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.     

Reynolds number effects in a simple planetary mixer

Planetary mixers are widely used in a diverse range of industrial applications. This paper presents an experimental investigation of mixing in a planetary mixer, and a comparison with numerical simulations based on a simple mathematical model of the flow. The model allows an exact expression for the velocity field in the Stokes flow regime, apparently the first for a mixer with genuinely moving parts, which permits accurate numerical tracking of material interfaces. Experiments performed at low Reynolds number (Re?1) show good agreement with corresponding numerical simulations, but as the Reynolds number is increased, the agreement between experiments and Stokes-flow numerics worsens, in a manner that reflects improving experimental mixing quality. Specifically, we find that islands of poor mixing shrink as Re increases. Our results suggest that, while numerical simulations in the Stokes flow regime may be used as a ‘sieve’ to select good mixing protocols at small Re, experiments or computational fluid dynamics simulations are required properly to evaluate mixing protocols operated at finite Reynolds numbers.     

新型同心双轴搅拌器功率与混合特性的数值模拟

基于同心双轴搅拌器的结构与运行特点,建立了兼顾其流动、混合过程的三维数学模型,并以过程工业应用较多的两种不同尺寸双层组合桨作为内桨、框式桨作为外桨构成的同心双轴搅拌器为研究对象,数值模拟了其在中高黏牛顿流体中同向及反向转动模式的功率特性、流场特性及混合特性。模拟结果表明,同向转动模式下,整个系统的搅拌功耗更小、混合效率更高;外桨功耗受内桨影响较大,一般随内桨转速的增大,恒速外桨的功耗同向转动时会减小、反向转动时会增大;对由桨式搅拌器构成的组合式内桨而言,当内桨直径与釜体直径之比为0.35左右时,相同Reynolds数下的单位体积混合能更小;中高黏牛顿流体中,同心双轴搅拌器的内桨采用上层六斜叶桨+下层六直叶桨的组合形式时更高效节能,仅在体系Reynolds数小于36时,上层二斜叶桨+下层二直叶桨的内桨组合形式才具有相对优势。     

剪切变稀体系同心双轴搅拌釜内的气液分散模拟

气液搅拌设备因其良好的适用性被广泛应用于过程工业中。为更好地比较不同工况下剪切变稀体系中的气液分散情况,通过实验研究整体气含率和相对功耗确定适宜的转动模式,进而模拟研究表观气速、体系黏度、搅拌转速对气含率和气泡尺寸的影响。结果表明,相同功率下内外双桨反向旋转模式在理想气液分散条件下,相较于单轴内桨和内外双桨同向旋转模式具有更高的气含率和更好的气体泵送能力;表观气速的增加有利于气泡的均匀分散,但气泡尺寸也会随之增大;有效黏度的增加使得搅拌桨的影响区域变小,不利于气泡的均匀分散,气泡尺寸也随之增大;搅拌转速的增加使得循环涡流的影响区域变大,高气含率区不断扩大。     

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.     

Mixing analysis in a coaxial mixer

The performance of a coaxial mixer in the laminar-transitional flow regime was numerically investigated with Newtonian and non-Newtonian fluids. These mixers comprised two shafts: a central fast speed shaft mounted with an open turbine, and a slow speed shaft fitted with a wall scraping anchor arm. To model the complex hydrodynamics inside the vessel, the virtual finite element method (POLY3D^TM software) coupled with a Lagrange multiplier approach to cope with the non-linearity coming from the rheological model was employed. Co-rotation and counter-rotation mode were compared, based on several numerical criteria, namely, mixing time, power consumption and pumping rate. It was found that co-rotating mode is more efficient than counter-rotating mode in terms of energy, pumping rate and homogenization time.     

Power-draw analysis of a coaxial mixer with Newtonian and non-Newtonian fluids in the laminar regime

The power consumption of a new coaxial mixer composed of a wall scraping arm and a series of rods and a pitched-blade turbine mounted on the same axis of revolution and operated in a contra-rotating mode has been characterized. The work is based on experimental measurements and 3D numerical simulations in the case of homogeneous Newtonian and non-Newtonian fluids in the laminar regime. Very good agreements between experimental and numerical results have been obtained. It has been shown that the Metzner-Otto concept can be extended to account for the speed ratio between the impellers, which allows to represent the power consumption results of the coaxial mixer on a single power master curve like with a single agitator mixer.     

