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Thesis Advisor王维
Degree Grantor中国科学院研究生院
Degree Discipline化学工程
Keyword广义流态化   气固两相流   漂移流   细网格模拟   介尺度结构   能量最小多尺度   聚团   气泡   多尺度cfd   数学建模
Abstract欧拉两相流模型经常用于分析气固流动系统。在本工作中,首先进行了关联长度的分析,使用一般曳力(如Wen-Yu曳力)的双流体模型定量分析了气固系统的特征关联长度。结果表明,不论颗粒浓度如何,介尺度结构在周期边界条件下均会出现,而且特征关联长度具有拐点的现象。 在实践中,气固系统可运行于多种方式,而它们都展示出了一定的介尺度非均匀性。这些非均匀结构影响了系统整体的性能。为了精确的模拟与分析气固系统,亚网格的介尺度非均匀性必须考虑。文中利用漂移通量的方法评估了不同的曳力模型。结果表明,能量最小多尺度(EMMS)方法得到的曳力系数是本征的凹函数,因而可以预测双态共存,并且对于网格分辨率的依赖很弱。之后,将EMMS模型与通用的双流体模型相结合在双周期边界条件下,分析了不同的广义气固流动系统(如提升管、下降管和湍动床)。并与实验结果进行了对比,发现符合较好。 最后,基于对气固流化床中气泡与聚团现象的认识,提出了一个基于结构的混合模型,初步模拟结果显示与实验较吻合,对于这种方法的网格依赖问题也进行了初步测试。
Other AbstractEulerian two-fluid model is used frequently to analyze gas-solid flow systems. In this work, first a correlation length analysis has been performed to quantify the characteristic length scale of such a gas-solid system using the two fluid model with the commonly used drag model i.e. Wen-Yu. The results reveal that meso-scale structures are present in both dense and dilute periodic systems. Moreover, the correlation length shows a turning point behavior. In real practice, gas-solid systems can operate in a variety of ways. All these systems display some level of meso-scale heterogeneity. These heterogeneous structures affect the overall performance of these systems. In order to accurately model and analyze these types of systems, effects of the sub-grid meso-scale heterogeneities must be taken into account. To asses the capabilities of several drag models, they were evaluated by the drift flux approach. It was found that the slip velocity curve of the Energy Minimization Multi-scale (EMMS) model was concave by nature and thus assured the existence of bi-stable states. Moreover, it was found to be very weakly dependent upon grid resolution. The EMMS drag model was then implemented with the commonly used two-fluid model in doubly periodic domains to analyze a variety of gas-solid flow systems such as risers, downers and turbulent beds. To do this, the approach of generalized fluidization introduced by Prof. Kwauk was followed. Experimental validation has also been carried out with reasonable agreement. Finally, based upon general experimental observations for gas-solid fluidized beds, a new structure dependent mixture model has been proposed towards the end. Preliminary results of the model are presented and its grid dependency is also tested.
Document Type学位论文
Recommended Citation
GB/T 7714
ATTA ULLAH. 广义气固流态化系统中的非均匀性研究[D]. 中国科学院研究生院,2013.
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