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The mixing characteristics of floating particle dispersion process in an eccentric stirred tank with dislocated fractal impellers were investigated using computational fluid dynamics (CFD) and experimental analyses. Solid concentration distribution, axial solid concentration profile, cloud height, solid integrated velocity, power consumption and just drawdown speed were investigated. Results showed that dislocated fractal impeller can enhance solid integrated velocity and fluid turbulent fluctuation intensity compared with dislocated pitched blade impeller, and eccentric agitation coupled with dislocated fractal impeller could destroy the typical circulation loops and symmetric flow field and improve the axial circulation efficiency of floating particles on the basis of dislocated fractal impeller. Eccentric agitation coupled with dislocated fractal impeller could further enhance the floating particle dispersion homogeneity (MI) and decrease the just drawdown speed (Njd) on the basis of dislocated fractal impeller and dislocated pitched blade impeller at the specific power consumption. Meanwhile, eccentric ratio of 0.3 or 0.4 was optimal for the floating particle mixing process in this work. [ABSTRACT FROM AUTHOR]

The solid-liquid mixing characteristics in a stirred tank with pitched blade impellers, dislocated impellers, and dislocated guide impellers were investigated through using CFD simulation. The effects of impeller speed, impeller type, aperture ratio, aperture length, solid particle diameter and initial solid holdup on the homogeneity degree in the solid-liquid mixing process were investigated. As expected, the solid particle suspension quality was increased with an increase in impeller speed. The dislocated impeller could reduce the accumulation of solid particles and improve the cloud height compared with pitched blade impeller under the same power consumption. The dislocated guide impeller could enhance the solid particles suspension quality on the basis of dislocated impeller, and the optimum aperture ratio and aperture length of dislocated guide impeller were 12.25% and 7 mm, respectively, in the solid-liquid mixing process. Smaller solid particle diameter and lower initial solid holdup led to higher homogeneity degree of solid-liquid mixing system. The dislocated guide impeller could increase solid particle integrated velocity and enhance turbulent intensity of solid-liquid two-phase compared with pitched blade impeller and dislocated impeller under the same power consumption. [ABSTRACT FROM AUTHOR]

• The hydrodynamics of floating and sinking particles mixing process was studied by CFD simulation. • Fractal impeller can reduce the just suspension speed of sinking particles. • Fractal impeller can reduce the just drawdown speed of floating particles. • Suspension quality of floating and sinking particles can be improved by fractal impeller. The hydrodynamics of floating and sinking particles mixing process in a stirred tank with four pitched-blade impellers and fractal impellers were investigated employing computational fluid dynamics (CFD) simulation. Axial solid concentration profile, solid concentration distribution, power consumption, trailing vortex, cloud height, water integrated velocity, just suspension speed (N js) and just drawdown speed (N jd) in the stirred tank were predicted. Results showed that the mixing quality of floating and sinking particles increased with the increase of impeller speed. Smaller density difference between solid-liquid two phases and smaller particle diameter resulted in higher suspension degree. Fractal impeller can enhance water integrated velocity and improve suspension quality compared with four pitched-blade impeller under the constant power consumption, and increasingly so with fractal iteration number. Meanwhile, fractal impeller can reduce the size of trailing vortex and decrease the power consumption based on four pitched-blade impeller at the same impeller speed. In addition, the just suspension speed (N js), just drawdown speed (N jd) and correlation equation for N js and N jd in the three different impeller systems were obtained. Fractal impeller could decrease the N js and N jd on the basis of four pitched-blade impeller, and decreasingly so with the fractal iteration number. [ABSTRACT FROM AUTHOR]

• Circle package fractal impeller was introduced to intensify floating particle drawdown process. • Hydrodynamics of circle package fractal impeller in floating particle drawdown process was studied. • Circle package fractal impeller can improve floating particles dispersion quality. • Circle package fractal impeller can reduce the just drawdown speed. The floating particle drawdown and dispersion characteristics in a stirred tank with four pitched-blade impellers, fractal impellers, and circle package fractal impellers were studied using computational fluid dynamics (CFD) simulation. The effects of impeller type, impeller speed, impeller spacing, impeller submergence, initial floating particle concentration, and floating particle diameter on the floating particle dispersion quality were investigated. Results showed that the floating particle dispersion degree was improved with an increment in the impeller speed, in term of axial solid concentration profile, solid concentration distribution, and cloud height. The impeller spacing of T 2/3 and impeller submergence of T /6 were appropriate for the floating particle drawdown process in this work. Higher initial floating particle concentration was easier to achieve a higher floating particle dispersion degree. Smaller particle diameter resulted in smaller buoyancy and more uniform distribution. Meanwhile, circle package fractal impeller can reduce the power consumption compared with four pitched-blade impeller and fractal impeller at the same impeller speed, and enhance the solid integrated velocity and the level of homogeneity for floating particle dispersion process under the constant power consumption. In addition, the just drawdown speed (N jd) and correlation equation for N jd in the three different impeller systems were obtained. [ABSTRACT FROM AUTHOR]

