Study On Fluid Chaotic Mixing Characteristics Enhanced By Pulse Jet In Rigid-flexible Impeller Stirred Reactor

Stirred reactor is widely used in chemical,metallurgical,and other process industries,and is the core equipment in process strengthening.The mixing efficiency of the fluid in the stirred reactor is closely related to the formation,migration,and evolution of its flow field structure.However,the structure characteristic of the stirred reactor makes it easy to produce the self-coherent pseudo-sequential flow field structure(mixing isolation zone),about 70%of the input energy of the impeller is consumed at the outer edge of the impeller blade and the trailing vortex behind the impeller blade,which reduces the mixing efficiency of the fluid.Pulse-jet coupled with rigid-flexible combination impeller is enriched the nonlinear terms in the fluid,improving the flow field structure and promoting the transformation of fluid in the mixing isolation zone to chaotic state.It has important value to improve the mixing effect of fluid.In this thesis,starting from the break mechanism of pulse jet coupled with rigid-flexible impeller to the mixing isolation zone,the fluid mixing characteristics,the chaotic characteristics and flow field characteristics of fluid in pulse jet coupled with rigid-flexible impeller system was studied.The effects of impeller type and jet flow type on the maximum Lyapunov exponent(LLE),gas-liquid dispersion and mass transfer performance were investigated in the ring-type gas distributor system.The flow field characteristic,gas-liquid dispersion,turbulent kinetic energy distribution and other regular were studied by CFD simulation.The effects of impeller type and jet flow type on the LLE and mixing time were investigated in the eccentric jet system.The fluid mixing time,vortex core velocity distribution and flow field characteristic were stu died by CFD simulation.The results showed that:(1)In the ring-type gas distributor system,the multi-flow coupled of the pulse jet flow field and the mechanical agitation flow field was formed by the combination of the pulse jet and the mechanical agitation,which can induce more fluid into chaotic state and enhance the chaotic mixing of the fluid.Compared with the no air jet rigid impeller system,the LLE of pulse jet coupled with rigid impeller system and pulse jet coupled with rigid-flexible combined impeller system respectively increases 56.5%,82.6%.(2)In the ring-type gas distributor system,the rigid-flexible combined impeller could significantly enhance the disturbance effect of the impeller on the surrounding fluid,increase the energy dissipation rate,reduce the thickness of the liquid film layer at the gas-liquid phase interface,and reduce the mass transfer resistance to enhance mass transfer.Compared with the rigid impeller system,the rigid-flexible combined impeller system K_La was increased by 13.5%,at V_g=80L/min and P_v=360W/m~3.Pulse-jet could enhance mass transfer,compared with the rigid impeller system,the rigid-flexible combined impeller system K_La was increased by 11.9%,at V_g=80L/min and P_v=360W/m~3.(3)The disturbance of the impeller to the fluid was enhanced by the flexible part multi-body motion of the rigid-flexible combined impeller and the pulse jet oscillation characteristics of the pulse jet coupled rigid-flexible combined impeller,the tail vortex-cavitation structure was significantly destroyed and the degree of gas-liquid dispersion was enhanced.(4)In the eccentric jet stirring system,eccentric jet,rigid-flexible combined impeller and pulse jet could effectively enhance the degree of fluid chaos and reduce the mixing time.Compared with the no jet rigid impeller system,the eccentric steady jet rigid impeller system mixing time was reduced about 55.8%,compared with the eccentric steady jet rigid impeller system,the eccentric steady jet rigid-flexible combined impeller system mixing time was reduced about 15.5%,compared with the eccentric steady jet rigid-flexible combined impeller system,the eccentric pulse jet rigid-flexible combined impeller system mixing time was reduced about 15.8%.The self-consistent mimetic structure of the stirred reactor flow field was destroyed by non-stationary means,which made the tracer particles diffused more quickly and the mixing efficiency of fluid was improved.(5)In the eccentric jet stirring system,the eccentric pulse-jet system with high impact frequency and low duty cycle had stronger ability to enhance fluid mixing.Compared with the eccentric steady jet rigid-flexible combined impeller system,the eccentric pulse jet rigid-flexible combined impeller system had many vortex ring structures,which promoted the flow,entrainment,mixing and diffusion of the entire flow field,the self-consistent mimetic structure of the stirred reactor flow field was further destroyed.In summary,pulse jet coupled rigid-flexible combined impeller could destroy the self-consistent mimetic structure of the stirred reactor flow field by the non-stationary jet oscillating disturbance and the multi-body motion of the flexible part,and strengthen the chaotic mixing of the fluid in the stirred reactor.