The numerical simulation of a new double swirl static mixer for gas reactants mixing

For the nitrogen oxide removal processes, high performance gas mixer is deeply needed for the injection of NH₃ or O₃. In this study, a new type of double swirl static mixer in gas mixing was investigated using computational fluid dynamics (CFD). The results obtained using Particle Image Velocimetry (PIV) correlated well with the results obtained from simulation. The comparisons in pressure loss between the experimental results and the simulation results showed that the model was suitable and accurate for the simulation of the static mixer. Optimal process conditions and design were investigated. When L/D equaled 4, coefficient of variation (COV) was <5%. The inlet velocity did not affect the distributions of turbulent kinetic energy. In terms of both COV and pressure loss, the inner connector is important in the design of the static mixer. The nozzle length should be set at 4 cm. Taking both COV and pressure loss into consideration, the optimal oblique degree is 45°. The averaged kinetic energy changed according to process conditions and design. The new static mixer resulted in improved mixing performance in a more compact design. The new static mixer is more energy efficient compared with other SV static mixers. Therefore, the double swirl static mixer is promising in gas mixing.     

Improving mixing performance by curved-blade static mixer

This article addresses design modification to a flat-blade static mixer to enhance mixing performance. The static mixer elements used in this work consist of four blades with curvature made to intensify turbulent-like flow, while reducing the pressure drop. The blades were mounted on a cylindrical housing with 45° rotation relative to the axial direction. The mixer assembly was used in three different arrangements of 8, 10, and 14 elements for a range of Reynolds number between 600 and 7,000. The coefficient of variance (COV) of samples was used to measure the mixing quality. The curved-blade mixer provides considerable improvement in mixing quality compared with the flat-blade mixer and comparable to the SMX mixer. Compared with the flat-blade static mixer, the new design reduces the COV by up to about 50%. This effect is more pronounced when the number of mixing elements increases. Furthermore, the friction factors for the modified mixer, obtained at a wide range of Reynolds number, were apparently smaller than those for the flat-blade, SMX, and SMV mixers. The correlation presented for the friction factor, when all mixer arrangements and aspect ratios were considered, supports the experimental data with ±15% deviation.     

仿柳叶形静态混合器的流动及混合特性

对仿柳叶形静态混合器内混合气流进行了速度场与浓度场的试验研究,结果表明该混合器内速度场与浓度场偏差均达到了非常理想的效果（优于国家标准偏差值）。同时采用CFD软件对该静态混合器内的流场进行了数值模拟,试验与模拟的数值结果以及两者的浓度云图分布都有着较好的一致性。随后的研究结果表明：在混合元件尾迹区域出现了纵向涡和发卡涡来促进混合;在经过混合元件区域时因为湍流动能耗散率增加形成的高湍流动能耗散率区能够使物质交换更加频繁;整个静态混合器的流动阻力也主要发生在该区域,随之出现的返混现象也在一定程度上加强了混合效果。     

稀释水掺混用薄板式静态混合器结构设计与优化

稀释水掺混是油田或炼油厂原油电脱盐系统的关键环节,直接影响电脱盐的效率和运行能耗。为进一步简化设备结构、助力系统降本增效,提出将薄板式静态混合器应用于稀释水掺混环节。在对薄板式静态混合器进行初步结构设计的基础上,对油水混合情况进行计算流体动力学(CFD)三维数值模拟,以离析强度的平方根(IOS0.5)、管路压降(Δp)等作为评价指标,考察了注水管长度、注水管?弯曲薄板间距、弯曲薄板导向位置、弯曲薄板厚度四个结构参数对油水两相混合程度及运行能耗的影响。以混合管路内径D为基准参照,借助响应曲面法(RSM)对关键结构参数进行优化,并对最优结构参数组合下的混合性能进行预测。优化所得最优结构参数组合为注水管长度为1/3D、注水管-弯曲薄板间距为4/25D、弯曲薄板导向位置为1/8π、弯曲薄板厚度为1/25D,结构优化后的IOS0.5相比优化前降低了43.06%。定性分析薄板式混合器内的水相速度云图和流线图可知,当注水比为2%时,油水两相在注水管下游3D处即可达到均匀混合状态,可见薄板式静态混合器能够在小注水比下快速实现油水均匀混合。     

