NMR Studies of Rapid Granular Flows

摘要CTS-0310006 D。马萨诸塞州阿默斯特大学的坎宁将用核磁共振（NMR）来研究颗粒状介质的性质，如沙子、珠子或种子，这些介质在重力或气流的作用下被迫进入运动状态。该项目将主要关注快速颗粒流，其中颗粒在相互碰撞之间以弹道方式飞行。三种专门的核磁共振技术将单独和协同使用：（a）磁共振成像（MRI），它产生显示物质密度的图像或轮廓，（B）脉冲场梯度（PFG），它提供关于物质在广泛的时间和空间尺度上移动的详细信息（c）超极化气体（HPG），这使得研究颗粒系统中的间隙气体成为可能。垂直通道流。一组实验将探测颗粒状物质通过垂直管道的流动。这是在工业应用中输送颗粒材料的常见方式之一，并且是颗粒剪切流的一个重要示例，其中流动的不同部分必须以不均匀的方式彼此移动。使用植物种子作为颗粒和MRI/PFG技术，将绘制出密度，速度和各向异性扩散张量与流体动力学理论进行比较。颗粒运动的统计数据将被检查，以寻找玻璃化、笼状运动和大波动的证据，正如在许多其他类型的颗粒剪切流中所发现的那样。第二组实验将研究流化床，在流化床中，向上流动的气体使颗粒状介质保持在流体状状态。流化床在化学工程和工业中有着广泛的应用，例如将石油裂解成汽油，以及开发更新、更清洁的燃料燃烧工艺。核磁共振测量的颗粒和气体运动在流化床的内部将提供数据，以比较与双流体动力学理论和大规模的模拟。颗粒的PFG NMR将用于研究细颗粒的均匀流化状态的性质和稳定性，并绘制出在较高气体流速下的波动不稳定性的色散关系。HPG NMR将用于研究气泡内部和周围的结构和流动模式，这对流化床的应用至关重要。(a)该项目将通过帮助阐明颗粒流和流化的基本物理过程来为社会提供长期效益。这将最终影响到用于设计许多重要工业设施的工程模型。(b)该项目进一步促进了科学和技术的跨学科发展，利用了马萨诸塞大学阿默斯特分校在软凝聚态核磁共振方面的专业知识和哈佛-史密森尼大学在超极化气体核磁共振方面的专业知识。(c)核磁共振的专业知识和设备存在于世界各地的许多实验室，因此，该项目的辅助好处是帮助开发核磁共振作为研究快速颗粒流的“下一代”标准工具。(d)学生将参与该项目的各个阶段，获得宝贵的培训和接触跨学科研究的兴奋和挑战。

AbstractCTS-0310006D. Candela, U of Massachusetts AmherstNuclear Magnetic Resonance (NMR) will be used to study the properties of granular media such as sand, beads or seeds, forced into a state of motion by gravity or gas flow. The project will focus mainly on rapid granular flows, in which the grains fly ballistically between collisions with each other. Three specialized NMR techniques will be employed, individually and in concert: (a) Magnetic Resonance Imaging (MRI), which produces images or profiles showing the density of material, (b) Pulsed Field Gradient (PFG), which gives detailed information on movements of material over a wide range of time and space scales (down to sub-micron), and (c) Hyperpolarized Gas (HPG), which makes it possible to study the interstitial gas in a granular system.Vertical Channel Flows. One set of experiments will probe the flow of granular materialthrough a vertical pipe. This is one of the common ways of transporting granular material in industrial applications, and is an important example of a granular shear flow, in which the different parts of the flow must move past each other in a nonuniform manner. Using botanical seeds as grains and MRI/PFG techniques, the density, velocity, and anisotropic diffusion tensor will be mapped out for comparison with hydrodynamic theories. Grain motion statistics will be examined for evidence of glassiness, caged motion, and large fluctuations-as have been found in many other types of granular shear flow.Fluidized Beds. A second set of experiments will study fluidized beds, in which an upwards flow of gas maintains the granular medium in a fluid-like state. Fluidized beds have widespread applications in chemical engineering and industry, e.g. to crack petroleum into gasoline and in the development of newer, cleaner-burning processes for combustion of fuels. NMR measurements of both grain and gas motion in the interiors of fluidized beds will provide data to compare with two-fluid hydrodynamic theories and large-scale simulations. PFG NMR of the grains will be used to study the nature and stability of the uniformly fluidized state of fine grains, and to map out the dispersion relation for wave-like instabilities at higher gas flow rates. HPG NMR will be used to study the structure and flow patterns in and around bubbles, which are crucial to applications of fluidized beds.Broader Impacts. (a) This project will provide long-range benefit to society by helping to elucidate the fundamental physical processes of granular flow and fluidization. This should ultimately affect engineering models used to design many important industrial facilities. (b) The project furthers the cross-disciplinary development of science and technology, making use of the expertise in soft-condensed matter NMR at UMass Amherst and the expertise in hyperpolarized gas NMR at Harvard-Smithsonian. (c) NMR expertise and equipment exist at many labs around the world; hence an ancillary benefit of this project is to help develop NMR as a "next generation" standard tool for studying rapid granular flows. (d) Students will participate in all phases of this project, gaining valuable training and exposure to the excitement and challenges of cross-disciplinary research.