The growing worldwide demand to reduce emissions from combustion calls for development of alternative approaches for high-efficiency and low-emission combustion. The blue whirl - a stable, spinning blue flame - is a recently discovered state of combustion that has an unexploited potential for clean, efficient combustion of liquid heavy hydrocarbon fuels, such as heptane, iso-octane, gasoline, and even crude oil. Since its discovery in 2016, the blue whirl has garnered significant interest from both academics and the media. Despite the interest, its fundamental structure and formation mechanism has remained unknown due to limitations in experimental diagnostics and simulation capabilities. The research in this project uses high-performance computing to simulate blue whirl from first principles, and then used the results to explain structure and dynamics of the blue whirl. Understanding the structure is the first step to controlling and using it for new ways for fuel-spill remediation, reduced-pollution combustion, and fluid mechanics research.

The blue whirl was discovered serendipitously while investigating the combustion and burning dynamics of fire whirls on water. A fire whirl, composed of flames and a strong spinning vortex, often occurs in fires in wildlands, urban environments, and natural disasters. When fire whirls form in the wild, they can be uncontrollable and threaten life, property and the surrounding environment due to their large flame structures, intense burning rates, and fast moving speeds. While researchers often focus on stopping or preventing fire whirls, a research group from the University of Maryland wanted to harness the power of fire whirls for a practical use. Fire whirls on top of water were studied to evaluate their potential for cleaning oil spills. During the experiments, a small, quiet, and stable blue spinning flame formed spontaneously from a large fire whirl. This blue spinning flame burns cleanly and efficiently until all of the fuel is consumed, forming no soot at all. The blue whirl consists of a blue spinning flame at the base, a bright-blue ring at the center, and a faint conical violet flame sitting above (Fig. A). This form of combustion was not reported previously. Even though many more experiments have been performed to understand this new flame, the structure and formation mechanism, however, remained unknown. In this project, we use a numerical approach to study and, for the first time, present the flame and flow structure of the blue whirl.

The numerical simulations show that the blue whirl is composed of three different types of flames, as summarized by a schematic diagram in Fig. B. The blue spinning flame at the base is a rich premixed flame. The upper conical violet flame is a diffusion flame. What cannot be seen easily in the laboratory experiments is the lean premixed flame surrounding the violet flame, that is, the upper region just outside of the diffusion flame. These three flames meet at the bright blue ring and form a fourth structure, which is a triple flame. The simulations also show that the flow structure emerges as the result of vortex breakdown, a fluid instability which occurs in swirling flows. A bubble mode of vortex breakdown is inside the blue whirl.

Simulating the blue whirl is difficult and numerically expensive. It requires three-dimensional calculations that are long enough to capture the dynamics. To accomplish this, we developed, refined, and implemented many new algorithms and combined these into a new reacting flow computational fluid dynamics (CFD) model. The underlying concept of the algorithm is based on BIC-FCT, the barely implicit correction to flux-corrected transport, which was designed for low-Mach-number flows, enabling large computational time steps. We developed and used a variable-stoichiometry chemical diffusive model (CDM) to describe the effects of diffusion and chemical reactions with heat release for fuel and air mixtures. The CDM is fast and make computations affordable by predicting the minimal number of species and reactions that are required to compute bulk flame properties. A number of test cases were calculated using this new CFD model and the results were compared to analytical, experimental, or other numerical reference results.

Revealing the structure of the blue whirl has broader impacts within and beyond the immediate subject area. With the worldwide need to reduce emissions from both wanted and unwanted combustion, discovery of this new combustion state points to new pathways for highly efficient and low-emission energy conversion and in situ fuel-spill cleanup. Understanding the flow and flame structures of the blue whirl serves as a fundamental step to controlling and utilizing it. New computational algorithms with high efficiency were developed, benefitting many other computational physics applications. In addition, these new algorithms provide a new approach for numerical analysis. 

Last Modified: 08/24/2020
Modified by: Carolyn Kaplan-Solomond