螺旋桨搅拌槽内湍流参数

在直径494和1600mm的平底有机玻璃槽内,利用一维恒温热膜风速仪测量了7种不同搅拌桨-槽体系中流体的时均速度、脉动速度、能谱函数、微分尺度以及湍流耗散等,对螺旋桨搅拌槽内湍流参数分布进行了详细的描述,研究了叶轮叶端安放角及离底距离对流动场的影响,并分析了放大过程对流动场的影响。     

轴流桨搅拌槽内完全湍流的研究

为研究轴流桨搅拌槽内完全湍流状况,采用相位多普勒粒子分析仪(PDPA)对MK和ZHX搅拌器进行流场测试,得到不同工况下的时均速度场分布.应用渐近不变性方法,选取适当的特征尺度,给出挡板处壁面射流的轴向速度相似剖面,确定用于评定槽内完全湍流界限的轴向速度分布曲线,建立了搅拌雷诺数与搅拌槽内完全湍流流动达到的高度之间的线性关系.结果表明,非全槽完全湍流状态下,槽上部会出现过渡流区;随雷诺数的增大,搅拌槽内完全湍流流动达到的高度增大;不同型式搅拌器的完全湍流流场所需的雷诺数不同,单层桨搅拌槽内达到全槽完全湍流需要很大的搅拌功率.     

搅拌槽内三维流场的LDV测量

利用三维激光多普勒测速系统(3D-LDV)对搅拌槽内流场进行了湍流测量和流场分析,同时对测量系统和测量原理进行了介绍,给出了在偏心搅拌和中心搅拌时搅拌槽内的时均速度场分布和湍流度等实验结果.这些结果为搅拌槽的设计和放大提供了很有价值的实验依据.     

两种不同挡板的双层桨搅拌槽三维流场数值模拟

采用CFD方法分别对三角形挡板和长方体空心挡板的双层斜直叶桨搅拌槽在水中产生的三维流场进行数值模拟,选取多重参考系法和标准k-ε湍流模型进行模拟,对两者的轴向平面速度场和时均速度分布进行了详细的对比分析,研究结果对搅拌槽的设计和实际应用具有重要的参考价值。     

板式螺旋桨搅拌槽内的流场及其流动特性

以板式螺旋桨叶轮为例,采用相位多普勒粒子分析仪测量了直径为300 mm的平底圆筒搅拌槽内的流场;分析了时均速度、脉动速度及湍流动能的分布,以及叶轮离底间隙变化和挡板对流场的影响。结果表明:随离底间隙增大,叶轮区脉动速度和湍流动能增大,时均速度和脉动速度最大值位置向槽中心方向移动;主循环区轴向速度最大值随离底间隙增大而减小;叶轮区湍流动能较高,随离底间隙增大,湍流动能最大值增大,位置靠近叶轮端部;挡板阻碍槽内切向流动,影响湍流动能的分布,挡板前流场反映了叶轮区的湍流动能分布。     

螺旋桨搅拌槽内湍流运动测量及数据处理

螺旋桨搅拌槽内湍流运动测量及数据处理侯拴弟，王英琛，施力田（北京化工大学化学工程系，北京１０００２９）关键词搅拌槽，湍流运动，螺旋桨１前言轴流式搅拌槽常用于物料混匀、结晶、悬浮及催化反应等过程，是化工生产中重要的单元设备之一。纵观以往文献［１～３］，...     

各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点,引入适用于强旋转流场的各向异性k-ε湍流模型,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟.利用文献中对搅拌槽内流场测定结果,给出了适用于Rushton 桨搅拌槽的各向异性湍流黏度系数值.模拟计算得到了搅拌槽内的流场分布和脉动速度分布,并同标准k-e湍流模型计算结果及文献数据进行比较.结果表明,各向异性k-ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量,明显优于标准k-ε湍流模型.     

各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点 ,引入适用于强旋转流场的各向异性k -ε湍流模型 ,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟 .利用文献中对搅拌槽内流场测定结果 ,给出了适用于Rushton桨搅拌槽的各向异性湍流黏度系数值 .模拟计算得到了搅拌槽内的流场分布和脉动速度分布 ,并同标准k -ε湍流模型计算结果及文献数据进行比较 .结果表明 ,各向异性k -ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量 ,明显优于标准k -ε湍流模型 .     

