The recent publications concerning temporal and spatial chaotic mixing were reviewed. As a result, it was found that the mixing time for temporal chaotic mixing of a single liquid, that is co-reverse periodic rotation, time-periodically fluctuating rotation and reciprocating, has been investigated extensively. However, the applications of temporal chaotic mixing have been tried only for suspension polymerization of styrene and preparation of silica microcapsules. On the other hand, the spatial chaotic mixing has been widely used in industry but the studies were relatively limited.
The mass transfer coefficient of cylindrical baffles in an agitated vessel has been measured for the first time with the constant potential method using aqueous solution of 1 N-KOH + 0.2 N-K4Fe(CN)6 + 0.01 N-K3Fe(CN)6. The average mass transfer coefficient on the baffles was three to five times larger than that of the vessel wall based on the power consumption per unit volume. The average mass transfer coefficient on the baffles increased with decreasing baffle diameter. The number of baffles, the clearance between the baffles and vessel wall, the position of the baffles and the position of the impeller did not affect the average mass transfer coefficient of the baffles under these experimental conditions. The average mass transfer coefficient of the cylindrical baffles measured herein agrees with the value obtained by an equation based on one published before. The distributions of the local mass transfer coefficient of the cylindrical baffles are shown graphically for various impeller speeds. The local mass transfer coefficient of the baffles near the impeller was larger than those in other positions, and that near the liquid free surface increased up to the same level as that of the baffles near the impeller as the impeller speed increased.
In this study, we quantitatively investigated the influence of the bottom shape of an agitated vessel stirred by dual impellers on the distribution of solid concentration and the optimal bottom shape of the vessel for the dispersion of solid particles was evaluated. Four types of bottom shapes were adopted, including a flat bottom tank (FBT), a profiled bottom tank (PBT), a semi-elliptical bottom tank (EBT) and a cone and fillet tank (CFT). It was found that a good dispersion of solid can be obtained in the order of EBT, PBT, FBT, and CFT at constant power consumption. The CFD simulation conducted in this work also showed that the area of lower velocity zone was small in the same order of EBT, PBT, FBT, and CFT. From the result obtained in this study, it is clarified that the distribution of solid concentration in the vessel is considerably influenced by the local flow formed at the bottom region of the vessel.
In hydrothermal synthesis of nano-size particles, in which an aqueous metal salt solution at a normal temperature and supercritical water are directly mixed, the mixing process of the two fluids is extremely crucial for obtaining a highly supersaturated crystallization field and growth process control of fine metal oxides. This paper reports on a newly developed swirling micro mixer as a replacement of a conventional mixing tee-union for improved mixing of the two fluids. The developed micro mixer was featured with segmented introducing of supercritical water, swirling and convergent nozzle flow, and was examined by using CFD simulation. In a practical experiment of boehmite (AlOOH) fine particle synthesis, the developed micro mixer demonstrated average particle sizing down from 100–200 nm to 60 nm (by TEM and DLS data) in comparison with the reference experiment data that used a conventional mixing tee. In addition, a reduced secondary aggregation and high dispersibility were also demonstrated. The experimental results show the swirling micro mixer works well for the synthesis of desired particles and the reduced possibility of channel blockage by particle aggregated solids.
The velocity distribution and circulation flow rate in a draft-tube stirred tank of 0.8 m diameter were studied experimentally and were numerically simulated using the FLUENT6.2 computational fluid dynamics (CFD) package. A renormalization group (RNG) k–ε turbulent model and multiple reference frame (MRF) were used in the simulation. The circulation flow number and liquid velocity predicted by CFD were in good agreement with experimental values. Both experimental and simulation results show that the axial velocity is far larger than radial and tangential velocities inside and outside the draft tube, and there is a secondary circulation zone close to the outside wall of the draft tube, which could affect the efficiency of the tank. The results are to be of reference for the industrial stirred tank with a draft tube.
The present study investigates hydrodynamic factors which affect the growth of maneb particles. Manganese ethylenebis, a crystalline product known as maneb, is an intermediate fungicide which is made through the reaction between ammonium ethylenebis and manganese sulfate. Crystal size distribution of maneb is under the influence of crystallizer hydrodynamics and is an effective parameter of product purity, ease of filtration, and health considerations. The present study investigates the effects of mixing intensity, location of feed entrance, use of draft tube and the flow direction in the tube on the particle size of product maneb. Experiments were carried out in a 7.5 L-jacketed reactor equipped with a 45° pitched 4-blade impeller, and removable draft tube and baffles. The results of the experiments showed that the mean size of particles was highly influenced by mixing intensity and had an optimum value as the mixing intensity changed. The results also suggested that the best location of feed entrance in order to produce large particles was underneath the impeller. The use of a draft tube when the flow direction in the tube was downward resulted in a larger mean size of product. Furthermore, based upon the Lagrangian mechanistic model both mesomixing and micromixing mechanisms were proved effective in the reactive crystallization of maneb.
