基于离散统一气体动力学的纳米颗粒分形团聚体多尺度流动直接数值模拟

The multiscale flow of fractal aggregates of nanoparticles widely exists in the fields of energy, environment, chemical industry, bioengineering. Investigation of the physical mechanism has important guiding significance for applications in nanoparticles fluidization, nanoparticles capture and control. Due to the complexity of the fractal structure and the multi-scale interactions between gas and solid, the mechanism of the gas-solid two-phase flow with nanoparticle aggregates is very complicated. The traditional methods are not suitable for studying this problem own to their limits. The discrete unified gas kinetic scheme (DUGKS) based on mesoscopic kinetic theory is a promising approach for the study of such a complicated problem. In this project, focusing on the particle-resolved aggregates, the multi-scale structure of nanoparticle aggregates is established by the fractal theory. With high-performance computing technology, the efficient particle-resolved direct numerical simulation (PR-DNS) of DUGKS is developed, where the DUGKS model is for the multi-scale flow simulation, and the boundary condition is constructed for describing the multi-scale interaction between the gas and solid. The developed PR-DNS of DUGKS is then used to analyze the effects of fractal dimension of aggregates, original particle size distribution and external flow conditions on the interactions of gas-aggregate and aggregate-aggregate, and furthermore, reveal the transport mechanism of multi-scale flow of fractal aggregates of nanoparticles.

纳米颗粒分形团聚体多尺度流动问题广泛存在于能源、环境、化工、生物工程等领域，研究其物理机理对纳米颗粒流化、纳米颗粒捕集和控制等有重要指导意义。由于涉及复杂分形结构和气固之间多尺度相互作用，纳米颗粒团聚体气固两相流动机理极为复杂。传统数值方法此时受到很大限制，基于介观动理学理论的离散统一气体动理论格式（DUGKS）是研究该类问题的有效方法。本项目从全解析团聚体出发，采用分形理论建立纳米颗粒团聚体的多尺度结构，建立多尺度流动模拟的DUGKS模型，构建气固多尺度相互作用的边界条件，发展高效DUGKS颗粒直接数值模拟算法；在此基础上，研究不同因素如团聚体分形维数、原始颗粒粒径分布、外部流场条件等对气体-团聚体以及团聚体-团聚体之间耦合机制的影响，揭示纳米颗粒分形团聚体多尺度流动的输运机理。

纳米颗粒团聚体两相流动是一个涉及多尺度效应的复杂流-固耦合系统，深入认识流体与颗粒团聚体以及团聚体与团聚体之间的动量传递和耗散特性和规律存在困难，并日益成为能源、环境、化工、生物工程等领域急需解决的一个共性基础科学问题。本项目针对纳米颗粒团聚体气固两相流动的多尺度输运特点，从全解析团聚体出发，着眼于不同尺度上的流动过程，构建准确描述团聚体多尺度流动特性的DUGKS 模型，建立描述气固多尺度相互作用的微观边界条件，发展高效DUGKS颗粒直接数值模拟算法，在一系列复杂多尺度气固流动问题中验证了其准确性和适应性。此外，选择纳米颗粒团聚体曳力关系式、单个团聚体在空气中的动态沉降过程以及团聚体相干沉降过程这三个气固两相流中的典型问题为研究对象，采用发展的DUGKS 方法研究团聚体多尺度流动过程及相互作用的影响，总结影响气体-团聚体以及团聚体-团聚体之间的耦合机制和输运规律的关键参数，为此类复杂气固两相流动的多尺度物理建模和输运微观机理提供新的研究手段，而且对涉及这一基本问题的纳米颗粒流化、纳米材料制备、纳米颗粒捕集和控制等均具有一定现实指导意义。