Effect of Impeller Spacing on the Flow Field of Yield-Pseudoplastic Fluids Generated by a Coaxial Mixing System Composed of Two Central Impellers and an Anchor

The three-dimensional flow field generated by a coaxial mixer composed of double Scaba impellers and an anchor in the mixing of the xanthan gum solution, a non-Newtonian yield-pseudoplastic fluid was investigated using the computational fluid dynamics (CFD) technique. The mixing time measurements were performed by a non-intrusive flow visualization technique called electrical resistance tomography (ERT). To evaluate the influence of the impeller spacing on the hydrodynamics of the double Scaba-anchor coaxial mixer, the upper impeller submergence was set to 0.140?m while the lower impeller clearance and the spacing between two central impellers were changed within a wide range. The experiments and simulations were conducted for both co-rotating and counter-rotating regimes at different impeller spacing. The analysis of the collected data with respect to the power number, flow number, mixing time, and pumping effectiveness proved that the co-rotating mode had superiority over the counter-rotating regime. Furthermore, the impact of the impeller spacing in the co-rotating mode was assessed with respect to the mixing time, power number, and mixing energy. The results demonstrated that a coaxial mixer with the impeller spacing of almost equal to the central impeller diameter (C₂?=?0.175?m) and the impeller clearance of C₃?=?0.185?m was the most efficient configuration compared to the other cases. Additionally, the influence of the impeller spacing on the flow pattern was assessed in terms of the radial velocity, tangential velocity, axial velocity, shear rate, and apparent viscosity profiles. When the impeller spacing (C₂) was varied, the merging flow and parallel flow patterns were observed.     

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。     

Influence of Impeller Diameter on Crystal Growth Kinetics of Borax in a Mixed Dual‐Impeller Batch‐Cooling Crystallizer

The influence of impeller diameter on crystal growth kinetics of borax decahydrate in a batch‐cooling crystallizer of non‐standard aspect ratio was evaluated. The dual‐impeller configuration consisted of a pitched‐blade turbine which was mounted below a straight‐blade turbine on a single shaft. Three different impeller‐to‐tank diameter ratios were investigated. In all experiments, mixing was conducted at just‐suspended impeller speed. To examine hydrodynamic conditions, mixing times were measured. The fluid flow pattern and velocity distribution were determined by computational fluid dynamics. Results showed that the smallest but also more regularly shaped crystals were produced in a system with standard diameter impellers. Product yield and power consumption were highest in this case.     

Hydrodynamics Characterization of an Ellipse Gate Impeller by Experimental and Numerical Studies

Hydrodynamics characteristics like flow pattern, shear rate distribution, power consumption, axial pumping capacity, mixing time, and mixing efficiency of an ellipse gate (EG) impeller were investigated by experimental and numerical methods. The numerical simulation results were validated by experimental data of power consumption and mixing time. Results indicate that the axial pumping number of the EG impeller is larger than that of any other reported large‐scale impeller under laminar regime, and that the shear rate formed by this impeller is less sensitive to Reynolds numbers. In‐depth analysis reveals the different function of each part of the EG impeller under different flow regimes. This impeller provides an almost similar mixing efficiency like the double‐helical ribbon impeller under laminar regime, but much higher mixing efficiency both under transitional and turbulent flow regimes.     

LDA Velocity Measurements of High‐Viscosity Fluids in Mixing Vessel with Vane Geometry Impeller

The object of this work was to measure the velocity field in non‐Newtonian fluids inside mixing vessel. The six‐bladed vane rotor used for mixing was designed from rotating vane geometry of a sensor system, commonly used for rheometrical measurements of complex fluids (Barnes and Nguyen, J. Non‐Newtonian Fluid Mech. 98 , 1‐14 (2001); Schramm, 1994). During mixing, the viscosity was determined by measuring the torque at different impeller speeds, and compared to rheologically obtained shear dependent viscosity. The velocity field was determined by LDA measurements at twelve places inside mixing vessel. It was observed that axial and radial component of the velocity were insignificant at all measurement points. On the other hand, the results showed the periodic nature of tangential component of the velocity, which was confirmed with computer‐aided visualization method.     

CFD Prediction of Flow and Homogenization in a Stirred Vessel: Part II Vessel with Three and Four Impellers

This article deals with CFD simulations of flow inside stirred vessels equipped with three and four radial or axial impellers mounted on the same shaft. A comparison was made between simulated data and experiments for one‐ and two‐impeller systems and was presented in Part I [1]. The effect of the lowest impeller off‐bottom clearance, number of impellers used, and impeller type on the tracer distribution was studied. The simulations were mainly focused on the grid size and type and the analysis of the concentration curves in each impeller section. The predicted velocity fields, power and pumping numbers, concentration curves, and mixing times were validated with experimental data. The simulation results show the significant influence of the grid density on the velocity profiles and power and pumping numbers in contrast to the low impact on the concentration curves. A better prediction of the concentration curves was reached when radial impellers were used; the mixing times were generally over‐predicted.     

