Numerical Simulation of Gas-Liquid Flow in a Stirred Tank with a Rushton Impeller

The gas-liquid flow field in a stirred tank with a Rushton diskturbine, including the impeller region, was numerically simulatedusing the improved inner-outer iterative procedure. Thecharacteristic features of the stirred tank, such as gas cavity andaccumulation of gas at the two sides of wall baffles, can be capturedby the simulation. The simulated results agree well with availableexperimental data. Since the improved inner-outer iterative algorithmdemands on empirical formula and experimental data for the impellerregion, and the approach seems generally applicable for simulatinggas-liquid stirred tanks.

Investigation on Eccentric Agitation in the Stirred Vessel Using 3D-Laser Doppler Velocimeter

The spatial structure of the velocity field in a stirred vessel with water has been measured and analyzed using the laser Doppler velocimeter technique, with the immersing depth and agitation speed of the impeller re-maining approximately constant. The experimental results were provided such as the mean velocity field, fluctuat-ing velocities, turbulent kinetic energy, Reynolds shear stress and time series of the velocity in the stirred tank. These results probably provided the valuable basis to further optimize and enlarge the stirred tank in the industrial process.

Numerical and experimental analyses of a stirred vessel for a large volumetric flow rate of sparged air

Computational fluid dynamics(CFD)and experimental analyses of some of the basic characteristics of air sparging in a tall stirred vessel equipped with a three-stage impeller are presented.The impeller was assembled from a radial ABT impeller as the lower,a turbine 6 PBT45 as the middle and an axial Scaba-type 3SHP1 impeller as the upper.All the impellers were of the same diameter,i.e.,225 mm,while the vessel diameter was 450 mm.The impeller’s rotational speed was 178 r·min-1.The aeration regime was established with an air volumetric flow rate of 28.3 m3·h-1.To the best of our knowledge,this study is the first to consider the very high gassing rate by means of CFD in a tank stirred by three-stage axial/radial impellers.The numerical simulation was performed using the ANSYS Fluent(R17.2,2016)code for solving the governing equations of fluid dynamics in single-and multi-phase systems.While discussing the bubble size distribution,a discrete population balance model(PBM)was used.Adopting CFD,the stirring power and the total void fraction(the total gas holdup)were calculated.The results were in good agreement with the measured values using a laboratory experimental device.

Effects of bubbly flow on bending moment acting on the shaft of a gas sparged vessel stirred by a Rushton turbine

The bending moment acting on the overhung shaft of a gas-sparged vessel stirred by a Rushton turbine,as one of the results of fluid and structure interactions in stirred vessels,was measured using a moment sensor equipped with digital telemetry.An analysis of the shaft bending moment amplitude shows that the amplitude distribution of the bending moment,which indicates the elasticity nature of shaft material against bending deformation,follows the Weibull distribution.The trends of amplitude mean,standard deviation and peak deviation characteristics manifest an "S" shape versus gas flow.The "S" trend of the relative mean bending moment over gas flow rate,depending on the flow regime in gas-liquid stirred vessels,resulted from the competition among the nonuniformity of bubbly flow around the impeller,the formation of gas cavities behind the blades,and the gas direct impact on the impeller when gas is introduced.A further analysis of the bending moment power spectral density shows that the rather low frequency and speed frequency are evident.The low-frequency contribution to bending moment fluctuation peaks in the complete dispersion regime.

CFD simulation of flow and mixing characteristics in a stirred tank agitated by improved disc turbines

To reduce the power consumption and improve the mixing performance in stirred tanks,two improved disc turbines namely swept-back parabolic disc turbine(SPDT)and staggered fan-shaped parabolic disc turbine(SFPDT)are developed.After validation of computational fluid dynamics(CFD)model with experimental results,CFD simulations are carried out to study the flow pattern,mean velocity,power consumption,pumping capacity and mixing efficiency of the improved and traditional impellers in a dished-bottom tank under turbulent flow conditions,The results indicate that compared with the commonly used parabolic disc turbine(PDT),the power number of proposed SPDT and SFPDT impellers is reduced by 43%and 12%,and the pumping efficiency is increased by 68%and 13%,respectively.Furthermore,under the same power consumption(0-700 W·m^(-3)),the mixing performance of both SPDT and SFPDT is also superior to that of Rushton turbine and PDT.

Experimental study of power consumption, local characteristics distributions and homogenization energy in gas–liquid stirred tank reactors

In this paper, the power consumption, the vertical local void fraction and the local gas–liquid interfacial area are investigated in the aerated stirred tank reactors(STRs) equipped with a rigid-flexible impeller. Meanwhile, the regressive correlation based on power consumption and interfacial area is proposed. Then a novel homogenization energy(HE = RSDPtm) expression based on power consumption and local interfacial area is redefined and used to indicate the mixing efficiency. The optimal operating mode is selected based on the change of the HE value. This paper can provide research ideas for structural optimization of stirred reactors.

