四大叶斜桨的流体动力性能

四大叶斜桨搅拌器适用于对剪切稀化流体的搅拌混合。本文对此种新型搅拌器的流体动力性能作了实验研究,测定了其Metzner常数、搅拌功率、排液量和混合时间。四大叶斜桨的密实度比为0.57,具有排液量大,混合速度快的优点,而且功耗适中。文中还给出了四大叶斜桨与常用的涡轮和螺旋桨的性能对比。     

双轴表面更新型搅拌釜中高粘牛顿流体与非牛顿流体的功耗

实验测定了双轴表面更新型卧式釜中采用三种不同形式的搅拌桨、多种桨间距、广泛的粘度范围内牛顿流体和非牛顿流体的功率特性;统一关联了牛顿流体与非牛顿流体的搅拌功耗,得到:N_PRe=C(l/D_f)~(-1)(V_L/V~*)~D式中的C、D是搅拌桨的结构参数.实验结果还表明,该装置的Metzner常数是转速的函数.     

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     

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。     

非牛顿流体搅拌流场的数值模拟研究进展

对非牛顿流体搅拌流场数值模拟过程中的控制方程、旋转桨叶的处理以及数值计算方法三个方面进行了综合论述。阐述了广义牛顿流体模型形式简单、计算量低,在非牛顿流体搅拌流场数值模拟过程中应用广泛;黏弹性流体本构方程具有高度的非线性,采用计算流体力学(CFD)方法对其搅拌流场进行数值模拟难度较高,目前仍处于起步阶段;通过合理简化黏弹性流体本构方程以及采用恰当的数值离散方法,有助于在黏弹性流体的搅拌流场数值模拟中取得进展。     

搅拌槽内非牛顿流体的微观混合特性

在直径为0·476m的搅拌槽内,分别选用三窄叶翼型搅拌桨和标准六直叶涡轮搅拌桨,采用羟乙基纤维素(HEC)水溶液非牛顿流体作为工作体系,以改进的硫酸铜沉降反应与氯乙酸乙酯水解反应的平行竞争反应作为研究微观混合的反应工作体系,在质量分数为0·1%~0·5%的HEC水溶液中,排除硫酸铜副反应的影响,利用铜离子质量浓度与可见光吸光度的线性关系,考察了进料时间、搅拌转速、溶液黏度、流体的非牛顿性以及搅拌桨类型等因素对副产物收率(XQ)的影响规律。结果表明,当进料时间超过临界进料时间3500s后,X_Q不再改变;随着搅拌转速的增加,X_Q减小,但其减小的趋势逐渐变缓;在相同转速下,X_Q随着流体黏度的增加而增加,但增加的趋势有所减缓;对于黏性流体不能仅仅采用雷诺数作为放大准则;在相同的能量耗散速率及桨叶区进料的情况下,Damkohler准数较大的标准六直叶涡轮搅拌桨的X_Q较小。     

纤维素酶水解反应器的开发与放大

以木质纤维素生产燃料乙醇为背景,在考察玉米秸秆同步糖化与发酵反应特征基础上,给出了合适的反应器构型:螺带型搅拌槽;直径0.2～0.8m,高径比1～2,转速20～120r/min,着重从冷模实验对搅拌槽性能进行了考察,为反应器放大提供必要的基础。实验结果发现,层流流动条件下,随流体剪切稀化性质增强,搅拌功率(Np)显著降低,而无因次混合时间(Ntm)则变化不大;给出适用于强剪切稀化流体的Metzner常数Ks关联式,且Ks与搅拌槽直径无关;对于几何相似的螺带桨,随着搅拌槽直径增大,若保持相同的单位体积功率,Ntm是相同的,这为我们提供了一个螺带型搅拌槽的放大准则;搅拌槽功耗及Ntm随着搅拌槽高径比的增大而增大;最后采用数值计算方法对搅拌槽内分散混合效率进行了考察,发现,根据混合效率的不同,可将螺带型搅拌槽分成几个不同的区域,搅拌槽内体积平均分散混合效率接近于0.5(简单剪切流)。纤维素酶水解反应器的放大,小试给出关键参数(单位体积生产能力),冷模实验提供混合性能、几何结构设计基础,经验判据与CFD相结合,关注基本流型的尺寸效应,兼顾单位体积功率及最大剪切速率。     

新型Maxblend搅拌器用于工业规模反应器混合特性的数值模拟

黄原胶溶液为研究体系,用CFD技术研究了Maxblend搅拌器在大高径比工业规模反应器中流体力学行为,重点考察了功率特性、死区分率、流场分布和剪切性能等流体动力学性能。结果表明:Maxblend搅拌器的功耗随转速提高而增加,死区分率随转速提高而减小;随着雷诺数增加,釜内平均剪切速率增加。工业规模反应器中,搅拌流场呈全釜大循环流型。     

