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Alternative TitleA two-phase structure-dependent multi-fluid model and its application in simulation of gas-solid fluidization
Thesis Advisor李静海
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword流态化   气固两相流   双流体模型   细网格模拟   介尺度结构   emms   团聚物   气泡   多尺度cfd   结构多流体模型   数学建模
Abstract气固流态化是典型的非线性非平衡系统,呈现出复杂的时空多尺度结构。其流动形式具体可表现为气体富集的稀相与颗粒聚集的密相共存,并伴有复杂的动态演化,甚至在某些临界条件下会发生突变或转折性变化。气泡和颗粒聚团是流态化系统中两种较有代表性的介尺度结构。这些结构对流动、热/质传递及反应等过程都有重要的影响,因而吸引了许多研究者通过计算流体力学(Computational Fluid Dynamics, CFD)方法来预测介尺度结构及其影响。现有文献中,对经典的双流体模型(Two-Fluid Model, TFM)细网格模拟能否准确描述非均匀气固流系统仍存在争议。 针对上述争议,论文第二章首先用传统的TFM对含有A类颗粒的鼓泡、湍动和循环流化床进行全面的二维细网格模拟。研究结果发现:(1)对鼓泡流化,网格细化预测的膨胀高度逐渐降低且趋于收敛,与实验值较接近;(2)而对湍动和循环流化,网格细化后的轴向颗粒分布与实验相差较大,特别是对循环流化床,预测的颗粒通量仍然远高于实验值。基于上述结果,我们认为单纯细化网格并不可行,必须合理量化结构(或考虑结构的影响)才有可能准确捕捉到多尺度现象。 论文第三章从颗粒团聚的机理出发,建立了一套含结构的基本方程,即结构多流体模型(Strcuture-dependent multi-Fluid Model, SFM)。其与传统TFM的区别在于考虑了结构对曳力、压力和应力等的影响,在网格内均匀分布假设条件下可以退化到TFM形式。而描述稳态和流化床整体行为时,则退成为EMMS(能量最小多尺度)模型的平衡方程。SFM的求解可以由TFM与EMMS曳力耦合来简化实现。这也从根源上合理地阐释了多尺度CFD方法的合理性,并将TFM和EMMS在底层相统一。 相比循环床中无定型的团聚物,鼓泡床中较规则的气泡更易于刻划与描述,有丰富的实验数据和关联式。因此,论文第四章从气泡出发,推导出类似的结构多流体模型(SFM)。它可以简化导出基于气泡的EMMS模型。因此,在SFM框架下,统一了基于聚团和基于气泡的两种不同介尺度描述下的EMMS模型,并指出SFM的关键在于介尺度曳力(Fdi),不同Fdi封闭模式有可能导致更多形式不同的EMMS模式。在此基础之上,完善和发展了基于气泡的EMMS模型,并进行了流化床模拟验证。结果初步表明,新的模型可以拓展应用到更宽的流域范围(从鼓泡、湍动到循环流化)。 第五章总结了本论文的主要成果和创新点,展望了基于结构的建模方法在其它体系中的应用前景,以及将来需要完善的研究内容。
Other AbstractGas-solid fluidization is a typical non-linear, non-equilibrium system, displaying complex structures over a wide range of scales with respect to time and space. The flow behavior manifests a coexistence of gas-rich dilute phase and particle-rich dense phase, which is accompanied with dynamic evolution and, under certain critical condition, sudden regime transition. The bubble and cluster are two typical forms of the meso-scale structures in a gas-solid fluidization. These structures have significant impacts on the flow behavior, mass/heat transfer and reaction, and hence have attracted many researches using CFD (Computational Fluid Dynamics) approach to investigate their effects. In literature, however, there still exists some arguments as to the feasibility of using traditional TFM (Two-Fluid Model) in investigating meso-scale structures. In Chapter 2, a series of fine-grid simulations using traditional TFM are performed for bubbling, turbulent and circulating fliudized beds with Geldart A particles. It is found that with grid refining the expansion height gradually reduces and converges to experimental value for bubbling fluidization. However, the profiles of axial solids concentration have big deviation in comparison with experiment for turbulent and circulating fluidization. For circulating fluidized bed, in particular, fine-grid resolution may help to predict the S-shape axial profile, though the predicted solids flux is still much larger than experimental data. According to the above results, it is possible that fine-grid TFM simulation is insufficient for predicting meso-scale structures of gas-solid flows. We need meso-scale modeling of heterogeneous structures to capture multi-scale behaviors. In Chaper 3, a structure-dependent multi-fluid model (SFM) is proposed. It is pointed out that the difference between the TFM and the reduced SFM only lies in the formulation of the stress, drag force and diffusion stress. SFM may reduce to the TFM if homogeneous distribution is assumed within each grid. Additionally, the balance equations of the EMMS (Energy-Minimization Multi-Scale) model can also be derived from SFM if they are used to describe steady-state, global reactor. Thus, the simplified solution of SFM can be realized through coupling of TFM and EMMS drag. In all, these efforts fundamentally explain the rationality of Multi-Scale CFD, and unify the TFM and EMMS models. Compared to cluster, study of bubble arouses less disputes and hence is easier to characterize with rich experimental data and correlations in literature. In Chapter 4, we further propose the Structure-dependent multi-Fluid Model (SFM) based on bubbles. The SFM may revert to the bubble-based EMMS model. Thus, both of the cluster-based and bubble-based EMMS models can be unfied under the umbrella of SFM according to different description of meso-scale structures. It is also noted that the key of SFM is how to close the meso-scale drag (Fdi), and different closures of Fdi may result in more forms of EMMS model. Furthermore, we improve and evaluate the bubble-based EMMS model with comparison to experimental data. The numerical results manifest that the new version of bubble-based EMMS model can be applied to wider range of flow regimes, from bubbling, turbulent to circulating fluidization. In Chapter 5, we summarize the main achievements in this thesis, and present the prospect of structure-dependent modeling approach in other systems and the future works on the SFM.
Document Type学位论文
Recommended Citation
GB/T 7714
洪坤. 基于两相结构的多流体模型及其在气固流态化模拟中的应用[D]. 中国科学院研究生院,2013.
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