Numerical optimization for blades of Intermig impeller in solid–liquid stirred tank

The multiphase flow in the solid-liquid tank stirred with a new structure of Intermig impeller was analyzed by computational fluid dynamics(CFD).The Eulerian multiphase model and standard k-ε turbulence model were adopted to simulate the fluid flow,turbulent kinetic energy distribution,mixing performance and power consumption in a stirred tank.The simulation results were also verified by the water model experiments,and good agreement was achieved.The solid-liquid mixing performances of Intermig impeller with different blade structures were compared in detail.The results show that the improved Intermig impeller not only enhances the solid mixing and suspension,but also saves more than 20% power compared with the standard one.The inner blades have relatively little influence on power and the best angle of inner blades is 45°,while the outer blades affect greatly the power consumption and the optimized value is 45°.     

Critical impeller speeds for a gas-inducing stirring tank loaded with solid particles

The influence of solid particles size,density and loading on the critical gas-inducing impeller speed was investigated in a gas–liquid–solid stirring tank equipped with a hollow Rushton impeller.Three types of solid particles,hollow glass beads with diameters of 300 μm,200 μm,100 μm,and 60 μm,silica gel and desalting resin,were used.It was found that the adding solid particles would change the critical impeller speed.For hollow glass beads and silica gel,whose relative densities were less than or equal to 1.5,the critical impeller speeds increased with the solid loading before reaching the maximum values,and then decreased to a value even lower than that without added solids.The size of the solids also had apparent influence on the critical impeller speed,and larger solid particles correspond to a smaller critical impeller speed.The experimental data also showed that the gasinducing was beneficial to the suspension of the solid particles.     

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.     

Large Eddy Simulations of Mixing Time in a Stirred Tank

Large eddy simulations （LES） of mixing process in a stirred tank of 0.476m diameter with a 3-narrow blade hydrofoil CBY impeller were reported. The turbulent flow field and mixing time were calculated using LES with Smagorinsky-Lilly subgrid scale model. The impeller rotation was modeled using the sliding mesh technique. Better agreement of power demand and mixing time was obtained between the experimental and the LES prediction than that by the traditional Reynolds-averaged Navier-Stokes （RANS） approach. The curve of tracer response predicted by LES was in good agreement with the experimental. The results show that LES is a reliable tool to investigate the unsteady and quasi-periodic behavior of the turbulent flow in stirred tanks.     

下沉颗粒气-液-固搅拌槽内微观混合特性研究（英文）

The parallel-competing iodide–iodate reaction scheme was used to study the micromixing performance in a multi-phase stirred tank of 0.3 m diameter. The impeller combination consisted of a half el iptical blade disk tur-bine below two down-pimping wide-blade hydrofoils, identified as HEDT+2WHD. Nitrogen and glass beads of 100μm diameter and density 2500 kg·m?3 were used as the dispersed phases. The micromixing could be improved by sparging gas because of its additional potential energy. Also, micromixing could be improved by the solid particles with high kinetic energy near the impeller tip. In a gas–solid–liquid system, the gas–liquid film vibration with damping, due to the frequent collisions between the bubbles and particles, led to the decrease of the turbulence level in the liquid and caused eventual y the deterioration of the micromixing. A Damping Film Dissipation model is formulated to shed light on the above micromixing performances. At last, the micromixing time tm according to the incorporation model varied from 1.9 ms to 6.7 ms in our experiments.     

搅拌槽内液-固两相体系的数值研究(Ⅰ)流场的数值模拟

Three-dimensional solid-liquid flow is mathematically formulated by means of the “two-fluid” approach and the two-phase k-ε-Ap turbulence model. The turbulent fluctuation correlations appearing in the Reynolds time averaged governing equations are fully incorporated. The solid-liquid flow field and solid concentration distribution in baffled stirred tanks with a standard Rushton impeller are numerically simulated using an improved “inner-outer” iterative procedure. The flow pattern is identified via the velocity vector plots and a recirculation loop with higher solid concentration is observed in the central vicinity beneath the impeller. Comparison of the simulation with experimental data on the mean velocities and the turbulence quantities of the solid phase is made and quite reasonable agreement is obtained except for the impeller swept volume. The counterpart of liquid phase is presented as well. The predicted solid concentration distribution for three experimental cases with the average solid concentration up to 20% is also found to agree reasonably with the experimental results published in the literature.     

