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。     

Effect of eccentricity on transitional mixing in vessel equipped with turbine impellers

In this paper, the results of the experimental studies of the mixing time, as well as the power consumption and baffle presence in the stirred tank with dual eccentrically located impellers are presented. The experiments were carried out in an unbaffled flat-bottomed cylindrical vessel. Three types of impellers were used: Rushton turbine, six pitched blade turbine and six flat blade turbine. The obtained data show that eccentricity of dual impeller systems leads to reduction of mixing time. Moreover, the experimental data confirmed the enlargement of power consumption in such systems. In the paper the analysis of relation between eccentricity ratio and mixing time has been performed.     

搅拌釡中层流流场的模拟

Stirred tanks are used extensively in process industry and one of the most commonly used impellers in stirred tanks is the R.ushton disk turbine. Surprisingly few data are available regarding flow and mixing in stirred-tank reactors with Rushton turbine in the laminar regime, in particular the laminar flow in baffled tanks.In this paper, the laminar flow field in a baffled tank stirred by a standard R.ushton turbine is simulated with the improved inner-outer iterative method. The non-inertial coordinate system is used for the impeller region, which is in turn used as the boundary conditions for iteration. It is found that the simulation results are in good agreement with previous experiments. In addition, the flow number and impeller power number calculated from the simulated flow field are in satisfactory agreement with experimental data. This numerical method allows prediction of flow structure requiring no experimental data as the boundary conditions and has the potential of being used to scale-up and design of related process equipment.     

Effect of eccentricity on laminar mixing in vessel stirred by double turbine impellers

Laminar mixing is often conducted in industrial processes, for example in polymerization reactors or in biotechnological processes. The laminar flow conditions caused problems of inefficient mixing due to some mixing anomalies like occurrence of the isolated mixing regions (IMR), segregation or compartmentalization phenomena. In this paper, flow visualization experiments are used to examine the size, positions and structure of the IMR regions as a function of Reynolds number and eccentricity ratio in the vessel equipped with double turbine impellers. It was found that the eccentricity brings deformation and reduction of the IMR volume. Moreover another benefit of using eccentrically located impeller systems is an improvement of axial flow. Two types of IMR regions are found: undulated IMR (UIMR) and ribbon-like IMR (RIMR). The structure of IMR depends on the eccentricity ratio defined as E/R. At the low eccentricity values the structure of single filament wrapped around core of the IMR is found. Additionally, the IMR region is inclined to the impeller plane.     

用CFD研究搅拌槽内的混合过程

在CFX软件的基础上开发了用于混合过程计算的程序,并在流动场计算的基础上对单层涡轮桨搅拌槽内的混合过程进行了初步的数值研究.对速度场和浓度场联立求解与单独求解两种处理方法分别进行了计算,计算得到的浓度响应曲线与文献数据趋势一致,两种方法计算的混合时间变化规律一致,联立求解计算得到的混合时间略小于单独求解,但是联立求解的计算量非常大.计算结果表明：混合过程与计算采用的流动场密切相关;混合时间大小不仅与监测点位置有关,还与加料位置有关,在搅拌桨附近加料混合时间最小,在槽底部加料混合时间最大.     

Investigation of impeller modification and eccentricity for non-Newtonian fluid mixing in stirred vessels

A shear thinning fluid (1% carboxymethyl cellulose) was used to investigate mixing under laminar flow conditions in an unbaffled vessel. The effects of impeller modification in addition to eccentricity were studied. Quantitative measurements such as percentage of uncovered area and coefficient of variance (CoV) of a tracer solution distributed inside the vessel were obtained using planar laser-induced fluorescence (PLIF) method. Increased eccentricity was found to be more effective than increasing rpm alone in reducing isolated mixing regions size (determined by the percentage of uncovered area). The dual-flow pitched blade turbine (DF-PBT), which was the modified version of a standard pitched blade turbine (PBT), was designed to provide both upward and downward flow at the same time to induce more chaotic flow. Though numerical analysis showed this type of flow generated, DF-PBT did not return lower values for the percentage of uncovered area and CoV than PBT did. Power consumption data were also compared between the two impeller types and eccentric locations. Further analyses focusing on the interactions between the impeller blades and fluid rheology is needed to improve laminar mixing in stirred vessels by impeller modification.     

