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基于EMMS 模型的CFD-PBM 方法模拟液-液流动体系
Thesis Advisor杨宁
Degree Grantor中国科学院研究生院
Place of Conferral北京
Degree Discipline化学工程
Keyword转子 -定子设备 破碎 计算流体力学 群平衡模拟 多尺度

转子-定子搅拌器被广泛应用在混合、多分散和乳化体系以获得所需的离散相粒径分布。离散相粒径分布是影响产品功能和质量的重要标准之一,通过优化设备结构和流动过程达到精确控制离散相粒径分布是工业上亟待解决的问题。近年来,越来越多的研究者用计算流体力学(Computational Fluid Dynamics, CFD)耦合群平衡模型(Population Balance Model, PBM)来模拟转子-定子体系的复杂湍流和离散相粒径分布。由于现阶段研究尚未对聚并或破碎过程有透彻的物理背景认识,所以现有的PBM破碎和聚并模型大多是统计模型、现象模型或者经验关联式。我们用CFD-PBM方法模拟了离散相弱聚并的Megatron 3-48转子-定子体系。通过对流场中湍动能耗散率的定量分析,结合理论推测出了系统中离散相破碎主要的场所;而模拟的出口离散相粒径分布与实验结果相比,基于液液搅拌槽体系的Alopaeus破碎速率模型严重低估了离散相的破碎速率。我们以能量最小多尺度(Energy Minimization Multi-Scale, EMMS)理论为指导,根据气液鼓泡塔体系的EMMS-PBM方法,提出了适用于液液体系的破碎或聚并速率的修正方法。该方法通过将系统能耗进行多尺度解析,利用介尺度能耗作为EMMS和PBM的结合点导出破碎速率的修正因子。经过EMMS-PBM方法的修正,模拟出的离散相粒径分布能够更好地符合实验数据,极大地改进了Alopaeus模型在转子-定子体系中对破碎的预测能力,表明EMMS-PBM方法具有良好的应用前景。

Other Abstract

Rotor-Stator mixing devices (RS) have found widespread application in mixing, dispersion and emulsification processes to acquire the desirable droplet size distribution (DSD) which is critical to the function and quality of products. Thus the precise control of DSD through the rational design and optimization of process or formulation is highly demanded. Computational fluid dynamics (CFD) becomes increasingly important to simulate the complex turbulence flow in RS devices which has significant impact on the final DSD. CFD can also be integrated with population balance equations (PBE) to predict the DSD as long as the breakage and coalescence rates could be accurately modeled by the kernel functions. However, the current kernel functions for droplet breakage and coalescence in population balance modeling (PBM) are either derived from statistical models or based on some phenomenological models or empirical correlations with adjustable parameters, since the underlying physics of droplet breakage and coalescence is complex and far from being well understood. We simulated the liquid-liquid two-phase flow and DSD for a weak coalescence emulsification system in a Megatron RS mixer with the CFD-PBM approach. The turbulence dissipation rate in the flow field was analyzed quantitatively, showing the regions where droplets are more likely to break up. The comparison of simulated DSDs and experimental results suggested that the Alopaeus breakage kernel predicted much larger droplet sizes and less droplet breakage than experimental data, although this kernel model was proposed for liquid-liquid mixing systems and took both the interfacial tension and viscous stress into account. We then tentatively proposed a novel approach for correcting the breakage kernels based on the Energy-Minimization Multi-Scale (EMMS) concept. This method features the multi-scale resolution of energy dissipation, and utilizes the so-called meso-scale energy dissipation to derive a correction factor for the breakage rate for PBE. The results showed that the new model can greatly improve the CFD-PBM simulation, and the DSD predicted was in good agreement with experiments, demonstrating the rationality and potential of this new approach. 

Document Type学位论文
Recommended Citation
GB/T 7714
覃成鹏. 基于EMMS 模型的CFD-PBM 方法模拟液-液流动体系[D]. 北京. 中国科学院研究生院,2016.
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