We tried in this study to numerically analyze the mixing process of high concentration slurry liquids having a plastic rheological property, and in which unevenness of both viscosity and concentration disappear along with agitation despite the initial condition in which the viscosities are different between the upper and lower halves of a stirred vessel. We verified the reliability of the analyzed results by comparing the viscosity and concentration with measurements of mixing time and the power consumption required to attain mixing. We clarified the relation between the mixing time and the initial concentration (viscosity) difference of the two layers or the final concentration of the mixture, both experimentally and analytically. We ascertained that a mixing performance index, NM′, can be well correlated with the Reynolds number, ReM, and the viscosity correction term for each initial concentration difference of the two layers. The numerical analysis enabled us to understand the correlation equations of the mixing process of a heterogeneously viscous system in which the initial viscosity varies through the vessel and to estimate the mixing performance in the vessel from a local point of view.
Mixing and segregation behavior of granular flow was investigated inside a sectorial shaped container, subjected to sinusoidal oscillations. A sectorial container containing particles was oscillated in the range of 1 to 4 Hz. The effects of different operating parameters such as frequency of oscillations, amplitude of oscillations, volume fraction, and the size ratio of particles were studied. Numerical simulations performed earlier were validated by experiments which indicated that the mixing and segregation of particles takes place in a particular range of frequency zone. A range of frequency has been identified for active and stagnant heaping of particulates. Good mixing was observed above and below a certain frequency range. Critical frequency region was also identified where the particulate matter was nearly symmetrical to the shape of the container and asymmetrical heaping was observed above and below this critical frequency range. The effect of initial arrangement of layers of different particulates was also investigated. Snapshots of experimental observations were compared and validated with the numerically simulated characteristics.
The kneading process of powders and highly vicious fluids is widely used in various industries. In recent years, it has become recognized that the performance of the kneader influences the quality of the products and that advancement of kneaders is much sought after. In this work, we studied the influence of kneading conditions, such as the rotational speed of the blade, the geometrical configurations of the kneader and the size of the vessel, on the mixing process of wet particles in a double-blade kneader based on the scale of segregation, SS, of the tracer concentration. An equation was proposed to express the influences of the kneading conditions using the power consumption per unit volume, apparent viscosity, size ratio of the kneader, dimensionless effective shearing area. It was found that this equation could estimate based on SS, the time and power consumption necessary to attain a desired mixed state for different types of kneaders, with an error of ±20%.
For the turbulent boundary layer at the side wall of an agitated vessel without baffles, new dimensionless variables u++ and y++ were derived as uθ+(βD/1.7d) and y+(βD/1.7d), respectively, instead of the dimensionless variables uθ+ and y+ in the universal velocity distribution law. The correlation of the measured tangential velocity at the side wall of an agitated vessel without baffles with u++ and y++ satisfied the logarithmic expression of the universal velocity distribution law in the range 20 < y++ < 70 for the paddle impeller having a larger value of the impeller similarity parameter than 0.1. This meant that the mixing length for an agitated vessel was βD/1.7d times that for a flat plate. Besides the tangential velocity at the turbulent boundary layer edge uD and the boundary layer thickness yD were roughly expressed as 0.85(d/D)UD and 0.030(1.7d/βD)L, respectively.
Continuous emulsification of low-viscosity liquids was investigated with three different types of motionless mixers, i.e., needle jetting mixer (NJM), Kenics Static Mixer® (KSM) and Ramond Supermixer® (RSM). Kerosene and n-heptane were used as the continuous phase, in which nonionic surfactant (Span80) was dissolved, and deionized water as the dispersed phase. All emulsification runs were carried out at the constant temperature of 303 K. The dispersed droplet diameters in the W/O emulsion were measured by means of microphotography. The size distributions of water droplets in emulsions were normalized by the Sauter mean diameter (d32), and then they obeyed a log–normal distribution function with an upper-limit. The ratio of the maximum droplet diameter (dmax) to d32 was estimated to be 2.30 for NJM and KSM and 1.86 for RSM, respectively. The correlations of d32 with the mean power input per unit mass of the media (PM) were derived. The slopes of the correlation lines on the d32–PM correlation chart took the almost same value of –0.4. This value agreed with the one derived from an isotropic turbulence law in low-viscosity liquids. The line for RSM was located below those for NJM and KSM.
It is well-known that the configuration of the impeller and the vessel has a significant influence on mixing characteristics. This paper, presents an investigation of mixing time and mixing power in a low viscosity liquid in the horizontal and vertical stirred vessels respectively. In this study the CFD software is used to simulate the flow pattern in the horizontal vessel. The difference in mixing characteristics of the two types of vessels is also discussed. It is found that the flow in the horizontal non-baffled vessel is possibly chaotic, while the mixing performance in the horizontal non-baffled vessel is a little better than that in the vertical one under the same experimental conditions. On the basis of this finding we propose an approach that can improve the performance of mixing and reduce the cost.
Three typical hydrodynamic characteristics (parallel, merging and diverging flow) in a baffled vessel stirred by a dual Rushton impeller agitator are investigated by large eddy simulation (LES) using two subgrid scale (SGS) models: the standard and dynamic Smagorinsky–Lilly models applied with the commercial CFD code FLUENT 6. The agitator motion is modeled using the sliding mesh approach. The results are compared with experimental and simulated data from the literature. It is concluded both SGS models used for LES can reproduce the complicated flow in the stirred vessel, while there are no remarkable differences between the results obtained by two SGS models though the dynamical Smagorinsky–Lilly model needs about 17% additional computing time.