Torque and bending moment acting on a flexible shaft agitated by disk turbines in a gas–liquid stirred vessel

The torque and bending moment acting on a flexible overhung shaft in a gas–liquid stirred vessel agitated by a Rushton turbine and three different curved-blade disk turbines(half circular blades disk turbine, half elliptical blades disk turbine, and parabolic blades disk turbine) were experimentally measured by a customized moment sensor. The results show that the amplitude distribution of torque can be fitted by a symmetric bimodal distribution for disk turbines, and generally the distribution is more dispersive as the blade curvature or the gas flow rate increases. The amplitude distribution of shaft bending moment can be fitted by an asymmetric Weibull distribution for disk turbines. The relative shaft bending moment manifests a "rising-falling-rising" trend over the gas flow number, which is a corporate contribution of the unstable gas–liquid flow around the impeller, the gas cavities behind the blades, and the direct impact of gas on the impeller. And the "falling" stage is greater and lasts wider over the gas flow number for Rushton turbine than for the curved-blade disk turbines.     

基于离散颗粒法模拟搅拌釜气含率分布

The discrete particle method was used to simulate the distribution of gas holdup in a gas-liquid standard Rushton stirred tank. The gas phase was treated as a large number of bubbles and their trajectories were tracked with the results of motion equations. The two-way approach was performed to couple the interphase momentum exchange. The turbulent dispersion of bubbles with a size distribution was modeled using a stochastic tracking model, and the added mass force was involved to account for the effect of bubble acceleration on the surrounding fluid. The predicted gas holdup distribution showed that this method could give reasonable prediction comparable to the reported experimental data when the effect of turbulence was took into account in modification for drag coefficient.     

Effects of rotational speed and fill level on particle mixing in a stirred tank with different impellers

The particle mixing was studied in a cylindrical stirred tank with elliptical dished bottom by experiments and simulations.The impeller types used were double helical ribbon(HR) + bottom HR,pitched blade ribbon + bottom HR,inner and outer HR + bottom HR,and pitched blade ribbon + Pfaudler + bottom HR labeled as impellers Ⅰ to Ⅳ,respectively.The quantitative correlations among the rotational speed,fill level and power consumption for impeller Ⅰ and impeller Ⅱ were obtained by experiments to validate the discrete element method(DEM) simulations.The particle mixing at different operating conditions was simulated via DEM simulations to calculate the mixing index using the Lacey method,which is a statistical method to provide a mathematical understanding of the mixing state in a binary mixture.The simulation results reveal that as the rotational speed increases,the final mixing index increases,and as the fill level increases,the final mixing index decreases.At the same operating conditions,impeller Ⅲ is the optimal combination,which provides the highest mixing index at the same revolutions.     

搅拌槽内的Rushton桨叶周围流场的PIV测量

In this paper, particle image velocimetry (PIV) was used to measure the mean and root mean square(RMS) velocity in the stirred tank with six-flat blade Rushton turbine and with no baffles. Two types of motion patterns were studied. One was that the impeller runs at constant speed, the other was that the impeller runs at time-dependent speed and in a periodic way. The emphasis of the paper was on the comparison of mean and RMS velocity vector maps and profiles between these two types of motion patterns, and especial attention was paid to the comparison of the mean velocity, time-averaged RMS velocity, phase averaged RMS velocity between the constant 3 RPS (revolution per second) and time-dependent operation. The Reynolds number was between 763 and i527. The study explained the mechanism that time-dependent RPS is more efficient for mixing than that of constant RPS.     

Flow Field Around Rushton Turbine in Stirred Tank by Particle Image Velocimetry Measurement

In this paper, particle image velocimetry (PIV) was used to measure the mean and root mean square(RMS) velocity in the stirred tank with six-flat blade Rushton turbine and with no baffles. Two types of motion patterns were studied. One was that the impeller runs at constant speed, the other was that the impeller runs at time-dependent speed and in a periodic way. The emphasis of the paper was on the comparison of mean and RMS velocity vector maps and profiles between these two types of motion patterns, and especial attention was paid to the comparison of the mean velocity, time-averaged RMS velocity, phase averaged RMS velocity between the constant 3 RPS (revolution per second) and time-dependent operation. The Reynolds number was between 763 and 1527. The study explained the mechanism that time-dependent RPS is more efficient for mixing than that of constant RPS.     

