快速流化床颗粒团聚机制及数理模型研究

Particle clustering is the typical characteristics of gas-solid flow in circulating fluidized beds, which has a direct effect on the gas-solid dynamic mechanism, heat transfer, mass transfer and chemical reactions in circulating fluidized bed reactors. However, the spatiotemporal evolution rules and mechanism of clusters is uncertain, leading to the lack of reliable theory support for the design, operation and numerical simulation of circulating fluidized beds. As a result, in this project, the dynamic evolution process of the formation, growth, motion, and break-up of clusters in the riser of the circulating fluidized bed will be captured with the 3D non-disturbance measurement technique of digital X-ray imaging technique. Accordingly, the mechanisms, which are responsible for the formation, growth, stability and break-up of these clusters, will be revealed, and the mathematical models of clusters will also be established. Then, the heterogeneous drag model and the particle-particle collision model will be modified based on the mathematical models of clusters. In order to reduce the number of particles to be tracked, particles will be treated as numerical particles, namely each numerical particle contains a certain number of real particles with identical properties, such as density, size, position and velocity. Finally, a cluster mechanism-dependent three-dimensional numerical simulation method for dense gas solid flow will be developed. The result of this project will be helpful for the design, improvement and scaling up of circulating fluidized beds, and promotes the establishment, modification and improvement of the simulation method for the large fluidization system.

颗粒团聚是快速流化床内气固流动的典型特征，直接影响床内气固流动机制以及反应器的传热、传质和化学反应性能。然而，目前对团聚物演化过程和存在机制尚不明确，以至于在快速流化床的设计、运行和数值模拟等方面缺乏可靠的理论支持。鉴于此，本项目拟采用X射线成像技术实现快速流化床内颗粒团聚形态、浓度和运动的三维非接触式测量，捕获颗粒团聚物形成、发展和解体的动态演化过程并揭示其内在动力学机理，建立颗粒团聚数理模型；基于颗粒团聚模型对非均匀曳力模型和颗粒碰撞模型进行修正，并引入“计算颗粒”将单元网格中具有相同物料属性的单颗粒打包，发展基于颗粒团聚机制的稠密气固耦合三维数值模拟方法。其研究结果将有助于快速流化床的设计、改进和大型化，并促进大型流态化系统模拟方法的建立、修正和完善。

以颗粒团聚为代表的非均匀气固流动是气固两相流的典型特征，直接影响气固流化设备的流动机制以及传热、传质和化学反应性能。颗粒团聚机制的研究对非均匀流动系统曳力模型的建立和数值模拟具有十分重要的意义，也是应用气固流态化技术的多个领域中的共性科学问题之一。本项目将试验研究、理论分析和数值模拟相结合，开展了快速流化床颗粒团聚机制和团聚物模型的相关研究：（1）颗粒团聚时空演化过程捕获：建立快速流化床气固流动可视化试验装置，采用高速摄像和图像处理系统对稠密气固两相流中颗粒团聚行为进行可视化非接触测量，并基于K-means 聚类算法实现对团聚物及其内部结构的自动识别，实现了团聚物动态演化过程的捕获，揭示其时空演化规律。（2）稠密气固两相流颗粒团聚机制的揭示：对气固流动图像进行多层分割，揭示了团聚物内部结构以及团聚物和团聚核形成的条件；基于互相关计算和质心跟踪，实现对快速流化床内团聚物浓度、形状、尺寸、运动轨迹、速度等特征参数的动态跟踪和记录，揭示颗粒团聚机制及其与物料属性和操作条件等的变化关系；建立透射式光纤测量系统，通过与可视化图像测量系统获得的试验结果比较，对两种测量方法进行探讨。（3）团聚物模型建立和非均匀曳力模型修正：基于团聚物浓度理论模型和整体非均匀指数，将试验数据与理论分析相结合，建立团聚物浓度计算公式；采用等效直径定义团聚物尺寸，基于“气泡模型”建立团聚物尺寸模型，并结合试验结果建立了团聚物尺寸计算公式；基于建立的团聚物浓度模型和尺寸模型对非均匀曳力模型进行修正。（4）建立基于颗粒团聚机制的气固双向耦合模型：将修正的非均匀曳力模型与MP-PIC算法相耦合，发展了气固非均匀数值模拟方法，揭示了非均匀流场中颗粒团聚物的发展过程。本项目促进了非均匀曳力模型和大型流态化系统模拟方法的建立、改进和完善，为气固流态化设备的应用提供可借鉴的理论基础。