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

雾化是指将液体流破碎成液滴集合的过程，然后通常将液滴喷射到流体流中或表面上以形成涂层。原子化发生在各种工业和自然系统中。 消费者熟悉使用喷雾技术生产的许多产品，例如奶粉、婴儿配方和涂漆表面，或者使用喷雾操作部署的许多产品，例如发胶、汽车燃料喷射器和鼻吸入器。 喷雾器的设计目的是将液体分散成细小的液滴，通过提高效率、降低成本和最大限度地减少浪费来改善产品。 在实际感兴趣的许多情况下，被喷射的流体是高粘性的，这对实现精细喷雾造成问题并且增加了用于产生喷雾的能量。 通常，高粘性流体的喷雾产生极大的液滴，这降低了雾化过程的有效性。 本项目将采用实验、理论和数值模拟相结合的方法对雾化过程进行研究。 研究小组将使用一个模型流动系统，并检查流动的不稳定性，这是雾化的开始。 本科生将通过明尼苏达大学的本科生研究经验计划招募到研究团队。研究人员将在科学博览会上为中学生制作一个流动展览，让他们了解应用于雾化的流体动力学原理。该项目将研究一种新设计的雾化器喷嘴的性能，该喷嘴可以显著降低能耗，同时可以将高粘度流体雾化成细小的液滴。该项目将结合理论、实验和计算流体动力学来确定增强性能的机制，这将使设计改进能够进一步提高能源效率。假设是在喷嘴内建立的两相逆流混合层负责高水平的湍流混合，产生直接作为喷雾从喷嘴出现的两相混合物。实验涉及表征的喷雾作为液体粘度的函数，逆流的空气-液体质量流量和动量比，和喷嘴的内部几何形状。同时，研究小组将使用平面逆流混合层的实验，允许光学访问，以详细研究具有逆流速度分布的液-气界面的动态。实验也将在阿贡国家实验室的X射线设施进行，以阐明喷嘴内的密度分布。这些实验将伴随着详细的线性稳定性分析，以确定混合层中是否存在绝对不稳定的剖面，这种剖面在实验中可能表现为自持振荡。高分辨率直接数值模拟（DNS）将阐明喷嘴内混合的物理过程，并为雾化过程中的工程实践者提供设计指导。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.