搅拌釜三维流场的雷诺应力模型数值模拟

引言 近年来,雷诺平均Navier-Stokes方程及其封闭方程组成的湍流模型在应用于搅拌釜复杂湍流现象的模拟上取得了成功,主要是采用标准k-ε[1]、RNG k-ε[2]等模型.     

双层桨搅拌假塑性流体混合特性的数值模拟

在直径为0.21 m的搅拌槽内,采用计算流体力学的方法,工作介质为质量分数1.0%的假塑性流体和1.5%的黄原胶水溶液,分别对错位六弯叶桨、六弯叶桨与45°三斜叶桨组成的双层桨混合性能进行了数值模拟,并做了对比分析,考察了层间距对搅拌流场的影响,探讨了不同形式的组合桨对混合特性的影响,确定了不同搅拌转速下较为适宜的层间距,并将错位六弯叶组合桨形式应用于FCC催化剂成胶搅拌过程。结果表明:在层流状态下,6PBT组合桨层间距为时可形成层间连接流,在过渡流区域时,层间距保持在左右比较适宜;6PBT组合桨在混合速率和混合效率方面都有一定优势。根据中试实验结果,催化剂颗粒耐磨损指数由2.6提高到了1.9,产品质量得到进一步提高,且节能在15%左右,因此是一种值得推广的双层组合搅拌器形式。     

侧进式搅拌釜内气液两相流的数值模拟

采用计算流体力学(CFD)技术对φ1.5 m×1.2 m侧进式气液搅拌釜内气液两相流场进行数值模拟,检验了3种气液分界面边界条件和两种相间曳力模型。通过UDF程序将上述模型分别与欧拉双流体模型和 dispersed k-ε 两相湍流模型进行耦合计算,得到搅拌功率准数、总体气含率和气相分布,并与冷模实验结果进行对比,得到能准确预测的CFD模型。研究结果表明,3种气液界面边界条件下采用标准S-N模型计算所得的功率准数和气体分布误差均较大,而Brucato-Tsuchiya模型的预测结果更接近实验结果;气液界面边界条件对总体气含率的预测影响较大,采用速度进口或脱气边界和Brucato-Tsuchiya模型耦合计算所得的结果误差比压力出口边界明显要小。     

错位六弯叶桨搅拌假塑性流体流场宏观不稳定性数值模拟

采用分离涡模型,对错位六弯叶（6PBT）搅拌槽内流场结构和宏观不稳定性（MI）进行了数值模拟。工作介质分别选用去离子水和不同质量分数的黄原胶水溶液,并将水的速度场分布与PIV实验结果进行了比较。通过采集监测点的速度时间序列,结合MATLAB软件编程,计算得到流场宏观不稳定频率的变化。结果表明：流场结构和速度矢量的计算值与PIV实验数据吻合较好,分离涡模型的计算结果可靠;提高转速,槽内假塑性流体MI频率峰值增大,脉动强度提高,当转速达到225 r·min^-1时,MI频率特征消失,频谱图呈现谱带现象,意味着流场进入混沌;流体的流变性对MI没有影响,MI现象是流体流动的共有特征。     

组合桨搅拌槽内高黏流体混合特性的数值模拟

利用计算流体力学数值模拟方法,对直段桨为双螺带式和Paravisc式以及底桨为锚式和变径双螺带式的组合桨功率和混合时间进行了研究,并对其中最优桨型进行了放大规律的探索。通过数值模拟得到了4种组合桨中各单桨及组合桨的功率准数关联式,提出组合桨中各桨之间功率上的反作用关系。在10种组合桨中,Paravisc-锚式组合桨达到完全混合时所转圈数最少,且在相同转速下量纲一剪切量较大,混合特性最优,将其从体积为100 L的搅拌槽内放大至200 L和500 L搅拌槽中,发现其符合桨端线速度的放大准则。文中对各桨型的功率特性、混合特性和放大规律的分析,可为工业设计和优化高黏度流体组合桨参数提供重要参考。     

Numerical simulation of a pitched‐blade turbine stirred tank with mirror fluid method

