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.     

轴流式翼型-透平组合桨在搅拌釜内的流动

<正>搅拌反应器是化工、医药、食品工业中广泛应用的基础设备之一,当搅拌釜内液深与搅拌釜直径相比较大时(通常比值大于1),往往采用双层或多层桨,以保证顶部到底部的循环流动。 透平桨产生的是径向流,形成以桨叶为中心线的上、下2个循环区,桨叶附近剪切速率高,气体分散能力强。翼型桨产生的流型是轴向流,形成全釜循环,混合均匀,剪切温和。为充分利用两者优势,加强全釜混合,剪切适度,特别是在粘稠物系中,可采用组合桨。目前尚未看到对翼型-透平组合桨流动状况的研究,而对于非牛顿流体中的组合桨流动状况则了解     

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.     

Oxygen absorption in a baffled tank agitated by delta paddle impeller

The power consumption and the volume-based mass transfer coefficient (k_la) for oxygen uptake from air into water have been measured for delta paddle impellers (10 cm and 20 cm diameter) in water in a 30 cm square sectioned baffled tank. Power consumption under ungassed conditions was measured in viscous mineral oil. For the 20 cm impeller, the effects of minor design modifications, such as sweeping the blade forward or back, have also been examined. The power numbers are approximately 2.0 in the absence of sparged gas, but are reduced by over 50% when gas is supplied beneath the impeller. Mass transfer data can be correlated with the power input per unit volume and the superficial gas velocity, showing that the performance with aeration of water is comparable to that reported by previous workers for the Rushton turbine impeller.     

Using positron emission particle tracking (PEPT) to study the turbulent flow in a baffled vessel agitated by a Rushton turbine: Improving data treatment and validation

Positron emission particle tracking (PEPT) is a relatively new technique allowing the quantitative study of flow phenomena in three dimensions in opaque systems that cannot be studied by optical methods such as particle image velocimetry (PIV) or laser Doppler anemometry (LDA). Here, velocity measurements made using PEPT in two sizes of baffled vessel (∼0.20 m and ∼0.29 m diameter) and two different viscosity fluids agitated by a Rushton turbine are compared for the first time directly in depth with some studies reported in the literature made by LDA for the turbulent regime in the equivalent geometry. Initially, the paper considers how the Lagrangian data obtained by PEPT can be converted into Eulerian in order to make the comparison most effective. It also considers ways of data treatment that improve the accuracy of both the raw PEPT data and the velocities determined from it. It is shown that excellent agreement is found between the PEPT and literature results, especially for the smaller vessel, except for the radial velocity just off the tip of the blade in the plane of the disc of the Rushton turbine. This difference is attributed to the very rapid changes in both magnitude and direction that occurs in that region and also to the different way of ensemble averaging in the two techniques. In addition, the results for the absolute velocities normalised by the impeller tip velocity for all the rectangular cross-section toroidal cells in each size of vessel and each fluid and a range of agitator speeds are compared in the form of frequency histograms. In this analysis, the velocities for each run are obtained from PEPT based on tracking a particle for 30 min and the mean and mode of the velocities each decrease slightly with decreasing scale and Reynolds number. The possible reasons for this variation in the mode and the mean are discussed. Overall, it is concluded that for the radial flow Rushton turbine the PEPT technique can be used to obtain accurate velocity data throughout the entire complex three-dimensional turbulent flow field in an agitated, baffled vessel except very close to the impeller in the radial discharge stream.     

涡轮桨搅拌槽内单循环流动特性的大涡模拟

利用大涡模拟方法研究了涡轮桨搅拌槽内的单循环流动特性,采用Smagorinsky-Lilly动力亚格子模式,与文献实验及模拟数据进行了详细的比较. 结果表明,叶片后方的双尾涡偏向槽底运动,上尾涡在30o处已开始衰减. 800000个非均匀分布的计算网格和30个桨叶旋转周期的样本数据统计可获得准确的大涡模拟数据. 时均速度、均方根速度和湍流动能的大涡模拟值与实验数据一致,而k-e模型的模拟值与实验不符. 桨叶区呈现较强的各向异性,这是导致k-e模型预测不准确的主要原因. 对于搅拌槽内的复杂流动,大涡模拟方法是一个非常有效的工具.     

