric flame forms, making a bright blue ring.

Knowing the flame’s structure might let researchers figure out how to scale the blue whirl up to a larger size or create it without passing through the dangerous firenado stage.