静态混合器中液液分散的实验及CFD模拟

在SK型静态混合器上进行甲苯-水两相混合实验,采用截面直接拍摄法获得分散混合性能指标Sauter平均直径（SMD）。利用Box-Behnken响应面分析设计实验,在Design Expert 7.0平台上拟合实验数据,获得SMD的多项式形式的表达式。建立了与实验相同的静态混合器物理模型,使用Mixture多相流模型、k-ε湍流模型进行了CFD模拟研究,获得了浓度场云图及分布混合指标不均匀系数。模拟所得压降与实验值的相对误差在15%以内,表明模拟结果与实验结果吻合较好。结果表明,静态混合器中液液分散过程是分散混合和分布混合共同作用的结果,两种混合经过6～8个混合单元后共同达到充分发展。充分发展后的SMD受表观流速、分散相分率和静态混合器直径三因素影响,且表观流速的影响最为显著;充分发展后的不均匀系数均达0.05以下,表明静态混合器自身具有较好的分布混合性能。     

Ross LPD型静态混合器内湍流传热与混合强化特性

在湍流状态Re=2640~17600下,采用恒热通量传热实验与数值模拟相结合的方法,系统研究Reynolds数Re和交错角对Ross LPD型静态混合器内湍流流动与传热性能影响,采用Nusselt数、Darcy摩擦系数、综合传热系数、速度场与温度梯度和压力梯度协同角等参数评价混合器内传热强化性能;基于CFD与LPT相耦合分析混合器内流体微元拉伸率。研究结果表明：SST k-ω模型预测Ross型静态混合器湍流阻力及传热结果与实验结果具有很好一致性;Ross混合器流场内形成与流场尺度较为接近的纵向涡,其涡心在圆形截面与半圆形截面中心间周期性迁移,横截面内湍流分散混合效率是Kenics的3.36~1.72倍;当Re>7040时,Ross LPD综合传热性能明显优于KSM;当叶片夹角为30°时,综合传热性能系数具有最大值;Ross LPD内插件具有高效低阻的技术优势和结构改进潜力。     

脉冲式静态混合器压力降分析

测定了雷诺数Ｒｅ从６８变化到１６５１时混合器压力降与雷诺数，摩擦系数与雷诺数的关系曲线和相应的曲线拟合方程。Ｒｅ＜５５０时，摩擦系数ｆ＝１３９．６６Ｒｅ－０．７４２，当Ｒｅ＞５５０时，流动基本处于湍流状态，ｆ＝５．２Ｒｅ－０．２０３。当雷诺数增大时，混合器内元件数量的改变对摩擦系数影响不大。     

Ross LPD型静态混合器内湍流传热与混合强化特性

在湍流状态Re=2640~17600下,采用恒热通量传热实验与数值模拟相结合的方法,系统研究Reynolds数Re和交错角对Ross LPD型静态混合器内湍流流动与传热性能影响,采用Nusselt数、Darcy摩擦系数、综合传热系数、速度场与温度梯度和压力梯度协同角等参数评价混合器内传热强化性能;基于CFD与LPT相耦合分析混合器内流体微元拉伸率。研究结果表明：SST k-ω模型预测Ross型静态混合器湍流阻力及传热结果与实验结果具有很好一致性;Ross混合器流场内形成与流场尺度较为接近的纵向涡,其涡心在圆形截面与半圆形截面中心间周期性迁移,横截面内湍流分散混合效率是Kenics的3.36~1.72倍;当Re>7040时,Ross LPD综合传热性能明显优于KSM;当叶片夹角为30°时,综合传热性能系数具有最大值;Ross LPD内插件具有高效低阻的技术优势和结构改进潜力。     

