Study On Experiment And Numerical Simulation Of Fluid Mixing Intensified By Rigid-flexible Impeller

An impeller is one of the most significant components of mixing equipment,because it provides energy and ensures a suitable flow state in mixing process. Thus, a rational design of impeller is an important factor to improve efficiency and conserve energy in fluid mixing. Research shows that the energy dissipation of stirred reactor exists at the outer edge of blade and behind trailing vortex, and accounts for approximately 70% energy input of impeller. A traditional rigid impeller is made up of rigid material, and transmits energy by fluid shearing and coalescing, which results in high energy consumption and low mixing efficiency. Meanwhile, a rigid-flexible impeller draws energy from vortex, which increases the mixing efficiency through interaction between flexible blade and surrounding fluid.A direct relationship between blade structure and fluid mixing efficiency has been observed, and a bonding force exists between fluid and flexible blade. The flexible blade deforms and moves because of fluid load. The deformation and movement of flexible blade affect flow field in turn, which causes a change in the fluid load and influences the fluid movement. Thus, fluid-structure coupling interaction analysis can be conducted to guide the design of rigid-flexible impeller and to study the fluid mixing process.Tap water and tap water-air were employed as working fluid. With the help of software Matlab and wavelet analysis, the LEmax of system was calculated with collected pressure fluctuation signals and torque time series, which was used to analyze the chaotic characteristics of fluid. The mixing time and the mixing behavior were observed by iodine liquid decolorization visualization technology, and the impeller power was calculated by shaft torque method, which was used to analyze the macro mixing performance of fluid. The velocity rate characteristics, the gas-liquid dispersion characteristics of flow field, and the total deformation, the equivalent stress of blade were simulated by two-way fluid-structure coupling method and tensor analysis with ANSYS Workbench platform. The specific contents were as follows:①The tap water was used as signal mixing system. For rigid impeller system and rigid-flexible impeller system, the LEmax, the mixing time, the velocity rate characteristics of system and the impeller power were comparative analyzed. The results showed that, under agitation speed 120 rpm, compared with rigid impeller, theLEmax of system was increased by 20%; the mixing time of system was decreased by32%; the power of rigid-flexible impeller was declined by 6%; the total deformation of blade tip of rigid-flexible impeller was 105 times of that of rigid impeller, while its equivalent stress of blade tip of rigid-flexible impeller was 83% greater than that of rigid impeller, which indicated that the rigid-flexible impeller had greater force to fluid under fluid-structure interaction and it contributed to energy transmission, liquid flow movement and mixing intensification.②The tap water-air was used as gas-liquid mixing system. For rigid impeller system and rigid-flexible impeller system, the LEmax, the mixing behavior, the velocity rate characteristics and the gas holdup of system were comparative analyzed. The results showed that, under agitation speed 180 rpm and air flow 1500 L/h, compared with rigid impeller, the LEmax of one-impeller system was increased by 21% and that of two-impeller system was increased by 5%; the bubble mixing degree was better than that of rigid impeller. Rigid impeller had greater shear force to fluid and it contributed to bubble crushing capacity and number improvement.③With the analysis of flow field, it could contribute to fluid axial flow strengthening, “dead zone” areas reduction, flow structure variation improvement and fluid mixing efficiency enhancement by adjusting layer spacing at 1800 mm, installing three-impeller or draft tube in leaching stirred reactor or adjusting layer spacing at1120 mm, mounting height at 400 mm, diameter at 800 mm in high-speed dispersion reactor. It indicated that, the numerical method could be used to optimize the stirred reactor structure of industrial production.