CFD Prediction of the Turbulent Flow Generated in Stirred Square Tank by a Rushton Turbine

The Computational Fluid Dynamics (CFD) have been used in the analysis and design of agitated vessel. Most of the researches done in this area are limited to the baffled or unbaffled stirred tank. In this paper, we have been interested in studying of the new design. Particularly, the flow and turbulence fields in square vessel stirred by a standard Rushton turbine have been simulated by means of CFD techniques. The Navier-Stokes equations governing the phenomenon of transfer of momentum are solved by a discretization method for finite volume. The MRF approaches can be used in simulation of the steady state problem. The numerical results from the application of CFD code Fluent with the stationary approach Multi Reference Frame (MRF) are presented in the planes containing the blade. The validation of CFD results with experimental measurements showed a good agreement.

THEORETICAL AND EXPERIMENTAL STUDIES ON BUBBLE DIAMETER AND GAS HOLDUP IN AERATED STIRRED TANKS

Experiments were carried out in geometrically similar vessels with diameters of 0.287, 0.495 and 1.1m respectively. Bubble diameter distribution was measured with a dual electric conductivity probe placed in the tanks. Gas holdup was measured by spillover method. Considering the coalescence of bubbles in the upper circulation region of the aeration stirred tank, introducing the concepts of turbulence decay and effective viscosity of gas-liquid system into this work, and taking into account the equilibrium between the surface energy of the bubbles and the energy supplied by agitation, mathematical models for bubble diameter and mean gas holdup were derived. The mathematical models were confirmed by experimental data.

HYDRODYNAMIC MODEL FOR THE IMPELLER REGION IN AN AERATED AND STIRRED TANK

The radial and axial distribution of mean 1iquid velocity were measured by a.hot-filmanemometer at the impeller region in an aerated and stirred tank 0.287m in diameter.The tangentialjet model for impeller discharge flow used for single phase flow was modified to conform with thecharacteristics of gas-liquid flow.The radial and axial velocity profiles at the impeller region in thegas-liquid stirred tank were calculated by the model The results predicted by the model were in goodagreement with those obtained in experiment.

转速及倾角对组合桨功率和微观混合性能影响研究

为了研究多层搅拌桨片倾斜角度以及转速对反应器功率和微观混合性能的影响规律,对装配不同多层搅拌桨的搅拌反应器内的流体流动和微观混合性能进行了实验和数值模拟研究,在反应器内引入碘化物-碘酸盐平行竞争反应体系来评估搅拌釜内的微观混合性能。此外,提出了表征反应器微观混合性能的离集指数(X_(S))和流场湍流特性之间的关联式,结果表明,模拟值与实验值吻合程度较高。当叶片倾角较小(φ≤30°)时,叶片倾角对釜内微观混合性能影响较小;而当叶片倾角较大(φ≥45°)时,釜内微观混合性能随叶片倾角的增加而显著减小。X_(S)与log(ε)之间基本呈线性关系。

Behavior of Liquid-Liquid Dispersions in a Stirred Vessel——Part 2: Phase Inversion Delay Time

A delay time was observed for both W/O inversion to O/W and the opposite inversion process. The delay time decreased with increasing stirring speed. For O/W inversion to W/O, the delay time was very sensitive to the oil volume fraction and the initial stirring speed. Increased oil volume fraction and increased initial stirring speed both reduced the delay time. Inversion from W/O to O/W was a gradual process. But inversion from O/W to W/O had two stages: the preparation stage and the phase inversion stage. The time needed in the first stage was the delay time.

Behavior of Liquid-Liquid Dispersions in a Stirred Vessel——Part 1: Uniformity Property

The conductivity of liquid-liquid dispersions in a stirred vessel was measured at different sampling positions inside the vessel. The results showed that both O/W dispersions and W/O dispersions are not spatially uniform inside the vessel, especially the W/O dispersions. The results provide further understanding of the properties of the liquid-liquid dispersions.

刚柔组合搅拌桨与刚性桨调控流场结构的对比

高黏度流体处于层流状态时,普遍存在的混合隔离区,降低了流体的混合效率。减小或消除隔离区,是实现流体高效混合的基本途径。采用实验研究与数值模拟相结合的方法,对刚性六直叶涡轮桨(刚性桨)和刚柔组合六直叶涡轮桨(组合桨)的流场结构进行研究,对比分析了两种桨叶在相同功耗(3 kW·m-3)时的轴向、径向和切向的速度矢量图、速度云图以及速度分布散点图。结果表明,刚性桨的能量集中在桨叶尖端部分,远离桨叶区域的流体速度很小甚至为0 m·s-1;而组合桨可将能量从桨叶尖端扩散至全槽,使槽内流体均具有一定的流速,提高了混合效率,且显色实验与数值模拟结果一致,组合桨体系的混合隔离区在短时间内就可消除,混合良好,而刚性桨体系的混合隔离区始终存在,混合效果不佳。

