Large Eddy Simulation of Liquid Flow in a Stirred Tank with Improved Inner-Outer Iterative Algorithm

In this study, the large eddy simulation technique was applied on the flow in a baffled stirred tank driven by a Rushton turbine at Re=29000. The interaction between the rotating impeller and the static baffles was accounted for by means of the improved inner-outer iterative algorithm. The sub-grid scale model was a conventional Smagorinsky model. The numerical solution of the governing equations was conducted in a cylindrical staggered grid. The momentum and the continuity equations were discretized using the finite difference method, with a third-order QUICK scheme used for convective terms. The phase-resolved predictions were compared with the experimental data of Wu and Patterson and good agreement was observed for both the mean and the turbulence quantities. They were much better than the Reynolds-averaged Navier-Stokes model including the Reynolds Stress Model for simulating the turbulence. The study also suggests the feasibility of LES in combination with the improved inner-outer iterative algorithm for the simulation of turbulent flow in stirred tanks.

Void Fraction Distributions in Cold-gassed and Hot-sparged Three Phase Stirred Tanks with Multi-impeller

Vertical distributions of void fraction in gas-liquid and gas-liquid-solid stirred tanks have been measured in a fully baffled dished base vessel of 0.48 m diameter, using a conductivity probe. The impeller configuration （a hollow half elliptical blade dispersing turbine below two up-pumping wide blade hydrofoils, identified as HEDT＋2WHu） recommended in previous work has been used in this work. The operating temperatures were 24℃ and 81℃, identified as cold and hot respectively. The effects of superficial gas velocity, agitator speed and the corresponding power input on the local void fraction in two-phase systems are .investigated and discussed. Results show thatth-e increasing of agitator speed or gas flow rate leads to an increase in local-void fraction at the majority of measurement points in both cold and hot systems. However, the unifo,rmity of gas dispersion does not always in crease as the raising of agitator speed and power input. In either cold or hot sparged conditions, the two- and three-phase systems.have similar vertical profiles for void fraction, with maxima in similar locations; however, the void fractions are significantly lower in hot sparging than with cold. In cold operation the presence of particles leads to a lower void fraction at most points, although the local void fractions increase a little with the addition of solid particles at high temperature, in good agreement with the global gas holdup results, and the possible reasons are discussed in this paper. This work can give a better understanding of the differences between cold-gassed and hot-sparged three phase＇stirred tanks.

Evolution of globular microstructures during processing of aluminium slurries

Slurry processing experiments were performed with AlSi7Mg0.6 to identify the globularization mechanisms.The melt sample water quenched slightly above the liquidus point is predominantly dendritic while that cooled into the semi-solid temperature range internally via stirring the melt with a rotating cylindrical block of the alloy itself becomes fully globular.The globules are much smaller when internal cooling and stirring are employed longer to achieve higher solid fractions before casting.Coarse dendrite fragments of various sizes are revealed,in the case of stirring after an initial fraction of solid is first formed without the benefit of additional internal cooling.

The Resident Time Distribution of Injected Dispersed Drops in Stirred Extraction Columns

A resident time model is proposed to evaluate the performance of agitated extraction columns. In this model, the resident time of dispersed drops is simulated with the discrete phase modeling, where the continuous phase and the dispersed phase （drops） are described by the single-phase Navier-Stokes （turbulence） model and Lagrangian model, respectively. The interaction of dispersed phase and continuous phase is neglected for the low concentration of drop in the cases studied. The statistical parameters of drops （the average resident time and standard deviation） under different operation conditions are computed for four columns. The relation of the above statistical parameters with the performance of columns is discussed and the criterions for an optimal compartment are outlined. Our results indicate that the resident time model is useful to evaluate the performance and optimize the design of extraction columns.

The Effect of Eddy-bubble Interaction Model on the Turbulent Dispersion of Gas Bubbles in Stirred Tanks

Based on trajectory equations of gas bubble,an eddy-bubble interaction(EBI)model was developed. This model considered the effect of non-drag forces and took the eddy-bubble interaction time as the refreshing time scale of turbulent fluctuations.The relationship between the crossing-eddy time and the eddy lifetime was discussed,and the predicted distributions of radial,axial velocities of bubbles and gas holdup were also given. Compared with eddy lifetime(EL)model,the EBI model gives somewhat smaller axial velocity in the upper circulation region and larger velocity in the lower circulation region,causing that fewer bubbles reach the lower circulation region and gas holdup becomes higher in the upper circulation region.The predicted gas holdup by the EBI model approaches closer to the experimental data in the discharge stream region.