The fractal impeller has fractal structure and self-similarity property. A series of high-speed jet flows can be generated in the flow field due to the existence of the gaps around the fractal impeller blade perimeter, which can break up the trailing vortex into small ones and improve the energy utilization rate. The more the number of fractal iteration, the lower the power consumption of fractal impeller. These conditions are beneficial to improve the solid particles suspension quality in the solid-liquid mixing process. • The hydrodynamics of fractal impeller in solid-liquid mixing process was studied by CFD simulation. • Fractal impeller can improve solid-liquid homogeneous degree. • Fractal impeller can reduce the power consumption. • Solid suspension quality increased with the increase of fractal iteration number of fractal impeller. The hydrodynamics of solid-liquid mixing process in a stirred tank with four pitched-blade impellers, fractal 1 impellers, and fractal 2 impellers were investigated using computational fluid dynamics (CFD) simulation. An Eulerian-Eulerian approach, standard k - ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate the solid-liquid two-phase flow, turbulent flow, and impeller rotation, respectively. The effects of impeller speed, impeller type, impeller spacing, impeller blade tilt angle, impeller blade shape, solid particle size and initial solid particle loading on the solid particle suspension quality were investigated. Results showed that the homogenous degree of solid-liquid system increased with the increase of impeller speed. The impeller spacing of T 5/6 and T and impeller blade tilt angle of 60° and 45° were appropriate for the solid-liquid suspension process. Fractal shape impeller was more efficient than jagged shape impeller in solid-liquid mixing process. Larger particle diameter and higher initial solid particle loading resulted in less homogenous distribution of solid particles. It was found that fractal impeller could improve the solid particle suspension quality compared with four pitched-blade impeller under the same power consumption, increasingly so with the fractal iteration number of fractal impeller. Moreover, fractal impeller reduced the size of impeller trailing vortex and consumed less power consumption compared with four pitched-blade impeller at the same impeller speed, and the more the number of fractal iteration, the higher the impeller energy utilization rate of fractal impeller. [ABSTRACT FROM AUTHOR]

The hydrodynamics of gas-liquid dispersion process in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers were investigated using a combined computational fluid dynamics (CFD) and population balance model (PBM) approach. A classical Eulerian-Eulerian approach coupled with standard k-e turbulence model was employed to simulate gas-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, flexible connection piece length, aperture size/ratio, impeller speed and superficial gas velocity on the local gas holdup and bubble size distribution for the gas-liquid dispersion process were investigated. Results showed that a long flexible connection piece length was conductive to the gas-liquid dispersion process. The optimum aperture ratio and aperture diameter were 12 % and 8 mm, respectively, for the gas-liquid dispersion process. Punched rigid-flexible impeller could reduce the differential pressure ahead and behind the impeller blade, decrease the size of low-pressure zone, and improve the water shear strain rate compared with rigid impeller and rigid-flexible impeller at the same Pg,v. It was found that punched rigid-flexible impeller was more efficient in terms of gas dispersibility than rigid impeller and rigid-flexible impeller. [ABSTRACT FROM AUTHOR]

Solid suspension characteristics were predicted by computational fluid dynamics (CFD) simulation in a stirred tank driven by a dual rigid-flexible impeller and a dual punched rigid-flexible impeller. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate the solid-liquid two-phase flow, turbulent flow, and impeller rotation in the stirred tank, respectively. The CFD results showed that dual punched rigid-flexible impeller could increase the axial velocity and turbulent kinetic energy dissipation rate, and decrease the quantity of sediment solid particles compared with dual rigid-flexible impeller. Less impeller power was consumed by dual punched rigid-flexible impeller compared with dual rigid-flexible impeller at the same impeller speed. It was found that punched rigid-flexible impeller was more efficient in terms of solid suspension quality than dual rigid-flexible impeller at the same Pw. The simulated results for the axial solid concentration were in good agreement with the experimental data. [ABSTRACT FROM AUTHOR]