Design and characterization of a Low-pressure-drop static mixer

Four-blade static mixer was designed for inline mixing of Newtonian fluids at Reynolds numbers from 700 to 6800. The mixer consists of four equally spaced blades mounted on cylindrical housing with 45° rotation relative to the circumference. It was tested in three different compartments of 6, 8, and 10 mixing elements; each element rotated 45° relative to the adjacent one. Multipoint sampling was used to measure concentration downstream the mixer. The mixing quality was measured by the coefficient of variance (CoV). The CoV decreases as the energy input per unit mass increases. This effect is more pronounced when the number of mixing elements increases. For the case of 10 mixing elements, a good mixing performance (typically more than 95% mixedness or CoV < 0.05) achieved, although a marginally good mixing performance could also be achieved by eight mixing elements. The friction factors were correlated as f = C₁/Re + C₂/Reⁿ with an average deviation of ±10% from experimental data. Furthermore, experimental friction factors were compared with existing models. For a wide range of Reynolds numbers, the friction factors are apparently smaller than those from SMV, KMX, and baffle-type static mixers. © 2018 American Institute of Chemical Engineers AIChE J, 65: 1126–1133, 2019     

Development of mixing time correlation for a double jet mixer

BACKGROUND: Jet mixing is one of the simplest methods to achieve mixing. There have been a number of experimental studies concerned with jet mixing; some of these studies report empirical correlations. The existing correlations are not useful where there are significant deviations from the idealized conditions. Most correlations reported in the literature deal with liquid flow with single or multiple jets, whereas the effect of radial angle on mixing time was not studied. This present study investigates the effect of operating parameters on experimental mixing time in a double jet mixer. Nozzle configuration for jet1 was fixed based on earlier studies (2/3rd position, nozzle angle 45° and nozzle diameter 10 mm). Mixing times were estimated for different jet2 configurations of jet angle (30°, 45° and 60°), radial angles (60°, 120°, 180°), jet diameter (5 mm and 3 mm) and located at different tank heights (2/3rd and 1/3rd from the bottom of the tank). RESULTS: A mixing time correlation was developed in terms of all the parameters using dimensional analysis. The constants and powers of the parameters involved in the correlation developed were estimated using a least square method to calculate the straight line that best fitted the mixing time data obtained during the experiments. The effects of change in angle of inclination of jet2 (θ₂), radial angle of jet2 with respect to jet1 (Φ₂) and diameter of jet2 (d₂) on mixing time were analyzed and compared with the experimental mixing time. CONCLUSION: The correlation developed based on the dimensional analysis and least square method predicts the mixing time for a double jet mixing tank. Copyright © 2009 Society of Chemical Industry     

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.     

Comparative analysis of different static mixers performance by CFD technique: An innovative mixer

The flow and mixing behavior of two miscible liquids has been studied in an innovative static mixer by using CFD,with Reynolds numbers ranging from 20 to 160. The performance of the new mixer is compared with those of Kenics, SMX, and Komax static mixers. The pressure drop ratio(Z-factor), coefficient of variation(CoV), and extensional efficiency(α) features have been used to evaluate power consumption, distributive mixing, and dispersive mixing performances, respectively, in all mixers. The model is firstly validated based on experimental data measured for the pressure drop ratio and the coefficient of variation. CFD results are consistent with measured data and those obtained by available correlations in the literature. The new mixer shows a superior mixing performance compared to the other mixers.     

Experimental evaluation of gas mixing with a static microstructure mixer

Mixing plays an important role in chemical reaction engineering. In the last years several types of static microstructure mixers have been developed. The characterization of microstructure mixing is difficult to perform as the dimensions are too small for conventional methods. Therefore, we report a method to characterize the mixing of two gases directly by measuring the concentration of the gases at the outlet of the mixer. The experiments have been carried out up to gas flows of 5000 ml/min STP per passage. The mixing degree and mixing length were determined as well as the mixing time was calculated. These values depend on the properties of the gases and other parameters as temperature and gas velocity. Thus complete mixing is achieved after a mixing length, i.e., the distance to the microchannel outlet, of only 300-800 μm. Corresponding mixing times are just 100-600 μs. Furthermore, discontinuities in the mixing characteristic can be explained with the results obtained. Also design parameters for a further improvement of the mixer geometry individually for various applications could be set up.     

