Rigid-flexible Impeller Research Articles

Study on mixing characteristics of viscoplastic fluid in a rigid-flexible impeller stirred tank

Abstract The rigid-flexible impeller (RF impeller) was used in the mixing process of viscoplastic fluid, and the mixing performance of RF impeller was explored by using numerical simulation and experimental analysis. Results indicated that RF impeller could reduce the power consumption (P) and demonstrate the advantage of energy-saving compared with Rushton turbine (RT). RF impeller demonstrated a more pronounced force coupling effect between the impeller and surrounding fluid, and exhibited superior adaptability in the flow field compared with RT. Meanwhile, the utilization of RF impeller can effectively enhance the expansion of high velocity region, expand the cavern zone, and decrease the mixing efficiency number while maintaining constant P compared with RT, and the size of high velocity region and cavern zone could be increased with an increase in impeller speed. Moreover, the cavern structure was obtained through the visualization experiment, and the results were similar to that in the simulation. The findings suggested that incorporating rigid-flexible combination structure design of impeller blades could effectively expand the cavern zone, reduce the stagnant zone, and enhance the mixing efficiency in the viscoplastic fluid mixing process.

Intensification of Solid–Liquid Suspension Performance in an Elliptical Uncovered Unbaffled Stirred Tank

The combination of an elliptical uncovered unbaffled stirred tank with a rigid-flexible impeller is a creative effort to improve the performance of solid–liquid suspensions. In this work, we proposed a new mixer combining an elliptical uncovered unbaffled stirred tank (E-UUST) with a rigid-flexible impeller. The just suspended speed (Njs), just suspended power consumption (Pjs), local axial solid concentration, and the largest Lyapunov exponent (LLE) in the E-UUST and the circular uncovered unbaffled stirred tank (C-UUST) were compared under the conditions of equal liquid height and volume. In addition, development of a secondary vortex structure was discussed based on the secondary flow theory by visualization. The results demonstrate that Njs and Pjs, in the E-UUST system, can be reduced significantly by 17 and 37.4%, respectively. The degree of homogeneity of the solids can be effectively increased using the E-UUST. Moreover, the E-UUST with a rigid-flexible impeller can achieve more chaotic mixing and reach a state of just suspension at lower power consumption. Finally, secondary motions in the E-UUST were observed, which may be the reason solid suspension in the E-UUST is better than in the C-UUST. This study shows that the E-UUST can be effectively used to intensify solid–liquid suspension performance and help understand the chaotic mixing of solid–liquid two-phase flows in stirred tanks.

Investigation of the Intensified Chaotic Mixing and Flow Structures Evolution Mechanism in Stirred Reactor with Torsional Rigid-Flexible Impeller

This investigation addresses the development of a Möbius strip with torsional structure as the flexible connection pieces for rigid-flexible impellers, enhancing the macroinstability of the flow field structure, the fluid transfer efficiency, and the chaotic mixing degree in the stirred tank. The effects of intrinsic parameters, including the impeller types and length and width of the flexible connection piece among impellers on the laminar flow field were explored. In addition, the chaotic characteristic parameters, helicity distribution, vorticity distribution, flow field structure evolution, power consumption, and fluid velocity of different rigid-flexible impeller systems were investigated as well. The experiment and computational fluid dynamics calculations demonstrated that the DRFMSC-PBDT system with torsional structure has better mixing characteristics at a lower level for power consumption, being clearly superior to DRFC-PBDT and DRFTSC-PBDT in the LLE and NTm values. The torsional structure of the Möbius strip could facilitate strengthening to form the axial circulation of the flow field, decreasing mass transfer resistance and destroying the stability of the flow field structure while contributing to improving the mixing efficiency of the system. In addition, the enhanced superior proportion of the stagnant zone and the mixing zone is also attributed to the increased secondary flow, as well as the generated perturbation wave with larger amplitude and smaller frequency, which are responsible for mixing in laminar flows.

