Dual Rushton Impellers Research Articles

An Effect of Fractal Baffles and Impellers with Double Stage 4-Blade Rushton Turbine to Fluid Flow Behaviour in Stirred Tank

The stirred tank is widely used in many industries to obtain the desired type of fluid mixing. In the context of mixing process, two different fluids and have a different properties will mix in a single equipment to produce another fluid with a new property. In this research, a new approach of stirred tank which is containing a new design of baffles and impellers was proposed for fluid mixing. The new design of baffles and impellers that proposed here are used a fractal pattern for both parts in the stirred tank. Implementing a fractal pattern for baffles and impellers in stirred tank believe will influence the flow characteristic inside the stirred tank, hence will improve a mixing performance. In order to investigate the kinds of flow properties, a Particle Image Velocimetry (PIV) technique with 1 μm seeding particle was used. Four configurations were tested which are normal baffles and normal impellers, normal baffles and fractal impellers, fractal baffles and normal impellers, and the last configuration is fractal baffles and fractal impellers. In this study, dual Rushton impellers with 4 blades were used with the configurations mentioned. The result shows the significant flow field capture by PIV measurement on each configuration. By using fractal impeller some vortex are shown in the tank and high velocity vector on flow field compare with normal impeller while normal baffles gives high velocity vector depends on the configuration were used. From the results, it was showed that the fractal design can give a certain level of mixing efficiency in stirred tank. The PIV technique also gives good flow visualization in order to determine the flow pattern in stirred tank with a new concept of baffles and impellers.

Comparison of flow patterns of dual rushton and CD-6 impellers

The flow patterns (parallel, merging and diverging) of dual CD-6 impeller has been modeled using computational fluid dynamics simulation in FLUENT software compared with dual Rushton impeller. Simulated results have been verified with experimental observations on Rushton impeller. Comparison of simulated and experimental results of Rushton impeller has been observed similar flow patterns, whereas CD-6 impeller exhibits different flow pattern from the Rushton impeller in merging flow pattern. Analysis has shown that formation of flow patterns of Rushton impeller and CD-6 impeller is different in the case of merging flow pattern.

Experimental and CFD investigation of power consumption in a dual Rushton turbine stirred tank

Power consumption of a mixing system is a key variable in chemical and bioprocess engineering, the determination of which is of interest of many processes. Besides, prediction of the flooding-loading transition in an aerated stirred tank is crucial for the correct design of aerated stirred tank reactors. In this research, laboratory investigation has been carried out on local and total power consumption of a single phase as well as gas–liquid phase systems in a fully baffled stirred tank equipped with dual six-blade Rushton turbines; moreover, the flow regime behavior of a gas–liquid system was investigated. Results have been compared with data obtained from CFD simulation of experimental setup and the data available in the literature. Reasonable agreement between the experimental and simulation results indicates the validity of the CFD model. Using predicted data some empirical correlations have been derived which present new relations in estimation of power consumption and flow regime transitions in stirred tanks with dual Rushton impellers.

Study of Flow Field, Power and Mixing Time in a Two-Phase Stirred Vessel with Dual Rushton Impellers: Experimental Observation and CFD Simulation

An agitated two-phase flow is studied numerically and experimentally in a mixing vessel agitated with two six-blade Rushton turbines. In Computational Fluid Dynamics (CFD), the full Eulerian multiphase approach coupled with the standard turbulence model is performed to deal with two-phase flow. The impeller rotation was modelled by the Multiple Reference Frame (MRF) approach. The simulation was used to investigate the flow field, power and mixing time in single and two-phase cases. The results of the calculations have been verified with the data that was measured by particle image velocimetry (PIV) and planar laser-induced fluorescence (PLIF) technique. The predicted results show good agreement with the experimental data. The computational model presented in this study could be useful for explaining the two-phase flow patterns on the mixing process and extending the applications of multiphase stirred reactors.

