Study On Chaotic Mixing Performance Of Fluid Intensified By Rigid-flexible Combined Impeller

The stirred reactor is the core component in the stirred tank，which is widely usedin chemical industry, metallurgy and other process industries. The impeller providesthe required energy to the fluid and forms a certain flow field, so as to strengthen heatand mass transfer, and improve production efficiency and ability. The rational designof impeller, contributed to forming suitable flow state, reduced the power consumption,and improved the mixing efficiency. Therefore, the design and optimization of theimpeller structure, the regulation of flow field has become the focus of fluid mixingprocess intensification.Fluid mixing process intensification is in the shortest time with less energy input,realizing fast mixing of micro-scale and low Reynolds number fluid. The rigid impellertransfers energy mainly through the shearing action into the surrounding fluid, about70%of the impeller input energy is concentrated in the blade tip and blade trailingvortex, which form turbulence area near the blade tip region, easy to form a cyclicaland symmetry flow field structure (" dead zones "), so as to reduce the mixingefficiency of the fluid. The rigid-flexible combination impeller enhances fluid mixing,the essence of which is in the form of rigid-flexible-flow, and coupling interact withthe surrounding fluid, which may induce chaotic mixing. It is necessary to study thechaotic mixing behavior of fluid enhanced by rigid-flexible impeller.Tap water and double rigid-flexible Rushton turbine impeller were employed asworking fluid and research object in this paper. With the help of wavelet analysis tooland software Matlab, the frequency of macro-instability﹑largest Lyapunovexponent and multi-scale entropy of the pressure fluctuation signals that collected byLabView software with the employment of sensors in the stirred reactor werecalculated. It was investigated that the chaotic characteristics of fluid in stirred vessel.The mixing time was measured and the evolution of regular region in stirred tank wasobserved by iodine liquid decolorization visualization technology. This work willprovide foundation for the design and application of new impeller in stirred vessel.This thesis mainly studied the following respects:①Pressure fluctuation signal in the stirred tank was collected by LabViewsoftware with the employment of sensors, and the signal was processed utilizationwavelet analysis. The frequency of macro-instability in stirred tank was calculated on the Matlab platform. Results showed that the frequency of macro-instability increasedlinearly with agitation speed. The macro-instability frequency of rigid-flexible impellerwas greater than rigid impeller under180rpm. At agitation speed180rpm,macro-instability frequency of rigid-flexible impeller followed by0.5096Hz and0.3459Hz at0.33T and0.5T off-bottom clearance respectively, and the macro-instabilityfrequency of0.25T off-bottom clearance disappeared and bands phenomenon appeared.The multi-scale structural character of the flow field was more pronounced, leading tochaotic fluid mixing characteristics enhanced.②The variation of the largest Lyapunov exponent in stirred tank was calculatedwith the collection of pressure fluctuation signal by pressure sensor and using Wolfanalysis of the collected singles in the Matlab. The difference between rigid andrigid-flexible impellers was analyzed in mixing performance. Results showed that therigid-flexible impeller enhances fluid movement and the fluid went into chaotic mixing.At210rpm, the fluid was chaotic mixing, and the largest Lyapunov exponent was0.041for rigid impeller, while it was0.048for rigid-flexible impeller. Flexible blade ofdouble rigid-flexible impeller disturbed fluid through own multi-body movement, andenhanced chaotic characteristics.③The variation of the multi-scale entropy in stirred tank was calculated on theMatlab platform with the collection of pressure fluctuation signal. The differencebetween rigid and rigid-flexible impellers was analyzed in energy transfer. Resultsshowed that the multi-scale entropy rate of rigid-flexible impeller at agitation speed150rpm is similar to that of rigid one at210rpm. The rigid-flexible impeller could transferenergy to low Reynolds number region effectively and make the energy distributeuniformly.④Mixing time was measured by iodine liquid decolorization visualizationtechnology. Experiment observed the evolution of regular region in stirred tank. Thecomparative mixing performance between rigid and rigid-flexible impeller wereintuitive analyzed. It showed that rigid-flexible impeller could shorten mixing time bydisturbing and adjusting regular region of flow field, and the mixing efficiency wasimproved.