运用粒子图像测速仪研究双层Rushton桨搅拌槽内湍流特性

Particle Image Velocimetry （PIV） has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height （ H = 1.4T ） of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter （ D = 0.4T ）. The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liq- uid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns.
Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different lo- cations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.     

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.     

非牛顿流体中的两并行运动气泡周围流场及气泡间相互作用的PIV研究

The flow fields surrounding two parallel moving bubbles rising from two identical orifices submerged in non-Newtonian fluid of carboxymethylcellulose (CMC) solution of three different mass concentration were measured experimentally by the use of particle image velocimetry (PIV). The influences of gas flowrate, solution mass concentration, orifice interval and the angle between two bubble centers line and vertical direction on the flow field surrounding bubbles were discussed respectively by analyzing the velocity vector, velocity contours as well as individual velocity components. The results show that the liquid velocity both in front of two bubbles and behind increases with gas flowrate duo to shear-thinning effect of previous bubbles, whereas decreases with the increase of CMC concentration due to the increase of drag force acting on bubbles. The effect of the orifice interval on the flow field around two moving bubbles becomes gradually obvious as the interval becomes closer. Moreover, two adjacent side-by-side bubbles repulse each other during rising, leading to the practical interval between them increased somewhat above the orifice interval. When the distance between bubbles is less than the orifice interval l0 mm, the interaction between two neighboring bubbles changed from mutual repellence to attraction with the decrease of the angle of the line of linking two bubble centers to the vertical direction.     

半椭圆管盘式涡轮桨搅拌槽内湍流结构的时间解析PIV研究(英文)

The turbulence structure in the stirred tank with a deep hollow blade (semi-ellispe) disc turbine (HEDT) was investigated by using time-resolved particle image velocimetry (TRPIV) and traditional PIV. In the stirred tank, the turbulence generated by blade passage includes the periodic components and the random turbulent ones. Traditional PIV with angle-resolved measurement and TRPIV with wavelet analysis were both used to obtain the random turbulent kinetic energy as a comparison. The wavelet analysis method was successfully used in this work to separate the random turbulent kinetic energy. The distributions of the periodic kinetic energy and the random turbulent kinetic energy were obtained. In the impeller region, the averaged random turbulent kinetic energy was about 2.6 times of the averaged periodic one. The kinetic energies at different wavelet scales from a6 to d1 were also calculated and compared. TRPIV was used to record the sequence of instantaneous velocity in the impeller stream. The evolution of the impeller stream was observed clearly and the sequence of the vorticity field was also obtained for the identification of vortices. The slope of the energy spectrum was approximately &;#61485;5/3 in high frequency representing the existence of inertial subrange and some isotropic properties in stirred tank. From the power spectral density (PSD), one peak existed evidently, which was located at f0 (blade passage frequency) generated by the blade passage.     

双层翼型桨搅拌槽内流动特性的PIV研究

在直径0.476 m的搅拌槽内,采用粒子图像测速技术对双层三叶CBY翼型桨搅拌槽内的流场进行了研究,考察了层间距、浸没深度和离底高度等参数对流场分布的影响. 结果表明,层间距H2≤0.6T(T为搅拌槽直径)时,槽内可形成整体的轴向循环流动,H2≥0.7T时槽内将产生分区流动现象. 浸没深度对桨叶排出流区域的速度影响很小. 降低下层桨的离底高度能加强下层桨的径向流动,并增大上层桨叶轮区和循环区流体的轴向流动.     

双层桨自吸式搅拌槽内的实验研究

应用Ansys12.0中的workbench建模,运用计算流体动力学(CFD)中的CFX对双层桨自吸式搅拌槽内的流场进行数值模拟,在数值模拟的基础上,利用实验室现有条件对双层桨自吸式搅拌槽内的气-液分散性能进行了研究.比较了两种不同的桨叶组合的气体吸入临界转速、功率消耗、气含率和传氧性能.结果表明,6P+6PDTU的功率消耗小于6DT+6PDTU,但是6DT+6PDTU的气含率和容积传氧系数均大于6P+6PDTU,因此具有更好的分散性能.     