柔性桨强化高黏度流体混合的能效分析

引言搅拌混合操作广泛应用于化工、食品、冶金及环保等过程工业领域[1-4]。搅拌槽内物料的混合程度及其功率消耗是影响产品质量和设备作业效率的关键因素。当物料黏度较高时,搅拌槽内的流     

Characterisation of three different impellers using the LDV-technique

A characterisation of three commonly used impellers was made in this study by measuring local mean velocities and the fluctuations of these velocities with the LDV technique. The data was used to estimate volumetric flow, velocity fluctuations and turbulent intensity in the impeller region of the tank. The impellers investigated were a high flow impeller, a pitched blade turbine and a Rushton turbine. The cylindrical vessel used was made of Perspex, had a dished bottom (DIN 28013), was equipped with four baffles and had an inner diameter of 0.45 m. It was found that the bulk velocities could be scaled with the tip-speed of the impeller (ND). The flow rate at constant impeller speed increased in the order high flow impeller — Rushton turbine — pitched blade turbine. The corresponding order for the turbulence fluctuation is: high flow impeller — pitched blade turbine — Rushton turbine. The velocity profile of the flow out from the high flow impeller was furthermore, not as smooth as could be expected.     

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.     

一种计算搅拌槽混合时间的新方法

基于对混合时间定义的思考,提出了一种新的定义方法,在湍流流场数值计算的基础上通过求解示踪剂的浓度输运方程,研究了单层涡轮桨搅拌槽内的混合过程。结果表明：搅拌转速和搅拌桨安装位置都影响混合时间的大小,而进料位置对混合时间的影响不大。对于不同的搅拌转速而言,随搅拌转速的增大,相同体积分数对应的混合时间逐渐减小。当搅拌桨安装在槽中间位置时所对应的混合时间最小。利用适宜的尺寸和安装位置的导流筒可有效降低混合时间。     

Effects of the Stirred Tank's Design on Power Consumption and Mixing Time in Liquid Phase

The influence of the stirrer type and of the geometrical parameters of both tank and agitator (clearance of an impeller from tank bottom, impeller diameter, draft tube and geometry of the tank bottom) on power consumption and mixing time in liquid phase under turbulent regime conditions (Re > 10⁴) have been studied. Different types of agitators have been used, namely Rushton turbine, 45° pitched‐blade turbine, MIXEL TT and TTP propellers and 1‐stage or 2‐stage EKATO‐INTERMIG propellers. All these stirrers were tested with the same power consumption per unit mass of liquid. On the basis of measured power consumption per unit mass, which is required to achieve the same degree of mixing, the results obtained in the present work show that the TTP propeller is the most efficient in liquid phase. Recommendations on the optimum geometric configuration have been made for each type of stirrer.     

一种新型搅拌桨多相混合性能研究

提出了一种新构型的搅拌桨一错位桨,并以空气-水-石英砂三相体系为研究对象,与传统的径流桨(Rushton桨)和轴流桨(斜叶桨)在功率消耗、混合时间、气体循环方面进行了比较.结果表明,错位桨相对于传统Rushton桨,功率消耗降低.适应气速范围广,轴向混合能力明显提升;在同等条件下与斜叶浆相比,气体分散能力强,混合时间少.这种新型桨能克服径向流叶轮在轴向混合方面能力的缺陷,有较好的潜在工业应用价值.     

Mixing times for process vessels with aspect ratios greater than one

Stirred tank reactors are one of the most important and common pieces of equipment used in speciality, pharmaceutical, and agrichemical processes. It is also typical for these to be operated at large aspect ratios; however, there is very little information in the open literature about mixing times in vessels with aspect ratios greater than one. This paper aims to provide new information in this area that will enable the design of better reactors.Electrical resistance tomography is used to monitor the mixing time of systems with aspect ratios higher than one. The mixing time has been measured on vessels of 0.914 and 0.610 m diameter with Cowles disc, Rushton turbine and mixed flow type impellers to aspect ratios of 2. The current correlation of choice, by Grenville and Nienow (2004), has been compared with the results and found to under predict the mixing time at aspect ratios greater than one. The exponent on the H/T term has been explored and it has been found that this varies with agitator type, this information has never been shown before.The affect of adding a second impeller on the mixing time and flow pattern is also investigated. Adding a second Rushton turbine creates zoning in the vessel which impedes the mixing; this can be visualised using electrical resistance tomography.     

搅拌槽中垂直列管外壁的传热膜系数分布

在一内径为 12 0 0mm容积为 1.5m3 的搅拌槽中 ,使用CBY螺旋桨、 4 5°斜叶桨和六直叶涡轮桨 ,对牛顿流体采用量热法 ,研究了列管外壁的局部传热膜系数的分布情况。实验结果表明 ,对轴向流桨和径向流桨 ,列管外壁的局部传热膜系数有不同的分布 ;列管外壁各点的局部传热膜系数均与ε0 .197成正比。此外 ,文中还研究了桨叶离底距离、桨径等因素对列管外壁局部传热膜系数的影响。     

Gas recirculation rate and its influences on mass transfer in multiple impeller systems with various impeller combinations