应用热膜风速仪测定搅拌槽内气—液两相流的流体力学参数

开发应用了恒温热膜风速仪（简称ＣＴＡ）来测定搅拌槽内气－液两相流的局部气含率、液体的时均速度及脉动速度的均方根值。根据柱形热膜探头以及气－液搅拌槽内流体流动特性，改进了用电压概率密度法＾［１］划分气－液搅拌槽内气液两相信号的方法，在此基础上得到了局部气含率、液体的时均速度及脉动脉动速度均方根值的计算公式。研究了热膜探头偏角及流体介质温度变化对热膜风速仪测量精度的影响，并对流体介质温度的影响进行了校     

轴流桨搅拌槽三维流场数值模拟

利用k -ε湍流模型预测了搅拌槽在不同操作条件下宏观速度场 ,模型成功预测了搅拌槽内速度分布 ,计算结果与实验结果吻合较好 .模型预测结果表明 ,搅拌槽内宏观流动场受搅拌桨槽径比影响较大 .对单层搅拌桨 -槽体系 ,挡板前后宏观流动场差别很大 ,在挡板以前区域 ,轴向流动较强 ,在整个r -z断面上形成一个整体循环 ;而在挡板后面区域 ,流体在桨叶安装位置高度附近转向轴心流动 ,槽体上半部区域形成二次循环区域 ,且二次循环区域内流体以向下流动为主 .     

SOLID-LIQUID FLOW AT DILUTE CONCENTRATIONS IN AN AXIALLY STIRRED VESSEL INVESTIGATED USING PARTICLE IMAGE VELOCIMETRY

Particle image velocimetry (PIV) has been used to quantify velocities and turbulence levels of the continuous and dispersed phases in a vessel agitated by a 45° pitched-blade turbine (PBT). To be able to measure velocities of both the continuous and dispersed phases in suspensions with a concentration of solids up to 1.5% by volume, a technique using matching of refractive indices in combination with image analysis to separate the phases was applied. It is found that the measurement technique works well and that 600 recordings are sufficient to estimate mean and RMS velocities. The mean velocities of both phases decrease while the RMS velocities increase as the solid content increases. The decrease in velocity is to a large extent a result of the impeller discharge being diverted towards the vertical tank wall at a distance from the tank bottom that increases with increasing volume fraction.     

Experimental determination and numerical simulation of mixing time in a gas-liquid stirred tank

In this work, mixing experiments and numerical simulations of flow and macro-mixing were carried out in a 0.24 m i.d. gas-liquid stirred tank agitated by a Rushton turbine. The conductivity technique was used to measure the mixing time. A two-phase CFD (computational fluid dynamics) model was developed to calculate the flow field, k and ε distributions and holdup. Comparison between the predictions and the reported experimental data [Lu, W.M., Ju, S.J., 1987. Local gas holdup, mean liquid velocity and turbulence in an aerated stirred tank using hot-film anemometry. Chemical Engineering Journal 35 (1), 9-17] of flow field and holdup at same conditions were investigated and good agreements have been got. As the complexity of gas-liquid systems, there was still no report on the prediction of mixing time through CFD models in a gas-liquid stirred tank. In this paper, the two-phase CFD model was extended for the prediction of the mixing time in the gas-liquid stirred tank for the first time. The effects of operating parameters such as impeller speed, gas flow rate and feed position on the mixing time were compared. Good agreements between the simulations and experimental values of the mixing time have also been achieved.     

Numerical simulation of flow field characteristics in a gas-liquid-solid agitated tank

Computational fluid dynamics simulation was carried out to investigate flow field characteristics in a gas-liquid-solid agitated tank. The Eulerian multifluid model along with standard k-ε turbulence model was employed in the simulation. A multiple reference frame approach was used to treat the impeller rotation. Liquid velocity, gas holdup and solid holdup distributions in the agitated tank were obtained. The effect of operating conditions on gas and solid distributions was investigated. The predicted flow pattern was compared with results in literature. The simulation results indicate that local hydrodynamic behaviors such as velocity, gas and solid holdup distribution, are strongly influenced by operating conditions. Within the scope of our study, increasing gas inlet rate caused liquid circulation to be weakened and was not in favor of gas dispersion. Solid holdup in the upper part of the tank, especially near the wall region decreased. Adding solid loadings resulted in liquid mean velocity near the surface region decreased, gas dispersion and solid suspension becoming worse.     

