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°.     

Scalar mixing in a turbulent stirred tank with pitched blade turbine: Role of impeller speed perturbation

Mixing of a passive scalar inside a pitched blade turbine (PBT) impeller stirred tank (STR) is studied using large-eddy simulation (LES) coupled with the immersed boundary method (IBM) for resolving moving interfaces. Mixing time is calculated based on the 95% homogenization of the scalar over the entire tank volume. Growth rate of the unmixed tracer volume is observed in order to identify the effects of low frequency macroinstability (MI) oscillations. Mixing time is significantly reduced when the STR flow is perturbed using a step-change in the impeller speed with a specific MI frequency. The enhancement in turbulent kinetic energy and changes in mean flow field due to the perturbation is observed. The spatio-temporal behavior of the large-scale mixing structures for the fixed impeller-speed case and the perturbed case are compared. The mechanism of mixing enhancement is further explored by observing dynamic changes in the concentration distribution and the velocity field over a perturbation cycle. Penalty in power requirement due to perturbation is calculated.     

Effect of pitched short blades on the flow characteristics in a stirred tank with long-short blades impeller

This work focuses on the design improvement of the long-short blades (LSB) impeller by using pitched short blades (SBs) to regulate the flow field in the stirred vessel. After mesh size evaluation and velocity field validation by the particle image velocimetry, large eddy simulation method coupled with sliding mesh approach was used to study the effect of the pitched SBs on the flow characteristics. We changed the inclined angles of the SBs from 30° to 60° and compared the flow characteristics when the impeller was operated in the down-pumping and up-pumping modes. In the case of down-pumping mode, the power number is relatively smaller and vortexes below the SBs are suppressed, leading to turbulence intensification in the bottom of the vessel. Whereas in the case of up-pumping mode, the axial flow rate in the center increased significantly with bigger power number, resulting in more efficient mass exchange between the axial and radial flows in the whole vessel. The LSB with 45° inclined angle of the SBs in the up-pumping mode has the most uniform distributions of flow field and turbulent kinetic energy compared with other impeller configurations.     

CFD simulations of gas-liquid-solid stirred reactor: Prediction of critical impeller speed for solid suspension

In this work, simulations have been performed for three phase stirred dispersions using computational fluid dynamics model (CFD). The effects of tank diameter, impeller diameter, impeller design, impeller location, impeller speed, particle size, solid loading and superficial gas velocity have been investigated over a wide range. The Eulerian multi-fluid model has been employed along with the standard k-ε turbulence model to simulate the gas-liquid, solid-liquid and gas-liquid-solid flows in a stirred tank. A multiple reference frame (MRF) approach was used to model the impeller rotation and for this purpose a commercial CFD code, FLUENT 6.2. Prior to the simulation of three phase dispersions, simulations were performed for the two extreme cases of gas-liquid and solid-liquid dispersions and the predictions have been compared with the experimental velocity and hold-up profiles. The three phase CFD predictions have been compared with the experimental data of Chapman et al. [1983. Particle-gas-liquid mixing in stirred vessels, part III: three phase mixing. Chemical Engineering Research and Design 60, 167-181], Rewatkar et al. [1991. Critical impeller speed for solid suspension in mechanical agitated three-phase reactors. 1. Experimental part. Industrial and Engineering Chemistry Research 30, 1770-1784] and Zhu and Wu [2002. Critical impeller speed for suspending solids in aerated agitation tanks. The Canadian Journal of Chemical Engineering 80, 1-6] to understand the distribution of solids over a wide range of solid loading (0.34-15 wt%), for different impeller designs (Rushton turbine (RT), pitched blade down and upflow turbines (PBT45)), solid particle sizes (120-) and for various superficial gas velocities (0-10 mm/s). It has been observed that the CFD model could well predict the critical impeller speed over these design and operating conditions.     

