Scientists discover a new kind of fire whirl

The “blue whirl” can greatly reduce pollution

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.

An air curtain used near Custer, SD in 2013 produced very little visible smoke. Photo by Bill Gabbert.

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.

air curtain
Air Burner Inc.

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.”

Using an air curtain to burn bug tree debris

Air CurtainI have heard of air curtains, but had not seen one being used until today. After Hurricane Andrew devastated a large section of south Florida in 1992, an air curtain was used to dispose of a huge pile of debris (we lovingly called Mount Trashmore) which for a while was the highest point in that part of the state.

An air curtain is not terribly sophisticated. It simply pumps air into the area where material is burning. The extra air flow and oxygen greatly shortens the time it takes to burn the fuel.Air-Curtain

Wednesday I visited a site near Custer, South Dakota where Bayfield, Colorado based NRG Consulting Services is conducting contract work on private land. They are thinning and reducing the fuel around structures as well as cutting and burning beetle-infested trees.

Some of the thinned and  bug trees they are cutting are green, partially green, or at least have a moisture content high enough to make it impossible to burn in a conventional slash pile right away. But if they wait for the wood to dry out enough to burn, the beetles will hatch and disperse to other trees.

Air CurtainThe air curtain makes it possible to burn the high-moisture content wood immediately.

The brand new portable unit they were using Wednesday has a movable ceramic-lined firebox mounted to a trailer. The box is raised and lowered by a hydraulic lift system. Once on site, the fire box, which has no bottom or floor, is lowered to the ground. So the burning occurs on the ground, surrounded by the box. If you notice the line of holes in the wall of the box in the photo below, that’s where the air comes out at a fairly high velocity. A small diesel engine powers the blower and the hydraulics that raises and lowers the fire box.Air Curtain

While I was at the site the system was producing virtually no smoke. The photo below shows what was left after a full day of use. At the end of the day they let it sit overnight, then the next morning they may raise the fire box and move it to a different location.

Air Curtain

While a fire in an air curtain is less likely than a burn pile to escape, there is some fire hazard associated with the system. At one point I happened to look down at the sleeve of my cotton shirt and saw a small burning ember about the size of a gnat. I brushed it off and saw that it left a small burn mark on my shirt. Then I noticed that the cotton shirt of the crew boss had at least a dozen holes just like the new one in my shirt. The gentleman closest to the machine, the person feeding it, was shrewdly wearing a Nomex shirt. I looked around and didn’t see any spot fires in the area. A burning permit is required to operate an air curtain in the Black Hills of South Dakota, which these folks had.Air Curtain