Semianalytical characterization of turbulence from radial impellers,with experimental and numerical validation

Based on conventional turbulent jet theory and the general theoretical framework of scalar dispersion in turbulent shear flows, a novel formulation of the radial impeller's jet in stirred tanks is introduced. Whereas previous studies considered the impeller's jet as developed, it is now comprised of two separate spatial regions along the radial axis: the zone of flow establishment (ZFE) and the zone of established flow (ZEF). This formulation is accompanied with semi‐analytical expressions for the prediction of turbulent key parameters including the random part of k and ε in the ZFE. The new theoretical framework is validated both with laser Doppler anemometry measurements and with 3‐D numerical simulations using the standard turbulent model. © 2015 American Institute of Chemical Engineers AIChE J, 61: 1413–1426, 2015     

不同搅拌系统气液氧传递的计算流体力学模拟

研究了搅拌槽中搅拌桨组合形式对气液氧传递特性的影响,主要采用Garcia-Ochoa和Gomez基于Higbie渗透理论提出的气液氧传递理论模型,在ANSYS CFX11.0软件基础上开发了用于模拟搅拌生物反应器中氧传递过程的方法。通过模拟比较了不同搅拌桨组合的气液氧传递系数,以及在有氧消耗和无氧消耗情况下反应器内饱和氧浓度的分布,发现上层为轴流式的三宽叶搅拌桨底层为径流式的六弯叶圆盘透平桨的组合形式氧供应能力最强,体积氧传递系数与桨的形式没有直接关系,同样的搅拌桨安装在不同的桨组合中或在不同的操作条件下所形成的体积氧传递系数是有差别的。     

轴流式搅拌器湍流运动特性

引言 搅拌混合是化工工业过程中最常见,也是最重要的单元操作之一,其主要目的是加速体系中传质或传热过程.     

不同组合桨搅拌槽内非牛顿流体的微观混合特性

在直径0.282 m的搅拌槽内,以羟乙基纤维素(HEC)水溶液为工作体系,以磷酸盐?碘化物?碘酸盐平行竞争反应为模型反应,比较了非牛顿流体体系中向心桨、Rushton桨、三斜叶桨的功率准数,考察了加料时间、桨型及双层桨组合对微观混合效果的影响。结果表明,随功率增大,功率准数基本不变,Rushton桨功率准数最大,是向心桨的两倍、斜叶桨的四倍。随加料时间增大,离集指数先减小后不变。在实验考察范围内,单位体积功耗相等的情况下,单层桨微观混合效果的顺序为Rushton桨?向心桨?斜叶桨,双层桨中高剪切的Rushton桨与强循环的斜叶桨组合的微观混合效率最高。     

Importance of using the correct impeller boundary conditions for CFD simulations of stirred tanks

Two experimentally determined sets of impeller boundary conditions were used to simulate the flow generated by a pitched blade turbine in a cylindrical baffled tank. Use of these two sets of boundary conditions in simulations with two different off bottom clearances led to the conclusion that the flow generated by a pitched blade impeller cannot be successfully predicted without considering the impeller location. Correct prediction of velocity fields in the tank required the correct specification of velocity boundary conditions. Successful prediction of the turbulent energy distribution required proper specification of the turbulence boundary conditions. There was almost no interaction between the velocity and turbulence fields. The turbulet kinetic energy dissipation rate was at a maximum close to the impeller in both geometries. Within this region the average dissipation rate was five and a half times greater that the average dissipation rate in the tank.     

搅拌槽内气液两相混沌混合及分散特性

传统Rushton刚性桨常应用于过程工业中搅拌反应器内的气液分散过程,但由于桨叶背后易形成较大的气穴,气液混合效果较差。为了提高搅拌槽内气液两相的混合效果,提出了一种刚柔组合桨强化气液两相的分散过程。利用LabVIEW软件处理刚性桨和刚柔组合桨体系中气液混合过程的压力脉动信号,通过Matlab软件编程计算最大Lyapunov指数（LLE）,分析气液混合体系的混沌混合行为,同时,对刚性桨和刚柔组合桨体系中的相对搅拌功耗、整体气含率、局部气含率进行测量。结果表明,在功耗为170 W,通气量为10 m³·h^-1条件下,与刚性桨相比,刚柔组合桨能够通过刚-柔-流的耦合作用促进桨叶能量的传递过程,提高搅拌体系的混沌混合程度,刚柔组合桨体系的LLE提高了8.89%。同时,在相同操作条件下,与刚性桨相比,刚柔组合桨能够有效提高相对搅拌功耗以及搅拌槽内的整体气含率和局部气含率,且搅拌槽内气体分散更为均匀。     

