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基于EMMS的介尺度传质模型及其在循环流化床锅炉燃烧模拟中的应用
其他题名EMMS-based meso-scale mass transfer model and its application to circulating fluidized bed combustion simulation
张楠
学位类型博士
导师李静海 ; 王维
2010-06-01
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工程
关键词气固传质 介尺度 循环流化床 燃烧反应 三维模拟
摘要循环流化床反应器内气固流动、传热、传质和反应过程与时空多尺度结构紧密耦合,具有典型的非线性非平衡特征。已有研究表明,对于此类多相复杂系统而言,传统的连续介质模拟方法,单纯依靠细化网格并不能准确捕捉其物理过程,必须建立合理的介尺度模型。 研究表明,亚网格层次的EMMS模型可以合理捕捉循环流化床内的流动特征。在此基础上,EMMS/mass传质模型将循环流化床中的气固传质进行了多尺度分解,并通过臭氧分解模拟验证了其合理性。然而,该模型中涉及的微尺度传质模型目前还有争议,特别是它随空隙率的变化关系以及在低雷诺数(Re)下的变化趋势还不明确。另外,EMMS/mass模型现有控制方程的复杂性使它很难与计算流体力学(Computational Fluid Dynamics, CFD)软件耦合,从而阻碍了它的推广和应用。为此,本文分别从微尺度和介尺度出发,对循环流化床内的传质过程进行了分析,对长期存在的努塞尔数(Nu)在低Re数下的分歧问题以及它随空隙率的变化关系进行了研究,为介尺度EMMS/mass模型的微尺度封闭提供了依据。随后,对EMMS/mass模型进行了简化,以方便其与CFD软件的耦合。最后,将以上多尺度流动、传质模型集成应用于150 MWe工业循环流化床锅炉的燃烧模拟。 论文第二章首先对微尺度上多颗粒均匀分布体系的传热进行了模拟研究。研究结果显示:低Re数时,Nu数随着速度和温度参考值选取的不同而变化,但其极限总是非零值,而实验结果存在争议主要是由于操作条件很难控制到足够低的Re数,以及参考速度和温度难以确定所导致;不同Re数范围下,空隙率对Nu数的影响不同,低Re数时,Nu数随空隙率的增加而增大,高Re数时,Nu数随空隙率的增加而减小,两者之间的转折点在Re=10附近。随后,根据循环流化床的流动特性,对介尺度EMMS/mass模型进行了简化。将简化的EMMS/mass模型与CFD软件Fluent耦合模拟了臭氧分解过程,结果与原模型预测值基本吻合,并且都比传统传质模型的结果更接近实验值。 论文的第三章至第五章将多尺度流动和传质模型逐步集成应用于循环流化床燃烧(Circulating Fluidized Bed Combustion, CFBC)模拟。其中,第三章首先对半工业循环流化床的流动进行了三维全循环模拟验证。研究结果表明,考虑介尺度结构的EMMS/Matrix模型与Fluent耦合后,可以合理预测循环流化床内的流动分布。在此基础上,第四章进一步对150 MWe工业循环流化床锅炉中固体颗粒主回路的流动进行了三维全循环模拟研究。在以上流动模拟的基础上,第五章将简化的EMMS/mass模型耦合到燃烧反应的组分质量守恒方程中,实现了150 MWe工业循环流化床锅炉燃烧室内三维燃烧模拟。 第六章总结了本论文获得的主要成果,并对介尺度传质模型的应用前景以及进一步开展研究的方向进行了讨论。
其他摘要The gas-solid flow, heat /mass transfer and chemical reactions in Circulating Fluidized Bed (CFB) reactors are inherently coupled with spatio-temporal multi-scale structures, and featured with non-linear non-equilibrium behavior. For such a multi-phase complex system, it is inadequate to reproduce the real physical process by refining the grid in the conventional Two-Fluid Model (TFM), calling for establishing meso-scale models. Since the sub-grid EMMS model has proved to be able to reproduce the hydrodynamics in a CFB, based on the multi-scale resolving of mass transfer behavior, the EMMS/mass model has been proposed and verified through the simulation of ozone decomposition. However, the micro-scale mass transfer mechanism involved in this model is still controversial, especially for its dependence on voidage and the variation in low Reynolds (Re) number region. In addition, the complexity of the current EMMS/mass model makes it difficult to couple with Computational Fluid Dynamics (CFD) software, thus hindering the spread and application. To solve this problem, the mass transfer in a CFB is first analyzed on the micro- and the meso-scale respectively, the long-lasting controversial issues of the variation of Nusselt (Nu) number with Re number in the low Re number region as well as its dependence on voidage are studied, which provides a basis for the choice of the micro-scale mass transfer model. To facilitate its coupling with CFD software Fluent, the EMMS/mass model is reduced, and it is successfully applied to the combustion simulation of a 150 MWe CFB boiler. The heat transfer behavior in multi-particle uniform system is first simulated on the micro-scale in chapter 2. The results show that Nu number varies with the reference velocity and temperature at low Re numbers, but its lower limit always equals non-zero. The divergence of the experimental results can be attributed to the difficulties in controlling experiments and the definition of the reference velocity and temperature. The voidage has different effects on Nu number in different Re number region. Nu number increases with increasing voidage at low Re numbers, but decreases with increasing voidage at high Re numbers, with a turning point roughly at Re =10. Further, the EMMS/mass model is reduced. Its coupling with the CFD software Fluent predicts similar results with the original model for the ozone decomposition. Both EMMS/mass models are better than the conventional. In chapter 3 to 5, the multi-scale flow and mass transfer models are integrated and applied to the simulation of Circulating Fluidized Bed Combustion (CFBC). Firstly, a three-dimensional (3D) full-loop simulation is carried out for a semi-industry scale CFB in chapter 3. The results manifest the powerful capability of the EMMS/Matrix model in predicting the hydrodynamics in a CFB. Further, a 3D full-loop simulation of a 150 MWe CFB boiler is carried out in chapter 4. Based on the above hydrodynamic simulations, the reduced EMMS/mass model is incorporated into the mass conservation equations for combustion species, and a 3D combustion simulation is realized for the furnace part of the 150 MWe CFB boiler in chapter 5. Chapter 6 summarizes the main achievements in this thesis, and discusses future work on meso-scale modeling.
页数126
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/1516
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张楠. 基于EMMS的介尺度传质模型及其在循环流化床锅炉燃烧模拟中的应用[D]. 北京. 中国科学院研究生院,2010.
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