Both the numerical and experimental approaches were used to study the effects of the gas recirculation and non‐uniform gas loading on the mass transfer rate for each impeller in a multiple impeller system. By combining the calculated gas velocity and local gas holdup, the gas recirculation rate around each impeller was estimated. The local mass transfer coefficients for systems equipped with various combinations of the Rushton turbine impeller (R) and pitched blade impeller (P) were determined by using the dynamic gassing out method. It is found that the Rushton turbine impeller has to be served as the lowest impeller in order to have a better gas dispersion and to give a higher overall K_La for a multiple impeller gas‐liquid contactor. The upper pitched blade impeller always enforces the circulating flow around the Rushton turbine impeller just beneath it and gives a higher overall average mass transfer rate. However, the system equipped with only the pitched blade impellers results in a much lower mass transfer rate than the other systems owing to the poor gas dispersion performance of the pitched blade impeller.     

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     

A CFD model for predicting the flow patterns of viscous fluids in a bioreactor under various operating conditions

Computational fluid dynamics simulation is becoming an increasingly useful tool in the analysis and design of simultaneous saccharification fermentation (SSF) and saccharification followed by fermentation process (SFF). To understand and improve mixing and mass transfer in a highly viscous non-Newtonian system, it was necessary to simulate the flow behavior in this bench scale bioreactor (BioFlo 3000). This study focused on designing a high concentration medium agitation system for such a process using the commercial computational fluid dynamics package Fluent (V. 6.2.20) and its preprocessor Mixsim (V. 2.1.10). The objective of this study is to compare performance of various designs of a bioreactor and identify the flow pattern and related phenomena in the bench scale tank. The configuration of the physical model for simulating a mixing tank with a Rushton impeller consists of an ellipsoidal cylindrical tank with four equally spaced wall mounted baffles extending the vessel bottom to the free surface, stirred by a centrally located six-blade Rushton turbine impeller. Simulations were performed with the original and a modified design in which the lower bottom shaft mounted a Lightnin A200 impeller. The results suggest that there is a potential for slow or stagnant flow between top impellers and bottom of the tank region, which could result in poor nitrogen and heat transfer for highly viscous fermentations. The results also show that the axial velocity was significantly improved for the modified geometry in the bottom of the tank.     

用激光多普勒测速计（LDA）分析粘弹性对Metzner和Otto系数的影响

The Metzner and Otto correlation is the single practical method for incorporating non-Newtonian effects in the mixing process. In this article, the Metzner and Otto' s idea, the role of viscoelasticity on the Metzner and Otto coefficient, ks, effects of flow regime on ks and the determination of ks for Rushton turbine impeller have been studied using the direct method of the laser Doppler anemometry (LDA) velocity measurement for the case of viscoelastic liquids. The normalized mean tangential velocity profiles are independent of Rushton turbine impeller speeds. Contrary to literature findings, it is shown that the variation of local shear rate against the impeller speed is better correlated by the power equation, i.e. γ= ks' · Nb' , in the transition region, i. e. ～ 30 < Re <～ 2000. Also, a correlation between improved coefficient, ks', and the elasticity number of viscoelastic liquids is given which is very helpful in designing of the mixing of both viscoelastic and inelastic non-Newtonian fluids t     

Numerical Investigation of the Effect of Impeller Design Parameters on the Performance of a Multiphase Baffle‐Stirred Reactor

The turbulent gas‐liquid flow field in an industrial 100‐m³ stirred tank was calculated by using computational fluid dynamics based on the finite‐volume method. Turbulent effects were modeled with the shear stress transport model, and gas‐liquid bubbly flow was modeled with the Eulerian‐Eulerian approach using the Grace correlation for the drag force interphase momentum transfer. The relative motion between the rotating impeller and the stationary baffled tank was considered by using a multiple frames of reference algorithm. The effects of Rushton and pitched‐blade impeller design parameters such as blade geometry, location, and pumping direction on the mixing performance were investigated. It was found that a combination of Rushton turbines with up‐pumping pitched‐blade turbines provides the best mixing performance in terms of gas holdup and interfacial area density. The approach outlined in this work is useful for performance optimization of biotechnology reactors, as typically found in fermentation processes.     

一种高效的双流道自吸气叶轮搅拌器(下)

对高效自吸气叶轮在长搅拌罐高位处向下喷射气液流的现象进行了实验研究。实验发现，搅拌器混合时间大大缩短，罐内传质均匀，叶轮兼具径流式和轴流式叶轮的优点。和十二叶Ｒｕｓｈ－ｔｏｎ透平及它在低位时的情况相比，比传质系数在１１７０～１７５０ｒ／ｍｉｎ范围内是十二叶Ｒｕｓｈｔｏｎ透平的１．９７～２．７４倍，是它在低位时的１．３３～２．０８倍。利用三维气泡运动方程分析了气泡的运动轨迹及下沉深度，计算结果和实验观测值基本相符。