微旋流混合器中液体混合特性CFD-PBM数值模拟

微旋流混合器是一种由微旋流气浮演化而来的设备,可以强化物料的混合与传质。文中研究利用CFD-PBM计算模型,研究微旋流混合器内气液两相分布,考察微旋流混合器内流体颗粒粒径分布及变化特性。分别以空气和液相作为气液两相,研究不同气、液相入口流速下的流场形态与相关流体力学参数。结果表明：通入气相会诱导设备内部产生新的循环流动形态,增强宏观混合,直径大于4.0 mm的气泡无法在内筒中的旋流环境中稳定存在,并随着旋流速度逐渐减小,气泡发生聚并,最终演化为4.0—10 mm直径的气泡流出。增加液相入口速度可以增加整体流动速度,同时增加气相体积分数,气相体积分数最高可达到18%。随着气相流速增加,在微旋流混合器内筒下端形成连续空腔。     

Kenics静态混合器的应用及研究进展

介绍了Kenics静态混合器的混合机理及其在流体混合和强化传热等操作过程中的应用,重点介绍了Kenics静态混合器的压力降计算方法以及拉伸混合和无序混合等方面的研究进展。     

Kenics型静态混合器中单气泡模拟研究

采用层流模型对Kenics型静态混合器内气液两相流单气泡流动进行模拟分析,应用多块结构化网格对整个流场进行划分,经过15.5s的计算得出:气泡在流动过程中,监测面处气相浓度会形成几个连续波峰;在气泡对应位置二次流速度相对较高,轴向速度也相对较高;气泡在入口对压力的影响大于出口侧,而且出口侧压力平稳。     

Kenics型静态混合器和GK型静态混合器流场的数值模拟及比较

采用计算流体动力学(CFD)的方法,分别计算了Kenics型静态混合器和GK型静态混合器内的流场。数值模拟的结果表明,Kenics型静态混合器内流场的湍动强度大于GK型静态混合器的,导致了Kenics型静态混合器的流体阻力和传热系数大于GK型静态混合器的。GK型静态混合器的压力降大约只是Kenics型静态混合器压力降的0.554~0.579倍,但两者的传热膜系数相差不大。GK型静态混合器具有较强的综合性能。     

幂律流体在Kenics型静态混合器中流动特性分析

为了探究Kenics型静态混合器内扭旋叶片剪切作用对幂律流体流动的影响,利用旋转流变仪测量了浓度为0.5wt%, 0.7wt%, 0.9wt%的羧甲基纤维素(CMC)水溶液的流变参数,采用数值模拟与实验研究了扭旋叶片作用下幂律流体流动阻力和剪切稀化特性。对流场研究表明,扭旋叶片诱导产生了内流涡旋、绕流涡旋和近壁面涡旋,有效强化了静态混合器内流体流动的剪切作用。受多个纵向涡旋分布的影响,扭旋叶片局域流场中周向45°位置速度最高,周向30°位置涡量与剪切应力最高而黏度最低。径向0.4倍半径位置速度最高,0.7倍半径位置黏度最高。静态混合器有效提高了流体的二次流流动速度和剪切应力,降低了幂律流体的黏度和流动阻力系数。     

Kenics型静态混合器的结构优化与数值模拟

基于三维有限元数值模拟方法,对应用于高黏聚酯熔体直纺工业丝过程中熔体管道输送的Kenics型静态混合器进行结构优化;对静态混合器内示踪粒子的流动轨迹进行统计学处理,通过停留时间分布、分离尺度、面积伸展率以及时间平均效率表征流动特性与混合性能。结果表明:相比传统静态混合器,当静态混合器的元件旋转角度为120°时,静态混合器的分离尺度降低,分散混合能力增强,拉伸作用增强,混合特性提高;相同管径下元件长径比为0.75时,静态混合器的混合能力较长径比为1.00或1.25时都有明显的提高;随元件长径比增加,混合效果逐渐变差,流动方式趋近于理想平推流动。     

Q型静态混合器内液滴群分散特性

静态混合器因具有结构紧凑、强化性能优异和连续性生产等优点被广泛应用于过程工业中,但Q型静态混合器(QSM)内多相流分散混合强化机理不完善制约了其在精细化工和原料药绿色生产中的应用。采用计算流体力学(CFD)耦合群体平衡方程对QSM内相含率α≤5%时液滴分散特性进行数值模拟,分析液液界面张力、动力黏度和相含率对液滴群d32的分散行为的影响。标准旋流静态混合器内的d32数值预测结果与实验结果有良好的一致性。模拟结果表明,在z/l=11.5处不同分散相d32减小73%~96%,RL-90-QSM对不同物性介质的分散混合具有高效性和普适性;在高雷诺数和低相含率下,不同分散相流过z/l=0~2时d32破碎速率最大,在z/l=2.5处d32减小51%~90%,d32随混合时间的增加逐渐减小且在z/l=10后趋于稳定;界面张力对混合结果的影响大于动力黏度。     

