Control of Vortex Breakdown and the Blue Whirl

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Fire whirls are dangerous, rapidly whirling flames that arise naturally in wildfires. When they do occur, they can be up to a kilometer tall and very destructive. Fire whirls can lift and throw burning material and generate fast winds that quickly spread the fire. Under the right controlled laboratory conditions, however, a fire whirl can transition into a beautiful, benign, small, blue flame called a blue whirl. These blue whirls can burn many different types of hydrocarbon fuels, either in gaseous or liquid form, while producing minimal pollutants and no soot at all. Such green-combustion properties are optimal for burning hydrocarbons, which make blue whirls a potential source of truly clean combustion energy. To use the blue whirl for practical applications, though, more needs to be understood about its structure, how it forms, and how to control it. This research project aims to understand how the blue whirl evolves naturally from a fire whirl and how to control it. The results of these investigations should reveal how to bypass the dangerous fire-whirl stage for creating the blue whirl, and the research should provide fundamental knowledge about swirling flows, which would benefit other related systems. This research project will train several early-career researchers, including a postdoctoral researcher and several graduate students.Previous studies led to the discovery of the existence of the blue whirl, its flame structure, and some of its dynamic properties. An important observation was that the blue whirl emerges from a fire whirl that goes unstable through a process known as vortex breakdown, which is a fluid transition leading to various possible modes of swirling flow. The objective of this research project is to focus on the vortex-breakdown transition and use large-scale numerical models to investigate whether it is possible to control the evolution to different modes. More specifically, the researchers will consider several different types of swirling flows subject to vortex breakdown. One type is a swirling flow with localized heat changes in the region of vortex breakdown. Here, the researchers hope to investigate and quantify how heat changes introduced into the vortex-breakdown process affect the transition to different modes. Another type of swirling flow to be investigated is a chemically reactive flow that produces flames such as the blue whirl. The researchers anticipate finding information on how to control the blue whirl and to enable its use for power generation and fuel spill clean-up. These studies will provide a fundamental understanding of the system that is needed to control vortex breakdown in other related applications, such as controlling instability on highly swept wings and swirl combustors. The new numerical algorithms and physical models developed from this research project will add to the general computational ability to simulate low-speed reactive complex flow systems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.