柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点,引入适用于强旋转流场的各向异性k-ε湍流模型,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟.利用文献中对搅拌槽内流场测定结果,给出了适用于Rushton 桨搅拌槽的各向异性湍流黏度系数值.模拟计算得到了搅拌槽内的流场分布和脉动速度分布,并同标准k-e湍流模型计算结果及文献数据进行比较.结果表明,各向异性k-ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量,明显优于标准k-ε湍流模型.     

各向异性k-ε湍流模型在Rushton桨搅拌槽三维流场整体数值模拟中的应用

根据搅拌槽内的流动呈各向异性的特点 ,引入适用于强旋转流场的各向异性k -ε湍流模型 ,用改进的内外迭代法对有挡板的Rushton桨搅拌槽进行了整体数值模拟 .利用文献中对搅拌槽内流场测定结果 ,给出了适用于Rushton桨搅拌槽的各向异性湍流黏度系数值 .模拟计算得到了搅拌槽内的流场分布和脉动速度分布 ,并同标准k -ε湍流模型计算结果及文献数据进行比较 .结果表明 ,各向异性k -ε湍流模型能成功反映Reynolds应力、湍流动能等湍流特征量 ,明显优于标准k -ε湍流模型 .     

柔性Rushton搅拌桨的功耗与流场特性研究

基于传统的Rushton桨,开发了一种柔性叶片Rushton搅拌桨。采用数值模拟方法研究了柔性桨的功耗及层流和湍流流场特性,并分别采用扭矩测量法和粒子图像测速法进行了实验验证。结果表明,对于实验规模的搅拌容器,当介质黏度与甘油接近时,可用橡胶作为柔性桨叶制作材料。Reynolds数≤100时,柔性桨的功耗大于刚性桨;Reynolds数大于该值后,柔性桨的功耗小于刚性桨。柔性桨叶对被搅拌流体具有自适应特性,流固耦合作用下产生的变形增加了流体的径向流动能力。搅拌低黏度流体时,柔性桨能提升近桨区流体的速度,增加桨叶远端流体的循环流动能力;搅拌高黏度流体时,近桨区和桨叶远端流体的速度均大于刚性桨。就尾涡而言,柔性桨产生的涡量较小,耗能少。     

柔性Rushton搅拌桨的振动特性

采用数值模拟的方法分析了课题组前期开发的实验室规模的柔性叶片Rushton搅拌桨的振动特性,并采用丹麦的Brüel & Kj?r及中国的东华振动测试仪进行了实验研究。结果表明,桨轴系统的第1~6阶振型为弯曲型,第7~12阶振型为扭转型。模拟得到的固有频率与实验结果吻合较好,均表明存在集聚现象;与干模态固有频率相比,湿模态固有频率有所降低。承受高频交变激励载荷时,桨轴系统存在明显的应力和应变谐响应。桨轴系统的固有频率随转速的增大而减小,随介质黏度的增大而增大。研究结果为柔性叶片Rushton搅拌桨的放大设计及工业应用奠定了基础。     

6种常规搅拌器的剪切性能研究

不同搅拌工况下剪切性能的需求是选用搅拌器的一个重要指标,文中基于对常规的6种搅拌器流场的CFD模拟,计算了搅拌器各自的剪切性能量纲一数C_3,分析了搅拌流场中剪应力的累计概率分布及统计平均结果。二者对比发现:从宏观平均效果上看,搅拌器剪切性能满足直叶桨>斜叶桨,圆盘涡轮>开启涡轮>桨式;而剪应力极大值影响作用则是直叶圆盘涡轮>斜叶圆盘涡轮>直叶桨式>斜叶开启涡轮>直叶开启涡轮>斜叶桨式。因此,对剪切作用敏感的搅拌过程,在采用常规的搅拌器剪切性能评价的基础上,还需要充分考虑搅拌流场中剪应力极大值的影响,合理选用搅拌器。     

固-液搅拌槽内桨型对颗粒悬浮特性影响的实验和模拟研究

以水为液相,玻璃珠为固相,在固-液搅拌槽内比较了传统径向流Rushton桨、轴向流下推式45°六斜叶桨以及新型半折叶搅拌桨的功耗、泵送能力和对固体颗粒的悬浮效果。并应用CFD （Computational fluid dynamics）方法研究了不同搅拌桨操作下颗粒的轴向速度分布。结果表明：在相同转速下,Rushton桨的功耗最大,新型半折叶桨与下推式45°六斜叶桨的功耗接近;新型半折叶桨的流量准数最大,泵送能力最强;在固-液体系中,新型半折叶桨与下推式45°六斜叶桨的流型类似,但3种桨中新型半折叶桨对固体颗粒的悬浮效果最好。     