刚柔组合搅拌桨强化流体混合的流固耦合行为

传统刚性搅拌桨通过对流体的剪切作用实现能量的传递,而刚柔组合搅拌桨可通过其多体运动行为强化能量传递。基于搅拌桨桨叶与流体之间的耦合运动作用,结合ANSYS Workbench仿真平台,采用双向流固耦合方法,模拟计算了刚性搅拌桨与刚柔组合搅拌桨桨叶的等效应力和总变形量,研究了流场的宏观结构;并通过测定混合时间和计算搅拌桨功耗对比分析了两种不同搅拌体系的混合行为。结果表明:刚柔组合搅拌桨使体系的混合时间缩短了近32%,搅拌桨功耗下降了7%,其桨叶尖端的变形量是刚性搅拌桨的10⁵倍,其应力比刚性搅拌桨增加了83%;与刚性搅拌桨相比,刚柔组合搅拌桨在流固耦合作用下对流体的作用力更大,能够更好地传递能量,增强流体运动,强化流体混合。     

高黏体系中最大叶片式搅拌桨直径的CFD优化

以黄原胶溶液为研究体系,借助于计算流体力学( CFD)方法研究了采用不同直径的最大叶片式搅拌桨时釜内流体速率分布、死区体积、剪切速率、表观黏度和总体流体状况等参数.研究发现:在保持功率不变的前提下,随着桨径的增大,釜内循环区影响范围变广,全釜平均液相速率逐渐增加,速度分布均匀度有所提高,死区体积明显下降.对于研究的搅拌...     

CFD analysis of several design parameters affecting the performance of the Maxblend impeller

A CFD characterization of the hydrodynamics of the Maxblend impeller with experimental validations has been carried out with viscous Newtonian and non-Newtonian inelastic fluids. The mixing cases investigated were the non-baffled configuration with Newtonian and shear-thinning fluids, and the baffled configuration with only Newtonian fluids. The study focused on the effect of the impeller bottom clearance and the Reynolds number on the power characteristics, the distribution of shear rates and the overall flow conditions in the vessel. It was found that the bottom clearance plays a significant role on the power consumption, and that the value of the Reynolds number and the power law index strongly affect the axial pumping efficiency and the shear rate profile. The best performance was obtained when the impeller Reynolds number is superior to 10.     

Finite element modeling of the laminar and transition flow of the Superblend dual shaft coaxial mixer on parallel computers

The macro-mixing mechanisms of the Superblend coaxial mixer consisting of a Maxblend impeller and a double helical ribbon agitator mounted on two independent coaxial shafts rotating at different speeds are numerically investigated. The simulations are based on the resolution of the Navier-Stokes equations with help of a parallel three-dimensional finite element solver exploiting the capabilities of high performance computers. To model the rotation of agitators a hybrid approach based on a novel finite element sliding mesh and fictitious domain method is used. The power consumption, the flow patterns, the shear rate distribution, the pumping capacity and the mixing time of the Superblend mixer are calculated from the simulated hydrodynamics. The simulations allow observing the flow as it evolves from deep laminar (Re=0.1) to transition (Re=520) regime. As Reynolds number increases, several recirculation zones above and below the middle of the tank are formed. It is found that operating the agitators in co-rotation mode requires less power consumption and exhibits equal or shorter mixing time than counter-rotation mode. The larger power consumption in counter-rotating mode is caused by the presence of high shear vortices generated between the two coaxial agitators. Furthermore it is shown that the shear distribution throughout the Superblend coaxial mixer operating in co-rotation mode is almost homogenous, which is highly desirable for shear sensitive products. In view of the results obtained in this work, the Superblend coaxial mixer is found as a good alternative for tough mixing applications.     

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.     

双螺带桨Metzner常数的模拟计算

Using the multiple reference frames （MRF） impeller method, the three-dimensional non-Newtonian flow field generated by a double helical ribbon （DHR） impeller has been simulated. The velocity field calculated by the numerical simulation was similar to the previous studies and the power constant agreed well with the experimental data. Three computational fluid dynamic （CFD） methods, labeled Ⅰ, Ⅱ and Ⅲ, were used to compute the Metzuer constant k5. The results showed that the calculated value from the slop method （method Ⅰ） was consistent with the experimental data. Method Ⅱ, which took the maximal circumference-average shear rate around the impeller as the effective shear rate to compute ks, also showed good agreement with the experiment. However, both methods suffer from the complexity of calculation procedures. A new method （method Ⅲ） was devised in this paper to use the area-weighted average viscosity around the impeller as the effective viscosity for calculating k5. Method Ⅲ showed both good accuracy and ease of use.     

