CAS OpenIR
基于格子Boltzmann方法的固-液搅拌槽直接数值模拟研究
许伟程
Subtype博士
Thesis Advisor杨超
2019-07-01
Degree Grantor中国科学院大学
Degree Discipline化学工程
Keyword固-液搅拌槽,格子boltzmann方法,全求解直接数值模拟,颗粒动力学,疏水微通道
Abstract

搅拌槽作为常见的单元操作设备在工业上有广泛的运用,其中固-液体系搅拌槽常用于化工、冶金、矿业、医药、环保等领域。固-液搅拌槽中颗粒动力学的研究对搅拌槽的设计、放大以及操作优化至关重要。该领域的研究主要针对固液混合特性和颗粒动力学两个方面的关键问题。其中,固液混合特性主要关注颗粒在搅拌槽中的运动规律,包括重质颗粒的悬浮、轻质颗粒的下拉,以及颗粒在搅拌槽中的聚集现象等。这一问题的研究与搅拌槽在工业上的应用密切相关。颗粒动力学则着眼于颗粒在搅拌槽中的受力情况,搅拌槽中颗粒动力学描述准确与否将直接影响搅拌槽数值模拟结果的正确性。本文将针对上述两个问题,重点研究颗粒聚集现象和搅拌槽中的颗粒动力学。目前二者面临的共同难点在于,实验和传统模拟方法在处理两个问题时均存在较大的局限。对于颗粒聚集现象,传统的E-E和E-L模拟方法无法处理体系中的有限体积颗粒,实验则受限于实验材料无法对各个变量的影响进行系统的考察。至于颗粒动力学的研究,实验和传统的模拟方法均无法测量或计算得到搅拌槽中颗粒的真实受力,也无从对其进行分析。因此,由于缺少合适的研究方法,对这两个问题的认识还十分有限。近年来,基于格子Boltzmann方法的全求解的直接数值模拟迅速发展,为搅拌槽的相关研究提供了新的路径,从而可以实现对搅拌槽中颗粒聚集和动力学的更深入的研究。有鉴于此,本文利用格子Boltzmann方法对固-液搅拌槽进行模拟,并在此基础上对搅拌槽中的颗粒聚集现象和颗粒动力学进行了系统的研究。具体工作及结论如下:(1)作为格子Boltzmann方法的运用,第一部分工作中利用该方法耦合Shan-Chen模型,研究了通道形状及壁面性质对微通道中液相流动的影响。首先,研究了缩口疏水微通道中二者对通量的影响。结果表明,在缩口微通道中,疏水壁面仍会提高通道的流体通过能力,且流量与滑移长度成线性正相关。其次,倾斜疏水壁面的存在还会改变通道中的流型,使疏水微通道中的液相流量呈现出独特的变化规律。此外,还对组合壁面性质的微通道进行了模拟。模拟结果表明,疏水直通道中的流量与疏水壁面的长度线性相关,壁面性质的交错形式对流量的影响较小。在缩口通道中也存在类似的规律,更多的疏水段也更有利于流体的通过。(2)在上述工作的基础上,进一步结合浸没边界法和硬球模型建立了搅拌槽的直接数值模拟程序框架。其中,浸没边界法用于模拟桨叶和颗粒的固壁边界,硬球模型则用于处理颗粒与不同固壁边界发生的碰撞事件。进而对低Reynolds数搅拌槽中的颗粒聚集现象进行了系统模拟,先后考察了颗粒初始位置、颗粒密度、颗粒直径、以及搅拌槽Reynolds数对搅拌槽中颗粒运动的影响。模拟结果表明,搅拌槽Reynolds数小于100的体系中颗粒有其特有的平衡区域,颗粒聚集现象的实质是颗粒群中具有相同性质的颗粒进入同样的稳定运动轨道。不同初始位置的颗粒均存在往平衡区域运动的趋势,但距离平衡区域较远的颗粒需要较长的诱导时间。颗粒平衡区域的位置与颗粒密度和搅拌槽Reynolds数相关。颗粒密度较小或搅拌槽Reynolds数较高的体系中,平衡区域将更靠近液相流动的涡的位置。反之亦然。当桨叶转速过快时,搅拌槽中的流动湍动较大,流体和颗粒运动呈现出一定的随机性,此时不会发生颗粒的聚集。另外,在本文研究的粒径范围内,粒径对平衡区域影响较小。(3)在前两项工作的基础上,将研究对象拓展含更多颗粒的体系,用以研究颗粒群在搅拌槽中的颗粒动力学行为。采用对模拟结果进行系综平均的方法,基于搅拌槽中流动的拟稳态假设对Eulerian-Eulerian方程进行简化,提出搅拌槽中的相间作用力分析方法。通过对比数值模拟方法得到的相间作用力和颗粒相的随体导数,验证假设的合理性和方法的可行性。在此基础上对搅拌槽中的压力梯度力以及曳力对颗粒运动的影响进行了探究。结果表明,搅拌槽中的压力分布与颗粒相性质无关,与搅拌槽Reynolds数相关,但压力梯度力对颗粒运动的影响程度却主要取决于颗粒性质,与搅拌槽Reynolds数无关。搅拌槽中颗粒受到的曳力与固液密度比、局部颗粒Reynolds数相关,随着局部颗粒Reynolds数的增加,颗粒受到的曳力呈现出先增加后平稳的变化趋势。根据模拟结果,本文提出了考虑固液密度比和局部湍动能影响的曳力模型。;Solid-liquid stirred tanks are widely used in chemical, metallurgical, pharmaceutical industry and environmental engineering. The study of particle dynamics in solid-liquid stirred tank is critical to its design, scale-up and operation optimization. The study of solid-liquid stirred tanks mainly includes two interrelated scientific problems: solid-liquid mixing characteristics and particle dynamics. To be more specific, the solid-liquid mixing characteristics mainly focus on the movement of particles in a stirred tank, including the suspension of heavy particles, the drawn-down of light particles and the aggregation of particles in a stirred tank. This kind of study is very significant to the use of stirred tanks in engineering; And the study of particle dynamics focuses on the interaction forces exerted on particles in a stirred tank, which provides important information for stirred tank simulation.For the research problems mentioned in the above, this thesis mainly focuses on the phenomenon of particles aggregation and particle dynamics in the stirred tank, because we notice that these two problems have the common ground that the experiment and traditional simulation methods have encountered great difficulties in studying these two problems. The lack of proper research methods leads to few studies about these subjects reported, and many technical problems still need to be solved. Recently, as the development of the lattice Boltzmann method (LBM), a novel high resolution direct numerical simulation was proposed to simulate the solid-liquid stirred tank, which is very suitable for studying these two scientific problems in stirred tanks. Therefore, the main idea of this work is to simulate the solid-liquid stirred tank based on the lattice Boltzmann method, aiming at a systematical study of the particle aggregation phenomenon and particle dynamics in a stirred tank. The specific scope and results are as follows:(1) In Chapter 2, as an application of LBM, the influence of microchannel shape and wall properties on the flow flux is studied by LBM coupled with the Shan-Chen model for modeling different hydrophobic channel walls. Firstly, the effect of channel shape and wall properties on the flux in the variable cross-section microchannel is studied. The results show that the hydrophobic surface in the microchannel can