多层翼形桨搅拌槽内混合过程的CFD模拟

The mixing process in a stirred tank of 0.476m diameter with single, dual and triple 3-narrow blade hydrofoil CBY impellers was numerically simulated by using computational fluid dynamics （CFD） package FLU-ENT6.1. The multi-reference frame （MRF） and standard k-ε turbulent model were used in the simulation. The shaft power and the mixing time predicted by CFD were in good agreement with the experiment. The effects of tracer feeding and detecting positions on mixing time were investigated. The results are of importance to the optimum design of industrial stirred tank/reactors.     

Experimental determination and numerical simulation of mixing time in a gas-liquid stirred tank

In this work, mixing experiments and numerical simulations of flow and macro-mixing were carried out in a 0.24 m i.d. gas-liquid stirred tank agitated by a Rushton turbine. The conductivity technique was used to measure the mixing time. A two-phase CFD (computational fluid dynamics) model was developed to calculate the flow field, k and ε distributions and holdup. Comparison between the predictions and the reported experimental data [Lu, W.M., Ju, S.J., 1987. Local gas holdup, mean liquid velocity and turbulence in an aerated stirred tank using hot-film anemometry. Chemical Engineering Journal 35 (1), 9-17] of flow field and holdup at same conditions were investigated and good agreements have been got. As the complexity of gas-liquid systems, there was still no report on the prediction of mixing time through CFD models in a gas-liquid stirred tank. In this paper, the two-phase CFD model was extended for the prediction of the mixing time in the gas-liquid stirred tank for the first time. The effects of operating parameters such as impeller speed, gas flow rate and feed position on the mixing time were compared. Good agreements between the simulations and experimental values of the mixing time have also been achieved.     

搅拌槽内单相湍流流场数值模拟研究进展

搅拌槽内的流场是决定混合、传热及传质等操作的基础,对流场的研究具有十分重要的意义,计算流体动力学是研究流场的重要方法。本文回顾了搅拌数值模型的发展历程,阐述了三十年来各种搅拌流场数值模拟方法的特点及其应用情况,对比分析了各种湍流模型的优缺点,并展望了未来搅拌槽内单相湍流模型的发展方向。     

计算流体力学用于搅拌器流场研究及结构设计

采用计算流体力学(CFD)模拟方法对现有同轴搅拌器的流场结构进行了模拟,同时进行了混合时间的数值研究,并与实验结果相比较,证实了模拟方法是可行的。针对现有搅拌器存在的问题,提出了3种改进型的搅拌器,并进行CFD模拟研究,发现桨Ⅲ所需混合时间最短,桨Ⅱ和桨Ⅰ次之;桨Ⅲ消耗的功率最大,而桨Ⅰ和桨Ⅱ所需功率最少。     

搅拌槽内三相流场混合时间的测定及模拟研究

混合时时间是评定搅拌设设备混合效率的重重要指标,为了了检测带有挡板和和导流筒的搅拌槽槽内液-固-固三相相流场的混合时时间,以甘油溶液液为液相,砂子和和赤泥为固相建立立了流场体系,采采用电导法测量体体系的混合时间。并对流场进行行了计算流体动力力学(CFD)模拟研研究,CFD模型采用基于欧拉多多相流模型和RNNGκ-ε湍流模型。流场混合时间间模拟结果与实验验结果的偏差较小小,说明该CFDD数学模型能很很好地预测流场的的混合时间。体系系的混合时间随随搅拌转速的增大大而减小,当流场场的轴向混合占主主导地位时,增设设导流筒可减小小流场的混合时间间。导流筒对于粒粒径和密度较小的的固体影响较大。。     

三轴搅拌釜不同桨径比下固-液悬浮特性的数值模拟

利用FLUENT软件对不同桨径比下立式三轴搅拌釜内单相和固-液两相混合过程分别进行三维数值模拟,对流场分析可知增大桨径比能改善立式三轴搅拌釜内流场的结构。利用修正速度判据得出不同桨径比下搅拌釜的临界悬浮转速,并通过计算临界悬浮转速下的搅拌功率,初步得出立式三轴搅拌釜桨径比的最佳取值范围在0.1～0.125之间。     

“波纹”内壁搅拌槽内的湍流流动特性

设计了一种“波纹”内壁搅拌槽,通过在内壁上安装一定数量的半圆管,以增强搅拌效果,并采用计算流体动力学方法对其内部流动特性进行了分析.通过与文献中的实验及模拟结果的比较,验证了所建数值模型及模拟方法的可靠性,随后对不同半圆管尺寸(半径)、数量和安装位置时搅拌槽内的流场结构和功率消耗进行了研究.结果表明,与标准搅拌槽相比,“波纹”内壁结构尽管不会明显增强流体的湍流程度,但能提高流体速度分布的均匀程度,而且功率消耗没有明显变化;当在搅拌槽内壁上均匀安装4根半径为搅拌槽直径1/20的半圆管时[LL1] ,能显著减少挡板后方及槽顶部流体的低速区.该结果为“波纹”内壁搅拌槽在过程工业中的应用奠定了基础.     