Investigation of the accuracy of the extrapolation method for the lattice Boltzmann simulation of viscous fluid flow in a Maxblend impeller system

This paper offers a thorough assessment on the performance of the extrapolation method for the lattice Boltzmann simulation of viscous mixing flows. This method appears to be well-suited for the treatment of the complex boundary conditions found in various mixing systems. Here, the ability to simulate accurate power consumption and pumping capacity is evaluated on several configurations of the Maxblend mixing system, which has proven efficient in a wide range of applications. First, the impact of the boundary conditions on the spatial convergence of the lattice Boltzmann method (LBM) is determined on the 3D Couette flow, clearly showing that small modifications of the boundary conditions may reduce the accuracy of the predicted shear rate and power. Second, a parallel LBM scheme was used to simulate fluid flow within a Maxblend mixing system. For the unbaffled configuration, the simulated power consumption and the pumping capacity are observed to be in good agreement with experimental data and finite element simulation results. The effect of the bottom clearance is also successfully evaluated, suggesting that the standard bottom clearance is not optimum in the transitional regime. Lastly, results for the most geometrically complex case (baffled configuration) indicate that the power consumption is affected by numerical perturbations appearing around the moving impeller. Overall, these results show that, when combined with the extrapolation method for the treatment of boundary conditions, the LBM is an efficient tool for the investigation of viscous flow in mixers of industrial relevance.     

Improving the dynamic performance of continuous-flow mixing of pseudoplastic fluids possessing yield stress using Maxblend impeller

The strategic approach of this article is to characterize the continuous-flow mixing of pseudoplastic fluids possessing yield stress in a stirred reactor with the Maxblend impeller. Dynamic experiments were carried out through the frequency-modulated random binary input of a brine solution to determine the extent of non-ideal flows. Mixing quality was determined on the basis of the extent of channeling and fully mixed volume. The effects of important parameters such as impeller speed (25–500 rpm), absence of baffles, fluid rheology (0.5–1.5%), fluid flow rate (3.20–14.17 L min⁻¹), and the locations of inlet/outlet on the dynamic performance of the continuous-flow mixing vessel were explored. The performance of the Maxblend impeller was then compared to the performances of various types of impellers such as close-clearance (an anchor), axial-flow (a Lightnin A320), and radial-flow (a Scaba 6SRGT) impellers. It was found when the channeling approached zero and the fully mixed volume approached the total fluid volume in the vessel, the power drawn by the A320 impeller and the Scaba impeller were about 2.9 and 4.3 times greater than that of the Maxblend impeller. Thus, the Maxblend impeller was able to drastically improve the performance of continuous-flow mixing with huge power savings. The mixing quality was further improved by optimizing the impeller speed, decreasing the fluid flow rate, decreasing the fluid concentration, and using bottom inlet- top outlet configuration. The flow non-ideality of the mixing system increased in the absence of the baffles. Thus, better mixing quality and more energy savings can be achieved by employing the findings of this study.     

On the hydrodynamics characterization of the straight Maxblend impeller with Newtonian fluids

The hydrodynamics generated by the straight version of the Maxblend^® impeller with Newtonian fluids in a baffled stirred vessel under the transitional and turbulent regime has been experimentally characterized by means of the particle image velocimetry technique. The flow fields obtained with the Maxblend were compared with those obtained with a double stage classical pitched blade turbine (PBT) and a double Ekato Intermig^® impellers under the same specific power draw. It is shown that these open impellers induce complex local flows in the radial and axial direction, with an intensity decreasing away from the blades. By contrast, the Maxblend impeller generates a more regular circulation pattern, with efficient top-to-bottom pumping.     

Using CFD and Ultrasonic velocimetry to Study the Mixing of Pseudoplastic Fluids with a Helical Ribbon Impeller

A commercial CFD package was used to simulate the 3D flow field generated in a cylindrical tank by a helical ribbon impeller. The study was carried out using a pseudoplastic fluid with yield stress in the laminar mixing region. Ultrasonic Doppler velocimetry (UDV), a noninvasive fluid flow measurement technique for opaque systems, was used to measure xanthan gum velocity. From flow field calculations and tracer homogenization simulations, power consumption and mixing time results were obtained. The torque and power characteristics remain the same for upward and downward pumping of the impeller, but the mixing times are considerably longer for the downward pumping mode. Overall, the numerical results showed good agreement with experimental results and correlations developed by other researchers. From the power and mixing time results, two efficiency criteria were utilized to determine the best pumping mode of the impeller.     

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.