Solids suspension in stirred tanks with pitched blade turbines

In view of developing a universal correlation for critical speed of suspension, extensive suspension experiments were conducted with tank scales in the range of 15-, D/T from 0.083 to 0.625, using four different sizes of spherical glass beads and employing Pitched Blade Turbines with four and six blades as the impellers. The periphery of the tank bottom was modified to include a permanent fillet in order to eliminate the effect of induced recirculation loop, which account for the formation of peripheral fillets of unsuspended solids. The critical speed of suspension N_c and power P_c were observed to vary independently both with D and T to give two correlations for each of the variables, N_c and P_c; one for the close proximity impeller operation where both N_c and P_c remained invariant with off-bottom impeller clearance and the second for the region where N_c and P_c were affected significantly by the impeller position. The effects of the physical characteristics of the solids were also included in the four correlations so proposed. It was clearly noticed that the correlations were valid up to a critical value of D/T beyond which the trapped particles in the stagnant zone below the impeller needed extra energy to be raked out and suspended, thus breaking the log-linear relationship between N_c (or P_c) and D/T hitherto maintained. Comparisons of the suspension speed and power have been made with open literature. More importantly, the reasons why the earlier studies differed with each other in N_c-predictions have been identified.     

Measurement of phase holdups in liquid-liquid-solid three-phase stirred tanks and CFD simulation

Phase holdup is an important hydrodynamic characteristic of multiphase systems relevant to optimization and scale-up of related process equipment. In the present article, measurements of phase distribution of solid particles and oil droplets are conducted in a lab-scale stirred tank by sample withdrawal under various operating conditions. A Eulerian-Eulerian three-fluid model is established for the prediction of phase distribution of two dispersed phases in the agitated liquid-liquid-solid dispersion system. The turbulence structure in the system is described by an extension of the standard k-ε turbulence model to three-phase flow including the influence of presence of two dispersed phases as an additional source of turbulent kinetic energy. Momentum exchange between continuous and dispersed phase as well as between the two dispersed phases are incorporated into the model formulation. Comparison of model predictions with experimental data suggests reasonable agreement for the dispersed oil phase. The predicted distribution of solid particles shows some discrepancies in comparison with the measurements, but the agreement is significantly improved for higher impeller speeds.     

Spectral and wavelet analysis of the flow pattern transition with impeller clearance variations in a stirred vessel

The double- to single-loop pattern transition in stirred vessels stemming from a change in the off-bottom clearance of a Rushton turbine has been investigated by laser doppler anemometry. Time-resolved data showed the transition occurring within a range of clearance values and allowed the distinction of three types of flow: the double-loop regime, the single-loop regime and an unstable one termed “transitional state”. Experiments of up to 3- duration showed that both the onset and the lifetimes of these types of flow were random; however, in the transitional state, the flow varied between the two circulation patterns in a periodic manner, with a frequency linearly related to the impeller rotational speed. The results have important implications for mixing process and vessel design as well as CFD predictions of the flows which are discussed.     

几种框式桨搅拌槽内流动特性的比较研究

用粒子成像测速（PIV）技术对传统框式桨、传统框式组合桨和新型框式组合桨的流动特性进行研究,对比了三种框式桨在相同工况下搅拌槽内的速度、流型和湍动能。结果表明：传统框式桨搅拌槽内流体流动以水平环流为主,在框式桨上方和框式桨中间区域流体流动不充分;传统框式组合桨搅拌槽内框式桨上方由于二折叶桨的作用使得框式桨上部流体流速变大,槽内流体上下部的流动得到加强,但在框式桨中心区域依旧存在流动死区;新型框式组合桨搅拌槽内两层桨叶间的连接流得到了加强,框式桨底部和中间区域物质和能量的交换更加充分。在考察的三种框式桨中,新型框式组合桨的混合效果更好。研究结果可为新型框式组合桨应用于化工合成工业中提供参考。     