Mixing of viscoelastic fluids with helical-ribbon agitators. I — Mixing time and flow patterns

Batch mixing of viscous fluids with helical-ribbon agitators in 2.4 liter and 13 liter vessels has been studied for agitator speeds up to 200 RPM. Seven different agitators of different dimensions were employed in this work. Mixing times were measured using a decoloration technique and circulation times were determined by the tracer bead method. In addition, velocity profiles were obtained from streak photographs using selective illumination of the vessel and PVC powder as tracer particles. It was found that the mixing times of Newtonian fluids, which agreed with previously published data, were considerably (3 to 7 times) shorter than those of the viscoelastic fluids. The mixing time was strongly affected by the fluids' elasticity; increasing as the fluid elasticity increased. The velocity profiles were qualitatively similar for all the fluids but showed decreased axial circulation and increased circumferential flow as fluid elasticity increased. However, mixing is not only a function of the axial circulation (impeller pumping rate) but also is a function of the perturbations superimposed on the main flow. A simple, first approximation model based on the impeller geometry and flow patterns is proposed to correlate the circulation capacity and mixing time data for the various geometries studied.     

An experimental study of double-to-single-loop transition in stirred vessels

The velocity characteristics of the flows in a fully baffled vessel of diameter T = 290 mm stirred by a Rushton impeller of diameter D = T/3 were investigated by means of laser-Doppler anemometry measurements. The effects of clearance and rotational speed on the flow patterns in the vessel were studied. It was found that at impeller clearances from the bottom of the vessel (C) around 0.2 T the characteristic double-loop flow pattern undergoes a transition to a single-loop one with the impeller stream direction becoming partly axial and being inclined at around 25 to 30° to the horizontal. The impeller stream inclination varied with radial distance from the impeller, as well as with angular position between blades (blade angle). Impeller speed was found to have no effect on the flow pattern or the mean velocities and turbulence levels normalized by V_tip for C/T > 0.20 or C/T ≤ 0.15. The flow structure measured with C = 0.15T is described in detail and the implications of the data for fluid mixing in stirred vessels are discussed.     

Effect of impeller type on continuous‐flow mixing of non‐Newtonian fluids in stirred vessels through dynamic tests

Mixing of non‐Newtonian fluids with axial and radial flow impellers is prone to a significant extent of nonideal flows (e.g., dead zones and channelling) within the stirred reactors. To enhance the performance of the continuous‐flow mixing of pseudoplastic fluids with yield stress, close‐clearance impellers were utilised in this study. We explored the effects of various parameters such as the type of close‐clearance impeller (i.e., the double helical ribbon (DHR) and anchor impellers), impeller speed (25–500 rpm), impeller pumping direction, fluid rheology (0.5–1.5% xanthan gum solution), fluid flow rate (3.20–14.17 L min^?1) and the locations of outlet (configurations: top inlet–top outlet, top inlet–bottom outlet) on the dynamic performance of the mixing vessel. The performance of the DHR impeller was then compared to the performance of various types of impellers such as axial‐flow (Lightnin A320) and radial‐flow (Scaba 6SRGT) impellers. The dynamic tests showed that the DHR impeller was the most efficient impeller for reducing the extent of nonideal flows in the continuous‐flow mixer among the impellers employed in this study. In addition, the mixing quality was further improved by optimising the power input, increasing the mean residence time, decreasing the fluid yield stress, using the up‐pumping impeller mode and using the top inlet–bottom outlet configuration. © 2011 Canadian Society for Chemical Engineering     

Study air–water system in horizontal loop geometry

To enhance the understanding of hydrodynamic of air–water multi-phase flow inside a toroidal geometry, experiments were carried out in horizontal torus reactor. Compared with vertical flow, the flow in horizontal milli torus reactor was characterized by one additional flow pattern. In vertical position two flow regimes are considered: not-dispersed and dispersed flow while in horizontal position three flow regimes have been distinguished: stratified flow, dispersed flow and mixed flow regimes. The mixing time is measured by a conductimetric method as described by (Benkhelifa et al., 2000). The effect of both superficial gas velocities and impeller rotation speeds has been studied. The mixing time has been decreased by increasing both the superficial gas velocity and the impeller rotation speed and has been shorter than the one given for the horizontal configuration. The axial dispersion inside the reactor was modelled by the Zhang's model. The obtained results are in a good agreement with Zhang's model.     

