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.     

Flow Compartments in Viscoelastic Fluids Using Radial Impellers in Stirred Tanks

The influence of viscoelastic flow properties on fluid dynamics using radial impellers is investigated. The use of transparent model fluids allows for the optical measurement of general flow behavior with a fluorescence dying technique. By varying viscoelastic flow properties, size of agitators and rotational frequency, the impact of these parameters on fluid dynamics is analyzed. Toroidally shaped, cavern‐like flow compartments form around the agitators in all fluids in specific rotational frequency ranges, preventing an efficient mixing. By balancing elastic with centrifugal forces, a simple model is developed with which compartment sizes can be predicted with good accuracy. The results indicate a good suitability of the elasticity number as a scale‐up criterion.     

带有内外组合桨的搅拌设备内流场的数值研究

利用滑移网格法计算了以一定转速比反向旋转的内外组合桨搅拌的搅拌设备内的流场,工作介质分别选取98％甘油和1．5％(wt)CMC水溶液。通过对单桨与内外组合桨产生的流场进行对比,研究了搅拌设备内不同高度处速度、速度变化率、表观粘度及剪切速率的分布特点。结果表明：采用内外组合桨,介质为中粘牛顿流体时,在罐内主体区远离罐壁区域径向速度受到一定削弱,但在某些局部区域被增强,而对于假塑性流体径向速度总体上得到增强。采用内外组合桨可以加强轴向循环,改善搅拌设备近壁区的流动状况,且对假塑性流体流动状况的改善要优于中粘牛顿流体,另外还可以显著地提高搅拌设备内的剪切速率,同时使剪切速率的分布比较均匀。     

螺带式搅拌器在假塑液中的混合及放大的研究

本文综合研究了螺带式搅拌器在假塑性液体中的混合特性及动力特性，测定了双螺带，内外螺带及螺带螺旋３种类型搅拌器的功率常数Ｋｐ和Ｍｅｔａｎｅｒ常数Ｋｓ，并得出混匀操作和传质工艺过程的放大准则。     

A new expression of the Ks factor for helical ribbon agitators

The purpose of this note is to present a new model that is able to predict an effective shear rate in a vessel equipped with helical ribbon agitators, when mixing shear‐thinning fluids. This model is based on well established results obtained for non‐Newtonian flow in cylindrical ducts.     

Mixing in the transition flow regime with helical ribbon agitators

The effects of the non-Newtonian properties on the effective deformation rate, mixing and circulation times and flow behaviour have been investigated in the transition flow regime of mixing systems. Based on the equivalent Couette flow, three models are proposed and are shown to predict similar and drastic increases of the effective deformation rate with the impeller rotational speed in the transition regime. The predictions are shown to fit very well data obtained for various non-Newtonian fluids mixed with helical ribbon agitators, and with literature data for anchor, blade turbine and flat disc agitators. The elasticity along with shear-thinning properties appear to have slight effects on the dimensionless mixing and circulation times in the transition regime, whereas their effects in the laminar regime are quite drastic, as reported by others.     

行星轮式搅拌器及其性能分析

介绍了行星轮式搅拌器的结构特征和工作原理,并且应用fluent软件模拟其流场。对锚式、推进式及行星轮式3种搅拌器的功率准数和相对混合效率数进行了对比分析。结果表明,在相同的工作条件下,行星轮式搅拌器的功率准数与锚式、推进式搅拌器的基本相等,而行星轮式搅拌器的相对搅拌混合效率远大于锚式和推进式搅拌器的。     

搅拌釜流场测试系统的开发

针对搅拌釜流场研究的需要,研究开发了基于粒子图像测速技术的搅拌釜流场测试系统,并用该测试系统对自行设计的搅拌釜流场进行了测定。     

Analysis of the Stability of Metal Working Fluid Emulsions by Turbidity Spectra

The physical stability of emulsions can be related to changes in the droplet size distribution over time. Stability control of emulsions used as metal working fluids is an important factor for the machining industry due to the decreased performance of aged and broken emulsions. Results of turbidimetric spectra measurements of metal working fluids for process control purposes and emulsion stability monitoring are discussed. Metal working emulsions were artificially destabilized by admixing salts which resulted in droplet coagulation. The destabilization process was investigated by measuring the droplet size distribution and the turbidity spectra over time. The results were evaluated based on quantitative criteria proposed in the literature. The applicability of these criteria to evaluate metal working fluids during machining operations is discussed.     

带有新型内外组合桨的搅拌设备内流场的数值研究

以滑移网格法的基本思想为出发点提出了滑移周期的概念.利用滑移网格法计算了以一定转速比反向旋转的新型内外组合桨搅拌的流场.通过对双层斜叶平桨、标准锚式桨和新型内外组合桨产生的流场进行对比,研究了搅拌设备内的宏观流动场、时均速度、速度变化率、剪切速率和轴向循环能力.结果表明：滑移周期概念的提出在一定程度上解决了滑移网格法计算周期长、计算成本高的问题.采用新型内外组合桨,加强了径向流动和轴向流动,改善了搅拌设备近壁区的流动状况,且对假塑性流体流动状况的改善优于牛顿流体.新型内外组合桨的剪切水平和轴向循环能力均优于双层斜叶平桨.     