轴流桨搅拌槽内轴向流动速度的研究

为研究轴流桨搅拌槽内轴向速度的分布,在直径为300 mm的平底圆筒搅拌槽内,采用相位多普勒粒子分析仪(PDPA)对轴流式搅拌器主流区的流场进行测量。在不同转速和离底间隙条件下,对挡板前轴向时均速度在径向和轴向的分布进行分析和研究,采用壁面射流理论,建立搅拌槽内壁面射流流动模型,揭示轴向流动特性。结果表明:轴向速度沿径向的分布具有相似性;轴向最大速度沿轴向衰减;轴向流边界沿轴向线性扩展。研究结果有助于进一步了解搅拌槽内部流动特性,为固-液悬浮操作、过程放大和搅拌设备的优化设计提供理论依据。     

双层涡轮桨偏心搅拌槽内的宏观不稳定性研究?

采用分离涡模型和 PIV 实验测试方法研究了双层桨偏心搅拌槽内的宏观不稳定性,通过对速度时间序列的频谱分析,讨论了搅拌桨的安装方式(单轴双桨和双轴双桨)及转动方向对宏观不稳定频率的影响,并与单层桨偏心搅拌进行了对比。研究发现,双层桨偏心搅拌的宏观不稳定频率稍低于单层桨偏心搅拌;搅拌桨安装方式及转动方向对双层桨偏心搅拌的宏观不稳定频率有明显影响：双轴双桨偏心搅拌时的频率稍高于单轴双桨偏心搅拌,搅拌桨反向旋转时的频率又高于同向旋转。     

几种框式桨搅拌槽内流动特性的比较研究

用粒子成像测速（PIV）技术对传统框式桨、传统框式组合桨和新型框式组合桨的流动特性进行研究,对比了三种框式桨在相同工况下搅拌槽内的速度、流型和湍动能。结果表明：传统框式桨搅拌槽内流体流动以水平环流为主,在框式桨上方和框式桨中间区域流体流动不充分;传统框式组合桨搅拌槽内框式桨上方由于二折叶桨的作用使得框式桨上部流体流速变大,槽内流体上下部的流动得到加强,但在框式桨中心区域依旧存在流动死区;新型框式组合桨搅拌槽内两层桨叶间的连接流得到了加强,框式桨底部和中间区域物质和能量的交换更加充分。在考察的三种框式桨中,新型框式组合桨的混合效果更好。研究结果可为新型框式组合桨应用于化工合成工业中提供参考。     

逆流桨强化搅拌槽内流体混沌混合及流场结构失稳研究

在传统三斜叶桨的基础上,结合逆流桨结构,提出三斜叶逆流桨,以破坏或者消除搅拌槽内稳定的对称性流场结构,提高流体传递效率及混沌混合程度。结合实验和模拟两种方法,主要研究了上推式三斜叶桨(PBTU)、外推内压式三斜叶逆流桨(PBTC-U)、外压内推式三斜叶逆流桨(PBTC-D)三种桨叶体系以及不同外层桨叶长度的PBTC-U桨体系内搅拌功耗、混合时间、混沌特性参数、流场结构以及流体速度分布。实验结果表明,N=130 r/min时,PBTC-U桨相对于PBTU桨和PBTC-D桨,体系混合时间分别从22.0、37.5 s缩短到16.5 s,功耗分别降低了5.6%和12.8%,LLE值分别提高了13.69%和37.01%。在确定PBTC-U桨适宜外层桨叶长度的研究中发现当外层桨叶长度D₂=0.375D时,搅拌功耗最低且混合时间最短。PBTC-U型逆流桨通过内外层桨叶的逆流作用,强化体系内流体的随机运动,使得流场的不稳定性得到增强,对称性被破坏,进而流场结构失稳,流体混合效率得到提高。另外,PBTC-U桨可以增强流体轴、径向速度分布的波动性,有利于提高体系的混合效率。     