基于CFD流场模拟的反应釜结构设计

反应釜是制药化工产品生产的主要设备之一,常以搅拌的形式出现。本文研究了搅拌器相关设计参数对反应釜内流场的影响,并以此为依据改进反应釜结构。首先通过工程设计方法得到小型制药搅拌釜的初始设计方案,利用CFD技术对搅拌釜内的流场进行数值模拟研究,所得结果与文献中的结果基本一致。然后调整反应釜搅拌器的安装高度与搅拌转速等设计参数,与初始设计方案进行模拟结果对比分析,根据混合效果与驱动输入功率等设计要求,修改设计参数得到更优的设计方案。     

Computational Fluid Dynamics Study of a Styrene Polymerization Reactor

The computational fluid dynamics (CFD) approach was adopted to simulate benzoyl peroxide (BPO)‐initiated styrene polymerization in a laboratory‐scale continuous stirred‐tank reactor (CSTR). The CFD results revealed the effects of non‐homogeneity and the short‐circuiting of the unreacted styrene and initiator on the reactor performance. The study also investigated the effects of the impeller speed and the residence time on the conversion and the flow behavior of the system. The CFD simulation showed that intense mixing remained confined to a small region near the impeller. With increasing impeller speed, it was found that the perfectly mixed region near the impeller expanded, thus reducing non‐homogeneity. Different contours were generated and exhibited the effect of the mixing parameters on the propagation rate and styrene conversion. The monomer and initiator conversions predicted with the CFD model were compared to those obtained with a CSTR model. The CFD model accounts for the non‐ideality behavior of the polymerization reactor, and hence conversion predictions are more realistic.     

橡胶沥青搅拌罐内混合过程的数值模拟

针对橡胶粉与基质沥青混合过程中出现的漂浮、沉底、粘壁及挂料现象,建立了橡胶沥青搅拌罐的几何模型,基于计算流体力学软件对罐内混合过程进行非定常固液两相流数值模拟,分析了影响混合均匀性的因素,如桨叶直径、桨叶位置、挡板及搅拌速度等. 结果表明,尺寸适宜的桨叶直径与合适的桨叶位置有利于形成循环的轴向流,并减少定常流现象,安装挡板有效减少了切向流,搅拌器转速不影响内部流场的基本形态,但适宜的搅拌转速提高了混合均匀性. 混合均匀度与模拟结果印证,且当搅拌器直径800 mm、桨叶距离罐底680 mm、桨叶宽100 mm、搅拌速度280 r/min时,优化后橡胶粉的分布较均匀,混合均匀度为0.24,处于完全离底悬浮状态,模拟结果与实验结果较吻合.     

改进型框式组合桨搅拌釜内流场特性

对5m³树脂反应釜及釜内改进型框式-二斜叶双层组合桨等比例缩小建立模型,基于计算流体力学（CFD）中的多重参考系法对该双层组合桨搅拌釜流场进行了模拟研究,并利用粒子图像测速（PIV）实验对模拟结果进行了验证。分析了桨叶离底间距、桨间距及组合桨安装角度的变化对流场产生的影响。随着离底间距的增大,搅拌釜下层框式桨横梁处产生往槽底的轴向流强度会逐渐减弱,不利于底部物料的混合;桨间距的增加导致两桨间对流减弱,不利于两桨间流体的混合,当桨间距与釜内径的比值为0.77时,搅拌釜内的整体流动情况较好。对上下层桨叶安装角度分别为0°、45°和90°这3种工况下的釜内流场特性研究表明,安装角度为90°时,斜叶桨产生的轴向流强度最大,此时搅拌釜内流体的混合效果最好。研究结果为改进型框式桨与二斜叶桨双层组合桨应用于树脂聚合反应实际工程提供参考。     