CFD Investigation of the Mixing of Yield‐Pseudoplastic Fluids with Anchor Impellers

The study was carried out to simulate the 3D flow domain in the mixing of pseudoplastic fluids possessing yield stress with anchor impellers, using a computational fluid dynamics (CFD) package. The multiple reference frames (MRF) technique was employed to model the rotation of the impellers. The rheology of the fluid was approximated using the Herschel–Bulkley model. To validate the model, the CFD results for the power consumption were compared to the experimental data. After the flow fields were calculated, the simulations for tracer homogenization were performed to simulate the mixing time. The effects of impeller speed, fluid rheology, and impeller geometry on power consumption, mixing time, and flow pattern were explored. The optimum values of c/D (impeller clearance to tank diameter) and w/D (impeller blade width to tank diameter) ratios were determined on the basis of minimum mixing time.     

Impeller geometry effect on crystallization kinetics of borax decahydrate in a batch cooling crystallizer

The effects of impeller type and diameter in a batch cooling crystallizer on the nucleation and crystal growth kinetics as well as on the shape and size distribution of borax decahydrate crystals were investigated. Two different types of impellers of various sizes were applied. Chosen impeller configurations generate completely different fluid flow patterns in the crystallizer what allows to investigate the influence of the axial and radial flow on the kinetic parameters as well. The nucleation in crystallizer was taking place by the heterogeneous nucleation mechanism at all mixing conditions. The number of crystals formed by this mechanism increases as ratio D/d_T decreases and it is higher when an axial flow pattern in crystallizer has been developed. The crystal growth rate increases with increasing the impeller size in observed supersaturation range. The radial impeller defined by ratio D₂/d_T = 0.58 could be considered as viable option for growth of borax crystal, since the further enlargement of this ratio does not increase growth rate and can only cause higher power consumption. The maxima in the coarser and finer fractions of CSD indicate a different influence of mixing conditions on the crystal grow and secondary nucleation. An axial flow pattern in crystallizer favors agglomeration of growing crystals increasing that way product mean crystal size, while radial flow results with more regular shape of borax crystals.     

Perturbed turbulent stirred tank flows with amplitude and mode-shape variations

Stirred tank (STR) flows at low and moderate Reynolds numbers show poor mixing behavior due to formation of segregated zones inside which both magnitude and fluctuation level of velocity components show lower values compared to the active fluid regime (i.e., impeller jet stream, circulation loops). Active perturbation of the STR flow using a time-dependent impeller rotational speed can potentially enhance mixing by breaking up these segregated unmixed zones and enhancing the turbulence level throughout the tank volume. In the present study, the effect of different perturbation cycles on an unbaffled turbulent stirred tank flow at a moderate Reynolds number (rotational speed N=3 rps) is studied using a large-eddy simulation (LES) technique coupled with immersed boundary method (IBM). The perturbation frequency (f) is chosen to correspond to a dominant macro-instability in the flow (f/N=0.022). Two different perturbation amplitudes (20% and 66%) and two perturbation shapes (square-wave and sine-wave) are investigated, and changes in the mean flow field, turbulence level and impeller jet spreading are examined. Large-scale periodic velocity fluctuations due to perturbations are noticed to produce large strain rates favoring higher turbulence levels inside the tank. Production of turbulent kinetic energy due to both the mean and periodic component of the velocity field is presented. Fluctuations in power consumption due to perturbation are also calculated, and shown to correlate with the perturbation amplitude.     

Using Tomography to Characterize the Mixing of Non‐Newtonian Fluids with a Maxblend Impeller

The flow field inside a cylindrical mixing vessel was visualized by electrical resistance tomography (ERT), a non‐intrusive measurement technique. Six tomography planes, each containing 16 sensing electrodes, measured the mixing time in the agitation of pseudoplastic fluid exhibiting yield stress. The effects of various parameters such as impeller types, impeller speed, fluid rheology, power consumption, Reynolds number, and absence of baffles on the mixing time were investigated. The Maxblend impeller was able to improve the mixing performance of non‐Newtonian fluids in a batch reactor. The mixing quality could be further enhanced by decreasing the xanthan gum concentration and using baffles in the mixing vessel.     

Investigation of laminar flow in a stirred vessel at low Reynolds numbers

Many mixing applications involve viscous fluids and laminar flows where the detailed as well as overall flow structures are important. In order to understand the fluid dynamic characteristics of low Re laminar flows in mixing vessels, the flow induced by a Rushton impeller for three Re namely, 1, 10 and 28, was studied both experimentally and computationally. It was found that for the highest Re, the flow exhibited the familiar outward pumping action associated with radial impellers under turbulent flow conditions. However, as the Re decreases, the net radial flow during one impeller revolution was reduced and for the lowest Re a reciprocating motion with negligible net pumping was observed. This behaviour has not been reported in the literature in the past and represents a highly undesirable flow pattern from the standpoint of effective mixing. The CFD results successfully reproduced this behaviour. In order to elucidate the physical mechanism responsible for the observed flow pattern, the forces acting on a fluid element in the radial direction were analysed. The analysis indicated that for the lowest Re, the material derivative of radial velocity near the blade tip is small thus a balance exists between pressure and viscous forces; the defining characteristic of creeping flow. The velocity and pressure forces are in phase because the velocity is driven by the pressure field generated by the rotation of the impeller. Based on these findings, a simplified analytic model of the flow was developed that gives a good qualitative as well as quantitative representation of the flow.     