刚柔组合搅拌桨强化搅拌槽中流体混沌混合

搅拌槽内普遍存在着两种不同类型的混合区域:混沌混合区和规则区。增大混沌混合区,是提高流体混合效率、降低搅拌过程能耗的重要途径。而合理设计搅拌桨有助于流体形成适宜的流动状态,实现混沌混合。柔性体与刚性体组合,可设计出具有多体运动行为的刚柔组合搅拌桨,可强化流体混沌混合行为。结合Matlab软件,实验研究了双层桨搅拌槽内自来水体系的最大Lyapunov指数(LLE)和多尺度熵(MSE)的变化规律,对比分析了刚性桨和刚柔组合桨两种桨叶对流体混沌混合的影响。结果表明,刚柔组合桨强化流体的运动特性,使更多流体进入混沌混合状态。在转速为210 r·min-1时,流体的混沌混合达到最佳状态,刚性桨体系的LLE为0.041,而刚柔组合桨体系的LLE为0.048;刚柔组合桨可有效耗散能量,使整个槽体的能量分布均匀,刚柔组合桨在150 r·min-1时的多尺度熵率与刚性桨在210 r·min-1时基本相近;刚柔组合桨体系的混合时间均低于刚性桨体系,在转速为120 r·min-1时,刚柔组合桨使流体的混合时间缩短了26%左右。刚柔组合桨可改变流场结构和能量耗散方式,强化流体混沌混合,实现高效节能操作。

湿法提钒浸出段搅拌反应器结构的优化

清洁提钒工艺中,熟料的湿法浸出是重要的操作单元。浸出搅拌反应器的合理设计与优化,可缩短浸矿时间以及提高浸矿效率。本文通过改变搅拌桨桨叶间的层间距、搅拌桨的安装层数以及安装导流筒等方法,对攀钢集团公司的浸出搅拌反应器进行优化和改进;并结合计算流体动力学(CFD)Fluent商业软件,分别模拟了原浸出搅拌反应器和改进后浸出搅拌反应器的宏观流场结构。结果表明:在层间距C2为1100mm的原双层搅拌桨浸出搅拌反应器内,流体轴向速度较小,"死区"现象较严重;与原反应器相比,调整双层搅拌桨桨叶之间的层间距C2为1800mm以及安装3层搅拌桨或导流筒,都可加强反应器内流体的轴向流动,减小"死区"范围,进而改善流场结构的均匀分布,有助于强化流体混合。

刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。

管输剪切模拟搅拌槽中流体平均剪切率的计算

计算搅拌槽内流体剪切率的Metzner_Otto方法只适用于计算某些叶轮 搅拌槽系统内层流时叶轮区的平均剪切率 ,不能满足原油管输过程剪切作用模拟的需要。根据流体流动的能量耗散率与剪切率的关系 ,提出了通过搅拌的轴功率计算搅拌槽全槽流体平均剪切率的方法。针对牛顿流体和幂律流体 ,提出了全槽平均剪切率的计算式。以实验室用的一个小型搅拌系统为例 ,使用所提出的方法 ,建立了槽内流体平均剪切率与搅拌转速、流体体积、幂律流体稠度系数及流动特性指数关系的经验数学模型。该模型预测的结果与直接由实测扭矩计算的平均剪切率相当接近。本方法适用于各种类型的叶轮 ,不受流体流态的限制 ,其原理还适用于符合其他流变模型的非牛顿流体。

顶吹气泡在两相间运动的形变过程对熔池搅拌效果的影响

针对云铜艾萨熔炼体系中贫化电炉柴油喷枪混合氮气顶吹熔池搅拌的优化,开展了两相分层液体浸入式气体顶吹的实验研究。实验结果显示:当顶吹喷枪插入熔渣层深度为熔渣层厚度2/3时即插入冰铜层深度大于冰铜层厚度的1/2时,气泡在二者中的运动符合传统的气泡在单一黏性液体中运动的规律;当气泡生成于渣层和冰铜层的分界面及冰铜层中时,在黏滞力的作用下气泡上升运动会将冰铜带入到渣层中;当喷枪口位于冰铜层后,气泡穿越两层流体的分界面后会形变成气泡刚刚产生时的球形,且冰铜被气泡带入到熔渣中的量同气泡在分界面之前的形变量成正比。

中心龙卷流型搅拌槽固液悬浮特性

开发出一种新型高效节能搅拌设备———中心龙卷流型搅拌槽 ,对其固 -液悬浮特性的研究 ,采用试验方法测试固 -液悬浮临界转速 ,关联出临界转速方程 ,得出其放大规律。试验表明 ,中心龙卷流型搅拌槽固 -液悬浮性能优于标准搅拌槽 ,在相同条件下其临界转速低 。

双层桨叶搅拌器流场的CFD模拟与PIV测量

利用CFD技术和PIV测量研究了搅拌器内双层桨叶不同位置的流场和浓度分布.基于Navier-Stokes方程和标准k-ε湍流模型,求解搅拌器的湍流场,考察了在不同转速和不同桨叶高度下搅拌器的流型变化和混合浓度情况.结果表明:改变双层桨叶位置,流场的流型基本不发生变化,均以桨叶为中心形成上下"双循环"流动;随着加料位置和桨叶高度的变化,对搅拌功率基本没有影响,但对不同监测点的浓度变化较大,对混合时间的影响也较大;采用桨叶部位加料可以充分利用流体的湍流特性,加快混合速度,缩短混合时间,节约成本,提高经济效益.