• The hydrodynamics of gas-liquid dispersion process was investigated based on CFD-PBM coupled model. • Self-similarity impeller could reduce the negative pressure zone behind impeller blades. • Self-similarity impeller could destroy gas cavities behind impeller blades. • Self-similarity impeller could improve gas-liquid mixing efficiency. In general, radial impeller has been the most widely employed for the gas-liquid mixing process in the stirred tank. However, gas cavities are easy to form behind the impeller blades, which results in the waste of power consumption, the limited gas handling capacity of impeller and the poor dispersion of gas. Based on self-similarity property of nonlinear theory, a type of self-similarity impeller was employed to strengthen gas-liquid dispersion process in this work. The hydrodynamics of gas-liquid dispersion process in a stirred tank with Rushton turbine and self-similarity impeller were comparatively investigated by computational fluid dynamics (CFD) simulation combined with a population balance model (PBM). The gas holdup distribution, gas cavities, relative power demand (RPD), bubble size distribution and turbulent intensity analysis in the gas-liquid dispersion process were predicted. Results showed that self-similarity impeller could destroy the gas cavity structure, reduce the negative pressure zone and gas accumulation, enhance the relative power demand (RPD) and improve the uniformity of gas holdup distribution compared with Rushton turbine, and the gas-liquid mixing efficiency can be further improved with the increase of the self-similar iteration number of self-similarity impeller. The proper increase of impeller speed was beneficial to overcome gas flooding and improve the gas-liquid two phase dispersion uniformity. The increase of gas flow rate obviously increased the gas holdup in liquid phase. Meanwhile, self-similarity impeller could increase the uniformity of bubble size compared with Rushton turbine, and increasingly so with the increase of self-similar iteration number of self-similarity impeller. The uniformity of bubble size was improved with the increase of impeller speed and was decreased with the increase of gas flow rate. In addition, self-similarity impeller could significantly enhance the turbulent intensity and turbulent kinetic energy dissipation rate by the jet flows through the concave-convex edges, which was beneficial to the gas dispersion process. [Display omitted] [ABSTRACT FROM AUTHOR]

• The self-similarity impeller based on Hibert curves was introduced in this work. • The hydrodynamics of self-similarity impeller was analyzed by CFD simulation with DES approach. • Self-similarity impeller can break up the large recirculation zones into smaller ones. • Self-similarity impeller can decrease the power number, and decreasingly so with the increase self-similar iteration number. Many studies reported that the nonlinear geometry could create a longer turbulence production region and higher turbulence intensities compared with conventional geometry. The self-similarity impeller based on Hibert curves was introduced to intensify the fluid mixing process in this work. The mixing performance of Rushton turbine (RT) impeller and self-similarity impeller were evaluated using experimental analyzes and computational fluid dynamics (CFD) with detached eddy simulation (DES) approach. The chaotic mixing characteristics, mixing time, power consumption, trailing vortices structures, turbulence intensity, turbulent kinetic energy dissipation rate and velocity magnitude distribution were investigated. Results showed that self-similarity impeller can effectively increase the largest Lyapunov exponent (LLE) and multiscale entropy (MSE) of mixing system, shorten the fluid mixing time and decrease the mixing number (η) compared with RT impeller under the constant power consumption, and an improvement was achieved in mixing efficiency with the increase of self-similar iteration number of self-similarity impeller. Meanwhile, the reductions in the power number (N p) of 34.6% with self-similarity 1 impeller and 51.9% with self-similarity 2 impeller were achieved in a wide Re range compared with RT impeller. Two large recirculation zones and trailing vortices were easy to form at the suction side of rectangular impeller blade, which can be destroyed and decomposed into smaller ones through the jet flows produced by the concave-convex structure of self-similarity impeller. In addition, self-similarity impeller can improve the uniformity of fluid turbulence intensity, enhance the turbulent kinetic energy dissipation rate and increase the axial circulation capacity in the stirred tank, and increasingly so with the self-similar iteration number. The power consumption is closely related to the trailing vortices of impeller blades. According to literature reports, most of the impeller energy was consumed in the region of trailing vortices and only a small part of the impeller energy was used in the fluid mixing process. The self-similarity impeller based on Hibert curves was introduced to intensify the fluid mixing process in this work. It was found that two large recirculation zones and trailing vortices were easy to form at the suction side of rectangular impeller blade, which can be destroyed and decomposed into smaller ones through the jet flows produced by the concave-convex structure of self-similarity impeller. The reductions in the power number (N p) of 34.6% with self-similarity 1 impeller and 51.9% with self-similarity 2 impeller were achieved in a wide Re range compared with RT impeller. Meanwhile, an improvement was achieved in mixing efficiency with the increase of self-similar iteration number of self-similarity impeller. [Display omitted] [ABSTRACT FROM AUTHOR]