准静态颗粒流介观结构的测量、表征及其在高温气冷堆调控中的应用

A granular material is a conglomeration of discrete solid particles. Rich types of mesoscopic structure are formed and energy would be transformed whenever the particles interact and the mesoscopic structures evolve. As a result, the complicated mechanics properties on macroscale are induced, as measured in many experiments and numerical simulations. The study on structures on macroscopic scale and constitutive relations on macroscopic scale is not only the frontiers in the meso-sciences, but also key technology of the exact flow controlling of fuel element spheres in the pebble-bed High Temperature Reactor (HTR). In this proposal, 1) we would firstly measured and quantify force chain networks and soft regions by using discrete element simulations, photo-elastic, acoustic detection techniques. A structure unit postulate would be proposed which includes force chains and soft regions, and and an evolution equation for the structure unit would be developed. After analysing the spatial and temporal evolution of the structural unit, the connection between mesoscopic and macroscopic scales would be established. 2) The the constitutive theory we developed on the basis of thermodynamics framework would be revised, especially determine the irreversible processes and the transport coefficients. After developing an appropriate numerical algorithm, the theory can be self-contained and feasible for engineering practices. 3) By using the Tsinghua HTR as a protocol, a large physical model would be build. The mesoscopic structure analysis, macroscopic theory calculations and the advanced measurement techniques are all employed for this study. The quasistatic deformation, quasistatic-flow transition would be well understood and controlled int the the pebble-bed HTR. The flow controlling techniques would be updated and the reactor will run more stably and safely. This proposal would benefit meso-science by providing a candidate methodology, and also become a successful example of solving HTR technology by using meso-science theories and methodology.

力链和软区是准静态颗粒流的两个基本介观结构，具有复杂几何形态和动力学特性，决定颗粒流宏观流变性质。本项目拟：1) 发展力链和易于发生非仿射位移的软区这两个重要介观结构的高时间和高空间分辨率测量技术，确定介观结构的几何特征和动力学特性，研究其随体系参数的动态演化规律；2) 建立考虑力链长度和颗粒自旋的应力和应变粗粒化方法，开展颗粒离散元DEM和物质点方法MPM的多尺度建模，解决准静态颗粒流的多力学状态描述、大变形计算难题。高温气冷堆堆芯颗粒流是反应堆设计和核燃料循环计算的基础，颗粒流精准计算与调控是核心技术难点，本项目拟 3) 开展大型堆芯颗粒流实验和DEM/MPM大规模计算，分析燃料颗粒参数、操作条件等对滞留区和交混区分布的影响，为精准计算和控制提供科学指导。该项目提供可参考的介尺度方法论，并成为介尺度科学在能源领域的应用范例。

本项目研究工作总体按照原计划进行，没有改变原案和计划。主要开展了3方面的研究：1). 在测量技术方面：发展和发明高时间、空间分辨率的先进光学SVS、声学谱和力学谱测量技术，实现对力链和“软区”等介观结构及其动力学响应特征尺度的有效探测，形成完整表征体系，并为颗粒物质宏观力学参数的确定提供了实验支持；2). 在本构理论方面：通过深入研究的力链和软区特征，完善Goldhirsh粗粒化理论，亦即引入力链长度和颗粒自旋来得到应力和应变的粗粒化方法，在此基础上，完善本构理论和发展算法，使得理论可靠、可行、易用，为解决高温气冷堆工程问题奠定基础；并发展DEM/MPM 多尺度建模；3). 在应用方面：取得气冷堆颗粒流静态─流态转变机制和流动规律的科学认识，解决燃料球滞留和流速控制等工程技术问题，为高温气冷堆稳定、安全运行提供指导。发表33篇期刊论文；申请和授权发明专利8项。2017年、2020年举办颗粒介质基本理论的学术会议2次，使本项目组的工作产生一定国际影响。毕业2位博士和15位硕士，在站1位博士后，从事颗粒介质力学基础研究。自主开发了用于多流态颗粒流计算GeoMPM 3D程序和颗粒流测试平台，开发了用于颗粒材料声学谱的测试平台，可用于研究堆芯颗粒流的预测和演进、颗粒流参数测量等。