Transitional flow in a Rushton turbine stirred tank

The way in which the single phase flow of Newtonian liquids in the vicinity of the impeller in a Rushton turbine stirred tank goes through a laminar‐turbulent transition has been studied in detail experimentally (with Particle Image Velocimetry) as well as computationally. For Reynolds numbers equal to or higher than 6000, the average velocities and velocity fluctuation levels scale well with the impeller tip speed, that is, show Reynolds independent behavior. Surprising flow structures were measured—and confirmed through independent experimental repetitions—at Reynolds numbers around 1300. Upon reducing the Reynolds number from values in the fully turbulent regime, the trailing vortex system behind the impeller blades weakens with the upper vortex weakening much stronger than the lower vortex. Simulations with a variety of methods (direct numerical simulations, transitional turbulence modeling) and software implementations (ANSYS‐Fluent commercial software, lattice‐Boltzmann in‐house software) have only partial success in representing the experimentally observed laminar‐turbulent transition. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3610–3623, 2017     

Validation of CFD simulations of a stirred tank using particle image velocimetry data

Methods for validating CFD simulations based on the Reynolds Average Navier-Stokes equation (RANS) against Particle Image Velocimetry (PIV) measurements are investigated and applied to one of the most common problems in the chemical process industry — the prediction of flow field in a stirred vessel. A total of 1024 sequential instantaneous 2D velocity fields along the central axial plane of a stirred vessel with a P-4 axial impeller are obtained through PIV measurement. From the PIV data, the mean velocity, turbulent kinetic energy, Reynolds stresses and dissipation rate fields are extracted. By introducing several tools to quantify the similarities and differences between two-dimensional fields, CFD predictions of the flow field are validated against PIV data. Furthermore, using PIV and LDV data, the effect of boundary conditions on CFD simulation results is examined. The effect of different Reynolds stress closures on the flow prediction is also studied.     

Hydrodynamic analysis of an axial impeller in a non-Newtonian fluid through particle image velocimetry

The rheology of non-Newtonian fluids in agitated vessels is complex making equipment sizing more an art than a science. To increase our understanding and to resolve the data gap for computational fluid dynamics validation, we present a detailed particle image velocimetry study of the hydrodynamics of Carbopol encountered in a 70 L mechanically mixed (A310) tank at three different rotational speeds (100, 250, 500 rpm). Bulk flow visualizations show that the flow field below the impeller is highly influenced by the rheological behavior of the fluid. Moreover, an analysis of the shear rate and viscosity revealed important spatial heterogeneities. An estimation of the Reynolds number classified the rotational speeds as the onset of the transitional regime (100), in the transitional regime (250) and turbulent conditions (500 rpm). This data set consists of local mean and fluctuating velocities at different locations below the impeller, which are available for validation and further study.     

Reynolds number scaling of flow in a Rushton turbine stirred tank. Part I—Mean flow, circular jet and tip vortex scaling

We consider scaling of flow within a stirred tank with increasing Reynolds number. Experimental results obtained from two different tanks of diameter 152.5 and 292.1 mm, with a Rushton turbine operating at a wide range of rotational speeds stirring the fluid, are considered. The Reynolds number ranges from 4000 to about 78,000. Phase-locked stereoscopic PIV measurements on three different vertical planes close to the impeller give phase-averaged mean flow on a cylindrical surface around the impeller. The scaling of θ- and plane-averaged radial, circumferential and axial mean velocity components is first explored. A theoretical model for the impeller-induced flow is used to extract the strength and size of the three dominant elements of the mean flow, namely the circumferential flow, the jet flow and the pairs of tip vortices. The scaling of these parameters with Reynolds number for the two different tanks is then obtained. The plane-averaged mean velocity scales with the blade tip velocity above a Reynolds number of about 15,000. However, parameters associated with the jet and tip vortices do not become Reynolds number independence until Re exceeds about 10⁵. The results for the two tanks exhibit similar Reynolds number dependence, however, a perfect collapse is not observed, suggesting a sensitive dependence of the mean flow to the finer details of the impeller.     

