结构双流体模型

In the numerical simulations of heterogeneous gas-solid flows, the gas and solids are treated as two interpenetrating continuua in state-of-the-art two-fluid models, and the structure inside each comutational cell is assumed as homogeneous during the derivation of governing equations, a proper sub-grid scale model is then established to consider the effect of heterogeneous structure on the constitutive laws. In this study, we derive the governing equations for heterogeneous gas-solid flow in risers directly according to its structural characteristics, that is, the coexistence of two-phase structure with particle-rich cluster phase and gas-rich dilute phase. A new two-fluid model is proposed by treating the cluster phase and the dilute phase as the two interpenetrating continuua. The interactions between heterogeneous gas-solid flows are then decomposed into the interactions within three homogeneous sub-systems according to the concept of scale decomposition proposed in Energy Minimization Multi-Scale (EMMS) model, which allows the utilization of knowledge of homogeneous fluidization to describe heterogeneous fluidization, determines the constitutive laws of the proposed two-fluid model and therefore significantly simplifies the mathematical description of heterogeneous gas-solid flows. Finally, the proposed two-fluid model is validated by comparing its simulation results with experimental data available in literature...In present study, a fluid in continuum model is defined according to the structural characteristics of complex systems and the concept of scale decomposition is then used to determine the constitutive laws, and therefore, simplify the mathematical description of complex multiphase systems. Present study offers a new possibility for studying complex multiphase flows.

在模拟非均匀气固两相流时，现有双流体模型把气体和固体颗粒看成是相互渗透的两流体，在推导控制方程时假设网格内的结构是均匀的，在得到控制方程后，再建立合理的亚格子模型来考虑非均匀结构对本构关系的影响。本项目提出在推导控制方程时直接考虑非均匀气固两相流的结构特性：根据提升管中固体颗粒由以单颗粒形式存在的稀相和以团聚物形式存在的密相共存的物理特性，提出把稀相和密相看成相互渗透的两流体，建立控制方程；再根据能量最小多尺度（EMMS）模型中尺度分解的概念，把气体－颗粒间非均匀的相互作用分解为三个均匀子系统内的相互作用，从而可以用均匀流态化的知识来描述非均匀气固系统，确定新模型的本构关系；最后通过模拟提升管内的流动特性验证新模型的可行性。本研究根据复杂系统的结构特性定义连续介质模拟中的流体，运用尺度分解的概念来确定本构关系，从而简化多相复杂系统的数学描述、为研究复杂多相体系提供了一条新思路。

循环流化床反应器广泛用于化工、能源等领域，然而由于其内部存在复杂的动态时空多尺度结构，人们对其的了解程度还不能满足工程设计与放大的要求。本研究以循环流化床提升管中的非均匀气固两相流为研究对象，根据循环流化床中固体颗粒由以单颗粒形式存在的稀相和以团聚物形式存在的密相同时存在的物理特性，在推导控制方程时直接考虑系统的结构特性，把稀相和密相看成相互渗透的两流体（现有双流体模型中把气体和固体颗粒看成相互渗透的两流体），建立结构双流体模型。再根据能量最小多尺度（EMMS）模型中尺度分解的概念，把复杂的非均匀气固相间作用简化为三个均匀的子系统内的相互作用（稀相内、密相内和稀相－密相相间相互作用），从而确定结构双流体模型的本构关系。并系统研究了输入参数敏感性、气固系统颗粒温度等关键参数的特性；最后通过模拟系统高通量循环流化床内的流动特性、出口效应并与实验比较,验证了结构双流体模型的可行性，同时将其应用于下行床反应器的模拟。结构双流体模型大大简化了描述复杂非均匀气固两相流的数学模型，为研究复杂多相系统提供一条新思路..在本项目的支持下已经在Chemical Engineering Science, Chemical Engineering Journal, Industrial & Engineering Chemistry Research和Powder Technology正式发表论文8篇，发表国际会议论文2篇。应邀在本领域权威会议Fluidization 14上做Keynote报告。