FLOW GENERATED BY PITCHED BLADE TURBINES II: SIMULATION USING κ-ε MODEL

Experimental data on average velocity and turbulence intensity generated by pitched blade downflow turbines (PTD) were presented in Part I of this paper. Part II presents the results of the simulation of flow generated by PTD

The standard κ-ε model along with the boundary conditions developed in the Part 1 have been employed to predict the flow generated by PTD in cylindrical baffled vessel. This part describes the new software FIAT (Flow In Agitated Tanks) for the prediction of three dimensional flow in stirred tanks. The basis of this software has been described adequately. The influence of grid size, impeller boundary conditions and values of model parameters on the predicted flow have been analysed. The model predictions successfully reproduce the three dimensionality and the other essential characteristics of the flow. The model can be used to improve the overall understanding about the relative distribution of turbulence by PTD in the agitated tank     

Detached eddy simulation on the turbulent flow in a stirred tank

A detached eddy simulation (DES), a large‐eddy simulation (LES), and a k‐ε‐based Reynolds averaged Navier‐Stokes (RANS) calculation on the single phase turbulent flow in a fully baffled stirred tank, agitated by a Rushton turbine is presented. The DES used here is based on the Spalart‐Allmaras turbulence model solved on a grid containing about a million control volumes. The standard k‐ε and LES were considered here for comparison purposes. Predictions of the impeller‐angle‐resolved and time‐averaged turbulent flow have been evaluated and compared with data from laser doppler anemometry measurements. The effects of the turbulence model on the predictions of the mean velocity components and the turbulent kinetic energy are most pronounced in the (highly anisotropic) trailing vortex core region, with specifically DES performing well. The LES—that was performed on the same grid as the DES—appears to lack resolution in the boundary layers on the surface of the impeller. The findings suggest that DES provides a more accurate prediction of the features of the turbulent flows in a stirred tank compared with RANS‐based models and at the same time alleviates resolution requirements of LES close to walls. © 2011 American Institute of Chemical Engineers AIChE J, 58: 3224–3241, 2012     

颗粒与液相间的湍流涡旋裂变传质模型

湍流在宏观上处于远程混沌无序状态, 而在介微观上处于近程有序状态。从分析湍流场中局 域中涡旋的串级结构出发,通过对N-S方程的涡旋输运形式进行求解, 导出一个具有分形意 义的涡旋群运动表达式,并得到涡旋掠过颗粒表面花费的时间, 然后同Higbie的传质渗透模型相联系,以涡旋的分布函数为权求其数学期望,得到搅拌湍流场中的传质系数表达式,结果与实验数据吻合较好。     

TURBULENT FLOW IN STIRRED TANKS: SCALE-UP COMPUTATIONS FOR VESSEL HYDRODYNAMICS

A numeric model for turbulent flow was used to compute the flow patterns in Rushton turbine agitated vessels. The cases considered cover two orders of magnitude for the three different scaling criteria of constant impeller Reynolds number, power input per unit mass, and impeller tip speed. The constant power input scale-up criteria maintains the turbulence levels throughout the vessel during scale-up. The circulation times, however, increase with vessel size for this scaling criteria. The other scaling criteria of constant impeller tip speed and impeller Reyonds number lead to decreasing turbulence levels in the tank along with further increases in circulation times.     

Azo Dye Ozonation Film Theory Utilization for Kinetic Studies

The ozonation kinetics of three azo dyes (Direct Yellow 27, Direct Blue 1 and Acid Black 52) in aqueous solution has been studied. Two types of reactors have been used, an agitated tank for studying the influence of variables and the stoichiometry, and an agitated cell for kinetic measurements. Both the film theory and the modes of ozone action on organic matter have been considered for obtaining the kinetic rate constants. A model of ozone absorption in the fast pseudo–m–th ozone order kinetic regime with two parallel reactions fits satisfactorily the experimental results. From it, the kinetic constants of both reactions have been evaluated.     

叶片形状对涡轮桨搅拌槽内尾涡特性的影响

Particle image velocimetry technique was used to analyze the trailing vortices and elucidate their rela-tionship with turbulence properties in a stirred tank of 0.48 m diameter,agitated by four different disc turbines,in-cluding Rushton turbine,concaved blade disk turbine,half elliptical blade disk turbine,and parabolic blade disk turbine.Phase-averaged and phase-resolved flow fields near the impeller blades were measured and the structure of trailing vortices was studied in detail.The location,size and strength of vortices were determined by the simplified λ2-criterion and the results showed that the blade shape had great effect on the trailing vortex characteristics.The larger curvature resulted in longer residence time of the vortex at the impeller tip,bigger distance between the upper and lower vortices and longer vortex life,also leads to smaller and stronger vortices.In addition,the turbulent ki-netic energy and turbulent energy dissipation in the discharge flow were determined and discussed.High turbulent kinetic energy and turbulent energy dissipation regions were located between the upper and lower vortices and moved along with them.Although restricted to single phase flow,the presented results are essential for reliable de-sign and scale-up of stirred tank with disc turbines.