自吸射流柔性搅拌桨振动特性

为了研究工况对新型自吸射流柔性搅拌桨的影响，运用实验和单向流固耦合数值模拟分析了装置的搅拌功耗、搅拌桨应力分布和振动特性。结果表明，相同工况下柔性Rushton搅拌桨的功率准数比自吸射流柔性搅拌桨高121.93%，并随Reynolds数的提高而减小。自吸射流柔性搅拌桨在支架与桨叶连接处应力最高，在本文研究范围内最大应力低于材料的许用应力。对自吸射流柔性搅拌桨在静模态和预应力模态下的固有频率和振型进行对比分析，两种模态前8阶为弯曲型，后4阶为扭变型；与静模态相比，预应力模态下固有频率和振型最大值的最大偏差分别为0.25%和27.56%。随着搅拌桨的转速和介质运动黏度提高，搅拌桨预应力模态固有频率增大，但同阶固有频率增长比变化较小。自吸射流柔性搅拌桨搅拌效率优势明显，本研究为多种工况下搅拌桨的强度和稳定性提供基础数据。     

刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。     

刚-柔组合搅拌桨强化流体混沌混合

合理设计搅拌反应器的桨叶,强化流体流动与混合行为,是实现流体高效、节能混合的重要手段。柔性体与刚性体组合,可设计出具有多体运动行为的刚-柔组合搅拌桨。结合PIV流场观测和CFD模拟,对比分析了刚性桨和刚-柔组合桨对流场结构及流体混沌混合行为的影响。结果表明,与刚性搅拌桨相比,刚-柔组合桨的柔性端强化能量传递,流体流速衰减速率降低25%,有利于搅拌桨输入能量在流场结构内的有效分配。传统刚性六凹叶和六直叶涡轮桨搅拌反应器内流体形成的流线结构具有明显的周期吸引子,其时均流场的分形维数分别为1.9046和1.9138。刚-柔组合六直叶涡轮桨搅拌反应器内流体流线呈明显的准周期性吸引子性质,其流场分形维数为1.9337,而刚-柔组合六凹叶涡轮桨搅拌反应器内流体流线具有典型的混沌吸引子性质,其流场分形维数为1.9545。刚-柔组合搅拌桨可改变流体流线的吸引子来调控流场的多尺度结构,强化流体混沌混合,实现高效节能操作。     

Gas-liquid hydrodynamics in a vessel stirred by dual dislocated-blade Rushton impellers☆

Towards the objective of improving the gas dispersion performance, the dislocated-blade Rushton impeller was applied to the gas–liquid mixing in a baffled stirred vessel. The flow field, gas hold-up, dissolved oxygen, power consumption before and after gassing were studied using the computational fluid dynamics (CFD) technique. Dispersion of gas in the liquid was modelled using the Eulerian–Eulerian approach along with the dispersed k–εturbulent model. Rotation of the impeller was simulated with the multiple reference frame method. A modified drag coefficient which includes the effect of turbulence was used to account for the momentum exchange. The predictions were compared with their counterparts of the standard Rushton impeller and were validated with the experimental results. It is concluded that the dislocated-blade Rushton impeller is superior to the standard Rushton impeller in the gas–liquid mixing operation, and the findings obtained here lay the basis of its application in process industries.     

Shear and skin friction on particles in power-law fluids agitated by flat-blade and fluid foil impellers

The shear rates that exert angular deformation on spherical particles have been measured. The particles are mimiced by a spherical probe. The probe has been immersed in various impeller-agitated power law fluids. The fluids are aqueous dispersions of polymers, e.g. CMC, xanthan gum and starch. The probe has been positioned in various points of a stirred vessel and at various angles. Angle-averaged shear rate distributions were produced. The distributions obtained are characteristic for the specific impeller flow patterns. The flow patterns have been identified by computational fluid dynamics (CFD). Two types of impellers representative for the flat and the fluid-foil blade design, i.e., a Rushton flat-blade turbine (RT) and a Narcissus impeller (NS) are studied. The effects of rheological properties and blade design on the ‘shear-rate-on-particles’ distribution are examined. The local shear field non-uniformity has been uncovered and compared in terms of the CFD-generated time-averaged velocity and deformation rate profiles. The ‘shear-rate-on-particles’ distribution apart from the impeller is found to follow qualitatively the time-averaged inner flow shear rate distribution. Referring to impeller speed 5-12.5 Hz, the dimensionless wall shear rate varied between 200 and 1000. In power law fluids, the shear rate on particles decreased up to 50%. The fluid-foil NS-generated shear field was found comparable to the shear field induced by conventional flat-blade turbines and appeared in cases less sensitive to polymer presence. The shear rate produced by the fluid-foil impeller in the highly shear-thinning model solution (n∼0.4) exceeded the flat-blade RT-imposed shear rate. The analysis has been extended to skin friction drag on particles. It is shown that, while exerting an undoubtedly greater angular deformation in water-like fluids, in polymer presence the conventional flat-blade turbine introduces a flow geometry that imposes particle drag that is close or in some cases even less than the one generated by the fluid-foil impeller. The fact implies a weak shape effect of radial turbines on shear-sensitive particles or particle dispersions in power law liquids.     

