纳米颗粒团聚流化的微观机理和模型研究

The understanding on fluidization of nanoparticles has been much improved in recent years. It is commonly accepted that the nanoparticles are not fluidized individually but as agglomerates, in the form of porous multistage structures, that are aggregates, simple agglomerates, and complex agglomerates. The complex agglomerates are studied extensively by the existing studies. The complex agglomerates are continuously coalescing and breaking due to fluid shear and inter-agglomerate collisions, which leads their properties hard to measure. In this project, the simple agglomerates are studied in detail, as a bridge to relate the fluidization behavior with the nanoparticle cohesion, since the simple agglomerates are rather consistent in composition. Based on the principle of sheath flow, a method of on-line sampling and focusing of complex agglomerates is developed. It is employed to study the drag force coefficient and collision behaviors (e.g. rebound, stick, crushing) of the complex agglomerates. The elasticity, plasticity and viscosity of the simple agglomerates are characterized. The comprehensive experimental data provides a solid base for modeling. Taking the simple agglomerates as the elements, we develop CFD-IBM-DEM and CFD-DEM models of adhesion and elastic-plastic particles. CFD-IBM-DEM model is used to investigate the relation between dynamic behavior and structure of the complex agglomerates and characteristics of the simple agglomerates. CFD-DEM models is used to investigate the hydrodynamic behaviors of the fluidized bed, e.g. agglomeration and bubble dynamics, size distribution of agglomerates, and the agglomeration and breakage rates. The relation between the particle cohesion, agglomerates and fluidization phenomenon can be revealed in this way. The next step is to study gas transfer process. The gas transfer within the aggregates, simple agglomerates, complex agglomerates, and inter-phases will be combined. Their effect on the final result of the gas transfer at different scales will be evaluated. To the end, new theories and models of nanoparticle fluidization are developed, based on the properties of the porous multi-stage agglomerate structure and the continuously coalescing and breaking agglomerates, which can provide a solid theoretical basis for optimization of nanoparticle fluidization reactors.

纳米颗粒流态化理论正在逐渐丰富，纳米颗粒以多级聚团（子聚团、简单聚团、复合聚团）形式存在于流化床，已形成共识。基于简单聚团具有稳定性和可测量性特征，本项目提出以简单聚团为切入点研究纳米颗粒流化的新思路。引入鞘流原理，发展复合聚团在线取样聚焦及观测方法，研究聚团曳力系数、聚团碰撞/反弹/破碎/粘结相图，提供全面实验数据。以粘塑性DEM颗粒表示简单聚团，构建单个复合聚团并开展运动和碰撞模拟研究，阐明复合聚团运动和碰撞及其与颗粒粘性的联系。以简单聚团为DEM基元，开展不同颗粒粘性/强度等条件下流化床模拟研究，揭示流化床中聚团-气泡运动、聚团尺寸分布及聚团生成破碎等。在结合聚团结构和流化形态研究基础上，综合子聚团、简单聚团、复合聚团和相间不同层次的气体传递过程，明确各层次对气体传递宏观效应的影响。最终，形成以聚团多级结构和动态生成破碎为基础的纳米颗粒流化理论和模型，为反应器优化设计提供理论基础。

纳米颗粒流化床作为一种新兴的纳米颗粒处理技术，在实现纳米颗粒宏量制备方面具有优势。纳米颗粒以松散聚团的形式流化，聚团是影响纳米颗粒流化床流动和反应的关键因素。本项目以聚团为核心，研究纳米颗粒粘性、聚团结构、流化特性以及它们的内在关联。利用原子力显微镜对典型的几μm量级的纳米颗粒聚团进行力-位移曲线测量，得到聚团的黏性力和杨氏模量。TiO2和Al2O3聚团间黏性力相当，大于SiO2聚团间黏性力。TiO2聚团杨氏模量最大，Al2O3聚团次之，SiO2聚团杨氏模量最小。随着法向荷载增加，聚团间接触面积增加，聚团黏性力和杨氏模量均增加。通过流化实验观测，明确了聚团具有多层级结构。流化聚团主要分布在直径几百μm、密度几十kg/m3范围，它们由更小单元的简单聚团（约几十μm）和初级聚团（约几μm）组成。结合离散颗粒模型和粘性颗粒碰撞模型，对单个聚团在剪切场中破碎过程进行数值模拟，发现随着剪切强度的增加，破碎重组后聚团的稳定尺寸减小，但结构更均匀密实，聚团稳定尺寸为几μm，分形维数可达到约2.5。进一步，以初级聚团为单元，耦合双流体和群平衡模型，模拟了不同粘性力条件下的流化，得到了均匀膨胀流化、鼓泡流化、沟流流化、失流化4类分区。在流化实验方面，提出了振动和搅拌复合作用辅助纳米颗粒流化方法。在振动或搅拌单独辅助作用下，增加单一参数，床层膨胀比的增加逐渐减弱。而复合作用辅助流化方法适用于改善粘性较大的纳米颗粒流化，使得床层膨胀比进一步增加，并能显著减轻纳米颗粒聚团沿床高方向的分层现象。本项目形成的以聚团多层级结构和动态生成破碎为基础的纳米颗粒流化理论和模型，为纳米颗粒流化床反应器优化设计提供理论基础。