Characterization of the Physics and Chemistry of the Blue Whirl for Clean Combustion

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This project is inspired by the blue whirl, a small, spinning blue flame that was first observed when it evolved naturally from a fire burning a liquid fuel puddle poured onto the top of a flat pan filled with water. Once the blue whirl forms, it remains in the steady, stable, completely blue state, in which no soot is formed. Once it forms, all of the turbulence in the original fire disappeared and the spinning top burned silently until there was no fuel remaining on the water. Since its discovery, there has been considerable theoretical conjecture about what this flame actually is, how it evolves, and how, if at all, it could be useful. Experiments have provided many clues. For example, almost any liquid hydrocarbon including crude oil, can create the blue whirl, it can be made on a smooth metal pan with no water, and the temperature in the purple haze at the top of the flame is very hot at 2000K. What is needed, however, is an understanding of the fluid dynamics and chemistry leading to and controlling the blue whirl. What is needed to further this understanding is a simulation that will show how the flame evolves and provide some guidance to experiments and theoretical analyses. This blue flame could potentially inspire an ideal burner that consumes any liquid fuel with no soot and minimal pollution.The objective of this proposal is to create a numerical model capable of simulating the blue whirl, from its inception as a small fire, through to the evolution into a fire whirl, then through the complex transition to the blue whirl. This is a complex physical and chemical problem involving evaporation of liquid fuel, formation of a fire whirl, and then the dynamic vortex breakdown of the fire whirl that leads to the blue whirl. The most stringent requirement of this model is that it must be optimized so that it can perform enough simulations to explore the physicochemical parameter space of the liquid-burning fire whirl and blue whirl. To accomplish this, it will be necessary to have a model that solves the governing reactive-flow conservation equations and adequately resolves the fluid dynamics, chemical reactions and energy release, heat and mass diffusion, and perhaps the multiphase problem of hydrocarbon liquid evaporation. The numerical model must be capable of simulating hydrocarbon flames through a range of stoichiometric from lean to rich. To accomplish this objective, a three-dimensional unsteady fluid model that allows transitions from flow regimes of incompressible to fully compressible flows, based on the barely implicit correct to flux-corrected transport, will be developed and tested. In addition, a chemical-diffusive model will be developed for liquid fuels and tested for diffusion flames. Intermediate steps in the project include simulations of fire whirls for benchmarks and three-dimensional vortex breakdown in compressible gases.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.