加压下填料塔中液相轴向反混的研究

Liquid phase axial mixing was measured with the tracer technique in a packed column with inner diameter of 0.15m,in which the structured packing,Mellapak 350Y,was installed.Tap water as the liquid phase flowed down through the column and stagnant gas was at elevated pressure ranging from atmospheric to 2.0MPa.The model parameters of Bo andθwere estimated with the least square method in the time domain.As liquid flow rate was increased,the liquid axial mixing decreased.under our experimental conditions,the effect of pressure on Bo number on single liquid phase was negligible,and eddy diffusion was believed to be the primary cause of axial mixing in liquid phase.     

Rushton桨气液搅拌槽内气泡流对搅拌轴弯矩的影响（英文）

The bending moment acting on the overhung shaft of a gas-sparged vessel stirred by a Rushton turbine, as one of the results of fluid and structure interactions in stirred vessels, was measured using a moment sensor equipped with digital telemetry. An analysis of the shaft bending moment amplitude shows that the amplitude distribution of the bending moment, which indicates the elasticity nature of shaft material against bending deformation, fol-lows the Weibull distribution. The trends of amplitude mean, standard deviation and peak deviation character-istics manifest an“S”shape versus gas flow. The“S”trend of the relative mean bending moment over gas flow rate, depending on the flow regime in gas–liquid stirred vessels, resulted from the competition among the non-uniformity of bubbly flow around the impeller, the formation of gas cavities behind the blades, and the gas direct impact on the impel er when gas is introduced. A further analysis of the bending moment power spectral density shows that the rather low frequency and speed frequency are evident. The low-frequency contribution to bend-ing moment fluctuation peaks in the complete dispersion regime.     

Influence of impeller diameter on overall gas dispersion properties in a sparged multi-impeller stirred tank☆

The impeller configuration with a six parabolic blade disk turbine below two down-pumping hydrofoil propellers, identified as PDT + 2CBY, was used in this study. The effect of the impeller diameter D, ranging from 0.30T to 0.40T (T as the tank diameter), on gas dispersion in a stirred tank of 0.48 m diameter was investigated by experimental and CFD simulation methods. Power consumption and total gas holdup were measured for the same impeller configuration PDT + 2CBY with four different D/T. Results show that with D/T increases from 0.30 to 0.40, the relative power demand (RPD) in a gas–liquid system decreases slightly. At low superficial gas velocity V_S of 0.0078 m·s^-1, the gas holdup increases evidently with the increase of D/T. However, at high superficial gas velocity, the systemwith D/T=0.33 gets a good balance between the gas recirculation and liquid shearing rate, which resulted in the highest gas holdup among four different D/T. CFD simulation based on the two-fluid model along with the Population Balance Model (PBM) was used to investigate the effect of impeller diameter on the gas dispersion. The power consumption and total gas holdup predicted by CFD simulation were in reasonable agreement with the experimental data.     

Spray and mixing characteristics of liquid jet in a tubular gas–liquid atomization mixer

For the design and optimization of a tubular gas–liquid atomization mixer,the atomization and mixing characteristics of liquid jet breakup in the limited tube space is a key problem.In this study,the primary breakup process of liquid jet column was analyzed by high-speed camera,then the droplet size and velocity distribution of atomized droplets were measured by Phase-Doppler anemometry (PDA).The hydrodynamic characteristics of gas flow in tubular gas–liquid atomization mixer were analyzed by computational fluid dynamics (CFD) numerical simulation.The results indicate that the liquid flow rate has little effect on the atomization droplet size and atomization pressure drop,and the gas flow rate is the main influence parameter.Under all experimental gas flow conditions,the liquid jet column undergoes a primary breakup process,forming larger liquid blocks and droplets.When the gas flow rate (Q_g) is less than 127 m~3·h~(-1),the secondary breakup of large liquid blocks and droplets does not occur in venturi throat region.The Sauter mean diameter (SMD) of droplets measured at the outlet is more than 140μm,and the distribution is uneven.When Q_g127 m~3·h~(-1),the large liquid blocks and droplets have secondary breakup process at the throat region.The SMD of droplets measured at the outlet is less than 140μm,and the distribution is uniform.When 127Q_g162 m~3·h~(-1),the secondary breakup mode of droplets is bag breakup or pouch breakup.When 181Q_g216 m~3·h~(-1),the secondary breakup mode of droplets is shear breakup or catastrophic breakup.In order to ensure efficient atomization and mixing,the throat gas velocity of the tubular atomization mixer should be designed to be about 51 m·s~(-1)under the lowest operating flow rate.The pressure drop of the tubular atomization mixer increases linearly with the square of gas velocity,and the resistance coefficient is about 2.55 in single-phase flow condition and 2.73 in gas–liquid atomization condition.     