双层涡轮桨搅拌反应器内混合时间的大涡模拟

采用计算流体力学(CFD)方法对直径为0.476m双层涡轮桨搅拌反应器内的流动及混合进行了数值模拟,并实验测试了混合过程。利用大涡模拟(LES)及Smagorinsky-Lilly亚格子模型求解湍流流动与示踪剂传递过程,桨叶区域采用滑移网格技术。研究结果表明,大涡模拟得到的示踪剂响应曲线和混合时间与实验结果吻合良好,其预测精度明显优于基于雷诺平均(Reynolds-averaged Navier-Stokes,RANS)的标准k-ε模型的模拟结果。大涡模拟是研究搅拌反应器内非稳态及周期性湍流流动的有效方法。     

CFD study on double-to single-loop flow pattern transition and its influence on macro mixing efficiency in fully baffled tank stirred by a Rushton turbine

For a fully baffled tank stirred by a Rushton turbine(RT), the flow pattern will change from double-to single-loop as the off bottom clearance(C) of the RT decreases from one third of the tank diameter. Such a flow pattern transition as well as its influence on the macro mixing efficiency was investigated via CFD simulation. The transient sliding mesh approach coupled with the standard k-ε turbulence model could correctly and efficiently reproduce the reported critical C range where the flow pattern changes. Simulation results indicated that such a critical C range varied hardly with the impeller rotation speed but decreased significantly with increasing impeller diameter. Small RTs are preferable to generating the single-loop flow pattern. A mechanism of the flow pattern transition was further proposed to explain these phenomena. The discharge stream from the RT deviates downwards from the horizontal direction for small C values; if it meets the tank wall first, the double-loop will form; if it hits the tank bottom first, the single-loop will form. With the flow pattern transition, the mixing time decreased by about 35% at the same power input(P), indicating that the single-loop flow pattern was more efficient than the double-loop to enhance the macro mixing in the tank. A comparison was further made between the single-loop RT and pitched blade turbine(PBT, 45°) from macro mixing perspective. The single-loop RT was found to be less efficient than the PBT and usually required 60% more time to achieve the same level of macro mixing at the same P.     

圆盘涡轮式桨叶对搅拌槽混合特性影响的CFD研究

利用CFD 技术对圆盘涡轮式搅拌槽内的浓度场进行数值模拟研究,主要考察了常见的平直桨叶（90°）、斜桨叶（60°和45°）的安装位置对混合时间θm、单位体积混合能Wr和浓度标准差σ的影响。在标准安装高度的平直桨叶下,对槽内速度进行分析,得到的数据与实验值非常吻合。研究表明：圆盘涡轮式桨叶由标准安装高度降低时,搅拌槽内的流型由径向流转变为轴向流,并且90°、60°和45°的转变为轴向流的相对安装高度（C/H）分别为0.20、0.233和0.267;混合时间是由槽内顶部和底部检测位置决定的;桨叶的标准相对安装高度（C/H=1/3）并不是混合性能最优的位置,针对90°、60°和 45°三种倾角的桨叶,在相对安装高度分别为0.213、0.267和0.320时的搅拌混合性能最佳;综合考虑省时、节能和混合均匀性的因素,倾角为45°的桨叶最佳,60°的桨叶次之。     

化工搅拌釜内流动测量技术的应用进展

搅拌釜内的物料混合是一个有限空间中的复杂非定常湍流问题,且常伴有强烈的传质、传热乃至反应过程。搅拌混合过程中影响因素多,理论分析难度大,实验获取搅拌釜内整场流动信息是其机理研究和搅拌混合设备优化设计的重要手段。本文归纳了在搅拌混合研究中传统流动测量技术的应用,分析其各自优缺点,着重探讨了新一代全场光学测速技术——粒子图像速度场仪（PIV）在搅拌混合实验中的应用,指出PIV在搅拌混合研究中具有广泛应用前景。PIV具有很高的空间分辨率和时间解析度,可以得到搅拌釜中混合流体的瞬时2D或3D速度场以及浓度场和温度场等信息,进行非定常湍流特性研究,有助于建立搅拌釜内多相流动模型,验证数值模拟结果,实现搅拌釜的优化设计,从而促进化工搅拌技术的进一步发展。     

Simulation of the population balances for liquid-liquid systems in a nonideal stirred tank. Part 2—parameter fitting and the use of the multiblock model for dense dispersions