Kenics型静态混合器充分发展段纵向涡演变分析

为分析Kenics型静态混合器内充分发展段沿轴向二次流纵向涡的形成诱因及演变过程,运用大涡模拟对混合器内流场进行研究,数值模拟结果与实验结果吻合较好. 结果表明,在第7个扭旋叶片所在区域内含3种旋涡,分别为叶片入口分割上一段流体后产生的合并旋涡、随扭旋叶片一起旋转的内流旋涡和绕流旋涡. 绕流旋涡是扭旋叶片高扭率产生的科氏力导致单侧流体压力不平衡,使边界层产生分离形成的诱导旋涡. 沿轴向将第7个扭旋叶片所在区域流场平均分成4段,第一和第二段的横截面上存在5个纵向涡,涡量和湍动强度平均值分别比第三和第四段高23.0%和8.93%. 在相邻叶片的分界面处,旋涡破碎和聚合产生能量损耗,使近壁面的涡量陡增,高出平均值73.0%.     

Kenics混合器混合性能的模拟研究

利用Fluent有限元分析软件计算了流体流过Kenics混合器过程中的应变速率,进而分析混合元件转速与旋转式Kenics混合器混合效率的关系,以及混合元件与机筒间隙对静态和旋转式Kenics混合器对混合效率的影响。模拟分析结果表明:旋转式Kenics混合器混合效率随转速增加而提高;减小混合元件与机筒间间隙有利于增加静态Kenics混合器混合效率,但间隙的减小对旋转式Kenics混合器混合效率的影响却很小。     

Kenics型静态混合器性能优化的数值与应用分析

采用计算流体力学方法研究了旋流元件对静态混合器内气相流场分布和混合性能的影响.结果表明:旋流元件的加入有效降低了管路外缘处气流切向速度和径向压强分布,管内流场分布较为均匀,其中以旋流元件数N=6时最为明显;在353K的管壁加热温度下,速度均方根值再次降低,明显提高了管内气相组分的质量分数均匀性.     

Liquid phase residence time distribution for gas–liquid flow in Kenics static mixer

The residence time distribution (RTD) of the liquid phase for co-current gas–liquid upflow in a Kenics static mixer (KSM) with air/water and air/non-Newtonian fluid systems was investigated. The effect of liquid and gas superficial velocities on liquid holdup and Peclet number was studied. Experiments were conducted in three KSMs of diameter 2.54 cm with 16 elements and 5.08 cm diameter with 8 and 16 elements, respectively, of constant L_e/D_e = 1.5 for different liquid and gas velocities. A correlation was developed for Peclet number, in terms of generalized liquid Reynolds number, gas Froude number and liquid Galileo number, where as for liquid holdup, a correlation was developed as a function of gas Reynolds number. The axial dispersion model was found to be in good agreement with the experimental data.     

Computational-fluid-dynamics study of a Kenics static mixer as a heat exchanger for supercritical carbon dioxide

The thermal efficiency of a Kenics^® KM static mixer used to pre-heat supercritical carbon dioxide, under high pressure conditions, is studied using computational fluid dynamics (CFD). A mesh sensitivity analysis is performed and the CFD model is validated against experimental results on heat transfer with conventional and supercritical fluids. Three turbulent models - standard k-?, RNG k-?, and k-ω - are employed to model the flow and heat transfer under high pressure conditions; the effects of large variations of the physical properties in the pseudo-critical region of the fluid are also studied. The RNG k-? model gives results that are closer to the experimental data than the other two turbulence models. The numerical results show that the static mixer has a thermal efficiency more than three times higher than that of a conventional empty pipe heat exchanger with similar heat transfer area.     

CFD modeling of immiscible liquids turbulent dispersion in Kenics static mixers: Focusing on droplet behavior

The present study is concerned with the computational fluid dynamics(CFD) simulation of turbulent dispersion of immiscible liquids, namely, water–silicone oil and water–benzene through Kenics static mixers using the Eulerian–Eulerian and Eulerian–Lagrangian approaches of the ANSYS Fluent 16.0 software. To study the droplet size distribution(DSD), the Eulerian formulation incorporating a population balance model(PBM) was employed. For the Eulerian–Lagrangian approach, a discrete phase model(DPM) in conjunction with the Eulerian approach for continuous phase simulation was used to predict the residence time distribution(RTD) of droplets.In both approaches, a shear stress transport(SST) k-ω turbulence model was used. For validation purposes, the simulated results were compared with the experimental data and theoretical values for the Fanning friction factor, Sauter mean diameter and the mean residence time. The reliability of the computational model was further assessed by comparing the results with the available empirical correlations for Fanning friction factor and Sauter mean diameter. In addition, the influence of important geometrical and operational parameters, including the number of mixing elements and Weber number, was studied. It was found that the proposed models are capable of predicting the performance of the Kenics static mixer reasonably well.     