涡流桨搅拌槽内流动非稳定性的大涡模拟

The aim of this work is to investigate the flow instabilities in a baffled, stirred tank generated by a single Rushton turbine by means of large eddy simulation （LES）. The sliding mesh method was used for the coupling between the rotating and the stationary frame of references. The calculations were carried out on the ＂Shengcao-21C＂ supercomputer using a computational fluid dynamics （CFD） code CFX5. The flow fields predicted by the LES simulation and the simulation using standard κ-ε model were compared to the results from particle image velocimetry （PIV） measurements. It is shown that the CFD simulations using the LES approach and the standard κ-ε model agree well with the PIV measurements. Fluctuations of the radial and axial velocity are predicted at different frequencies by the LES simulation. Velocity fluctuations of high frequencies are seen in the impeller region, while low frequencies velocity fluctuations are observed in the bulk flow. A low frequency velocity fluctuation with a nondimensional frequency of 0.027Hz is predicted by the LES simulation, which agrees with experimental investigations in the literature. Flow circulation patterns predicted by the LES simulation are asymmetric, stochastic and complex, spanning a large portion of the tanks and varying with time, while circulation patterns calculated by the simulation using the standard κ-ε model are symmetric. The results of the present work give better understanding to the flow instabilities in the mechanically agitated tank. However, further analysis of the LES calculated velocity series by means of fast Fourier transform （FFT） and/or spectra analysis are recommended in future work in order to gain more knowledge of the complicated flow phenomena.     

Simulation of the hydrodynamics and mass transfer in a falling film wavy microchannel

The flow in a liquid falling film is predominantly laminar, and the liquid-side mass transfer is limited by molecular diffusion. The effective way to enhance the mass transfer is to improve the liquid film flow behavior. The falling film behaviors of water, ethanol and ethylene glycol in nine different wavy microchannels were simulated by Computational Fluid Dynamics. The simulation results show that the falling film thickness exhibits a waveform distribution resulting in a resonance phenomenon along the wavy microchannel. The fluctuation of liquid film surface increases the gas–liquid interface area, and the internal eddy flow inside the liquid film also improves the turbulence of liquid film, the gas–liquid mass transfer in falling film microchannels is intensified. Compared with flat microchannel, the CO_2 absorption efficiency in water in the wavy microchannel is improved over 41%. Prediction models of liquid film amplitude and average liquid film thickness were established respectively.     

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.     

改善气液传质的新型表面曝气装置

A novel surface aeration configuration featured with a self-rotating and floating baffle (SRFB) and a Rushton disk turbine (DT) with a perforated disk has been developed. The SRFB, consisted of 12 fan blades twisted by an angle of 30° to the horizontal plane, is incorporated onto the impeller shaft to improve gas entrainment, bubble breakup, mixing in a φ154 mm agitated vessel. This new configuration is compared to the conventional DT surface aeration experimentally. The results suggest that the critical impeller speed for onset of gas entrainment is lower for the new configuration and it demands greater power consumption. Moreover, the SRFB system produces 30%-68% higher volumetric mass transfer coefficient per unit power input than that obtained in the conventional DT surface aerator under the same operation conditions.     

折流式旋转床的流体力学行为(英文)

As a high gravity (HIGEE) unit, the rotating packed bed (RPB) uses centrifugal force to intensify mass transfer. Zigzag rotating bed (RZB) is a new type of HIGEE unit. The rotor of RZB consists of stationary discs and rotating discs, forming zigzag channels for liquid-gas flow and mass transfer. As in RPBs, some hydrodynamic behavior in RZB is interesting but no satisfactory explanation. In this study, the experiments were carried on in a RZB unit with a rotor of 600 mm in diameter using air-water system. The gas pressure drop and power consumption were measured with two types of rotating baffle for RZB rotors, one with perforations and another with shutter openings. The circumferential velocities of gas were measured with a five-hole Pitot probe. The pressure drop decreased rapidly when the liquid was introduced to the rotor, because the circumferential velocity of the liquid droplets was lower than that of the gas, reducing the circumferential velocity of gas and the centrifugal pressure drop. The power consumption decreased first when the gas entered the RZB rotor, because the gas with higher circumferential velocity facilitates the rotation of baffles.