Experimental investigation on micromixing characteristics of coaxial mixers in viscous system

An experimental investigation into the micromixing performance of coaxial mixers in a viscous system is reported, in which the iodide-iodate reaction system is chosen to quantitatively characterize the product distributions. The effects of feeding time, feeding position, impeller speed, inner impeller configuration, and operation mode on the segregation index, Xs, are examined. It is revealed that the feeding near the inner impeller benefits micromixing and should be regarded as the preferred position. The presence of the rotating outer impeller causes the micromixing performance of the coaxial mixer to be significantly better than the single-shaft mixer. While an increase in the outer impeller speed has a limited influence on micromixing, the inner impeller speed is the dominant influencing factor, that is, the Xs decreases obviously when the inner impeller speed is increased. On the other hand, the coaxial mixers with multiple and axial inner impellers have a better micromixing performance at the same specific power consumption, P_V, than that with single and radial inner impellers. Among the configurations consisting of a Rushton impeller (RT), six-straight-blade turbine impeller (SBT), and six-pitched (45°)-blade turbine impeller (PBT), the Xs of the coaxial mixer is always the smallest at the same P_V when the PBT + RT configuration is used as the inner impeller. In addition, it is found that the difference in Xs that results from various operation modes is small in terms of power consumption; however, the co-rotation mode is still recommended for the micromixing of the coaxial mixer due to its excellent performance in general.     

Numerical investigation of the mixing efficiency of the Ekato Paravisc impeller

The viscous mixing characteristics of the Ekato Paravisc are compared with those of an anchor and a double helical ribbon. The methodology is based on 3D CFD finite element-based simulations. The predictions are first validated by comparing the Newtonian and non-Newtonian power consumptions and mixing times against literature experimental data. Then, the computed 3D laminar flow patterns and several mixing performance criteria (power consumption, pumping capability, intensity of segregation, mixing time, mixing efficiency and specific energy) of the impellers are investigated. It is shown that the Paravisc mixer characteristics lie between that of the other impellers at low Reynolds number.     

同心双轴复合式搅拌釜用于牛顿流体时的功耗及混合特性

在直径0.48 m的椭圆底搅拌槽内,液位与槽径比(H/T)为0.6,采用不同粘度的牛顿流体糖浆溶液,研究了分别以CBY, 45o四斜叶桨及Rushton涡轮桨作为快速分散桨、锚式桨作为慢速桨构成的同心双轴搅拌系统,在快、慢速轴同向和异向2种旋转方式操作时的功率特性和混合性能. 结果表明,分散桨对锚式桨的功率消耗影响较大. 两轴同向旋转时,分散桨会使锚式桨的功耗降低,转速比RN增加,降低幅度也增大,RN=14时,锚式桨功率可降至单独旋转时的约10%;异向旋转时锚式桨的功率随RN增加而增加,RN=14时,锚式桨功率可增至单独旋转时的2倍左右. 但锚式桨对分散桨的功率消耗影响很小,在±5%以内. 计算同心双轴复合搅拌系统的复合功率准数和复合雷诺数关系时考虑了RN的影响,使在实验条件下不同转向及RN的功率曲线较好吻合. 混合效果同向旋转优于异向旋转,在牛顿流体中,达到相同混合效果时,CBY桨的能量消耗仅为其他2个分散桨的20%~30%.     

Hydrodynamics characterization of the Maxblend impeller

The hydrodynamic characteristics of the Maxblend^TM impeller have been investigated in the case of viscous Newtonian fluids. Both laboratory experiments and 3D finite element based computational fluid dynamics (CFD) simulations have been carried out. The power consumption, the mixing evolution yielding the mixing time, and the effect of baffles in the laminar and transition flow regimes have been determined. It was found that the limit Reynolds number between the laminar and transition regimes is approximately 25 and 38 for the unbaffled and baffled configurations, respectively. Based on the range of Reynolds numbers studied in this work, the best window performance of the Maxblend^TM mixer where fast and homogenous mixing is achieved is the end of the laminar regime and the early transition regime with baffles.     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

Mixing energy investigations in a liquid vessel that is mixed by using a rotating magnetic field

The purpose of the present report is, first, to develop the basic equations and similarity criteria governing the mixing system with the magnetic particles under the effect of a rotating magnetic field (RMF); second, to carry out the experimental investigations dealt with the power consumption and mixing time under the action of RMF. Dimensionless correlations are proposed to predict power consumption and the mixing time in the analyzed mixing system. The study relates to the mixing energy defined as the product of the power input and the mixing time in a tested experimental set-up. The data obtained for mixing efficiency was correlated in terms of the modified Reynolds number which can be used for predictions of mixing process in systems with the magnetic particles. The magnetic particle may be treated as a miniaturized mixer and it may offer a unique alternative approach to mixing. The satisfactory comparison indicates that the mixing process under the action RMF has the significantly higher of the energy consumption than the classical agitators.     

