CAS OpenIR
双分散气固流化床的介尺度模型及CFD模拟
NOUMAN AHMAD
Subtype博士
Thesis Advisor王维
2018
Degree Grantor中国科学院大学
Degree Discipline化学工程
Keyword双分散,介尺度,鼓泡床,曳力系数
Abstract

气固鼓泡床反应器在能源、化工等领域均有重要应用,如烯烃聚合,煤/生物质燃烧,矿石焙烧等。气固流态化中往往呈现颗粒聚团或气泡等介尺度结构,且实际应用中颗粒通常具有多分散性(涉及不同颗粒尺寸或密度),这使得介尺度结构的形成机制更加复杂。这种多分散性还会导致床内出现颗粒分层等现象,在低速的密相床中尤为显著,影响了相间动量传递和反应行为。深入理解多分散流态化中的颗粒分离和混合现象对于合理设计反应器以及确定最佳的操作条件至关重要。近年来,随着多相流理论和计算流体力学(CFD)技术的迅速发展,特别是介尺度理论的兴起,使得CFD成为研究多相复杂流动行为的强大工具。在各种模拟方法中,连续介质模型(又称为欧拉模型)由于其较少的计算量被广泛用于工程计算。而在模拟气固流动时,曳力系数对于准确捕捉气固流动中的介尺度结构等典型特征起到关键作用。很多研究者指出,在低速多分散密相床(如鼓泡床)中曳力是影响不同颗粒分离和混合的关键因素。在过去的十几年中,以能量最小多尺度模型(EMMS)为代表的介尺度模型被成功应用于气固流化床反应器的模拟,但是绝大部分的研究都基于均一颗粒的流态化系统(或称单分散流态化系统)。由于单分散系统和多分散系统中形成的介尺度结构具有显著区别,因而有必要在建模中考虑颗粒的多分散性特征。作为初步尝试,本论文着眼于发展一种针对双分散鼓泡床反应器的介尺度模型,相关研究内容描述如下。论文第一章是文献综述,介绍气固流态化的主要特征,尤其是以鼓泡床为代表的低速密相床流态化中的一些典型现象,以及多分散流态化和单分散流态化之间的异同点。论文第二章通过比较双周期边界区域(局部平衡系统)和实际流态化反应器(非平衡系统)的模拟,分析了气固流态化中的非平衡流动特性,指出针对气固流态化建立介尺度模型的必要性。在论文第三章中,对适合于单分散鼓泡流态化的EMMS/bubbling介尺度模型进行了系统的参数分析,重点考察气泡直径公式的影响。并将该模型作为双分散流态化系统建模的基础。论文第四章在EMMS/bubbling介尺度模型的基础上,建立一个针对双分散鼓泡流态化系统的新模型。提出针对稳定流动的求解方法,通过比较鼓泡流化床整体的膨胀系数等宏观参数对模型进行了初步验证。在论文第五章中,通过在力平衡方程中引入不同颗粒和气泡等的加速度项,将上述模型用于计算非稳态条件下双分散流动的气固曳力系数。并引入一些假设用于简化该模型的求解算法,通过一系列参数敏感性分析考察这些假设的合理性。在论文第六章中,将模型与连续介质模型耦合,对两种双分散鼓泡床反应器进行了模拟。这两种双分散系统涉及的颗粒分别是不同尺寸相同密度的两种颗粒,以及尺寸和密度均不同的两种颗粒。随后通过比较实验数据和模拟结果,详细讨论了模型的适用性。论文第七章总结了本论文获得的主要成果,展望了针对双分散流态化系统建模方面值得深入研究的几个方向。;Gas-solid bubbling fluidized beds have various industrial applications including olefin polymerization, coal/biomass combustion, mineral calcinations etc. In a gas-fluidized bed, particle clusters and bubbles are typical mesoscale structures. In particular, solid particles involved in practical applications usually have a wide size distribution or different densities, showing polydisperse characteristics and making mesoscale mechanisms more complex. Such polydispersity of solid particles leads to segregation phenomena in fluidized beds especially in dense fluidization operated in low velocity, thus greatly influencing the flow hydrodynamics and reaction performance. Understanding the segregation and mixing phenomena is crucial for the proper design and operation of fluidized bed units.Nowadays, Computational fluid dynamics (CFD) is emerging as a powerful tool to help understand the complex hydrodynamics of multiphase flow. Among various approaches, the continuum model also called Eulerian approach is the most commonly used method for industrial applications due to its acceptable computational cost. In simulating the gas-solid flows, drag force plays a key role in capturing mesoscale structures. Many researchers pointed out that the drag is the important and dominant force in influencing the segregation and mixing behaviors of binary mixtures at low gas velocities.In the last decade EMMS (Energy Minimization Multiscale method) based drag modeling has been applied successfully to gas-solid fluidization. But most of the previous work is based on monodisperse systems. Further efforts considering the polydispersity is very necessary to truly understand the key features in binary fluidization because there is an evident difference in formation of mesoscale structures between monodisperse and polydisperse flow systems. As a preliminary exploration in this regard, this work aims to develop the EMMS based drag model for binary fluidization at low velocity. The related contents are described as follows.Chapter 1 is a literature review where a brief discussion about challenges in gas-solid fluidization like heterogeneity and typical features associated with polydispersity is presented. Chapter 2 discusses non-equilibrium features of a typical gas-solid fluidized system in terms of solid velocity distribution and how it breaks the local equilibrium assumption in modeling based on numerical experiments of a doubly periodic system and actual fluidized beds, and points out the necessity of mesoscale modeling for heterogeneous gas-solid fluidization.In Chapter 3, as the EMMS/bubbling model for the monodisperse system is considered as the basic framework for further modeling for the fluidized binary particle mixture, a thorough parameter analysis of the model is performed. The investigation of bubble diameter correlations is emphasized due to the importance of bubble characteristics in segregation and mixing phenomena in binary particle system.In Chapter 4, a new model extended from the EMMS/bubbling is established to describe the binary fluidization. Model equations are formulated and a solution scheme for the steady state flow is proposed. The preliminary model validation for the global flow behavior is then performed.In Chapter 5, the new model is used to calculate the drag coefficient for unsteady flow of binary fluidization by introducing the particle and bubble accelerations in force balance equations. The numerical scheme based on some assumptions is proposed and related parameter sensitivity analysis is performed to investigate the reasonability of the assumptions.In Chapter 6, the validation work for the new model by integrating it into the continuum model is presented where two bubbling fluidized beds having different segregations (i.e., one system where solid components have different sizes and equal densities, the other system where solid components have different sizes and densities) are simulated.At last, conclusions and future work are presented in Chapter 7. 

Language英语
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40728
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
NOUMAN AHMAD. 双分散气固流化床的介尺度模型及CFD模拟[D]. 中国科学院大学,2018.
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