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.     

不同型式搅拌桨对黄原胶水溶液搅拌效果的CFD数值模拟

使用FLUENTa软件对黄原胶溶液在搅拌槽内的流动特征、桨叶搅拌效果和功率消耗进行了数值模拟. 计算采用多重参考系方法和标准k-e湍流方程. 黄原胶浓度为0~2.0%(w),桨型为直叶圆盘涡轮、非对称抛物线圆盘涡轮和四斜叶桨. 结果表明,不同桨型下溶液的粘度分布有较大差异,且搅拌效率随溶液浓度增加急剧下降,转速增加能有限提高搅拌效率. 径流桨和轴流桨产生的功率消耗随溶液浓度改变呈相反的变化趋势. 在黄原胶浓度2.0%(w)、搅拌转速7.5 r/s时,所有桨型下有效搅拌体积所占比例均低于60%;与在水中相比,直叶圆盘涡轮的功率消耗降低约7%,而四斜叶桨的功率消耗增加29%.     

Power and mean flow characteristics in mixing vessels agitated by hyperboloid stirrers

The mean flow and energy consumption in vessels powered by hyperboloid stirrers was investigated. The Newton number followed an inverse linear law for Reynolds numbers below approximately 200, which had values more than twice higher the corresponding Newton number for a standard Rushton turbine. At high Reynolds number flows the Power number varied between 0.5 for a D/T = 0.78 impeller to 0.95 for a D/T = 0.24 impeller, as compared to a value of 5 for the standard Rushton stirrer, and to values of 0.31 and 1.58 for the Chemineer and Prochern hydrofoils. The power consumption did not change with the fluid height and was double for the double-stack configuration. The shear ribs below the impeller were the main contributor to the increased energy loss relative to a non-shear ribbed impeller and the small clearance had no major effect upon the power consumption. For the D/T= 1/3 hyperboloid stirrer the flow in the whole tank was rather gentle, defining a circulation number of 0.57, thus leading to a circulation efficiency more than 7 times lower than that of the hydrofoils.     

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.     

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。     

Power Consumption in Gas‐Liquid Contactors Agitated by Double‐Stage Rushton Turbines

The dependence of power consumption on impeller spacing in unaerated and aerated gas‐liquid contactors agitated by dual Rushton turbine systems was studied, and the gas flow rate and viscosity effects were measured in relation to these parameters. The experiments were carried out in a 0.19 m i.d. vessel stirred by two Rushton turbines with a diameter d = 0.10 m; with blade length and blade height 0.25 d and 0.2 d, respectively. In tap water the impellers acted independently for spacings greater than 1.65 d, while in glycerol solutions the two impellers already acted independently at an impeller spacing equal to 1.2 d. In aerated systems, a notable increase in the power consumption with increasing impeller spacing could be detected for small gas flow rates and low viscosities, while a decrease in the Newton number with increasing Froude number could be observed at constant impeller spacing. The Newton number was not affected by flow number at high viscosity values.     

A study on some effects of a drag-reducing agent on the performance of a stirred vessel

The influence of the addition of a drag reducing agent (100 ppm PAA) to a pure liquid (water) in a stirred vessel has been studied. The vessel was stirred with two types of agitators, a Rushton turbine and an axial A310 impeller.This study was meant to implement previous findings with new data and various types of measurements were performed to determine a number of parameters. It is confirmed that the drag-reducing agent decreases power draw and increases mixing time. Mean flow and the fluctuating velocities were determined (with the PIV technique) and notable reduction in turbulence intensity was detected. The mechanism of tracer dispersion (followed by the PLIF technique) was also documented qualitatively. Finally, the influence of the drag-reducing agent on the particle size distribution of the solid product of a precipitation reaction (reaction of barium chloride and sodium sulphate) was shown.