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。     

多层新型桨搅拌槽内气-液两相流动的实验与数值模拟

对三层新型组合桨气-液两相搅拌槽内的流体流动进行了实验研究,并采用计算流体力学(CFD)的方法对气-液两相搅拌槽的通气搅拌功率、流场、局部气含率及总体气含率进行了数值模拟,数值模拟采用了欧拉-欧拉方法,数值模拟结果与实验值吻合良好,同时考察了通气流量和搅拌转速对通气搅拌功率和气含率的影响规律. 研究结果表明,欧拉-欧拉方法能较好地模拟搅拌槽内气-液两相流的流动状况.     

LASER-DOPPLER ANEMOMETRY IN A BAFFLED MIXING TANK

Experiments were conducted for extremely high turbulent mixing operations in a baffled mixing lank. Three components of the mean and root-mean-square velocities were measured using a laser-Doppler anemometer. The impeller rotation rate, the impeller type, and the impeller location were considered.

Due to the complexity of flow in a baffled mixing tank, there are still several fundamental modeling and computational difficulties in simulating the flow, Thus, these experimental data based on the sophisticated and extensive measurements may improve fundamental modeling methods.

The results show that the flow patterns depend on the impeller type. The effect of the impeller location on flow was small. The axial direction of the How was caused by the baffles. The flow was three-dimensional, rotational periodic, and statistically random. This information successfully characterizes the flow in the baffled mixing tank.     

Rushton桨搅拌槽中气液两相流动的全流场数值模拟

The gas-liquid flow field in a stirred tank with a Rushton disk turbine,including the impeller region,was numerically simulated using the improved inner-outer iterative procedure.The characteristic features of the strirred tank,such as gas cavity and accumulation of gas at the two sides of wall baffles,can be captured by the simulation.The simulated results agree well with available experimental data.Since the improved inner-outer iterative algorithm demands no empirical formula and experimental data for the impeller region,and the approach seems generally applicable for simulating gas-liquid stirred tanks.     

LASER-DOPPLER ANEMOMETRY IN A BAFFLED MIXING TANK

Experiments were conducted for extremely high turbulent mixing operations in a baffled mixing lank. Three components of the mean and root-mean-square velocities were measured using a laser-Doppler anemometer. The impeller rotation rate, the impeller type, and the impeller location were considered.

Due to the complexity of flow in a baffled mixing tank, there are still several fundamental modeling and computational difficulties in simulating the flow, Thus, these experimental data based on the sophisticated and extensive measurements may improve fundamental modeling methods.

The results show that the flow patterns depend on the impeller type. The effect of the impeller location on flow was small. The axial direction of the How was caused by the baffles. The flow was three-dimensional, rotational periodic, and statistically random. This information successfully characterizes the flow in the baffled mixing tank.     

Numerical Study of Solid-Liquid Two-Phase Flow in StirredTanks with Rushton Impeller(Ⅱ) Prediction of Critical Impeller Speed

The critical impeller speed, NJS, for complete suspension of solid particles in the agitated solid-liquid two-phase system in baffled stirred tanks with a standard Rushton impeller is predicted using the computational procedure proposed in Part I. Three different numerical criteria are tested for determining the critical solid suspension. The predicted NJS is compared with those obtained from several empirical correlations. It is suggested the most reasonable criterion for determining the complete suspension of solid particles is the positive sign of simulated axial velocity of solid phase at the location where the solid particles are most difficult to be suspended.     

搅拌槽内液-固两相体系的数值研究(Ⅱ)临界搅拌转速的预测

The critical impeller speed, NJS, for complete suspension of solid particles in the agitated solid-liquid two-phase system in baffled stirred tanks with a standard Rushton impeller is predicted using the computational procedure proposed in Part Ⅰ. Three different numerical criteria are tested for determining the critical solid suspension. The predicted NJS is compared with those obtained from several empirical correlations. It is suggested the most reasonable criterion for determining the complete suspension of solid particles is the positive sign of simulated axial velocity of solid phase at the location where the solid particles are most difficult to be suspended.