Study On Chaotic Mixing Characteristics Of Fluid In Perturbed Rigid-flexible Impeller Stirred Reactor

Stirred reactor is an important device for fluid mixing in chemical,pharmaceutical,biological fermentation and other processes.The stirred impeller is used as the core part of fluid mixing in the reactor for the energy transfer,and the change of its structure can form different flow fields,and fluid mixing efficiency is closely related to the flow field structure,migration and evolution performance.Therefore,designing and optimizing stirred impeller structure can enhance the chaotic flow of fluids and improve the efficiency of fluid mixing.In the stirred reactor with traditional rigid blade,the mixing of single phase is easy to form symmetrical flow field structure,the flow field structure interface is stable,so it is difficult to induce chaos,and it is not conducive to the full mixing of the fluid.In the process of gas-liquid mixing,the cavitation structure behind the blade is difficult to destroy,which resulting in low gas dispersion and uneven bubble size distribution,it is not conducive to gas-liquid biphase mass transfer.Therefore,it is necessary to establish a method for the stirred impeller to induce the instability of the flow field boundary in the isolation region and destroy the cavitation structure behind the blade.Therefore,in this thesis,the Perturbed rigid-flexible(PRF-RT)impeller was used to enhance the chaotic mixing of single phase fluids by inducing the instability of the flow field boundary in the isolation region through the perturbed blade structure and the flutter of the flexible blade.And the Perturbed rigid-flexible fractal(PRFF-RT)impeller was used to enhance the gas-liquid dispersion and destroy the cavitation structure behind the stirred impeller.The chaotic characteristics,mixing time,power consumption and flow field structure in the single phase system were investigated,and the chaotic characteristics,local gas holdup and local bubble size distribution of the gas-liquid biphase system were investigated.The main conclusions are as follows:1.Single phase mixing system(1)The experimental results showed that the PRF-RT impeller could strengthen the energy dissipation of the blade through the perturbed blade structure and the flutter of the flexible blade,destroy the boundary of the isolated regions,improve the instability of the flow field,destroy the fluid vortex ring structure,and reduce or eliminate the isolated mixing regions,improve the fluid mixing efficiency,and increase the LLE of the system.At 90 r/min,the LLE of the PRF-RT impeller system compared with the R-RT impeller system and PR-RT impeller system increased 13.29%and 7.25%respectively,and the mixing time of the PRF-RT impeller system compared with the R-RT impeller system and PR-RT impeller system decreased 37.50%and 23.08%respectively.Meanwhile,the fitting relation between Mixing time(θm)and Power consumption per unit volume(PV)was obtained.R-RT impeller system,θm=8.14+23.52 e(-Pv209.72),PR-RT impeller system,θm=5.48+20.64 e(-Pv/290.98),PRF-RT impeller system,θm=4.89+18.02e(-Pv/183.50).(2)The numerical results showed that the PRF-RT impeller could enhance the movement,collision process of the fluid vortex ring structure,promote the separation and coalestion of the fluid vortex ring structure,and reduce or eliminate the fluid mixing isolated regions,improve the mixing efficiency of the fluid.In addition,PRF-RT impeller could improve the velocity of the fluid at the bottom,top and wall of the stirred tank,strengthen the overall flow velocity of the fluid in the stirred tank.2.Gas-liquid biphase mixing system(1)The experimental results showed that PRFF-RT impeller could destroy the cavitation structure through the small jets formed by the flexible fractal impeller,enhance the gas-liquid biphase dispersion.At 110 r/min and Vs=0.2 m/s,the LLE of the PRFF-RT impeller system compared with the R-RT impeller system increased 5.67%,and the LLE of the fractal-2 impeller system compared with the fractal-1 impeller system increased 5.16%.At Vs=0.2 m/s and Pv,g=490 W/m3,the bubble size distribution range of PRFF-RT impeller system was from 4.0 mm to 10.0 mm,which was smaller than that of R-RT and PR-RT impeller systems.(2)The numerical results showed that the gas dispersion of PRFF-RT impeller system was more uniform,and the bubble size distribution range was smaller than that of R-RT and PR-RT impeller systems,and the cavitation structure behind the impeller was significantly reduced.In summary,in the single phase system,the symmetrical flow field structure was destroyed by PRF-RT impeller,and PRF-RT impeller induced the instability of the interface of the isolated region.In the gas-liquid biphase system,the cavitation structure behind the blade was destroyed by the perturbed fractal rigid-flexible impeller,and PRFF-RT impeller improved the gas-liquid dispersion and reduced the bubble size distribution range.In this thesis,a new method was established to induce the instability of the flow field boundary in the isolation region by the PRF-RT impeller and destroy the cavitation structure behind the blade by the PRFF-RT impeller.The research results in this work can provide a theoretical reference for high efficiency and energy saving mixing operation in perturbed rigid-flexible impeller stirred reactor.