Mirror fluid method [Yang and Mao, Phys. Rev. 2005; E 71:036704] combined with local grid refinement is proposed to deal with the numerical simulation of turbulent flow in a pitched‐blade turbine stirred tank. By such a novel method, the domain occupied by the impeller is assigned suitable flow parameters explicitly by the mirror relation, so that the correct shear and normal forces on the fluid side of an interface segment is eventually guaranteed. Satisfactory agreement between our predictions and the reported experimental data is achieved both in single‐phase baffled or unbaffled stirred tanks and solid–liquid two‐phase systems. © 2012 Canadian Society for Chemical Engineering     

Numerical simulation of a dissolution process in a stirred tank reactor

A dissolution process of solid particles suspended in a turbulent flow of a Rushton turbine stirred tank is studied numerically by large eddy simulations including passive scalar transport and particle tracking. The lattice-Boltzmann flow solver and the Smagorinsky subgrid-scale model are adopted for solving the stirred tank flow. To the LES a finite volume scheme is coupled that solves the convection-diffusion equation for the solute. The solid particles are tracked in the Eulerian flow field through solving the dynamic equations of linear and rotational motion of the particles. Particle-particle and particle-wall collisions are included, and the particle transport code is two-way coupled. The simulation has been restricted to a lab-scale tank with a volume equal to . A set of 7×10⁶ spherical particles in diameter are released in the top part of the tank (10% of the tank volume), resulting in a local initial solids volume fraction of 10%. The particle properties are such that they resemble those of calcium chloride beads. The focus is on solids and scalar concentration distributions, particle size distributions, and the dissolution time. For the particular process considered, the dissolution time is found to be at most one order of magnitude larger than the time needed to fully disperse the solids throughout the tank.     

大型侧进式搅拌釜内湍流流场的数值模拟

采用计算流体力学(CFD)技术对直径和高度均为13 m的大型侧进式搅拌釜内均相宏观流场进行数值计算。结果表明,将计算域划分为大约90万网格时,计算得到的搅拌功率曲线与实验数据吻合较好;考察不同操作转速、搅拌桨安装角度及个数对釜内低速死区分布的影响,发现增大搅拌转速很难有效地消除水平面上的死区;搅拌桨垂直向下5.71°或水平偏转11°安装能明显改善流体运动。三桨和四桨搅拌体系对釜上部流场的优化要好于两桨体系;但在相同转速下,双桨、三桨和四桨搅拌釜的搅拌功耗分别是单桨搅拌釜的1.2倍、2.3倍和3.4倍。综合考虑,三桨体系搅拌效率较高。最后采用组分模型计算得到不同转速下三桨釜的混合时间。     

Numerical evaluation of mass transfer coefficient in stirred tank reactors with non-Newtonian fluid

In fermentation processes, a constant supply of oxygen is fundamental for cell growth. The supply rate is controlled by the volumetric mass transfer coefficient. The literature reports few numerical studies evaluating the volumetric mass transfer coefficient for aerated systems with non-Newtonian fluids in stirred tanks. The aim of this work was to undertake a numerical study of the main hydrodynamic and mass transfer parameters, including average gas hold-up, and power number. Xanthan gum solutions were used to simulated. The simulations were performed with different impeller rotational speeds (600 to 1000 revolution per minute) and specific gas flow rates (0.4 to 1.2 volume of gas per volume of liquid per minute), adopting an Euler-Euler approach and assuming uniform spherical bubbles. The turbulence was simulated with k?ε turbulence model and sst shear stress transport turbulent model. The numerical results were compared with experimental values available in the literature. The results showed good agreement between the numerical and experimental values of gas hold-up, power number, and volumetric mass transfer coefficient. The sst shear stress transport turbulence model provided better results, compared to the standard k?ε model, for simulation of volumetric mass transfer coefficient in a non-Newtonian fluid under the conditions used. Simulations for uniform bubbles with 3 millimeters diameter gave mass transfer coefficient values that were close to the experimental data.     