Computation of the flow and reactive mixing in dual-Rushton ethoxylation reactor

Two ethoxylation reactors, powered with dual-Rushton impeller, were investigated in this paper using computational fluid dynamics (CFD). The flow was described as an incompressible, turbulent single-phase liquid (hydrophobe substrate) mixing with chemical species (ethylene oxide). The first reactor, simulated for the purpose of validation of hydrodynamics, was based on the literature with experimental (K. Rutherford, K.C. Lee, S.M. Mahmoudi, M. Yianneskis, Hydrodynamic characteristics of dual-Rushton impeller stirred vessel, AIChE J., 42 (1996) 332–346.) and numerical (G. Micale, A., Brucato, F., Grisafi, M., Ciofalo, Prediction of flow fields in a dual-impeller stirred vessel, AIChE J., 45 (1999), 445–464.) data on velocities and turbulent kinetic energy. After the successful validation exercise with reasonable accuracy, simulations were extended to predict the flow hydrodynamics and ethoxylation kinetic in a 6 m 3 industrial ethoxylation reactor equipped with a dual-Rushton turbine. The ethoxylation kinetics were taken from the experiments previously conducted by the authors (Y.N. Chiu, A. Easton, K.F. Ngian, K.C. Pratt, Kinetics of a catalysed semi-batch ethoxylation of nonylphenol, in: Proceedings of the 4th Biennial Engineering Mathematics and Applications Conference, Melbourne, Australia, 2000.). Reasonably good agreement was obtained between the simulated and experimental data on the time-dependent changes of ethylene oxide mass fraction in the bulk liquid in the industrial ethoxylation operations.

Particle Image Velocimetry Study of Turbulence Characteristics in a Vessel Agitated by a Dual Rushton Impeller

Particle Image Velocimetry (PIV) has been used to investigate turbulence characteristics in a 0.48 m diameter stirred vessel filled to a liquid height ( H = 1.4 T) of 0.67 m. The agitator had dual Rushton impellers of 0.19 m diameter ( D = 0.4 T). The developed flow patterns depend on the clearance of the lower impeller above the base of the vessel, the spacing between the two impellers, and the submergence of the upper impeller below the liquid surface. Their combinations can generate three basic flow patterns, named, parallel, merging and diverging flows. The results of velocity measurement show that the flow characteristics in the impeller jet flow region changes very little for different positions. Average velocity, trailing vortices and shear strain rate distributions for three flow patterns were measured by using PIV technique. The characteristics of trailing vortex and its trajectory were described in detail for those three flow patterns. Since the space-resolution of PIV can only reach the sub-grid rather than the Kolmogorov scale, a large-eddy PIV analysis has been used to estimate the distribution of the turbulent kinetic energy dissipation. Comparison of the distributions of turbulent kinetic energy and dissipation rate in merging flow shows that the highest turbulent kinetic energy and dissipation are both located in the vortex regions, but the maxima are at somewhat different locations behind the blade. About 37% of the total energy is dissipated in dual impeller jet flow regions. The obtained distribution of shear strain rate for merging flow is similar to that of turbulence dissipation, with the shear strain rate around the trailing vortices much higher than in other areas.

Investigation of Fluid Flow in a Dual Rushton Impeller Stirred Tank Using Particle Image Velocimetry

The flow fields in a dual Rushton impeller stirred tank with diameter of 0.48 m ( T) were measured by using Particle Image Velocimetry (PIV). Three different size impellers were used in the experiments with diameters of D = 0.33 T, 0.40 T and 0.50 T, respectively. The multi-block and 360° ensemble-averaged approaches were used to measure the radial and axial angle-resolved velocity distributions. Three typical flow patterns, named, merging flow, parallel flow and diverging flow, were obtained by changing the clearance of the bottom impeller above the tank base ( C 1) and the spacing between the two impellers ( C 2). The results show that while C 1 is equal to D, the parallel flow occurs as C 2≥0.40 T, C 2≥0.38 T and C 2≥0.32 T and the merging flow occurs as C 2≤0.38 T, C 2≤0.36 T and C 2≤0.27 T for the impellers with diameter of D=0.33 T, 0.40 T and 0.50 T, respectively. When C 2 is equal to D, the diverging flow occurs in the value of C 1≤0.15 T for all three impellers. The flow numbers of these impellers were calculated for the parallel flow. Trailing vortices generated by the lower impeller for the diverging flow were shown by the 10° angle-resolved velocity measurements. The peak value of turbulence kinetic energy ( k/V 2 tip = 0.12−0.15 or above) appears along the center of the impeller discharging stream.

Large Eddy Simulation of Flow Fields in Vessels Stirred by Dual Rushton Impeller Agitators

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.

Simulation of flows in stirred vessels agitated by dual rushton impellers using computational snapshot approach

Flow in baffled stirred vessels involves interactions between flow around rotating impeller blades and stationary baffles. When more than one impeller is used (which is quite common in practice), the flow complexity is greatly increased, especially when there is an interaction between two impellers. The extent of interaction depends on relative distances between the two impellers and clearance from the vessel bottom. In this paper we have simulated flow generated by two Rushton (disc) impellers. A computational snapshot approach was used to simulate single-phase flow experiments carried out by Rutherford et al. (1996). The computational model was mapped on the commercial CFD code FLUENT (Fluent Inc., USA). The simulated results were analyzed in detail to understand flow around impellers and interaction between impellers. The model predictions were verified using the data of Rutherford et al. (1996). The results presented in this paper have significant implications for applications of computational fluid mixing tools for designing multiple impeller stirred reactors.

