Control of Vortex Breakdown and the Blue Whirl

火旋风是危险的，在野火中自然产生的快速旋转的火焰。当它们真的发生时，它们可以高达一公里，非常具有破坏性。火焰漩涡可以举起并抛出燃烧的物质，并产生快速的风，迅速蔓延火势。然而，在适当控制的实验室条件下，火焰漩涡可以转变成一种美丽的、良性的、小的蓝色火焰，称为蓝色漩涡。这些蓝色漩涡可以燃烧许多不同类型的碳氢化合物燃料，无论是气态还是液态，同时产生最小的污染物，根本不会产生烟尘。这种绿色燃烧特性对于燃烧碳氢化合物是最理想的，这使得蓝色漩涡成为真正清洁燃烧能源的潜在来源。然而，要将蓝色漩涡用于实际应用，还需要更多地了解它的结构、形成方式以及如何控制它。这个研究项目旨在了解蓝色漩涡是如何从火漩涡自然演变而来的，以及如何控制它。这些研究结果将揭示如何绕过危险的火漩涡阶段来产生蓝色漩涡，并为漩涡流动提供基础知识，这将有利于其他相关系统。该研究项目将培养几名早期研究人员，包括一名博士后研究员和几名研究生。先前的研究发现了蓝色漩涡的存在，它的火焰结构，以及它的一些动态特性。一个重要的观察结果是，蓝色漩涡是从一个火漩涡中出现的，这个火漩涡在一个被称为漩涡破裂的过程中变得不稳定，这是一个流体转变，导致各种可能的漩涡流动模式。本研究项目的目标是关注涡-击穿转变，并利用大尺度数值模型来研究是否有可能控制其向不同模态的演变。更具体地说，研究人员将考虑几种不同类型的旋涡流受到旋涡破坏。一种是涡旋流动，在涡旋破裂区有局部的热变化。在这里，研究人员希望调查和量化引入涡击穿过程的热量变化如何影响向不同模式的转变。另一种要研究的旋涡流是产生火焰的化学反应流，如蓝色旋涡。研究人员期望找到关于如何控制蓝色漩涡的信息，并使其能够用于发电和燃料泄漏清理。这些研究将为在其他相关应用中控制涡流击穿所需的系统提供基本的理解，例如控制高后掠翼和涡流燃烧器的不稳定性。从该研究项目中开发的新的数值算法和物理模型将增加模拟低速反应复杂流动系统的一般计算能力。该奖项反映了美国国家科学基金会的法定使命，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。

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.