涡轮桨搅拌槽内流场特性的V3V实验

用体三维速度测量技术（volumetric three-component velocimetry measurements,V3V）实验研究了涡轮桨搅拌槽内桨叶附近流场。通过速度数据得到三维流场特性,确定尾涡三维结构;分析了叶片后方30°截面轴向、径向和环向速度沿径向分布规律;对比了V3V和2D-PIV（particle image velocimetry）径向和轴向速度,发现速度分布吻合较好,特别是尾涡所在的射流区。用2D-PIV方法对尾涡发展规律进行研究,发现受流体自由液面影响,尾涡轨迹向上倾斜,并与水平方向成10°,上、下尾涡运动轨迹不对称,下尾涡运动比上尾涡稍快,衰减亦较快,这与V3V实验结果一致;叶片后方60°尾涡依然清晰可见。用V3V和2D-PIV方法对桨叶附近湍流各向同性假设进行了分析,发现桨叶区和尾涡所在位置湍动能被各向同性假设近似法高估了25%~33%,桨叶区和尾涡所在位置趋向于各向异性。     

Reynolds number scaling of flow in a stirred tank with Rushton turbine. Part II — Eigen decomposition of fluctuation

We consider the question of scaling of flow within a stirred tank with increasing Reynolds number. Experimental results obtained from two different tanks of diameter 15.25 and 29.21 cm with a Rushton turbine operating at a wide range of rotational speed is considered for the scaling analysis. The resulting Reynolds number of the flow ranges from 4000 to about 80,000. Phase locked stereoscopic PIV measurements have been performed in order to obtain all three components of velocity on three different vertical planes close to the impeller. The scaling of plane-averaged mean flow was explored in the companion part-I of the paper. The scaling of rms and skewness of fluctuation about the mean is investigated here. The fluctuation about the mean is explained in terms of time-dependent oscillation of the impeller-induced jet and tip vortex components of the flow. The spatial structure of the instantaneous fluctuation about the mean is investigated in terms of eigenmodes obtained using proper orthogonal decomposition with the method of snapshots. The scaling of energy content of the dominant eigenmodes with Re is investigated.     

Experimental and CFD investigation of power consumption in a dual Rushton turbine stirred tank

Power consumption of a mixing system is a key variable in chemical and bioprocess engineering, the determination of which is of interest of many processes. Besides, prediction of the flooding-loading transition in an aerated stirred tank is crucial for the correct design of aerated stirred tank reactors. In this research, laboratory investigation has been carried out on local and total power consumption of a single phase as well as gas-liquid phase systems in a fully baffled stirred tank equipped with dual six-blade Rushton turbines; moreover, the flow regime behavior of a gas-liquid system was investigated. Results have been compared with data obtained from CFD simulation of experimental setup and the data available in the literature. Reasonable agreement between the experimental and simulation results indicates the validity of the CFD model. Using predicted data some empirical correlations have been derived which present new relations in estimation of power consumption and flow regime transitions in stirred tanks with dual Rushton impellers.     

基于粒子图像测速技术的流浆箱阶梯扩散器性能试验

采用粒子图像测速(PIV)系统对两种出口段型式阶梯扩散器的内流场进行了测试,得到了多种流量工况下阶梯扩散器轴向水平截面的速度分布、阻力损失及阶扩处空化等情况。测试结果表明,入口浆速为8～24m.s-1时,出口段横向速度分布在长径比为3～5时达到均匀,随后圆管出口段横向速度分布再由均匀发展到不均匀,而方管出口段的横向速度在达到均布后,后续流动没有明显不均匀的横向速度分布。入口浆速越大,对纤维的解絮越有利,但随着入口浆速的增大,流动阻力急剧增加,阶扩处空化区域增大,故在纤维充分分散的前提下,入口浆速以不超过空化的临界浆速为宜。研究表明,方管出口阶梯扩散器更适用于高速纸机,适当高的浆速和合适的形状尺寸是获得定量均匀、匀度好纸张的保证。