流化床内颗粒流体两相流的CFD模拟

采用先进的CFD模拟技术分析流化床内两相复杂体系的非线性流体动力学特征已得到普遍认同,但是由于不同研究者对颗粒与颗粒以及颗粒与流体之间相互作用力认识的差异,导致欧拉-欧拉框架下动量守恒方程的不同表达形式。本文在总结文献中有关颗粒黏性力、固相压力和两相间作用力的基础上,从双流体理论出发,提出了一个考虑拟平衡态下固体颗粒对流体相和固相动量守恒方程均有影响的简捷流体动力学模型。该模型的主要特点是表征颗粒离散属性的特征长度视为颗粒直径的同一数量级。随后在CFX4.4商业化软件平台上通过增加用户自定义子程序,对网格尺度、时间步长和最大颗粒堆积率的无关性进行检验,介绍了作者近年来采用该模型模拟二维/三维流化床内液固体系的散式流态化、气固Geldart A类物料的散式/聚式流态化和床层塌落特性以及Geldart B/D类物料的鼓泡/射流流态化和床层塌落的研究进展。模拟的主要结果与经典理论、本研究实验和文献报道数据相一致,说明该模型可以用来预测流化床内密相颗粒流体体系的动力学特性。

CFD simulation of particle-fluid two-phase flow in fluidized beds

Advanced CFD simulation has been regarded as one of important tools for dealing with the complicated non-linear fluid dynamics in the fluidized beds. Different momentum balances of particles and fluid phases in Eulerian-Eulerian models have been proposed due to different opinions on fluid-particle and particle-particle interactions. Particle viscous force, solid pressure force and inter-phase drag force are discussed in this paper. Then, authors provided a simple hydrodynamic model based on the two-fluid theory, which considered the effect of discrete particles on both the fluid- and solid-phase momentum equations under a quasi-equilibrium state. One of its main features is that the characteristic length in the model is of the order of the particle diameter. Numerical simulations were conducted in the platform of a commercial software package, CFX 4.4, by adding user-defined Fortran subroutines. After the independence of grid size, time step and the maximum solid volume fraction were explored, case studies including homogeneous fluidization of liquid-solid system, homogenous/aggregative fluidization of Geldart A particle and bubbling/collapsing/jetting fluidization of Geldart B/D particle for gas-solid system were conducted in 2D/3D fluidized beds. The majority of results were compared with classic theories, experimental data and simulations in the literature and good agreements were obtained, which indicates that this model can be used to predict the fluid dynamics of the dense particle-fluid two-phase flow in the fluidized beds.