搅拌槽内液-固两相体系的数值研究(Ⅰ)流场的数值模拟

Three-dimensional solid-liquid flow is mathematically formulated by means of the “two-fluid” approach and the two-phase k-ε-Ap turbulence model. The turbulent fluctuation correlations appearing in the Reynolds time averaged governing equations are fully incorporated. The solid-liquid flow field and solid concentration distribution in baffled stirred tanks with a standard Rushton impeller are numerically simulated using an improved “inner-outer” iterative procedure. The flow pattern is identified via the velocity vector plots and a recirculation loop with higher solid concentration is observed in the central vicinity beneath the impeller. Comparison of the simulation with experimental data on the mean velocities and the turbulence quantities of the solid phase is made and quite reasonable agreement is obtained except for the impeller swept volume. The counterpart of liquid phase is presented as well. The predicted solid concentration distribution for three experimental cases with the average solid concentration up to 20% is also found to agree reasonably with the experimental results published in the literature.     

Rushton桨搅拌槽中气液两相流动的全流场数值模拟

The gas-liquid flow field in a stirred tank with a Rushton disk turbine,including the impeller region,was numerically simulated using the improved inner-outer iterative procedure.The characteristic features of the strirred tank,such as gas cavity and accumulation of gas at the two sides of wall baffles,can be captured by the simulation.The simulated results agree well with available experimental data.Since the improved inner-outer iterative algorithm demands no empirical formula and experimental data for the impeller region,and the approach seems generally applicable for simulating gas-liquid stirred tanks.     

Numerical Study of Solid-Liquid Two-Phase Flow in Stirred Tanks with Rushton Impeller (Ⅰ) Formulation and Simulation of Flow Field

Three-dimensional solid-liquid flow is mathematically formulated by means of the "two-fluid" approach and the two-phase k-ε-Ap turbulence model. The turbulent fluctuation correlations appearing in the Reynolds time averaged governing equations are fully incorporated. The solid-liquid flow field and solid concentration distribution in baffled stirred tanks with a standard Rushton impeller are numerically simulated using an improved "inner-outer" iterative procedure. The flow pattern is identified via the velocity vector plots and a recirculation loop with higher solid concentration is observed in the central vicinity beneath the impeller. Comparison of     

Numerical Simulation of Macroscopic Mixing in a Rushton Impeller Stirred Tank

The macroscopic mixing in a stirred tank with different tracer injection locations, impeller speeds and impeller positions is simulated numerically by solving the transport equation of the tracer based on the whole flow field in the baffled tank with a Rushton disk turbine numerically resolved using the improved inner-outer iterative procedure. Predicted mixing time is compared well with the literature correlations. The predicted residence time distribution of the stirred tank is very close to the present experimental results. The effect of the installation of a draft tube on the mixing time and residence time distributions is addressed.     

用各向异性代数应力模型数值模拟搅拌槽中的三维全流场

In accordance to the anisotropic feature of turbulent flow, an anisotropic algebraic stress model is adopted to predict the turbulent flow field and turbulent characteristics generated by a Rushton disc turbine with the improved inner-outer iterative procedure. The predicted turbulent flow is compared with experimental data and the simulation by the standard κ-ε turbulence model. The anisotropic algebraic stress model is found to give better prediction than the standard κ-ε turbulence model. The predicted turbulent flow field is in accordance to experimental data and the trend of the turbulence intensity can be effectively reflected in the simulation. The distribution of turbulent shear rate in the stirred tanks was simulated with the established numerical procedure.     

Large Eddy Simulations of Mixing Time in a Stirred Tank

Large eddy simulations （LES） of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Smagorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agreement of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes （RANS） approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.     

Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass trans-fer performance under al operating conditions. At high uG and a given power input, the impel er combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.     

Power consumption and flow field characteristics of a coaxial mixer with a double inner impeller

A coaxial mixer meeting the actual demand of a system with high and variable viscosity is investigated. It has an outer wal-scraping frame and a double inner impeller consisting of a four-pitched-blade turbine and Rushton turbine. The power consumption and flow field characteristics of the coaxial mixer in laminar and transitional flow are simulated numerically, and then the distribution of velocity field, shear rate and mass flow rate are analyzed. The simulation results indicate that the outer frame has little effect on the power consumption of the double inner impeller whether in laminar or transitional flow, whereas the inner combined impeller has a great effect on the power consumption of the outer frame. Compared with the single rotation mode, the power consumption of the outer frame will decrease in co-rotation mode and increase in counter-rotation mode. The velocity, shear rate and mass flow rate are relatively high near the inner impeller in all operating modes, and only under double-shaft agitation wil the mixing performance near the free surface be improved. In addition, these distributions in the co-rotation and counter-rotation modes show little difference, but the co-rotation mode is recommended for the advantage of low power consumption.     

3D hydrodynamics involving multiple eccentric impellers in unbaffled cylindrical tank

In this paper, the numerical predictions of 3D hydrodynamics and power consumption in a vessel stirred by mul-tiple eccentrical y located impel ers are presented. The vessel is a flat-bottomed cylindrical one equipped with six-curved bladed impel ers. Aqueous solutions of xanthan gum are used, which have a shear thinning behavior with yield stress. The influence of several parameters on the mixing efficiency has been investigated, namely:the stirring rate, fluid rheology, impeller number and impeller clearance from the tank bottom. Our predicted results are compared with other experimental data and a satisfactory agreement is found.     

