CAS OpenIR
复杂气固流动的EMMS建模
赵梦
Subtype博士
Thesis Advisor葛蔚
2019-07-01
Degree Grantor中国科学院大学
Degree Discipline化学工程
KeywordEmms,变径流化床,气固流动,稳态建模 ,流动密封阀
Abstract

随着日益严格的环境要求和逐渐丰富的产品需求,过程工艺在反应器结构和操作方式上逐渐呈现多样化和复杂化,同一工艺中的各反应器或同一反应器中的各反应区段的构型、尺寸以及操作模式都有可能不同。其中的非均匀动态结构对于单个反应器甚至整个工艺的流动、传递和反应过程都有着重要的影响。能量最小多尺度(EMMS)模型经过三十多年的发展,已经在气固快速床、气固鼓泡床、气液、气液固三相等系统中得到了成功的应用。以“先整体分布、后局部模拟、再细节演化”为特征的EMMS多尺度计算模式推进了虚拟过程工程的实现。本论文基于EMMS理论,通过全局动力学的建模及验证并与CFD模拟耦合来研究具有操作条件复杂性的流动密封阀和具有几何结构复杂性的变径流化床内的动力学规律,从而完善复杂气固系统的全系统稳态建模理论。针对流动密封阀体系,先通过稳态方法对流动密封阀内各区域的宏观流体动力学特征进行理论预测,其中刘新华等[1]提出的改进的EMMS气固鼓泡理论模型的预测精度明显优于现有经验关联式的结果。然后将此气固EMMS鼓泡理论模型通过修正曳力系数的方式与双流体模型耦合,实现对流动密封阀更精细的非稳态模拟研究。模拟考察了流动密封阀返料室、供给室和立管各区域内的非稳态时空多尺度流动特征,模拟结果与实验数据吻合良好。基于非稳态模拟总结了操作参数(如操作气速)变化对流动密封阀不同区域流动的影响规律。针对变径流化床体系,考虑操作参数随高度连续变化这一特点,在轴向EMMS模型中引入团聚物动态演化方程,建立了适用于变径流化床的轴向EMMS稳态模型,将其与径向EMMS稳态模型耦合,实现了变径流化床轴径向流动特征的稳态预测,预测结果与实验数据吻合较好。另外,本研究还分别考察了结构参数和操作参数对变径流化床轴径向分布的影响,结果表明改变结构参数对提升管和过渡段轴向空隙率的影响远大于改变操作参数的影响。在渐缩和渐扩两种类型的变径提升管中均可以捕捉到径向空隙率呈现环核分布结构,并在渐缩提升管底部壁面附近和渐扩提升管顶部壁面附近观察到有颗粒向下移动的现象。在上述稳态模型的基础上,论文采用三维瞬态欧拉-欧拉模型,并耦合不同的曳力模型研究了渐缩和渐扩提升管的瞬态和时均非均匀结构特征。研究发现提升管直径变化导致其内流动参数(如表观气速)实时变化,造成采用基于全床平均流动参数的传统均匀曳力和平均EMMS曳力的模拟结果误差较大。由此本研究提出用考虑轴向流动参数差异的插值EMMS曳力来模拟渐缩/渐扩提升管,模拟结果比传统均匀曳力和平均EMMS曳力的结果更准确,且与实验数据吻合更好。基于此模拟方法,进一步考察了不同操作参数和结构参数条件下,渐缩和渐扩提升管内气固流动特性及演变规律。然后采用CFD-DEM方法耦合EMMS曳力分别对渐缩过渡段和渐扩过渡段进行了精细的模拟,其非稳态模拟时均结果与稳态模拟结果相吻合。与变径CFB提升管的轴向空隙率分布相似,变径过渡段也存在复杂的轴向非均匀分布。最后,论文总结了本研究的主要工作、结论、创新点,并对该方向未来发展提出了一些展望。;With the increasingly stringent environmental requirements and abundant product diversification, process technology is becoming further diversified and complex in reactor structure and operation mode. Novel reactors are designed for various objectives such as high volumetric yield, minimum by product production, less energy consumption and minimal impact on the environment. The configuration, size and operation mode of different reactors in the same process or the sections in the same reactor may be different, with the heterogeneous structures playing an important role in the flow, transfer and reaction process of a single reactor or even the whole process. After three decades of development and evolution, the Energy-Minimization Multi-Scale (EMMS) model has been successfully applied to gas-solid fast fluidized bed, gas-solid bubbling fluidized bed, gas-liquid two-phase and gas-liquid-solid three-phase systems. The EMMS multi-scale computing paradigm characterized by "first global distribution, then local simulation, and last detailed evolution" promotes the realization of virtual process engineering (VPE). In this thesis, based on the EMMS theory for heterogeneous structure, steady –state modeling and unsteady-state simulations were carried out for the dynamics of the loop seal with complex operating conditions and the tapered fluidized bed with geometric complexity, so as to perfect the full-loop steady-state modeling method of gas-solid systems.For the loop seal, various steady-state methods were used to predict theoretically the macro-hydrodynamic characteristics of each region in the loop seal. Among them, the improved gas-solid EMMS bubbling model proposed by Liu et al.