Numerical investigation of blade shape in static mixing

The mixing performance of the KMX and SMX static mixers have been compared using 3D high-resolution computational fluid dynamics (CFD) simulations. Although these mixers have a similar design composed of layers of blades, their blade shape is different: curved for the KMX and flat for the SMX. The flow of a Newtonian fluid in steady laminar regime has been considered as the benchmark of the study. The simulation was first validated by assessing the pressure drop vs. the number of mixer elements and the results were found to be in good agreement with experimental data. To evaluate the mixing quality, cross-section stream function, extensional efficiency, mean shear rate, residence time, intensity of segregation, stretching, and Lyapunov exponent have been selected. Analysis of the flow pattern and mixing parameters shows differences between the mixers and it appears that the curved blade is more efficient than the flat blade design at the expense of a slightly higher pressure drop. In practice, the KMX mixer should provide a higher mixing rate at high viscosity ratio than the SMX mixer. © 2004 American Institute of Chemical Engineers AIChE J, 51: 44–58, 2005     

拉伸流动静态混合器混合性能的实验研究

设计了1种能够增强聚合物混合效果的拉伸流动静态混合器（EFM）,以高密度聚乙烯/聚苯乙烯（PE⁃HD/PS）作为混合体系,根据共混体系扫描电子显微镜（SEM）照片及分散相的平均粒径,研究了不同盘形结构和不同盘棱间隙（δ）（0.125~2.0 mm）下EFM的混合性能。结果表明,EFM的盘形入口结构对其混合性能影响较小,混合能力随盘棱顶端圆角半径的增大而有所提升,随δ的增大出现先降低再升高又降低的变化趋势。     

Chaotic advection in a twin cam mixer under various operating conditions

A twin cam mixer (TCM) system equipped with two triangular cams is built in this study. The effect of operating conditions on chaotic advection in TCM is investigated. Particle Image Velocimetry (PIV) is employed to investigate the flow characteristics under co- and counter-rotating modes. Based on the good agreement between simulation and experimental results, chaotic advection under various operating conditions is investigated numerically. To evaluate the chaotic advection, time series Poincaré section, recording point density, and combined mixing index are proposed. The results show that counter-rotating mode is superior to co-rotating mode regarding chaotic advection. A more global chaotic advection can be obtained with the cams in TCM rotating at a higher average angular velocity, higher disturbance quantity, and lower disturbance frequency. The chaotic advection in TCM under different waveform protocols has no significant difference, but it is all better than that under constant protocol.     

Laminar mixing of shear thinning fluids in a SMX static mixer

Flow and mixing of power-law fluids in a standard SMX static mixer were simulated using computational fluid dynamics (CFD). Results showed that shear thinning reduces the ratio of pressure drop in the static mixer to pressure drop in empty tube as compared to Newtonian fluids. The correlations for pressure drop and friction factor were obtained at Re_MR?100. The friction factor is a function of both Reynolds number and power-law index. A proper apparent strain rate, area-weighted average strain rate on the solid surface in mixing section, was proposed to calculate pressure drop for a non-Newtonian fluid. Particle tracking showed that shear thinning fluids exhibit better mixing quality, lower pressure drop and higher mixing efficiency as compared to a Newtonian fluid in the SMX static mixer.     

新型静态混合器湍流特性数值模拟

结合新型静态混合器的结构特点,利用CFD软件采用标准的k-ε湍流模型对新型静态混合器内的湍流状态下的三维不可压缩流场进行数值模拟。通过研究新型静态混合器脉动速度分布的对称性及其间歇性发现:新型静态混合器内3个方向速度分量的偏斜因子和平坦因子分布具有周期性;x和z2个方向的速度概率密度分布存在较小不对称性且其平坦因子数值在2.3—5.7变化,径向偏斜因子的数量级均较轴向小1个数量级。采用新的数据处理方法计算和分析得到了不同长径比下新型静态混合器湍流流动阻力统一特性曲线及其关联式。     

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.     

随动式动态混合器的混合性能

介绍了随动式动态混合器的结构原理,采用截面直接拍摄法在光管和随动式动态混合器中进行了甘油-水两相混合实验,获得甘油的轴向分布图。此外,采用FLUENT中的Mixture多相流模型、RNG k-ε 湍流模型进行了混合性能数值模拟研究,获得了甘油的浓度场云图及混合不均匀系数。数值模拟与实验所得甘油在光管中的流动状态具有很好地一致性,证明了模拟方法的可靠性。研究结果表明,随动式动态混合器对分散相具有“分割-分隔、扰流、摩擦-混合”的作用,模拟所得甘油沿流动方向不均匀系数的变化,表明了随动式动态混合器具有较好的混合性能。