Gas-liquid hydrodynamics in a vessel stirred by dual dislocated-blade Rushton impellers☆

Towards the objective of improving the gas dispersion performance, the dislocated-blade Rushton impeller was applied to the gas–liquid mixing in a baffled stirred vessel. The flow field, gas hold-up, dissolved oxygen, power consumption before and after gassing were studied using the computational fluid dynamics (CFD) technique. Dispersion of gas in the liquid was modelled using the Eulerian–Eulerian approach along with the dispersed k–εturbulent model. Rotation of the impeller was simulated with the multiple reference frame method. A modified drag coefficient which includes the effect of turbulence was used to account for the momentum exchange. The predictions were compared with their counterparts of the standard Rushton impeller and were validated with the experimental results. It is concluded that the dislocated-blade Rushton impeller is superior to the standard Rushton impeller in the gas–liquid mixing operation, and the findings obtained here lay the basis of its application in process industries.     

用改进的内外迭代法数值模拟Rushton涡轮搅拌槽流场

在有挡板的搅拌槽中，受搅拌桨驱动的液体在挡板的作用下会产生复杂的三维湍流流动.利用“快照”法思路和改进的内外迭代法及k–e湍流模型对Rushton涡轮有挡板的搅拌槽进行了整体数值模拟. 同文献中的实验数据进行了比较，模拟值同实验值基本吻合. 改进后的内外迭代法不依赖经验公式和实验数据，有一定的通用性.     

Numerical simulation of fluid dynamics in the stirred tank by the SSG Reynolds Stress Model

The Speziale, Sarkar and Gatski Reynolds Stress Model (SSG RSM) is utilized to simulate the fluid dynamics in a full baffled stirred tank with a Rushton turbine impeller. Four levels of grid resolutions are chosen to determine an optimised number of grids for further simulations. CFD model data in terms of the flow field, trailing vortex, and the power number are compared with published experimental results. The comparison shows that the global fluid dynamics throughout the stirred tank and the local characteristics of trailing vortices near the blade tips can be captured by the SSG RSM. The predicted mean velocity components in axial, radial and tangential direction are also in good agreement with experiment data. The power number predicted is quite close to the designed value, which demonstrates that this model can accurately calculate the power number in the stirred tank. Therefore, the simulation by using a combination of SSG RSM and MRF impeller rotational model can accurately model turbulent fluid flow in the stirred tank, and it offers an alternative method for design and optimisation of stirred tanks.     

用大涡模拟结合改进的内外迭代法数值模拟搅拌槽中的液相流动

1 INTRODUCTION Stirred tank reactors are widely encountered in the chemical, pharmaceutical, and hydrometallurgical proc- esses. The fluid motion in stirred tanks is three-dimensional, complex, and covers a wide range of spatial and temporal scales. In the area surrounding the impeller, the flow is highly turbulent and swirling. The numerical simulation of such reactor systems is helpful in quantifying the effects of the impeller type, geometry, and the operational conditions in order to …     

涡轮桨搅拌槽流动场数值模拟

在旋转坐标系下 ,采用k-ε湍流模型模拟了两个时间瞬间六直叶涡轮搅拌槽内流体流动状态 ,模型成功地再现了六直叶涡轮搅拌槽内“双循环”流动形式 .模拟结果表明当桨叶离底距离C/T =0 .16 7时 ,槽体内流动从径向流动转化为轴向流动 .模拟计算搅拌功率准数及桨叶排出流量准数与实验结果相差很小 ,数值模拟速度分布与实验测量结果吻合较好 ,且预测速度数值明显优于“黑箱”模拟方法     

SHEAR STRESSES ON THE WALLS OF AGITATED VESSELS

Characteristics of flow near the walls of baffled vessels equipped with a six blade Rushton type turbine were measured by an electrochemical method. A directionally sensitive probe was used for determination of velocity gradient vector fields. It was found that the flow conditions near the wall could be described by a meridional profile of the friction coefficient. An extensive set of experimental data obtained by the limiting current technique showed that the profile depends on the Reynolds number, on the flow index of the (power-law) fluids and on the d/D simplex. The maximum values of the friction coefficient characterizing flow near the impeller plane can be correlated by a simple relation of the laminar boundary layer type.     

SHEAR STRESSES ON THE WALLS OF AGITATED VESSELS

Characteristics of flow near the walls of baffled vessels equipped with a six blade Rushton type turbine were measured by an electrochemical method. A directionally sensitive probe was used for determination of velocity gradient vector fields. It was found that the flow conditions near the wall could be described by a meridional profile of the friction coefficient. An extensive set of experimental data obtained by the limiting current technique showed that the profile depends on the Reynolds number, on the flow index of the (power-law) fluids and on the d/D simplex. The maximum values of the friction coefficient characterizing flow near the impeller plane can be correlated by a simple relation of the laminar boundary layer type.     

大涡模拟搅拌槽中的液相流动

采采用大涡模拟湍流模型对有档板的Rushton 桨搅拌槽进行了数值模拟研究。控制方程采用控制容积法进行离散，对流项用三阶QUICK格式，扩散项是二阶中心差分。压力 速度耦合方程在交错网格上采用SIMPLE算法进行求解。小尺度流动的模化采用动力学（dynamic）亚格子模型。搅拌桨与档板之间的相互作用采用改进的内外迭代法进行处理。计算结果和文献值吻合得很好。