The blue whirl consists of three different flame structures that swirl together into one otherworldly blue ring, according to new research.
Stable blue whirl observed in an experiment. Image credit: H. Xiao, University of Science and Technology of China.
The blue whirl is a small, stable, spinning blue flame initially discovered in 2016 in experimental studies of turbulent, sooty fire whirls, also known as fire tornados.
It burns a range of different liquid hydrocarbon fuels cleanly with no soot production, presenting a previously unknown potential way for low-emission combustion.
“Blue whirls evolve from traditional yellow fire whirls,” said Professor Elaine Oran, a researcher in the Department of Aerospace Engineering at Texas A&M University.
“The yellow color is due to radiating soot particles, which form when there is not enough oxygen to burn the fuel completely.”
“Blue in the whirl indicates there is enough oxygen for complete combustion, which means less or no soot, and is therefore a cleaner burn.”
“We now know that blue whirl will burn all of the fuel available as it exits a burner or from a surface.”
To simulate how blue whirls emerge, Professor Oran and colleagues coupled 3D, time-dependent equations that describe the motion of viscous fluid substances to a model for fuel conversion and chemical energy release.
They started by simulating experimental conditions, then tweaked physical parameters such as fuel and air size and velocity in their calculations until a blue whirl materialized.
They showed that a blue whirl is composed of three different flames — a diffusion flame and a premixed rich and lean flame — all of which meet in a fourth structure, a triple flame that appears as a whirling blue ring.
They also found that vortex breakdown — a fluid instability that occurs in swirling flows — enables the blue-whirl structure to emerge.
“The flame and flow structure revealed by the simulations serves as a fundamental base to further investigate how to create the blue whirl in a more controlled way,” said Xiao Zhang, a PhD student in the Department of Aerospace Engineering at the University of Maryland.
“It leads pathways to answering more complex questions.”
“Examples of such questions are: How can we create blue whirls on different scales? Can we bypass the transitional, sooty, dangerous fire whirl stage and create the stable and clean blue whirl directly?” said Joseph Chung, a PhD student in the Department of Aerospace Engineering at the University of Maryland.
“The newly developed algorithms and models also provide great exploring tools to find these answers.”
The findings were published in the journal Science Advances.
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Joseph D. Chung et al. 2020. The structure of the blue whirl revealed. Science Advances 6 (33): eaba0827; doi: 10.1126/sciadv.aba0827
Source link: https://www.sci.news/physics/blue-whirl-structure-08744.html
