Structure of the blue swirling flame revealed by supercomputers

Lightning struck a bourbon warehouse, setting fire to an 800,000 gallon liquor depot in Kentucky’s Bardstown countryside in 2003. Some spilled into a nearby creek, generating a huge fire tornado, or “bourbonado,” such as reported locally.

The aerial video of it inspired scientists to investigate fire swirls, fire tornadoes, as something promising for oil spill repair because hydrocarbons burned with relatively little soot.

Their investigation of the vortex of fire in the laboratory led them to find something that amazed them. The chaotic and dangerous vortex of fire transformed into a clean and docile burning flame is called the “blue vortex”.

One of its discoverers is now on a science team that uses supercomputers assigned by the Extreme Science and Engineering Discovery Environment (XSEDE) to reveal the structure of the blue vortex, a new type of flame that consists of four separate flames. Scientists hope that one day the blue eddies can be used to burn fuels in a cleaner way.

“The main discovery of this new computational study is that we now know the main structure of the blue vortex,” said Elaine Oran, professor and VI chair of the O’Donnell Foundation, Department of Aerospace Engineering, Texas A&M University. Oran is a co-discoverer of the blue vortex and a co-author of a study on its structure published in PNAS, August 2020. “We know it’s a combination of many types of flames coming together and forming in the probably most ideal configuration for combustion that we had seen before.”

A blue vortex is similar to a rotating blue flame that looks like a child’s shirt. Oran says the top is the same shape as the Harry Potter sorting hat. Most of its burning occurs along a very bright blue edge that rotates.

The researchers used experimental data from the 2016 study that first discovered the blue vortex. The experimental setup consisted of two half cylinders and a cylindrical stainless steel pot filled with water. A liquid fuel, n-heptane, was poured onto the surface of the still water in the center of the pot and then ignited. Two quartz half-cylinders hung above the pan. The offset of the half cylinders created two vertical slits that allowed air to be drawn tangentially to the flame region, a method commonly used to create fire swirls for laboratory study.

At first a chaotic pool fire formed. The cold air drawn into the chamber created a strong vertical flow, subsequently creating a high and intense vortex of fire. Then, unexpectedly, he collapsed into the calm blue structure of the vortex flame.

“We studied the structure of this new flame through numerical simulation and discovered the type of combustion and where it occurs,” said study co-author Xiao Zhang, postdoctoral researcher, Department of Aerospace Engineering, Texas A&M University, who works for Oran.

Supercomputer simulations helped uncover the blue vortex structure, which turns out to be composed of three types of flames. Below is a rich premixed flame, crowned at the top by a diffusion flame in the shape of a purplish cap. The simulations revealed a hidden flame surrounding the purple haze, just outside the diffusion flame. The three flames combine into a triple flame which forms its luminous edge.

Scientists faced some challenges in simulating the flames.

“The blue vortex in the [laboratory] the experiments evolved and developed on their own, “Zhang said. There was limited diagnostic data from the experiments that didn’t give us enough conditions to use to start the calculations. We started with a numerical hunt.”

They developed new algorithms capable of simulating low Mach number flows efficiently and implemented the algorithms in computational fluid dynamics code that solves reactive, unstable and compressible Navier-Stokes flow equations. Using this code, they explored the effects of controlling parameters such as the size and speeds of the air intakes and fuel. Eventually, they were able to capture the blue vortex in their simulations.

“These simulations of the blue vortex involved multiple scales in time and space,” Zhang said. “We also needed to model the multiple physics and chemistry of heavy hydrocarbons. These can be very difficult and expensive to calculate. Furthermore, we wanted to maintain the three-dimensional dynamics of this new flame. These three-dimensional aspects added more cost to the computation.”

Scientists received supercomputer assignments on XSEDE, funded by the National Science Foundation. Through XSEDE, they used the Stampede2 supercomputer and the Ranch data storage system at the Texas Advanced Computing Center.

The simulations for numerical hunting and the final blue vortex simulation consumed 4 million CPU hours deployed on the University of Maryland’s Deep Thought2 system; the Thunder system of the Air Force Research Laboratory; and Stampede2, which represents approximately 23,000 node hours on its Skylake nodes.

In addition to the flame structure, the scientists also examined the flow structure of the blue vortex which involved a fluid dynamics phenomenon called vortex rupture. Basically, the chaotic, swirling yellow flame collapses into a vortex-breaking “bubble mode” and forms the blue vortex.

“What surprised me the most was how it evolved from the fire vortex,” Oran explained. “A vortex of fire is a monster, a devastating thing. Then all of a sudden it turns into this little silent little flame without turbulence. In the process of formation, you have seen all these fluid dynamic modes of breaking the vortex, which is a beautiful fluid phenomenon. that you could see from the vortexes that come off the wing of an airplane. “

The researchers hope that further understanding of the blue vortex will help scientists develop ways to burn fuels more cleanly. “It can potentially be a new way to extract energy from traditional fossil fuels with minimal soot, reduced pollution and environmental impact,” said Zhang.

Oran pointed out that serendipity played an important role in the discovery of the blue vortex phenomenon.

Oran said: “I think it is important to explore, follow your curiosity and try new ideas. If we had never seen, for example, the fire on the lake in Kentucky, when all the bourbon spilled over the lake and the lightning did it. ignited and formed fire swirls on the lake, we would never find the blue vortex. Every time you look under the carpet, you find something new. A new bug, a new flame. ”