Flooding Characteristics of Hydrofoil Impeller in a Two- and Three-phase Stirred Tank

The flooding characteristics of hydrofoil impeller were systematically investigated in a two-and three-phase 383 mm i.d. stirred tank operated on air, water and spherical glass beads. The volumetric solid concen-tration Cs was varied from 0 to 25%. And the superficial gas velocity Ug was at the range of 0-0.096 m·s-1. A fast and objective method for identifying flooding point NF is developed based on the statistical analysis of the pressure fluctuation signals. It is found, the effect of solid concentration on the flooding point NF depends on the gas velocity. At the lower gas velocity (Ug = 0.010 m·s-1), the solid concentration has only a minor effect. However, it displays a very significant effect on the flooding point NF at the medium and high gas velocity. The flooding point NF linearly increases with the gas velocity Ug, at lower solid concentration (Cs = 0, 10%). When Cs = 20%, the behavior of NF versus Ug becomes more complex. The correlations of the flooding characteristics in the slurry stirred tank are proposed by considering the solid concentration effect.

Power demand and mixing performance of coaxial mixers in a stirred tank with CMC solution

Experimental investigation was carried out in an elliptical based stirred tank with a diameter of 0.48 m to explore the power demand and mixing performance of coaxial mixers.Syrup and CMC solution(sodium carboxy methyl cellulose)were used as the Newtonian and non-Newtonian fluids,respectively.Four different coaxial mixers were combined with either CBY or Pfaudler impeller as the inner one,and anchor or helical ribbon(HR)as the outer one.Results show that Pfaudler-HR is the optimized combination among four coaxial mixers in this work,which provides the shortest mixing time given the same power consumption.Compared with the syrup solution,the increase of power input can make the mixing time decreasing more obviously in the CMC solution.The quantitative correlations for both syrup and CMC solutions were established to calculate the power draw and the mixing time of four coaxial mixers.

Warm asphalt rubber:A sustainable way for waste tire rubber recycling

Recycling end-of-life tire rubber as asphalt modifier is known as a sustainable paving technology with merits including enhanced pavement durability,waste tire consumption and noise reduction.However,the criticisms on the high construction emissions of asphalt rubber(AR)have limited its application.Warm mix asphalt(WMA)effectively reduces the mixing and compaction temperatures of conventional hot mix asphalt mixtures.The combination of AR and WMA,called warm asphalt rubber(WAR),is a promising paving material which achieves pavement sustainability from principles to practices.Many studies have demonstrated that WMA technologies work effectively with AR pavement in different ways,alleviating the concerns of potential higher emissions of AR by decreasing mixing and paving temperatures.A comprehensive literature review about WAR brings a better understanding of this promising paving technology.The findings of 165 publications were summarized in this review.It summarized the recent developments of WAR in various aspects,including rheological properties,mix design,mixture mechanical performance,field application,construction emission,and asphalt-rubber-WMA additive interaction.It is expected that this review is able to provide extensive information to explore further research development and application of WAR.

Parametric study of gas−liquid two-phase flow field in horizontal stirred tank

Based on Fluent software,the gas−liquid two-phase flow in the horizontal stirred tank was simulated with SST k−ωturbulence model,Eulerian−Eulerian two-fluid model,and multi-reference flame method.The mixing process in the tank was calculated by tracer method.The results show that increasing the rotating speed or gas flow is conducive to a more uniform distribution of the gas phase and accelerates the mixing of the liquid phase.When the rotating speed exceeds 93 r/min,the relative power demand remains basically constant.The change in the inclination angle of the upper impeller has minimal effect on the gas phase distribution.When the inclination angle is 50°,the relative power demand reaches the maximum.An appropriate increase in the impeller distance from the bottom improves the gas holdup and gas phase distribution but increases the liquid phase mixing time.

