Study On Chaotic Mixing Performance Of High-viscosity Fluid Intensified By Flexible Impeller

The fluid mixing in stirred reactor is a common unit operation of industry, which iswidely used in rubber, food, oil, paper and other industries. The impeller is the corecomponent in the stirred tank that provides the required energy to the fluid and forms acertain flow field, so as to achieve fluid mixing. The rational design of impeller,contributed to forming suitable flow state, reduced the power consumption, andimproved the mixing efficiency. Therefore, the design and optimization of the impellerstructure, the regulation of flow field became the focus of fluid mixing strengthen study.With the development of the mixing theory and summary of working experience, avariety of rigid impellers were developed. The shear effect was the main way for theenergy transportation by these rigid impellers to enhance the low-viscosity fluid mixing.However, the rigid impeller energy consumption was high in the high-viscosity fluid,stirring shaft was prone to be fractured. Related studies showed that the flexibleimpeller in the form of rigid-flexible coupling interact with the surrounding fluid anddraw energy from the vortex by the effective deformation of the flexible end. It couldeffectively increase the chaotic mixing regions of the flow field, reduce the isolatedmixing regions (IMR) of the flow field, and intensify chaotic mixing behavior toachieve energy-saving operation. But there are no researches about flexible impeller forhigh-viscosity fluid chaotic mixing. It is necessary to study the chaotic mixing behaviorof high-viscosity enhanced by flexible impeller through the experiment and simulationmethod.Glycerin and flexible Rushton turbine (RT) impeller were employed as workingfluid and research object, respectively. With the help of wavelet analysis tool andsoftware Matlab, Kolmogorov entropy and largest Lyapunov exponent of the torquetime signals that collected with the employment of torque sensors in the stirred werecalculated. It was investigated that the chaotic characteristics of fluid in stirred tank.The mixing time was measured and the evolution of IMR in stirred tank was observedby acid-alkali visualization technology, and the flow field structure was reviewed bycomputational fluid dynamics (CFD) simulation. Combined with mixing and powerconsumption, the mixing efficiency of impeller was analyzed. This work will providefoundation for the design and application of high-viscosity fluid stirred tank. This thesismainly studied the following respects: ①Torque signal in the stirred tank was collected by torque virtual instrumentsoftware, and the signal was processed utilization wavelet analysis. The variation of theKolmogorov entropy and largest Lyapunov exponent in stirred tank were calculated onthe Matlab platform. By comparing of the kolmogorov entropy and largest Lyapunovexponent between rigid RT impeller and flexible RT impeller, it showed that bothKolmogorov entropy and largest Lyapunov exponent of flexible RT impeller weregreater than rigid one at the same agitation speed (N=120r·min-1, the K=0.175, λ max=0.046of flexible, K=0.085, λmax=0.032of flexible), and chaotic mixingcharacteristics of high-viscosity fluid was enhanced by perturbation of flexible impeller.②Power consumption and mixing time were measured, together with shaft torquemethod and acid-alkali visualization technology. The comparative power characteristics(PV) and mixing performance between rigid RT impeller and flexible RT impeller wereanalyzed with different flexible materials. It showed that, the PVof rigid and flexibleimpeller didn’t have significant difference at the same agitation speed (N=60r·min-1,the PV=340W·m-3of rigid, PV=346W·m-3of flexible). However, energy waseffectively transferred to the surrounding fluid by the disturbance of flexible RTimpeller. Mixing characteristics of fluid was enhanced and mixing time of fluid instirred tank was drastically reduced. Compared with the rigid RT impeller, mixingefficiency of the silicone rubber flexible RT impeller was improved52.8%, reflectingthe superiority of flexible impeller to energy-saving.③The CFD fluent software was employed to simulate the fluid structure of theflexible RT impeller mixing system by multiple reference frames (MRF) method. Thesimulation results verified that the velocity distribution and characteristic of the fluidwas strengthened by interactions of flexible impeller and fluid in the stirred tank. Theflexible impeller induced chaotic mixing by destroyed IMR of the fluid, and the mixingefficiency was improved.