Enhanced Atomization of Viscous Liquids Using Insights from Global Instabilities of Two-Phase Countercurrent Mixing Layers

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Atomization refers to the process of breaking up a liquid stream into a collection of droplets, which are then usually sprayed into a fluid flow or onto a surface to form a coating. Atomization occurs in a variety of industrial and natural systems. Consumers are familiar with many products that have been produced using a spraying technique, such as powdered milk, infant formula, and painted surfaces, or are deployed using a spraying operation, such as hair spray, automotive fuel injectors, and nasal inhalers. The sprays are designed to atomize the fluids into fine droplets that improve products by enhancing efficiency, reducing costs, and minimizing wastes. In many cases of practical interest, the fluids that are sprayed are highly viscous, which poses problems to achieving fine sprays and increases the energy used to produce a spray. Often, sprays of highly viscous fluids produce extremely large droplets that diminish the effectiveness of the atomization process. This project will use a combination of experiments, theory and numerical simulation to study the atomization process. The research team will use a model flow system and examine the instability in the flow that is the start of atomization. Undergraduate students will be recruited to the research team through the University of Minnesota's Research Experience for Undergraduates program. The researchers will create a traveling exhibit that will be used at science fairs for middle-schoolers to acquaint youngsters with principles of fluid dynamics applied to atomization.This project will investigate the performance of a newly designed atomizer nozzle that dramatically reduces energy consumption, while enabling the atomization of highly viscous fluids into sprays of fine droplets. The project will use a combination of theory, experiments and computational fluid dynamics to identify the mechanisms responsible for the enhanced performance, which will enable design enhancements to further improve energy efficiency. The hypothesis is that a two-phase counterflow mixing layer established inside the nozzle is responsible for high levels of turbulent mixing, creating a two-phase mixture that emerges from the nozzle directly as a spray. The experiments involve characterization of the spray as a function of liquid viscosity, counterflowing air-liquid mass flow and momentum ratios, and nozzle internal geometry. In parallel, the research team will use experiments on planar countercurrent mixing layers that allow optical access to examine in detail the dynamics of a liquid-air interface with counterflow velocity profiles. Experiments will also be performed at the X-Ray facility at Argonne National Labs to elucidate the density profile inside the nozzle. These experiments will be accompanied by a detailed linear stability analysis to identify the presence of absolutely unstable profiles in the mixing layer, which may appear in experiments as self-sustained oscillations. High-resolution Direct Numerical Simulations (DNS) will clarify the physics of mixing inside the nozzle and provide design guidance to engineering practitioners in atomization processes.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.