Collaborative Research: Grain and Gas Motion in Dense Granular Flows

国家科学基金会-化学和运输系统多相过程计划(1415年)提案编号：0651397首席研究员：坎德拉，唐纳德隶属：马萨诸塞大学阿默斯特分校提案标题：合作研究：密集颗粒流中的颗粒和气体运动核磁共振和磁共振成像将被用于非侵入性地研究两种稠密颗粒流系统中的颗粒和气体的运动：气体流态化和垂直通道流。该项目将在两个关键方面超越以前对这些系统的核磁共振/核磁共振研究：(A)将使用同步气泡生成技术来探测流态化床中气泡的时空结构，包括气体和颗粒核磁共振；以及(B)将在比以前可能用于核磁共振研究的大得多的流态化床中探索气体运动和交换，作为迈向具有直接工程意义的床研究的一步。作为一个合作项目，拟议的工作将结合在阿默斯特大学马萨诸塞州坎德拉实验室的颗粒流中颗粒的核磁共振专业知识，以及哈佛大学沃尔斯沃斯实验室的超极化气体核磁共振专业知识。这一合作对于同步气泡实验将是至关重要的，以确保颗粒和气体数据可以获得相同的颗粒流动状态。该项目的先前工作使用颗粒和超极化气体核磁共振来探索气态化床层中的速度场和气乳交换，但由于气泡的混沌随机发生，对模型进行尖锐测试的能力一直受到限制。在这个新项目中，气泡将与核磁共振采集同步产生，从而建立起颗粒和气体运动的详细图像。当前关于稠密颗粒流体玻璃性、历史依赖性的理论思想将直接面对，气体流动和交换机制的模型也将面临挑战。用低场超极化气体核磁共振研究大型沸腾床。常规核磁共振在大型系统中的适用性受到了严重限制，因为整个样品体积都需要一个强大的、均匀的磁场。相比之下，超极化气体核磁共振可以在低得多的磁场下工作，适用于工业规模的应用。作为将核磁共振方法从颗粒介质放大到工业规模的一步，将在现有的低场磁共振装置上建造一个大规模的沸腾床，并使用超极化技术来研究气体的运动和交换。这次合作是独一无二的，因为它有能力执行这一突破性研究，将超极化气体核磁共振应用于大规模沸腾床。重力驱动的垂直通道(料斗或筒仓)流是一种基本的颗粒剪切流，但由于缺乏无创探头，有关其三维结构的信息很少。本文将利用核磁共振技术研究重力驱动垂直槽流的流场和时空相关结构。将测试提出的稠密颗粒剪切流的本构定律，并探索随着流动参数的变化，诸如力链等相关结构的增长和衰减。广泛的影响本科生(包括许多来自代表不足的群体)参与研究的强大传统将继续，以及传统的研究生研究和培训。现有的跨学科、机构间合作将得到加强，以将超极化气体等新的核磁共振技术应用于软物质物理问题。这个大规模的低场核磁共振项目应该为非侵入性核磁共振技术的真正工业规模应用铺平道路。

National Science Foundation - Division of Chemical &Transport Systems Particulate & Multiphase Processes Program (1415)Proposal Number: 0651397 Principal Investigators: Candela, Donald Affiliation: University of Massachusetts Amherst Proposal Title: Collaborative Research: Grain and Gas Motion in Dense Granular Flows Nuclear magnetic resonance and magnetic resonance imaging (NMR/MRI) will be used to noninvasively study the motions of grains and gas in two dense granular flow systems: the gas fluidized bed and vertical channel flow. This project will extend beyond previous NMR/MRI studies of these systems in two key aspects: (a) a synchronous bubble-generation technique will be used to probe the space-time structure of bubbles in a fluidized bed, both with gas and grain NMR; and (b) gas motion and exchange will be explored in fluidized beds much larger than previously possible for NMR studies (up to 50 cm diameter), as a step towards studies of beds of direct engineering relevance. As a collaborative project, the proposed work will combine the expertise for NMR on grains in granular flows in the Candela lab at UMass, Amherst, with the expertise for hyperpolarized-gas NMR in the Walsworth lab at Harvard. This collaboration will be crucial for the synchronous-bubble experiments, to ensure that grain and gas data can be taken for identical granular flow states.Intellectual MeritPrevious work on this project has used grain and hyperpolarized-gas NMR to explore the velocity field and gas-emulsion exchange in gas-fluidized beds, but due to the chaotic random occurrence of bubbles the ability to sharply test models has been limited. In this new project bubbles will be created synchronously with NMR acquisition, permitting a detailed picture of grain and gas motion to be built up. Current theoretical ideas on the glassy, history-dependent nature of dense granular fluids will be directly confronted, as will models of the gas flow and exchange mechanisms. Large-scale fluidized bed studied by low-field hyperpolarized-gas NMR. The applicability of conventional NMR to large systems is severely constrained by the need for a strong, uniform magnetic field over the entire sample volume. Hyperpolarized-gas NMR by contrast can function with much lower magnetic fields, practical for industrial-scale applications. As a step towards scale-up of NMR methods for granular media to industrial scales, a large-scale fluidized bed will be constructed in the existing low-field MRI setup and use hyperpolarized techniques to study the motion and exchange of gas. This collaboration is unique in its ability to perform this groundbreaking research of applying hyperpolarized gas NMR to a large-scale fluidized bed. Gravity-driven vertical-channel (hopper or silo) flow is one of the basic granular shear flows, yet little information exists on its structure in 3D due to the lack of noninvasive probes. NMR/MRI will be used to study the flow field and space-time correlation structure of gravity-driven vertical-channel flows. Proposed constitutive laws for dense granular shear flows will be tested and the growth and decay of correlated structures like force chains will be explored as the flow parameters are changed.Broader ImpactsA strong tradition of involvement of undergraduates (including many from underrepresented groups) in research will be continued, alongside traditional graduate research and training. Existing cross-disciplinary, inter-institutional collaboration will be enhanced to bring novel NMR techniques such as hyperpolarized gas to bear on soft-matter physics problems. This large-scale low-field NMR project should pave the way for true industrial-scale applications of noninvasive NMR techniques.