In the previous part of this work (Chem. Eng. Sci. 54 (1999) 5887), a multiblock simulation model was developed in order to allow the close examination of different regions of a stirred tank for drop size distribution calculations. In this paper, that model is tested in a parameter fitting procedure. The drop breakage and coalescence parameters are fitted against drop size measurements from dense liquid-liquid dispersions, which were assumed fully turbulent. Since the local turbulence and flow values of a stirred tank are used in the present model, the fundamental breakage and coalescence phenomena can be examined more closely. Furthermore, the present model is capable of predicting inhomogeneities occurring in a stirred tank. It is also to be considered as an improved tool for process scale-up, compared to the simple vessel-averaged population balance approach, or use of correlations of dimensionless numbers only. The present model can use two sources of data for fitting parameters in the drop rate functions. One is to use transient data of the measured drop size distribution as the impeller speed is changed. The other is to use time-averaged data measured at different locations of the stirred tank. It is shown in this paper that the different flow regions can be chosen from the CFD simulations in a straightforward manner. CFD flow simulation results can be used to select the flow regions when no experimentally obtained flow conditions are available. This is especially useful for non-standard vessels, such as reactors containing cooling coils. After fitting the parameters with a multiblock model, the population balance model can be rather easily incorporated into a commercial CFD program to investigate different flow conditions.     

搅拌槽内黏性流体流动的DPIV测量与CFD模拟

搅拌槽是化学工业及其相关工业广泛应用的设备之一,由于其内部流动的复杂性,搅拌混合操作目前尚未形成完善的理论体系．对搅拌槽的设计和放大,主要是依赖半经验的方法,对其内部流场有必要进行更深入的研究．目前对不同黏性流体的流动测量及计算流体力学模拟工作见诸报道较少,而     

Hydrodynamics of upflow anaerobic sludge blanket reactors

The hydrodynamic characteristics of upflow anaerobic sludge blanket (UASB) reactors were investigated in this study. A UASB reactor was visualized as being set‐up of a number of continuously stirred tank reactors (CSTRs) in series. An increasing‐sized CSTRs (ISC) model was developed to describe the hydrodynamics of such a bioreactor. The gradually increasing tank size in the ISC model implies that the dispersion coefficient decreased along the axial of the UASB reactor and that its hydrodynamic behavior was basically dispersion‐controlled. Experimental results from both laboratory‐scale H₂‐producing and full‐scale CH₄‐producing UASB reactors were used to validate this model. Simulation results demonstrate that the ISC model was better than the other models in describing the hydrodynamics of the UASB reactors. Moreover, a three‐dimensional computational fluid dynamics (CFD) simulation was performed with an Eulerian‐Eulerian three‐phase‐fluid approach to visualize the phase holdup and to explore the flow patterns in UASB reactors. The results from the CFD simulation were comparable with those of the ISC model predictions in terms of the flow patterns and dead zone fractions. The simulation results about the flow field further confirm the discontinuity in the mixing behaviors throughout a UASB reactor. © 2008 American Institute of Chemical Engineers AIChE J, 2009     

Numerical Simulation of Macroscopic Mixing in a Rushton Impeller Stirred Tank

The macroscopic mixing in a stirred tank with different tracer injection locations, impeller speeds and impeller positions is simulated numerically by solving the transport equation of the tracer based on the whole flow field in the baffled tank with a Rushton disk turbine numerically resolved using the improved inner-outer iterative procedure. Predicted mixing time is compared well with the literature correlations. The predicted residence time distribution of the stirred tank is very close to the present experimental results. The effect of the installation of a draft tube on the mixing time and residence time distributions is addressed.     

半圆管曲面涡轮搅拌槽内混合特性的数值模拟

在商业化软件ANSYS CFX 10.0平台上,采用多重参考系法来解决挡板与桨叶之间的相对转动问题,由标准k-ε模型对半圆管曲面涡轮搅拌槽内流动和混合过程进行了详细的数值模拟,本模拟所得的功率准数和设计值以及相关文献值吻合良好。结果表明：当搅拌桨离底距离由搅拌槽直径的1/2处变为1/3处时,搅拌槽内的流型均为典型的“双循环流型”,而当搅拌桨离底距离由搅拌槽直径的1/3处降低至1/6处时,槽内流型由典型的“双循环流型”转变为“单循环流型”;通过对不同时刻不同桨叶离底距离下的示踪剂浓度分布图分析表明槽内的混合过程与流动场密切相关;加料点位置对于最终的流场混合效果有着显著影响,对于混合时间数据的采集应注意不同加料位置时监测点的选取。CFD模拟结果表明本文所采用的模型可以很好的预测半圆管曲面涡轮搅拌槽内的混合特性,为进一步改进和优化半圆管曲面涡轮的设计提供了一定的参考。