Agitation of Herschel–Bulkley fluids with the Scaba–anchor coaxial mixers

The design of the coaxial mixers depends on many interrelated parameters including the geometry and dimensions of the mixing vessel, the location and type of the impellers, speed ratio, impeller diameter, rotation mode, and fluid rheology. No study has been reported in the literature regarding the mixing performance of the coaxial mixers in the agitation of yield-pseudoplastic fluids. Thus, the main objective of this study was to evaluate the performance of a Scaba–anchor coaxial mixer (a novel configuration) in the mixing of xanthan gum solutions (yield-pseudoplastic fluids). The Herschel–Bulkley model was used to describe the rheological behavior of the xanthan gum solutions. To develop new correlations for the generalized Reynolds and power numbers of the coaxial mixers employed in the agitation of this class of non-Newtonian fluids, we utilized numerous experimental and computational fluid dynamics (CFD) data. The new correlations were tested successfully at different operating conditions (e.g. speed ratio, fluid rheology, and operation mode).     

Flow field analysis of a cavity transfer mixer

The cavity transfer mixer (CTM) was primarily designed as a distributive mixing device to be used as an add-on unit to existing extruders. In determining the CTM overall mixing efficiency as well as its potential use for various applications, the flow patterns/characteristics within this mixer must be well understood. In this work, a fluid dynamics analysis package, FIDAP, based on the finite element method was employed to simulate the flow patterns in a CTM with 6 rows and 4 cavities per row. A 3D, isothermal flow analysis was carried out and the fluid of choice was a flexible polyvinylchloride whose rheological behavior is described by a power-law model fluid. The flow field was characterized in terms of velocity profiles, pressure distributions, shear rates/shear stresses generated and a parameter λ quantifying the elongational flow components. The results of the flow simulations were compared with experimental data. The CTM potential use for dispersive mixing was discussed.     

Effects of rotor and stator geometry on dissolution process and power consumption in jet-flow high shear mixers

The jet-flow high shear mixer(JF-HSM)is a new type of intensified equipment with special configurations of the rotor and the stator.The mass transfer property and power consumption were studied in the solid-liquid system for a series of JF-HSMs involving different configuration parameters,such as rotor diameter,rotor blade inclination,rotor blade bending direction,stator diameter,and stator bottom opening diameter.The flow characteristics were examined by computational fluid dynamic simulations.Results indicate that the turbulent power consumption of the JF-HSM is affected by the change in rotor blade inclination and stator bottom opening.With the increase in the shear head size and the change in the rotor into a backward-curved blade,the solid-liquid mass transfer rate can be remarkably increased under the same input power.Dimensionless correlations for the mass transfer coefficient and power consumption were obtained to guide the scale-up design and selection of such a new type of equipment to intensify the overall mixing efficiency.     

圆盘涡轮式搅拌器的搅拌性能比较分析

在椭圆封头搅拌槽中根据行业标准建立了4种圆盘涡轮式搅拌器的几何模型,利用标准k-ε湍流模型和多参考系(MRF)方法,研究了叶片形状、搅拌速度、旋转方向和流型转变对搅拌功率、排出流量、泵出效率和剪切速率的影响。结果表明：叶片形状对功率、排出流量、泵出效率和流型转变的临界安装高度均有影响;径向流条件下,泵出效率排序为圆弧叶(反)>弯叶(正)>箭叶(反)>圆弧叶(正)>平直叶>箭叶(正)>弯叶(反);流型由径向流转变为轴向流后,搅拌功率及径向排出流量均下降。对标准搅拌器的性能评估为工业搅拌设备的选型和设计提供了参考依据。     

双转子连续混炼机转子混炼段流场的数值研究

采用有限元分析软件POLYFLOW对非牛顿聚合物熔体在ECM30双转子连续混炼机转子混炼段的三维等温流动进行了数值模拟，得到了转子混炼段流场的压力、各速度分量、剪切速率的分布。通过对这些流场参数分布的分析。发现在该类混炼机的轴向存在着一定的反向流动。使得该设备具有较强的轴向分布混合特性；另一方面，转子螺棱顶部与机筒内壁的问隙处存在较大的剪切速率，保证了对物料进行有效的分散。     

Computational study of convective–diffusive mixing in a microchannel mixer

Scalar mixing due to convection and diffusion in a microchannel mixer is studied using CFD. A method is developed to quantitatively measure the effect of false diffusion on scalar decay rate. This method computes an average false diffusivity from a given numerical solution and it is not limited to any particular numerical scheme. It is found that a range of molecular diffusivity exist in which average false diffusion is smaller than molecular diffusion and scalar decay rates can be computed accurately with CFD in the mixer. This range of molecular diffusivity covers most of the liquid solutions encountered in chemical and biochemical engineering. When effective diffusivity is used, this range can be further expanded. The predicted mixing structures agree well with experimental results in literature. The classical lamellar structures of the baker's transformation are strongly affected by diffusion. The striation doubling process is destroyed by diffusion broadening at very early stage in the mixer. The optimal mixing is achieved at low Re when the mixing mechanism in the mixer is the baker's transformation. At higher Re, secondary flow is generated and the mixing mechanism is the competition of the kinematics of the baker's transformation and the dynamics of the cross sectional flow. Results show that the secondary flow hinders mixing and the scalar decays at lower exponential rates than when the mixing is due to the baker's transformation alone.