Experimental and numerical study of gas hold-up in surface aerated stirred tanks

Although the distribution of gas hold-up in stirred tanks is a key factor to their design and operation, systematic experimental data on local gas hold-up of surface-aerated stirred tanks are not available in open literature. In this work, turbulent two-phase flow in a surface aeration stirred tank with a diameter of 0.380 m was investigated experimentally and numerically. The gas hold-up was measured with a conductance probe at various operating conditions. A surface baffle to improve the efficiency of surface aeration of a Rushton disk turbine was designed and tested. The experimental data suggest that the gas hold-up distribution in the surface aeration tank is very non-uniform, and the surface baffle improves the aeration rate particularly at a high agitation speed. A three-dimensional in-house computational fluid dynamic (CFD) two-fluid model with the standard k?A_p turbulence model was used to predict the gas-liquid flow, and the impeller region was handled using the improved inner-outer iterative procedure. Based on Kolmogoroff's theory of isotropic turbulence, a constitutive equation for surface aeration strength was proposed. The numerical prediction, in combination with the measurements, gives insight to the surface aeration performance of stirred tanks. It was found that the simulation reasonably predicted the gas hold-up distribution in the upper tank, but underestimated it in the region below the stirrer.     

Importance of using the correct impeller boundary conditions for CFD simulations of stirred tanks

Two experimentally determined sets of impeller boundary conditions were used to simulate the flow generated by a pitched blade turbine in a cylindrical baffled tank. Use of these two sets of boundary conditions in simulations with two different off bottom clearances led to the conclusion that the flow generated by a pitched blade impeller cannot be successfully predicted without considering the impeller location. Correct prediction of velocity fields in the tank required the correct specification of velocity boundary conditions. Successful prediction of the turbulent energy distribution required proper specification of the turbulence boundary conditions. There was almost no interaction between the velocity and turbulence fields. The turbulet kinetic energy dissipation rate was at a maximum close to the impeller in both geometries. Within this region the average dissipation rate was five and a half times greater that the average dissipation rate in the tank.     

CFD predictions of flow near impeller blades in baffled stirred vessels: Assessment of computational snapshot approach

The present article summarizes simulations of turbulent flow generated by a Rushton turbine (six blades with disc) and a downflow pitched blade turbine (four blades, 45° inclined) using a computational snapshot approach. The computational snapshot approach proposed by Ranade and Dommeti was extended and generalized to suit impellers of any shape. The approach was implemented using a commercial CFD code, FLUENT (Fluent Inc., USA). Mean flow and turbulence characteristics were computed by solving the Reynolds averaged Navier-Stokes equations combined with the standard k - l turbulence model. The QUICK discretization scheme (with SUPERBEE limiter function) was used to discretize all the governing equations. Preliminary numerical experiments were carried out to identify adequate grid resolution. The predicted results were compared with the comprehensive data set available in the literature. Simulated results show a pair of trailing vortex behind the blades of a turbine. The results were also compared quantitatively in the near-impeller region with the published experimental data and published simulated results using other approaches. The simulations have captured most of the key features of near-impeller flows with sufficient accuracy. The results and conclusions drawn from this study will have important implications for extending the applicability of CFD models to simulate complex stirred reactors.     

Design rules for suspending concentrated mixtures of solids in stirred tanks

This study examines complete off-bottom suspension conditions for slurries containing mixtures of solids at a wide range of concentrations. Binary combinations of five different particles; bronze, two sizes of glass beads, ion exchange resin, and nickel were tested with two impellers: the Lightnin A310 and the down pumping 45° PBT. The effect of particle size ratio and density ratio of the two solid phases on N_js was investigated. Solids loadings were varied from 3 to 56 wt% (weight percent) with water as the liquid phase. The results showed that the highest particle N_js in a mixture of solids is not a sufficient design specification to ensure suspension of the mixture. For one of the cases studied, the particle–particle interactions became significant at high loadings and resulted in a decrease in N_js. Different behaviours were observed for the other mixtures. The performance of the two impellers was also compared. The A310 impeller consumes significantly less power at N_js than the PBT. Both impellers showed an effect of off-bottom clearance on N_js, but the effect of off-bottom clearance depended on both the solids concentration and on the solids used.     