increase the fluid flux through the channel, and the flow flux is linearly and positively correlated with the slip length. Moreover, the presence of the inclined hydrophobic wall changes the flow pattern in the channel, so that the flux variation law in the hydrophobic microchannel exhibits a different tendency from a macrochannel. Subsequently, the microchannels with wetting and non-wetting walls are simulated. The results show that the flow flux in a straight hydrophobic channel is linearly related to the length of hydrophobic wall, and has little relation to the staggered arrangement of surface properties. A similar result can be found in the variable cross-section channel, and more non-wetting surface also can improve the flux in such channels.(2) Based on the LBM simulation of single-phase flow mentioned above, a direct numerical simulation program for a stirred tank is established by combining the immersed boundary method and the hard sphere model. The immersed boundary method is used to simulate the solid wall of impeller and particles. The hard sphere model is used here to deal with the collision events of a particle with other particles and solid wall. On this basis, the particle aggregation phenomenon in the low Reynolds number agitation tank is simulated. The effects of initial position of the particles, particle density, particle diameter and stirred tank Reynolds number on particle motion are investigated. The simulation results show that the particles in low Reynolds number agitation tank have their unique equilibrium regions. And the essence of the particle aggregation is that each individual particle in particle swarm approaches gradually the same stable orbit of motion. Particles at different initial positions all have the tendency to move toward to the same equilibrium region, but the particles far from this region require a longer induction time. The location of equilibrium region is related to the particle density and stirred tank Reynolds number. The light particle and high Reynolds number induce an equilibrium region, which is close to the center of vortex of liquid circulation flow, and vice versa. Besides, when the rotation speed of impeller is too fast, the flow in the stirred tank will fluctuate violently, the movement of fluid and particles exhibits more and stronger random fluctuations, and the aggregation of particles does not occur at this condition. In addition, within the particle size range studied in this paper, the particle size has little influence on the equilibrium region.(3) Base on the direct numerical simulation program for stirred tanks developed above, we extend it to simulate a system with more particles for studying the particle dynamics in a stirred tank. Based on the assumption of quasi-steady flow in a stirred tank, the simplified Eulerian-Eulerian equation and the ensemble averaged simulation results are used to analyze the interaction forces exerted on particles in a stirred tank. By comparing the interaction force calculated by the immersed boundary method and the material derivative of solid phase, the rationality of the hypothesis and feasibility of this analysis method are verified. Then the pressure gradient force and drag force in a stirred tank can be obtained through simulation. And the results show that the pressure distribution in a stirred tank is independent from the solid phase properties, but related to the stirred tank Reynolds number. It is interesting to find that the degree of influence of pressure gradient force on the particle motion depends mainly on the particle properties. The drag force in a stirred tank is related to the solid-liquid density ratio and the local particle Reynolds number. With the increase of local particle Reynolds number, the drag force increases firstly and then tends to an asymptotic value. Based on the drag force simulation results, an empirical correlation for stirred tanks is proposed by considering the influence of solid-liquid density ratio, local particle Reynolds number and local liquid energy dissipation. 

Language中文
Document Type学位论文
Identifierhttp://ir.ipe.ac.cn/handle/122111/40656
Collection中国科学院过程工程研究所
Recommended Citation
GB/T 7714
许伟程. 基于格子Boltzmann方法的固-液搅拌槽直接数值模拟研究[D]. 中国科学院大学,2019.
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