PIV Measurement and CFD Simulation of Liquid-Liquid Mixing in Mixer Settler with Rigid-Flexible Impeller

Abstract Particle image velocimetry (PIV) was used to measure the flow field in the mixing chamber of a mixer settler. An Eulerian-Eulerian approach, standard k-ε turbulence model, and multiple reference frames (MRF) technique were employed to simulate liquid-liquid two-phase flow, turbulent flow, and impeller rotation in the mixer settler, respectively. The effects of impeller type, impeller clearance and baffle configuration on the flow field were analyzed. Results showed that rigid-flexible impeller can spread the impeller energy more uniformly in the mixing chamber and increase the velocity of the continuous phase through the multi-body motion of flexible sheet compared with rigid impeller. The flow numbers of RF-RT, RF-PBT and RF-CBT systems were increased by 46 %, 54 % and 43 %, respectively, compared with RT, PBT and CBT systems. Meanwhile, the suction capacity of impeller was enhanced with a decreased impeller clearance, and the backflow was eliminated by the installation of baffles.

Numerical Simulation of Gas-Liquid Dispersion in A Stirred Tank Agitated by Punched Rigid-Flexible Impeller

Abstract 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.

Design of Impeller Blades for Intensification of Gas-Liquid Dispersion Process in a Stirred Tank

Abstract Gas-liquid dispersion characteristics were experimentally investigated by measuring largest Lyapunov exponent (LLE), relative power demand (RPD), and local gas holdup in a stirred tank with rigid impellers, rigid-flexible impellers, and punched rigid-flexible impellers. Results showed that punched rigid-flexible impeller could enhance the value of LLE, namely, the chaotic mixing degree of gas-liquid system compared with rigid impeller and rigid-flexible impeller. RPD for punched rigid-flexible impeller was higher than that for rigid impeller and rigid-flexible impeller. The local gas holdup of punched rigid-flexible impeller system was higher than those of rigid impeller system and rigid-flexible impeller system at the same Pg,m. In addition, a long flexible connection piece length could improve the chaotic mixing degree, RPD, and local gas holdup. The aperture diameter of 8 mm and free area ratio of 12 % of punched rigid-flexible impeller were particularly suitable for the gas-liquid dispersion process in this work.

Experimental study of power consumption, local characteristics distributions and homogenization energy in gas–liquid stirred tank reactors

In this paper, the power consumption, the vertical local void fraction and the local gas–liquid interfacial area are investigated in the aerated stirred tank reactors (STRs) equipped with a rigid-flexible impeller. Meanwhile, the regressive correlation based on power consumption and interfacial area is proposed. Then a novel homogenization energy (HE = RSDPtm) expression based on power consumption and local interfacial area is redefined and used to indicate the mixing efficiency. The optimal operating mode is selected based on the change of the HE value. This paper can provide research ideas for structural optimization of stirred reactors.

Gas-liquid mixing performance, power consumption, and local void fraction distribution in stirred tank reactors with a rigid-flexile impeller

In this work, the mixing performance of gas–liquid was characterized by the largest Lyapunov exponent (LLE), relative power demand (RPD), power number NP and local void fraction in a rigid-flexible impeller reactor equipped with a tube sparger. The effects of impeller types, gas flow rates, agitation speed, flexible piece lengths, axial spacing distances lh and angles θ were investigated. Meanwhile, global variable analysis (GVA) based on the mixing index (MI) was proposed to characterize the mixing process. The results show that the increases in flexible piece length, agitation speed, and gas flow rate could enhance the mixing degree. Meanwhile the reductions of axial spacing, and angle were also able to improve the mixing performance of gas–liquid two phases to a certain extent. These findings were in conformity with the results of investigation into MI. The results of this study could provide theoretical guidance for the optimal design and engineering scale-up of STRs.

CFD Simulation of Solid Suspension in a Stirred Reactor Driven by a Dual Punched Rigid-Flexible Impeller

Abstract 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.

Intensification of chaotic mixing in a stirred tank with a punched rigid-flexible impeller and a chaotic motor