Theoretical prediction of gas-liquid mass transfer coefficient, specific area and hold-up in sparged stirred tanks

A prediction method for calculating the volumetric mass transfer coefficient, k_La, in gas-liquid sparged stirred tanks is proposed. A theoretical equation based on Hibie's penetration theory and the isotropic turbulence theory of Kolmogoroff is used for k_L determination. The values of the interfacial area have been calculated from a hold-up theoretical equation and the mean size of the gas bubble. Both Ostwald-De Waele and Casson models are used to describe the rheological properties of the fluid. The model predicts the mass transfer coefficient and the interfacial area values in stirred tank reactors, analysing the influence of different variables. The values of the volumetric mass transfer coefficient can be calculated for different geometries of the reactor, different physicochemical properties of the liquid and under different operational conditions. The capability of prediction has been examined using experimental data available in the literature for Newtonian and non-Newtonian fluids, for very different vessel sizes, different numbers and types of stirrers and a wide range of operational conditions, with very good results.     

Turbulence kinetic energy transport processes in the impeller stream of stirred vessels

In the present study ensemble-averaged and phase resolved estimations of the production, diffusion and convection of kinetic energy in the impeller stream of a vessel stirred by a Rushton turbine are provided. To determine those terms all Reynolds stresses (normal and shear) as well as most of the triple-velocity correlations terms were also estimated. The measurements were obtained with a two-channel laser Doppler anemometer, LDA, for and the region investigated was delimited by the ranges 0.18?r/T?0.35 and 0.278?z/T?0.387. A Reynolds triple decomposition has been employed to determine the amount of turbulent kinetic energy convected by the organised and mean motions and extracted by the turbulent Reynolds stresses from the organised and mean motions. The data presented can provide valuable information for the design of mixing processes and crucial data to validate computational fluid dynamics simulations of stirred vessel flows.     

The confined impeller stirred tank (CIST): A bench scale testing device for specification of local mixing conditions required in large scale vessels

A new stirred tank geometry, the confined impeller stirred tank (CIST), was designed to provide repeatable testing of the effect of mixing on the performance of chemical additives at the bench scale. The CIST (T = 0.076 m, H = 3T) is filled with five or six impellers. Three impeller geometries were tested: A310, Rushton and Intermig. This paper presents the following hydrodynamic characteristics of the CIST: power number, flow number, momentum number, velocity profiles at different locations in the tank and the transition point from fully turbulent to transitional flow. Based on the scaled velocity profiles, the CIST was able to keep the flow turbulent at Re < 2000 for Rushton turbines and 3200 for Intermigs. The ratio ?_max/?_average was lower for the CIST than for a conventional stirred tank, indicating that the energy dissipation is more uniformly distributed in the CIST. The CIST consistently maintains turbulent flow down to a Reynolds number 10× smaller than that needed in a conventional stirred tank.     

Mixing in multiple-impeller stirred tank reactors with boiling liquids

Mixing in a boil-off mechanically stirred tank reactor with multiple impellers was examined. Power consumption and gas hold-up were measured in boiling water in a 0.2 m i.d. stirred tank reactor with three four-pitched blade downflow disk turbines. Vapour was generated from both the immersed ring heater and the impellers. At low vapour generation rates, vapour was mainly generated from the impellers rather than from the heater, whereas nucleation occurred at the heater instead of the impeller at higher vapour generation rates. The mechanical power consumption decreased due to vapour generation. The change in boiling-to-non-boiling mechanical power ratio with varying impeller rotational speed and boiling rate was complicated and not monotonous except at higher impeller speeds and boiling rates. The gas hold-ups increased with increasing vapour generation rate but were rather small as compared to those in cold gas dispersing systems. Empirical correlations for power consumption and gas hold-up in boiling liquids were developed using the present experimental data.     

