Chaotic Mixing Performance Intensification In Stirring Tank By Flexible Impeller With CFD Simulation

Stirred reactor is typical process equipment, which is widely used in the chemical,pharmaceutical, petrochemical industry and food processing areas. Because of thecomplex association between fluid mixing behavior and the stirred reactor structure, aswell as the nonlinear association between the highly sensitive dependence of the mixedfluid and complex space-time chaotic characteristics, it still shows a lack of a completetheoretical support for the stirred reactor design and enlargement. Currently, the designand amplification of the stirred tank strongly relies on experience. The stirring blade isthe core devices of the stirred reactor, which provides the required energy by the fluid toform a certain flow field to achieve the mixing of the fluid. Therfore, the reasonabledesign of stirring impeller should be foucesed on in oder to enhance the mixing effect inthe stirring reactor.Research indicates that isolation area is widely distributed in the whole tank whichis the worst obstacle to mixing. With the development of nonlinear and chaos theory, thechaos phenomenon started to be used to control the flow structure in order to strengthenthe mixed performance of the fluid. Then it is very important to find methods to destroythe isolated mixing regions and will be significant in engineering.The conventional methods to strengthen the chaotic mixing in the vessel includevariable speed mixing, eccentric stirring and reciprocating mixing. These methods candestroy the periodicity and symmetry the fluid particle trajectory by non-lawdisturbances to induce the chaotic mixing. The disturbance which come form theoutside of the fluid system can effectively induce the chaotic mixing. However, itsoperation is relatively cumbersome with complicated device structure which will leadthe cost of the energy and equipment machining will be increase deeply. It is notconductive to the industrial applications. The blade design to strengthen the chaoticmixng is lack and deeply needed. Flexible impeller can be designed by combiningflexible body and rigid body, which posseses the characters of multibody movement andenergy saving. The numerical simulation, developed form dynamical systems theory,was employed in the stirred tank which was equipped with flexible impeller. The resultsproved that the chaotic mixing in the local and whole of the vessel was enhancedthrough the “slef-excitation”disturbance of the flexible impeller by the interactionbetween the impeller and the fluid. The PIV experiment was employed to verifaced the accuracy of the simulation.Then further studied, such as macroscopic flow field structure, velocity character,turbulent performance, shear effect, and power consumption, of the flexible impellerwas carried out. The conclusions were obtained as following:①Through the comparison of the PIV experiment and CFD simulation results, thetime-mean flow field structure and the velocity characteristic was at the basic agreementwithin10%error. The numerical simulation method in this paper was reliable. The flowfield processing method(MRF), the standard k-εturbulent equation and the boundarycondition set were well suitable for the physical model used in the experiment.②The double-loop flow pattern was occurred when the vessel equipped with thetraditional rigid turbine blade (PBT-6, RDT-6). The flow field had the trend to change asthe overall circulation flow pattern (one-loop flow pattern), when the blades werechanged as the flexible ones, especially for the flexible PBT-6impeller. The resultsshowed that the traditional RDT-6and PBT-6impellers with radial flow generated sheareffect in order to transfer engnergy to the fluid, while, the flexible RDT-6and PBT-6impellers with axial flow chiefly enhance the wave and flow interaction to strengthenthe energy transformation with rich scale structure. The results of PIV experimentsshowed that the fartal dimension was enhanced when the stirring impeller changed fromthe rigid ones to the flexible ones and the increase of the fractal dimension presentedlinear correlation with the rotating speed. The equation was controlled by the geometricparameters.③The turn-style lobe formed by the tangle of two kinds of invariant manifolds,which are called the “stable manifold” and “unstable manifold”, played as a templatefor formation of the mixing pattern and consequently chaotic mixing was produced. Inthe process of generating chaotic mixing, the stream line will be stretched and deformed.Thorgh the analysis of the chaotic mixing model it can be known that the wave andstream interaction was the key role for the chaotic mixing enhancement and thegenerating of the stream line was the expression for the chaotic mixing. The contrase ofthe results by the CFD simulation of the flow field with the rigid impellers and theflexible impellers showed that it was good for the stream line stretching and deformingwith the flexible paddle stirring. The flexible PBT-6impeller formed the “pseudodouble-loop”flow pattern which presented a nice performance for the axial mixingeffect by the fluctuations of the flexible body. While the stream line of the flow fieldstirred by the flexible RDT-6impeller shown a clear “S” shape. These indicated that the flexible empeller can help the energy tansfermation to enhance the chaotic mixing in thevessel.④The turbulence properties of fluid in stirred reactor were mainly produced in therigid blade area and the energy dissipation also began at this area. It was showed thatthe turbulent degree was much stronger and mixing effect is much better in this regionthan in the others. But, the large gradient of the turbulent kinetic energy distribution,which would produce large exclusion zone, was not helpful for the energy transfer in thewhole vessel. The flexible impeller can effectively improve the turbulent kinetic energydistribution and the energy transfer to strengthen the chaotic mixing of the fluid.⑤The impleller which generates radial flow have high shearing effect and hightturbulent degree which is suitable for breaking and scattering material in the vessel. Thepaddle which forms axial flow pattern can strongly enhance the entirely circulationcapability in the tank with relative low power consumption. The flexible stirring bladeand the rigid on had the same shear capacity. But the flexible impeller can enhance theebturekt circulation deeply. Therefore, the flexible impeller can effecitively combine theadvantaged of the axial and radial flow pattern which provide the new impeller a broadindustry prospects.