Computational fluid dynamics-discrete element method simulation of stirred tank reactor for graphene production

Liquid phase exfoliation(LPE) process for graphene production is usually carried out in stirred tank reactor and the interactions between the solvent and the graphite particles are important as to improve the production efficiency. In this paper, these interactions were revealed by computational fluid dynamics–discrete element method(CFD-DEM) method. Based on simulation results, both liquid phase flow hydrodynamics and particle motion behavior have been analyzed, which gave the general informati...     

半圆管曲面涡轮搅拌槽内混合特性的数值模拟

在商业化软件ANSYS CFX 10.0平台上,采用多重参考系法来解决挡板与桨叶之间的相对转动问题,由标准k-ε模型对半圆管曲面涡轮搅拌槽内流动和混合过程进行了详细的数值模拟,本模拟所得的功率准数和设计值以及相关文献值吻合良好。结果表明：当搅拌桨离底距离由搅拌槽直径的1/2处变为1/3处时,搅拌槽内的流型均为典型的“双循环流型”,而当搅拌桨离底距离由搅拌槽直径的1/3处降低至1/6处时,槽内流型由典型的“双循环流型”转变为“单循环流型”;通过对不同时刻不同桨叶离底距离下的示踪剂浓度分布图分析表明槽内的混合过程与流动场密切相关;加料点位置对于最终的流场混合效果有着显著影响,对于混合时间数据的采集应注意不同加料位置时监测点的选取。CFD模拟结果表明本文所采用的模型可以很好的预测半圆管曲面涡轮搅拌槽内的混合特性,为进一步改进和优化半圆管曲面涡轮的设计提供了一定的参考。     

计算流体力学用于搅拌器流场研究及结构设计

采用计算流体力学(CFD)模拟方法对现有同轴搅拌器的流场结构进行了模拟,同时进行了混合时间的数值研究,并与实验结果相比较,证实了模拟方法是可行的。针对现有搅拌器存在的问题,提出了3种改进型的搅拌器,并进行CFD模拟研究,发现桨Ⅲ所需混合时间最短,桨Ⅱ和桨Ⅰ次之;桨Ⅲ消耗的功率最大,而桨Ⅰ和桨Ⅱ所需功率最少。     

搅拌槽内非牛顿流体的微观混合特性

在直径为0·476m的搅拌槽内,分别选用三窄叶翼型搅拌桨和标准六直叶涡轮搅拌桨,采用羟乙基纤维素(HEC)水溶液非牛顿流体作为工作体系,以改进的硫酸铜沉降反应与氯乙酸乙酯水解反应的平行竞争反应作为研究微观混合的反应工作体系,在质量分数为0·1%~0·5%的HEC水溶液中,排除硫酸铜副反应的影响,利用铜离子质量浓度与可见光吸光度的线性关系,考察了进料时间、搅拌转速、溶液黏度、流体的非牛顿性以及搅拌桨类型等因素对副产物收率(XQ)的影响规律。结果表明,当进料时间超过临界进料时间3500s后,X_Q不再改变;随着搅拌转速的增加,X_Q减小,但其减小的趋势逐渐变缓;在相同转速下,X_Q随着流体黏度的增加而增加,但增加的趋势有所减缓;对于黏性流体不能仅仅采用雷诺数作为放大准则;在相同的能量耗散速率及桨叶区进料的情况下,Damkohler准数较大的标准六直叶涡轮搅拌桨的X_Q较小。     

搅拌反应器中计算流体力学数值模拟的影响因素研究进展

回顾了搅拌反应器中计算流体力学（CFD）数值模拟方法的研究历程,包括搅拌桨叶边界条件法、内外迭代法、滑移网格法、多重参考系法、快速照相法等方法;结合各种模拟方法的特点,对机械搅拌反应器数值模拟过程中的挡板的存在、尾涡的消除、复杂桨叶类型的模拟、网格精度的影响、湍流模型的选择、离散格式的差异等几个因素的影响作了分析。提出了搅拌反应器的CFD数值模拟仍有待深入研究。     