Chaotic mixing performance enhanced by rigid-flexible impeller with long-short blades in stirred tank

The presence of a mixing isolation regions in a stirred reactor is a major obstacle to enhancing fluid mixing. Breaking the symmetrical flow field structure in the stirred tank and destroying the mixing isolation area can improve the fluid mixing efficiency. The Matlab software was used to calculate the maximum Lyapunov exponent (LLE) and multi-scale entropy (MSE). The effects of different blade types, flexible blade length, flexible blade number, blade height from bottom and rotation speed on fluid mixing were compared. The results show that the rigid-flexible impeller with long-short blades (RF-LSB) can enhance the flow field structure more unstable and asymmetric with deformation and random vibration of flexible pieces, destroy the symmetry flow in the process of fluid mixing, induce the asymmetric flow field, and make more fluid into the chaotic state. When at 90 r/min and three pieces of flexible, the LLE of the RF-LSB is larger than that of rigid impeller and rigid-flexible impeller RF-LSB with increase of 20.22% and 7.98% respectively. The mixing time (θ_m) of the three systems [RF-LSB (three pieces), rigid impeller, rigid-flexible impeller] has an exponential relationship with the power consumption per unit volume (P_v). When P_v is constant, θ_m of the RF-LSB system is the smallest. Results showed that the RF-LSB (three pieces) is superior to rigid impeller and rigid-flexible impeller, which is more conducive to fluid chaotic mixing.     

长短叶片复合型刚柔桨强化搅拌槽内流体混沌混合行为

搅拌反应器中混合隔离区的存在是强化流体混合的主要障碍。打破搅拌槽中的对称性流场结构,破坏混合隔离区,可以提高流体混合效率。采用Matlab软件编程计算最大Lyapunov指数(LLE)和多尺度熵(MSE),比较了不同桨叶类型、柔性片长度、柔性片数量和桨叶离底高度以及转速对流体混合的影响。结果表明,长短叶片复合型刚柔桨(RF-LSB)桨叶通过刚柔耦合错位连接,柔性片的形变与随机振动对流体的非稳态扰动,使流场结构不稳定性和不对称性增强,强化了流体混合效果。当柔性片数量为3,搅拌转速为90 r/min时,RF-LSB体系比刚性桨和刚柔桨体系的LLE值分别提高了20.22%和7.98%;三种体系[RF-LSB(柔性片数量为3)、刚性桨和刚柔桨体系]的混合时间(θm)与单位体积功耗(Pv)呈指数型关系,当Pv相同时,RF-LSB(柔性片数量为3)的θm最小,表明RF-LSB(柔性片数量为3)更有利于流体混沌混合。     

Estimation of the turbulent kinetic energy dissipation rate from 2D-PIV measurements in a vessel stirred by an axial Mixel TTP impeller

The turbulent dissipation rate is a key parameter in stirred tanks and its local values may have a strong influence on the performance of many processes. However, the local dissipation rate estimation is far from easy in a stirred tank, especially near the impeller discharge where maximum values are encountered. The aim of this work is to estimate the dissipation rate in a vessel used for animal-cell cultures and stirred with a down-pumping axial impeller (Mixel TTP) from velocity fields measured by 2D-PIV. Special attention is paid to the assumptions necessary to estimate the dissipation rate from 2D measurements and to the influence of measurement spatial resolution on the estimated values. The analysis of isotropy ratios measured on vertical, horizontal and tangential planes shows that the turbulence in the impeller discharge is far from isotropic. Isotropy assumptions classically used to estimate the dissipation rate from 2D measurements may thus lead to erroneous values. Based on the measured isotropy ratios, a new relationship is proposed to estimate the dissipation rate in the impeller discharge. This relationship is then used to estimate the dissipation rate on a vertical plane located in the impeller discharge zone. In order to analyze the influence of the measurement spatial resolution on the estimated values of the dissipation, a total of 12 spatial resolutions are tested. Results show that if the spatial resolution is divided by a factor 2, the dissipation rate increases by 220%. For the smallest spatial resolution value used, the maximum dissipation rate estimated is 50 times higher than the mean overall dissipation rate and the corresponding minimum value of the Kolmogorov scale is nearly 3 times smaller than the Kolmogorov scale computed from the mean overall dissipation rate.