Gas-filled cavity structures and local void fraction distribution in vessel with dual-impellers

An experiment was carried out in a pilot-size aerated mixing vessel using dual Rushton impellers. The two-phase mixture was composed of air and deionized water. Different gas-filled cavity structures were recognized close to the outer edge of the impeller blade by frequency transformation of the time-domain structural function. The majority of hydrodynamic regimes for the upper impeller were of VC structure, while the lower impeller operated over the VC, 1L, 2L, S33, L33, and RC structures respectively. Local void fraction α was also measured by a resistivity probe at 250 modes in the vertical half-section plane of the vessel. Spatial distributions of α are presented for the aforementioned hydrodynamics regimes. A transition from quite homogenous local void fraction distribution in a VC structure to a rather nonhomogeneous void distribution with a high concentraion of bubbles around the shaft was detected.

Prediction of flow fields in a dual‐impeller stirred vessel

AbstractNumerical simulations were conducted for the flow field in a baffled tank stirred by a dual Rushton impeller. For this geometry, LDV measurements show a characteristic dependence of the flow patterns upon the position of the impellers. Two advanced modeling approaches were tested. In the first, the vessel was divided into two concentric blocks, coupled by a sliding grid technique, and simulations were conducted in time‐dependent mode. In the second approach, the vessel was modeled as two partially overlapping regions, the inner one rotating with the impeller and the outer one stationary; simulations were run in steady‐state mode for each of the two regions, while information was iteratively exchanged between them after azimuthally averaging and transforming for the relative motion. A third set of simulations was conducted for comparison purposes by using a more conventional method, in which the impellers were not explicitly simulated, while their effects were modeled by imposing suitable values of velocities and turbulence quantities (derived from single‐impeller experiments) at the blade periphery. The first two techniques gave similar predictions and successfully reproduced the dependence of the flow patterns on the position of the impellers. The latter method required far less computational effort. On the other hand, the impeller boundary conditions technique failed to reproduce the experimental flow patterns, because of the inadequacy of single‐impeller boundary conditions for the present geometry.

Hydrodynamic characteristics of dual Rushton impeller stirred vessels

AbstractThe flows generated in vessels stirred by two Rushton impellers were investigated in two vessels of diameter (T) 100 and 294 mm with impellers of diameter D = T/3 using flow visualization, power consumption, mixing time, and 360° ensemble‐averaged and 1° angle‐resolved LDA measurement techniques. The flows depended strongly on the clearance of the lower impeller above the base of the vessel (C1), the separation between the impellers (C2), and the submergence (C3) of the upper impeller below the top of the liquid column height (H). When these distances were varied, three stable and four unstable flow patterns were observed. Comparisons between the two LDA techniques showed that while the 360° ensemble‐averaged measurements are useful for characterizing the overall flow structure and turbulence levels in the vessel, care must be exercised when interpreting such data, since in the impeller region they include periodic variations in the mean velocity in addition to the turbulent fluctuations. The trailing vortex structure and flow periodicity produced by the Rushton impellers is shown to decay significantly within a cylindrical region of height 1.2D and radius 1.0D centered around the middle of the vessel, when C1 = C2 = T/3. The turbulence structure within this region is anisotropic, while outside this region it might be considered mostly isotropic. The main flow features scaled well between the vessels.

Gas–liquid dispersion with dual Rushton impellers

Aerated and unaerated power consumption and flow patterns in a 0.56 m diameter agitated vessel containing water with dual Rushton turbines have been studied. Under unaerated conditions with a liquid height-to-diameter ratio of 2, an impeller spacing of 2 to 3 times the impeller is required for each to draw an amount of power equal to a single impeller. For aerated conditions, if a similar spacing is used, equations for the flooding-loading transition and for power consumption for a single Rushton impeller can be extended relatively easily to dual systems. All results for this spacing are explained by reference to bulk flow patterns and gassed-filled cavity structures and the proportion of sparged gas flowing through the upper impeller is also estimated. Such a spacing is generally recommended since it maximizes the power draw and hence the potential for oxygen mass transfer. Data are presented for other spacings but the results do not fit in easily with single agitator studies because strong impeller-impeller flow pattern interactions occur.