Numerical Study of Solid-Liquid Two-Phase Flow in Stirred Tanks with Rushton Impeller (Ⅰ) Formulation and Simulation of Flow Field

The critical impeller speed, N_(JS), for complete suspension of solidparticles in the agitated solid-liquid two-phase system in baffled stirred tanks with a standardRushton impeller is predicted using the computational procedure proposed in Part Ⅰ. Three differentnumerical criteria are tested for determining the critical solid suspension. The predicted N_(JS)is compared with those obtained from several empirical correlations. It is suggested the mostreasonable criterion for determining the complete suspension of solid particles is the positive signof simulated axial velocity of solid phase at the location where the solid particles are mostdifficult to be suspended.

Effect of precipitate evolution on corrosion behavior of friction stir welded joints of AA2060-T8 alloy

Friction stir welding was used to join two AA2060-T8 plates,and then the effect of precipitate evolution on microstructure and corrosion behavior of the joint was investigated.The evolution of precipitates on the top surface of the joint was characterized by scanning electron microscopy and transmission electron microscopy.The corrosion behaviors of different regions in the joint were investigated by an electrochemistry method and an alternating salt spray exposure.The corrosion was mainly dependent on the nature of precipitates in each region of the joint.The shoulder affected zone had the worst corrosion resistance as a result of the re-dissolved ofθ′(Al2Cu),T1(Al2CuLi)andδ′(Al3Li)phases,the formation of intergranular precipitates and precipitate-free zones.However,the thermomechanically affected zone had a slightly improved corrosion resistance because it had no intergranular precipitates.The heat affected zone and base metal had the best corrosion resistance.

Dispersion of SiC particles in mechanical stirring of A356-SiC_p liquid

In order to clarify the dispersion of SiC particles in straight-blade mechanical stirring of A1-SiCp liquid, the dispersion of SiC particles in A356-3.5% SiCp （volume fraction） liquid in a cylindrical crucible was studied. The relationship between rotating speed of stirrer and radial relative deviation of SiCp content in A356 liquid between the center and the periphery of crucible was established in the conditions of 35° for the gradient angle a of blade and 10 mm/s for the speed of moving up and down of stirrer. The results show that the radial relative deviation of SiCp content increases gradually with increasing the rotating speed of stirrer. When the rotating speed of stirrer is 200 r/min, the vertical dispersion of SiC particles in A356 liquid is even, but the radial relative deviation of SiCp content is 0.24. Consequently, the northomogeneous dispersion of SiC particles in A356 liquid is mainly resulted from the nonhomogeneous radial dispersion of SiC particles.

Computational fluid dynamics simulation of gas-liquid two phases flow in 320 m^3 air-blowing mechanical flotation cell using different turbulence models

According to the recently developed single-trough floating machine with the world's largest volume(inflatable mechanical agitation flotation machine with volume of 320 m3) in China, the gas-fluid two-phase flow in flotation cell was simulated using computational fluid dynamics method. It is shown that hexahedral mesh scheme is more suitable for the complex structure of the flotation cell than tetrahedral mesh scheme, and a mesh quality ranging from 0.7 to 1.0 is obtained. Comparative studies of the standard k-ε, k-ω and realizable k-ε turbulence models were carried out. It is indicated that the standard k-ε turbulence model could give a result relatively close to the practice and the liquid phase flow field is well characterized. In addition, two obvious recirculation zones are formed in the mixing zones, and the pressure on the rotor and stator is well characterized. Furthermore, the simulation results using improved standard k-ε turbulence model show that surface tension coefficient of 0.072, drag model of Grace and coefficient of 4, and lift coefficient of 0.001 can be achieved. The research results suggest that gas-fluid two-phase flow in large flotation cell can be well simulated using computational fluid dynamics method.

Comparison of the Reynolds-averaged Turbulence Models on Single Phase Flow Simulation in Agitated Extraction Columns

The flow field of liquid phase (water) of agitated extraction columns is simulated with the help of computational fluid dynamics (CFD). Four kinds of Reynolds-averaged turbulence models, i.e. the standard k-ε model, the RNG (renormalization group) k-s model, the realizable k-ε model and the Reynolds stress model, are compared in detail in order to judge which is the best model in terms of the accuracy, less CPU time and memory required. The performance of the realizable k-s model is obviously improved by reducing the model constant from C2 = 1.90 to C2 = 1.61. It is concluded that the improved realizable k-e model is the optimal model.

