基于EMMS模型的气固两相流模拟

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题名	基于EMMS模型的气固两相流模拟
作者	刘雅宁
学位类别	博士
答辩日期	2011-05-31
授予单位	中国科学院研究生院
导师	葛蔚 ; 王军武
关键词	气固两相流多尺度宏尺度模型介尺度模型 DEM
其他题名	EMMS-based Simulation of Gas-Solid Flow
学位专业	化学工程
中文摘要	气固两相流反应器在工业界得到了广泛的应用，它处于典型的非线性非平衡状态，呈现出复杂的时空多尺度结构，相应的，描述该过程的模型也应该是多尺度的。各尺度上的模型都有各自的优点和局限性，例如宏尺度模型计算效率高，但是分辨率低，无法完成复杂动态演化过程的模拟；微观模型计算量过大，无法对实际工业过程进行模拟。为此，本论文将以能量最小多尺度(EMMS)模型为基础，采用多尺度计算方法，提高计算的精度和有效性。论文第一章总结了气固流动过程中各个尺度上的特征现象，并介绍了各个尺度上计算模型的特点及研究现状，分析了采用多尺度模型来描述气固流动中多尺度现象的必要性。第二章研究了宏尺度和介尺度EMMS模型与双流体模型的耦合模式，以宏尺度EMMS模型预测的初场分布成功加速了双流体模型计算达到统计定态的过程。研究发现这种计算模式与初始为全床均匀分布和初始为固定床状态的模拟相比，将到达统计定态所需的计算时间缩短了数倍。该模式充分结合了宏尺度模型计算量较少和介尺度模型较准确的优势，并可以扩展到对其他系统的模拟。第三章改进了介尺度EMMS模型，结合隐式团聚物直径描述和两步法求解的优点，得到了考虑微元非均匀结构的合理的曳力修正系数，完善了其理论基础，扩大了其应用范围，并实现了其与双流体模型的耦合。以此模拟了实际提升管反应器，比较了该方法与基于网格内均匀假设的传统曳力模型在计算固体循环量、固体体积分率的轴径向分布等方面的差异，发现本模型得到的计算结果与实验结果更加吻合。第四章进一步实现了介尺度与微尺度离散模型的耦合计算，建立了基于固相离散描述的离散单元法(DEM)和光滑粒子动力学(SPH)方法与FLUENT的耦合模式，研究了不同数据映射方式及在FLUENT中不同实现模式对模拟结果的影响。模拟中以介尺度EMMS模型考虑了DEM与FLUENT耦合模拟中网格内非均匀结构对气－固相间曳力的影响，与常规曳力模型相比得到了更接近实验的轴向空隙率分布，说明此类计算中也需要考虑介尺度结构才能较准确描述气固流动规律。第五章对本论文进行了总结并展望了未来的研究方向。
英文摘要	Gas-solid two-phase reactors are widely used in industries, which are typically in non-linear and non-equilibrium states characterized by spatio-temporal multi-scale structures. In response to this physical nature, the physical models describing these reactors should also be multi-scaled. Each model has its own strengths and weaknesses. For example, macro-scale models are computationally very efficient, but are insufficient in describing the dynamic evolution of the flow. On the other hand, micro-scale models have a very good resolution with respect to time and space scales but require extremely high computational cost for simulating industrial processes. The object of this study is, therefore, to use the Energy-Minimization Multi-scale (EMMS) model to improve the accuracy and efficiency of computer simulations of gas-solid two-phase flow. In chapter 1, the typical features of gas-solid two-phase flow at different scales are summarized and a state-of-the-art review of various models at each scale is presented. Based on the features of each model, it is concluded that multi-scale models should be adopted to match the multi-scale nature of gas-solid two-phase flow. In chapter 2, in order to simulate the hydrodynamics of circulating fluidized bed risers efficiently and accurately, the macro-scale EMMS model was coupled with a two-fluid method (TFM) elaborated by the meso-scale EMMS model considering the effect of sub-grid scale heterogeneity on inter-phase drag force. The overall flow distribution under the steady state was approximately predicted by the macro-scale EMMS model, which serves as the initial condition for TFM simulations reproducing the dynamic behavior of heterogeneous gas-solid flows. Using the solid circulation flux as a criterion, it was shown that this coupling approach can significantly reduce the time required to reach the statistically steady state, as compared to the initial conditions of packed bed or homogeneous dispersion. In chapter 3, the meso-scale EMMS model was modified and used as the sub-grid scale model for inter-phase drag force in two fluid modelling to simulate the gas-solid flow in riser. The simulation results are in better agreement with corresponding experiment data, including solid circulation flux, radial and axial solid concentration profiles, than that from the standard drag model. The applicability and accuracy of the meso-scale EMMS model is therefore validated. In chapter 4, discrete description of the solid phase such as the discrete element method (DEM) and smooth particle hydrodynamics (SPH) were integrated into the comercial computaional fluid dynamics (CFD) software—FLUENT. As in this approach the discrete solid phase and the continumm gas phase are resolved at different scales, the parameters used to calculate the inter-phase interaction force have to be mapped from Eulerian frame to Lagranian frame and vice versa. Therefore, the effect of two widely used mapping methods and three implementation methods in FLUENT on the results are studied. The meso-scale EMMS model considering the effect of heterogeneous structures inside grids are integrated into the DEM-CFD method. The simulation results of lab-scale riser show that compared to the data obtained from tranditional DEM-CFD method, simulations with meso-scale EMMS drag model gives a better agreement with experiment, indicating that the meso-scale structure should also be considered in DEM-CFD simulaiton. Chapter 5 outlines the main conclusions of present study and presents my personal perspective on multiscale simulation on gas-solid flow.
语种	中文
公开日期	2013-09-23
页码	103
内容类型	学位论文
源URL	[http://ir.ipe.ac.cn/handle/122111/1689]
专题	过程工程研究所_研究所（批量导入）
推荐引用方式 GB/T 7714	刘雅宁. 基于EMMS模型的气固两相流模拟[D]. 中国科学院研究生院. 2011.