Rushton涡轮搅拌槽内流场特性及颗粒运动行为数值模拟

基于计算流体动力学（CFD）方法,采用大涡模拟（LES）和拉格朗日颗粒追踪技术计算了Rushton涡轮搅拌槽内流场特性及三种St颗粒的运动行为。平均流场（切向速度、轴向速度和径向速度）、颗粒速度及浓度分布方面与实验值的吻合度较好,验证了数值模拟的可靠性。结果表明,搅拌流场及颗粒运动均呈现循环流特性,当转速N=313r/min不变时,St=0.24的小颗粒几乎实现了均匀分布;而St=37.3的大颗粒与流体的跟随性较差,底部沉积率较高,容器顶部会出现一定的颗粒空白区。叶轮附近产生一系列的湍流涡结构,并且由于剧烈的颗粒-壁面碰撞,该位置颗粒拟温度最高;小颗粒（St=0.24）的运移主要受叶片后方尾涡的控制,均匀分布在低涡量区;而大颗粒（St=37.3）由于具有较大的惯性,运动不再由涡主导,很快被叶轮甩向边壁,穿过了尾涡所形成的高涡量区,故而叶轮对附近大颗粒的搅拌效果较差。     

偏心组合桨搅拌槽内层流混合过程的数值模拟

目前,处理高黏流体和对剪切敏感介质的层流搅拌槽的报道并不多见。本文建立了描述双层组合桨搅拌槽内高黏非牛顿流体层流流动、混合过程的数学模型,利用Laminar模型、多重参考系法（MRF）和示踪剂浓度法对其流场特性、示踪剂扩散过程进行数值模拟,分析搅拌槽内轴向速度曲线、示踪剂浓度响应曲线和混合时间。结果表明：中心搅拌中间面将介质阻隔在各自的半层内运动,偏心搅拌介质作全局运动,轴向混合能力突出;转轴中心搅拌依靠上下半层浓度差的增大向下扩散,转轴偏心搅拌通过不对称结构扩散示踪剂,叶轮相对转轴偏心搅拌则利用叶片的不对称分布;距离加料点较近和较远的监测点浓度响应曲线因振荡和调整,混合时间较长,处于中间面的监测点拥有最短的混合时间。     

搅拌槽内假塑性流体洞穴研究进展

介绍了在假塑性流体层流搅拌中广泛存在的洞穴现象,表明其对物料的传质和传热极为不利;说明了假塑性流体剪切变稀的流变特性,阐述了预测洞穴大小的3种数学模型：球形模型、圆柱形模型和环形模型,同时综述了不同搅拌器洞穴形状及大小变化的最新研究成果,认为圆柱形模型（EN模型）能更好地描述假塑性流体洞穴的形状及其随雷诺数的变化,洞穴边界速度定义为 0.01U_tip;最后指出了利用 CFD 技术研究洞穴变化以及流场特征是必然趋势。     

Intensification of viscous fluid mixing in eccentric stirred tank systems

The mixing process of highly viscous liquids in a small stirred tank for High Throughput Experimentation (HTE) apparatus was investigated using a visualization approach, i.e., the Planar Laser-Induced Fluorescence (PLIF) technique. Segregated regions were detected despite a long period of agitation, indicating that the mixing homogeneity can be hardly attained for the mixing of highly viscous liquid. Based on the image analysis of PLIF results, we defined a desegregating time to quantify the mixing process of viscous liquids and to compare the mixing performances under different operating conditions. Eccentric agitation proved to be an efficient way to intensify the mixing of viscous liquid for the fact of significantly reduced desegregating time under the same stirring rate.     

On the detailed dynamics of coupled continuous stirred tank reactors

The dynamic behaviour of coupled parallel and serial arrays of cells is analysed. Phenomena such as synchrnization of oscillations at a common frequency, rhythm splitting, amplitude amplification, jump between oscillatory regimes and quasi-periodic oscillations of a torus type have been observed for an autocatalytic reaction with product inhibition. The possibility of steady-state operation driven by the coupling of two cells, each of which is oscillatory by itself, has been detected. The configuration of the cell arrays does not change the bifurcation pattern drastically.