[1] can effectively improve the accuracy of the predictions than that of the existing empirical correlations. Then a heterogeneity index was calculated from an improved EMMS bubbling model to measure the interphase drag coefficient and further integrated into the two-fluid-model (TFM) approach to simulate the loop seal for a circulating fluidized bed (CFB) system. This index was dependent on superficial gas velocity in bubbling fluidization, so a region-specific drag correction scheme is proposed to allow the application of various heterogeneity index correlations to different zones of the loop seal, since superficial gas velocity may differ much from the recycle to supply chamber. The simulation results were in good agreement with the experimental data. Based on the unsteady state simulation, the effects of operating parameters on the flow in different regions of the loop seal were summarized.For the tapered bed, considering the operating parameters varying continuously with height, the axial EMMS model by introducing a general cluster evolution equation was upgraded and then, together with the radial EMMS model, was used to predict the hydrodynamics of tapered fluidized bed. The predicted results were in good agreement with the experimental data. Depending on the operating and configuration parameters, voidage may increase monotonously with height, or increase firstly but then decrease to be even lower than the entrance voidage in the tapered-in or tapered-out bed. The axial voidage profiles can be influenced much more significantly by the wall inclined angle than by operating conditions in the tapered fluidized bed. In addition, radial voidage distribution may take on a typical core-annulus structure in both the tapered risers, and the particles even become downward moving near the bottom wall in tapered-out riser or the top wall in tapered-in riser.Then, three-dimensional gas-solid flow in tapered-out and tapered-in risers was simulated by the two-fluid model using an improved structure-dependent drag based on the EMMS model. The EMMS model was solved at different axial levels to determine different correlations of heterogeneity index with voidage, which were then interpolated between these levels to improve the prediction of varying interphase drag in the tapered risers. Considering the axial variation of the EMMS drag, the simulation predicts much more reasonable flow dynamics in the tapered risers than those coupled with an average EMMS drag or homogeneous drag laws, and is in good agreement with the experimental data. Based on this simulation method, the axial and radial heterogeneities as well as the parametric effects on the flow dynamics in the tapered risers were further discussed in detail. Then, a so-called CFD-DEM method coupling with the EMMS drag law was used to simulate the transition sections of tapered fluidized beds at higher resolution than traditional continuum methods. The results of the unsteady simulation are in good agreement with those of the steady-state simulation. Similar to the tapered risers, axial non-uniform distribution also exists in the transition sections.Finally, the main results, conclusions and novelties are summarized in the thesis and some aspects of future development are put forward in this direction.

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40685
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
赵梦. 复杂气固流动的EMMS建模[D]. 中国科学院大学,2019.
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