A general correlation for pressure drop in a Kenics static mixer

A new pressure drop correlation in a Kenics static mixer has been developed. Pressure drop data were generated from computational fluid dynamics (CFD) calculations, avoiding the experimental limitations in obtaining comprehensive data enough for developing a reliable pressure drop correlation. Dimensional analysis reveals that the pressure drop characteristic of the Kenics static mixer can be described by three dimensionless groups, i.e., the friction factor, Reynolds number (Re), and aspect ratio of a mixing element (AR). A systematic graphical analysis led to a single master curve governing the pressure drop behavior of the Kenics static mixer, which had never been achieved before. We derived a pressure drop correlation fitting well with the obtained master curve in a general form into which the AR effect on the pressure drop is directly incorporated. Unlike the already existing correlations available in the literature, the correlation proposed in this study can cover the whole range of Re from laminar to turbulence. The reliability of the proposed correlation was validated by the comparison with various pressure drop data reported in the literature.     

Development of positron emission particle tracking for studying laminar mixing in Kenics static mixer

Positron emission particle tracking (PEPT) is a flow visualisation technique that has found application in a wide range of processes. In this work, PEPT has been used to study laminar flow of a high viscosity Newtonian and non-Newtonian fluid in a Kenics static mixer (KM). Through analysis of the trajectories of many hundreds of passes of the tracer particle through the mixer, it is possible to compute the overall flow field and to visualise how the fluid twists and folds as it passes along the mixer. Eulerian velocity maps plotted for the Newtonian and non-Newtonian fluids showed that the length required for the flow to develop is shorter for the non-Newtonian fluid than the Newtonian. The stretching and folding mechanism of mixing was observed by grouping the trajectories into clusters according to whether the trajectory passes to the left or right of the blade at the transition between elements. Those trajectories making the same L–R–L decision tended to remain in the same striation through two or three elements until that striation became stretched and folded back on itself, sandwiching other layers. It is clear that the PEPT data is rich and powerful. We are hopeful that the techniques we develop for the flow and mixing in the Kenics mixer will be applicable to studying more complex laminar flows.     

带有Kenics静态混合器的水平液固两相流化床换热管的压降研究

以饱和硫酸钙为介质,在安装Kenics静态混合器的水平液固两相流化床换热管上进行实验研究,考察介质流速、Kenics静态混合器扭率、颗粒体积分数及颗粒尺寸对压降的影响,并与冷态实验条件下的压降变化规律进行比较。结果表明：同等操作条件,安装Kenics静态混合器后压降比安装前提高20%～140%;压降随雷诺数的增大而增大,随Kenics静态混合器扭率的增大而减小;颗粒体积分数对压降也有影响。根据实验数据,得出了稳定操作条件下压降与上述影响因素之间的经验关联式,为带有Kenics静态混合器的水平液固两相流化床换热器的设计提供计算依据。     

SK型静态混合器壁压脉动信号的多尺度多分形特性

为了探讨静态混合器内非线性、非均匀性和混沌特性机理,利用高速数据采集系统对SK型静态混合器管壁处脉动压力进行测量。结合小波变换模极大值理论,对采集的不同进口流量下壁压波动时间序列用Daubechies二阶小波提取1～7尺度下的特征信号,并分别对提取的信号进行R/S分析。通过对压力波动信号不同尺度下的细节信号与概貌信号研究,发现在不同尺度下其表现出不同的分形结构,且随着进口流量的增大,分形结构的变化趋势基本一致。此外,该系统不仅存在确定性非周期成分,而且不同尺度的旋涡之间的能量交换导致混沌的产生。各尺度信号的能量分布表明,压力波动信号主要体现了宏观尺度的旋涡级串的相互作用。     

Single and two-phase flows in a horizontal pipe with a Kenics static mixer: Effect of pressure drop on mixing

Single and two-phase flows pressure drops through a Kenics static mixer were investigated, for liquid and gas Reynolds numbers ranging from 8110 < Re_L < 18 940 to 1730 < Re_G < 8680, respectively. New friction factor correlations were established for single and two-phase flows, showing better agreement than those available in the literature. Dissipated energy and characteristic time constants were estimated from experimental data. For instance, a dissipated energy with a maximum value of 510 W/kg was calculated in two-phase flow with the drift-flux model. The dispersed phase reduced the characteristic mixing times and its influence was more important than the continuous phase for all the characteristic mixing time investigated. Furthermore, the macroscopic characteristic mixing time was shown to be the governing mixing process for almost all gas and liquid flow rates explored.