Particle Diffusion and Mixing during Silo Drainage

摘要CTS-0334587A。库德罗利，克拉克大学将使用高帧速率数字成像和直接粒子跟踪实验来研究颗粒流中的混合和分离。除了测量筒仓侧壁附近颗粒的扩散，PI还建议使用折射率匹配的流体研究筒仓内部颗粒的运动。这项调查将为困扰模块化卵石床反应堆设计的一个问题提供严格的答案，这是核工程师目前感兴趣的问题。一个核心问题是台球大小的燃料鹅卵石之间的扩散量，这些鹅卵石含有放射性物质，它们缓慢地循环通过反应堆。这项研究对许多其他工业系统来说很重要，在这些系统中，颗粒物的混合和分离之间的竞争非常重要。在与麻省理工学院研究生Choi Jaehyuk Choi进行的初步实验中，在筒仓内排水时，使用高帧频数字成像技术对颗粒的位置进行了长期跟踪。尽管根据堆积几何和空穴运动建立的运动学模型是对筒仓内颗粒平均流动的单参数描述，但它们并不能正确地预测实验中观察到的混合速度。从塑性理论衍生的其他建模方法不涉及扩散和混合速率。PI建议对扩散速率进行实验研究，不仅作为流速的函数，而且还可以通过改变孔口的性质来改变剪切梯度。然后研究了料仓形状和料斗半角的影响。边界条件的不连续性和其他障碍物会产生激波前锋，从而增强混合。当两种物质的大小或密度不同时，将研究分离机制对混合速度的影响。实验将是了解慢速颗粒流的合作努力的一部分。除了实验，还将与麻省理工学院应用数学系的马丁·巴赞特和鲁本·罗萨莱斯教授一起进行建模和模拟。一个新的包含观测到的关联的SPOT模型正在开发中，它是对这项实验研究的补充。用涨落耗散定理定义的有效温度等新概念对减缓颗粒流动的适用性将得到检验。拟议的活动还将涉及重要的教育和科学宣传部分。博士后助理将接受培训，他有兴趣在鼓励本科生参与研究的环境中追求职业生涯。这项拟议的工作还将影响两名女研究生的研究。这笔助学金将增强本科生的暑期研究经验，并帮助他们在实验室完成荣誉论文项目。PI还计划继续在邻近的高中演讲，介绍从纳米到宏观的颗粒技术带来的挑战，并激发人们对科学的兴趣。

AbstractCTS-0334587A. Kudrolli, Clark UniversityMixing and segregation in particulate flows will be investigated using experiments with high-frame rate digital imaging and direct particle tracking. Besides measuring diffusion of the particles near the side walls of a silo, the PI proposes to also investigate the motion of the particles deep inside the silo using index-matching fluids. The investigation will provide rigorous answers to a problem that plagues the design of modular pebble-bed reactors, which is of current interest to nuclear engineers. A central question is the amount of diffusion among billiard ball sized fuel pebbles containing radioactive material that are slowly cycled through a reactor. The study is important to many other industrial systems where competition between mixing and segregation in particulates is important.In preliminary experiments conducted with MIT graduate student Jaehyuk Choi, the positions of particles were tracked using high frame rate digital imaging over a long period inside the silo as it drains. Although kinematic models formulated in terms of packing geometry and void movement have served as one-parameter descriptions of the mean flow of the grains inside a silo, they do not correctly predict the mixing rate observed in the experiments. Other modeling approaches derived from plasticity theory do not address diffusion and mixing rates. The PI proposes to experimentally investigate diffusion rates not only as a function of flow rates but also shear gradients that can be varied by changing the nature of the orifice. Then the effect of the shape of the silo and the half angles of the hopper will be studied. Discontinuity in the boundary condition and other obstacles induces shock fronts which can enhance mixing. The effect of segregation mechanisms on mixing rates will be investigated when two species that differ in size or density are present.The experiments will be part of a collaborative effort to understand slow granular flow. In addition to the experiments, modeling and simulations with Professors Martin Bazant and Ruben Rosales of the Applied Mathematics Department at MIT will be performed. A new spot model incorporating the observed correlation is being developed which complements this experimental study. Applicability of new concepts such as \effective" temperature defined using the fluctuation dissipation theorem to slow particulate flow will be tested. The proposed activity will also involve significant educational and science outreach components. A post-doctoral associate will be trained who is interested in pursuing a career in an environment where undergraduate research participation is encouraged. The proposed work will also impact the research of two female graduate students. The grant will enhance summer research experience of undergraduate students and help accomplish their honor thesis projects in the laboratory. The PI also plans to continue speaking at neighboring high schools to introduce the challenges posed by particulate technology from nano to macro scale, and to excite interests in the sciences.