Laminar mixing of non-Newtonian CMC solution was investigated experimentally in a stirred tank with a rigid impeller, a rigid impeller coupled with a chaotic motor, a rigid-flexible impeller, a rigid-flexible impeller coupled with a chaotic motor, a punched rigid-flexible impeller, and a punched rigid-flexible impeller coupled with a chaotic motor. Visualization experiment was used to reveal the presence of segregated regions, and observe the structure changes of segregated regions. The mixing performance was characterized by the largest Lyapunov exponent (LLE) and dimensionless mixing time (NTm). Results showed that punched rigid-flexible impeller could further enhance the value of LLE, namely, the chaotic extent of mixing system compared with rigid impeller and rigid-flexible impeller, and the NTm for punched rigid-flexible impeller was shorter than that for rigid impeller and rigid-flexible impeller at the same consumption per unit volume (Pv). The chaotic motor could further increase LLE value, and reduce the NTm on the basis of the normal motor. Moreover, the optimum aperture diameter and aperture ratio of punched rigid-flexible impeller were 8mm and 12%, respectively, and the variable speed period of 10s and constant speed period of 5s of chaotic motor were particularly suitable for the laminar mixing process.

Numerical simulation of solid-liquid suspension in a stirred tank with a dual punched rigid-flexible impeller

The hydrodynamics of solid-liquid suspension process in a stirred tank with a dual rigid impeller, a dual rigid-flexible impeller, and a dual punched rigid-flexible impeller were investigated using computational fluid dynamics (CFD) simulation. A classical Eulerian-Eulerian approach coupled with standard k-ε turbulence model was employed to simulate solid-liquid turbulent flow in the stirred tank. The multiple reference frame (MRF) approach was used to simulate impeller rotation. The effects of impeller type, impeller speed, flexible connection piece width/length of dual rigid-flexible impeller, aperture size/ratio of dual punched rigid-flexible impeller, particle diameter, and liquid viscosity on the homogeneity degree of solid-liquid system were investigated. Results showed that the homogeneity degree of solid-liquid system increased with an increase in impeller speed. A long and wide flexible connection piecewas conductive to solid particles suspension process. Larger particle diameter resulted in less homogenous distribution of solid particles. An increase in liquid viscosity was beneficial to maintain solid particles in suspension state. The optimum aperture ratio and aperture diameter were 12% and 8mm, respectively, for solid particles suspension process. It was found that dual punched rigid-flexible impeller was more efficient in terms of solid particles suspension quality compared with dual rigid impeller and dual rigid-flexible impeller under the same power consumption.

Design of impeller blades for efficient homogeneity of solid-liquid suspension in a stirred tank reactor

Computational fluid dynamics (CFD) was used to investigate the hydrodynamics of solid-liquid suspension process in a stirred tank with rigid impellers, rigid-flexible impellers and punched rigid-flexible impellers. The effects of impeller type, impeller speed, flexible connection piece length, impeller spacing, particle size, and aperture size/ratio on the mixing quality were investigated. Results showed that the degree of solid-liquid homogeneity increased with an increase in impeller speed. A long flexible connection piece was conductive to solid particles suspension process. The solid particles could not obtain enough momentum to suspend to the upper region of stirred tank with small impeller spacing. Larger particle size resulted in less homogenous distribution of solid particles. The optimum aperture ratio and aperture diameter of punched rigid-flexible impeller were 12% and 8mm, respectively, for solid particles suspension process. It was found that punched rigid-flexible impeller was more efficient in suspending solid particles compared with rigid impeller and rigid-flexible impeller at the same power consumption. In addition, less impeller power was consumed by punched rigid-flexible impeller compared with rigid impeller and rigid-flexible impeller at the same impeller speed.

Solid-liquid mixing performance in a stirred tank with a double punched rigid-flexible impeller coupled with a chaotic motor

The mixing performance of solid-liquid system was investigated in a stirred tank with a double rigid impeller, a double rigid-flexible impeller, a double punched rigid-flexible impeller, and a double punched rigid-flexible impeller coupled with a chaotic motor. The effects of the impeller types, the flexible connection piece width/length, the aperture size/ratio, and the variable/constant speed periods were investigated. The mixing performance was characterized by the largest Lyapunov exponent (LLE) and axial local solid concentration profile. The results indicated that the double punched rigid-flexible impeller and chaotic motor could enhance the LLE value of solid-liquid mixing system on the basis of the double rigid-flexible impeller and traditional motor under the same power consumption. The variation in solid homogeneous degree was in good agreement with that of LLE. It was found that the double punched rigid-flexible impeller coupled with a chaotic motor could further improve the solid suspension quality and energy efficiency compared with the double rigid impeller or double rigid-flexible impeller coupled with a traditional motor in the solid-liquid mixing process.