用于腈纶废丝溶解的往复回转式搅拌器的研究

介绍往复回转式搅拌器的特性 ,研究三角形截面往复回转式搅拌器的桨叶尺寸、釜体尺寸、桨间距、搅拌转速和搅拌功率之间的关系 ,通过对单向回转扭矩法测得的功率与日本扭矩法比较 ,验证了往复回转功率表法的可靠性 ,并进行了模拟腈纶长丝搅拌实验 ,长丝长度达釜径的 18倍 ,仍不缠结 ,当加入的长丝束质量比为 11.8%时 ,搅拌功率约增加 1.2倍     

Flow of a viscoelastic shear-thinning fluid between two concentric rotating spheres

The flow of a viscoelastic fluid, which is simulated by a Criminale-Ericksen-Filbey model, between two concentric rotating spheres is analyzed theoretically. This problem has wide applications in practice. The assessment of the performance of a polymerization reactor, for example, requires knowledge about the rheological properties of the reacting fluids and flow patterns around agitators. We show that the shear-thinning effects of both the viscosity and the elasticity of a fluid may lead to double-vortex flows. The origin of these vortices are discussed, and the effect of the relative sizes of the inner and the outer spheres on the vortex flows is investigated. The influence of the elasticity on the torque required to rotate the spheres is also evaluated.     

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。     

筒式搅拌器性能试验研究

介绍了获得实用新型专利的筒式搅拌器的结构和工作原理,对筒式搅拌器的搅拌性能进行了试验研究和分析,并与推进式搅拌器和涡轮式搅拌器进行了比较。试验结果表明,3种搅拌器在同等条件下,筒式搅拌器的能耗最小,且其排液量和循环流量最大。     

高黏度流体混合研究进展

综述了近20年来高黏度流体混合领域的研究进展,阐述了文献中常用实验物系的选择、流变特性的描述及混合性能的研究评价方法等;介绍了高黏度物系搅拌混合设备的主要类型:立式、卧式及单轴、双轴等,以及各种形式搅拌混合设备的研究成果。指出今后仍需进行深入研究的领域:同轴异速、双轴异速、立式搅拌设备及卧式搅拌装置中的桨型优化及混合性能评价;CFD在高黏度混合领域的研究仍有待深入。     

Scaling factors in granular flow—analysis of experimental and simulations results

The relationship between the design of powder mixers and their performance is of crucial importance for many industrial processes and yet is not well understood. Here the flow patterns generated by an agitator carrying a long flat blade were compared to those generated by a single ploughshare using positron emission particle tracking and DEM. The performances of these agitators were also assessed against those of multi-blade and multi-plough agitators. The powder flows created by the different designs had some qualitative similarities. Indeed, all the agitators were shown to generate a loop of circulation below the free surface of the bed. The radial support of each mixing element was also observed to act as a separatrix for the axial flow, leading to the formation of a loop of circulation on each side of the mixing elements, thus inhibiting axial convection of material. The single-blade agitators, be the flat blade or the plough, were found to induce a pulsing regime, this feature being also observe with the multi-blade agitators but with a lesser amplitude. For moderate agitator frequencies of rotation where inertial effects are negligible, fundamental characteristics of the flow patterns extracted from power spectra of axial displacements could be scaled with the number of blade passes.     

Einfluss der Rührwerksposition auf den Mischprozess in Biogasanlagen anhand eines Paddelrührwerks

The development of agitators in biogas plants is based on empirical values. Here, the influence of altered agitator positions and fluid properties on the flow field of a paddle agitator was examined by particle image velocimetry (PIV) measurements. The viscosity characteristics of fermenter slurries were analyzed by a pipe viscometer to adapt the viscosity characteristics of a model fluid with regards to similarity laws. An altered position reduces the dead zones significantly by up to 55 %.     

Heat transfer rates to newtonian fluids in aUSn anchor-agitated kettle

Heat transfer rates to Newtonian fluids in a jacketed kettle using anchor agitators have been measured. Experimentally evaluated liquid-film heat transfer coefficients at the vessel walls can be correlated by the modified Sieder-Tate equation as used by previous workers but the results of the present and previous work are shown to be better correlated by the application of the theory of local isotropic turbulence. An attempt has also been made to establish the effect of wall clearance on the heat transfer efficiency of the contactor. The results indicate that high heat transfer rates obtainable with agitator-scrapers can be realized with anchors without scrapers.     

筒式搅拌器及其开发

介绍了实用新型专利——筒式搅拌器的结构和设计原理。筒式搅拌器主要有筒体和内弯叶片组成,搅拌时能同时产生强大的径向流和轴向流,具有较大的排液量和循环流量,混合效果极强。筒式搅拌叶轮的各尺寸设计取决于液体性质、混合要求、容器直径和转速等因素。     

Development and use of positron emitting particle tracking (PEPT) for velocity measurements in viscous fluids in pilot scale equipment

Positron emitting particle tracking (PEPT) can be used to trace the path of a radioactive particle within opaque fluids in pilot scale equipment. Isokinetic tracers can be used to characterize flow fields inside such equipment. A pilot scale heat exchanger was designed and used with PEPT to evaluate the accuracy and applicability of the technique; measurements were performed over a range of flows found in food processing plant, including extruder flows. Velocity distributions were estimated from the measured tracer locations for a range of barrel speeds, flow rates and rheological properties. The flow field is 3D; barrel speed and flow rate had a more pronounced effect on the velocity distributions than the rheological properties of the fluids. The flow field was simulated using a simple 2D approach and a full 3D simulation. The simulated velocity distributions compared favourably to the experimentally determined profiles: trends were correctly predicted by the 2D model whilst statistical agreement was found between data and the 3D simulation. The agreement between PEPT and CFD reinforces the validity of both methods.