Characterization on the hydrodynamics of a covering-plate Rushton impeller

A modified Rushton impeller with two circular covering-plates mounted on the upper and lower sides of the blades was designed. There are gaps between the plates and the blades. The turbulent hydrodynamics was analyzed by the computational fluid dynamics (CFD) method. Firstly, the reliability of the numerical model and simulation method was verified by comparing with the experimental results from literature. Subsequently, the power consumption, flow pattern, mean velocity and mixing time of the covering-plate Rushton impeller (RT-C) were studied and compared with the standard Rushton impeller (RT) operated under the same conditions. Results show that the power consumption can be decreased about 18%. Compared with the almost unchanged flow field in the lower stirred tank, the mean velocity was increased at the upper half of the stirred tank. And in the impeller region, the mean axial and radial velocities were increased, the mean tangential velocity was decreased. In addition, the average mixing time of RT-C was shortened about 4.14% than the counterpart of RT. The conclusions obtained here indicated that RT-C has a more effective mixing performance and it can be used as an alternative of RT in the process industries.     

Design and Analysis of a Submerged Membrane Reactor by CFD Simulation

CFD was applied to demonstrate the effect of reactor configurations on the fluid flow pattern in submerged membrane reactors. A mixture model, a realizable k‐? model, and the multiple reference frame (MRF) technique were employed to simulate the solid‐liquid turbulent flow. Influences of the introduction of a ceramic membrane, the relative position between ceramic membrane and impeller, and the types of impeller on velocity profiles and concentration distributions were systematically discussed. These simulation results were validated qualitatively with experimental data for various reactor configurations.     

最大叶片式桨在假塑性流体中的搅拌流场模拟

为研究最大叶片式桨在高黏假塑性流体中的搅拌流动行为,以黄原胶溶液为研究体系,采用计算流体力学方法重点研究了釜内流体的功耗特性、速率分布、剪切速率、表观黏度分布和总体流动状况。结果表明:最大叶片式桨具有与大多数径流桨相似的"双循环"流型结构,且预测的功耗特性与实验数据一致性良好。最大叶片式桨适用于高黏假塑性流体的混合,而对于高黏牛顿流体的混合则效果不佳。釜内的剪切速率分布较宽泛,且受转速影响较大。转速可作为该桨改善黄原胶体系混合效率的重要参数之一。     

刚柔组合搅拌桨增强混合澄清槽内流体宏观不稳定性

流体宏观不稳定性可以反映流体轴向能量和质量的传递行为。为揭示刚柔组合搅拌桨（简称柔性桨）作用下混合澄清槽中油水液-液两相非稳态流动规律,采用频谱分析和小波分析组合法研究混合澄清槽内宏观不稳定性,并进行模拟验证。研究表明,柔性桨在转速低于250 r·min^-1时,流体宏观不稳定频率与转速呈线性关系,而转速超过250 r·min^-1,流体因界面卷吸行为吸入空气,宏观不稳定频率谱图呈现功率谱带,流场结构呈多尺度结构特征,流体宏观不稳定频率消失,液-液混合体系出现明显的乳化现象。与刚性桨相比柔性桨能增强宏观不稳定性,提高流体混合效率,强化能量传递行为。计算模拟发现,柔性桨能明显提高桨叶的抽吸能力,增强流体轴向运动的行为,避免流体过度搅拌,有利于流体澄清。     

错位刚柔桨强化搅拌槽内流体混合实验及数值模拟

为消除搅拌反应器中混合隔离区,对标准刚性桨（R-RT）、错位刚性桨（PR-RT）和错位刚柔桨（PRF-RT）三种桨叶体系的流体混沌特性参数、流场结构以及流体运动速度进行了探讨。采用Matlab软件编程计算最大Lyapunov指数（LLE）和多尺度熵（MSE）,通过计算流体力学研究了三种桨叶体系流场结构和流体运动速度的差异。实验及计算结果表明,错位刚柔桨通过柔性桨叶的随机扰动破坏了隔离区介稳态流场边界,较大程度地消除了混合隔离区。PRF-RT的LLE相比于R-RT和PR-RT分别提高了13.29%和7.25%,MSE也较PR-RT和R-RT大;PRF-RT增强了流场不稳定性,形成了不对称性流场结构,减少了隔离区分布范围;PRF-RT强化桨叶能量耗散,提高了搅拌槽底部、顶部液面以及搅拌槽壁区域流体运动速度,减小了流体混合时间。