Above: A “blue whirl”.
Scientists attempting to develop a new method for mitigating oil spills by burning the oil were hoping to find a way to reduce the air pollution as the petroleum product burns. We’ve all see the thick, black smoke at an oil fire. They may be a step closer to their goal with the discovery of a new type of fire behavior — a previously unseen type of flame. They call it a “blue whirl”.
A paper published online August 4, 2016, in the peer-reviewed journal Proceedings of the National Academy of Sciences describes this previously unobserved flame phenomenon.
A yellow flame is a sign of very incomplete combustion and produces more particulates and air pollution than a blue flame like you see on a gas fueled stove.
So far they have only created the blue whirl in a chamber which has slits in the side that cause the air to rotate as it enters. Over a layer of water they injected a liquid fuel, n-heptane, and then ignited it. The flame at first is yellow but eventually transitions to a small, swirling blue flame.
“Blue whirls evolve from traditional yellow fire whirls. The yellow color is due to radiating soot particles, which form when there is not enough oxygen to burn the fuel completely,” said Elaine Oran, Glenn L. Martin Institute Professor of Engineering and co-author of the paper. “Blue in the whirl indicates there is enough oxygen for complete combustion, which means less or no soot, and is therefore a cleaner burn.”
“This is the first time fire whirls have been studied for their practical applications,” said Michael Gollner, co-author of the paper and assistant professor of fire protection engineering at the A. James Clark School of Engineering at the University of Maryland.
One of the principles that reduces the pollution in the blue whirl is that plenty of oxygen is available for the fuel, helping it to burn more completely. Another is that the partially burned fuel remains in the flame longer, burning more completely.
Land managers sometimes use an “air curtain” to burn woody debris from fuel reduction operations. We wrote about this in 2013 after visiting one near Custer, South Dakota. The one we saw was trailer-mounted. The key to the system is pumping large amounts of compressed air into the fire box or open trench. Some of the devices create a vortex which traps the particulates keeping them in the burn zone longer, causing them to more completely burn while reducing their size and the visible smoke.
Some oil spill remediation techniques include corralling the crude oil to create a thick layer on the water surface that can be burned in place, but the resulting combustion is smoky, inefficient, and incomplete. However, the Clark School researchers say blue whirls could improve remediation-by-combustion approaches by burning the oil layer with increased efficiency, reducing harmful emissions into the atmosphere around it and the ocean beneath it.
“Fire whirls are more efficient than other forms of combustion because they produce drastically increased heating to the surface of fuels, allowing them to burn faster and more completely. In our experiments over water, we’ve seen how the circulation fire whirls generate also helps to pull in fuels. If we can achieve a state akin to the blue whirl at larger scale, we can further reduce airborne emissions for a much cleaner means of spill cleanup,” explained Gollner.
Beyond improvements to fuel efficiency and oil spill remediation, there are currently few easy methods to generate a stable vortex in the lab, so the team hopes their discovery of the ‘blue swirl’ can serve as a natural research platform for the future study of vortices and vortex breakdown in fluid mechanics.
“A fire whirl is usually turbulent, but this blue whirl is very quiet and stable without visible or audible signs of turbulence,” said Huahua Xiao, assistant research scientist in the Clark School’s Department of Aerospace Engineering and corresponding author of the paper. “It’s really a very exciting discovery that offers important possibilities both within and outside of the research lab.”