《燕子赋(一)》“密箄相■”试解

结合中土文献和佛经材料对《燕子赋(一)》中"密箄相"作了尝试性的延证考释。"密箄相■"当为"蜜蓖相伴",意为酥蜜酪油麻油相和细研,是密教为消除灾障作护摩法时拌和供养食物的行为,转指为作护摩法,狱使用此来指称雀儿"口中念佛,心中发愿"这种临时抱佛脚的行为。

Influence of Sb modification on microstructures and mechanical properties of Mg_2 Si/AM60 composites

The semi-solid antiburning AZ61-1.0%Y magnesium alloy slurry with fine circular solid phase was fabricated by a novel type continuous mechanical stirring in this work.The microstructure of the semisolid slurry was characterized by a metallography microscope.The results show that the fine circular solid phase distributes uniformly in the slurry when the stirring temperature ranges from 600 to 605℃.With the increase of the stirring velocity,the size of the solid phase becomes smaller and smaller.With the increase of the stirring time,the size of solid phase gets finer,but if the stirring time is longer than the critical time,it will be coarsened abnormally.The mechanical properties of semi-solid AZ61-1.0%Y alloy are superior to those of the normally casting magnesium alloy.

Experimental Study on Aqueous Phase Entrainment in a Mixer-settler with Double Stirring Mode

The mixer-settler is a core device of solvent extraction for separating rare earth elements. There are some adverse effects like high rare earth accumulation and poor production efficiency during industrial production. Current researches usually focus on changing the structure of the mixer-settler without making a breakthrough towards gravity clarification. In this paper, in order to improve the efficiency of clarification, a mixer-settler with double stirring mode was designed and manufactured by adding a stirring device in the settler after reducing the volume of the settler. The innovation of this research involves adopting the ultraviolet-visible spectrophotometer to investigate the quantity of aqueous phase entrainment at the settler outlet in order to measure the clarification degree. Experimental results show that the clarification effect with stirring is better than that without stirring. The clarification effect is ameliorated as the stirring speed increases. Generally, the clarification effect shows a best condition when the offset distance is 12.5 cm, making the phase entrainment reduced to less than 0.1%. When the clearance over the tank bottom is 7 cm and 10 cm, respectively, the quantity of aqueous phase entrainment is better than the case with a clearance of 4 cm. The results show that the stirring paddle close to the mixed phase zone can better promote the two-phase separation.

Mass Transfer Enhancement of Gas Absorption by Adding the Dispersed Organic Phases

Mass transfer enhancement of gas absorption by adding a dispersed organic phase has been studied in this work. Various dispersed organic phases （heptanol, octanol, isoamyl alcohol, heptane, octane, and isooctane） were tested respectively in the experiment. According to the theoretical model and experimental data, the overall volumetric mass transfer coefficient and enhancement factor were obtained under different dispersed organic phase volume fraction and stirring speed. The experimental results indicate that gas-liquid mass transfer is enhanced at different level by adding a dispersed organic phase. The best performance of enhancement were achieved with the dispersed organic phase volumetric fraction of 5% and under an intermediate stirring speed of 670 r·min^-1. Among the organic phases tested in the experiment, alcohols show better performance, which gave 20% higher enhance-ment of overall volumetric mass transfer coefficient than adding alkanes.

Determination of Trihalomethanes in Water Samples Using Headspace Solid-Phase Microextraction Gas Chromatography

The chlorination process is one of the water treatment method used for the disinfection of water. The disinfection by products are trihalomethanes such as chloroform, dichloromethane, dibromochloromethane and bromoform. A headspace solid-phase microextraction method has been developed for determination oftrihalomethanes in water samples. The experimental parameters such as the stirring rate, extraction time, extraction temperature and desorption time were investigated. The linearity, detection limits and percentage recovery were evaluated. The optimum conditions were stirring rate 800 rpm/min, extraction time 6 min, extraction temperature 20 ~C, desorption time 2.5 min and desorption temperature 220 ~C. The detection limits were 0.01 ~g/L and the recoveries were in the range of 86-110 %, The proposed method was successfully applied to determination of THM4 in tap water samples. The THM4 contents were varied depending on the sample sites and the season. The total THM4 contents in cool, summer and rainy season were in the range of 27.58-41.89, 32.06-60.73 and 46.26-69.87 p.g/L, respectively. Confirmation of the detected compounds in water samples were performed by gas chromatograph-mass spectrometer. The mass spectra of the target compounds in water samples is in good agreement with trihalomethanes standard spectra.