Study On Chaotic Mixing Behavior Of Non-newtonian Fluid Enhanced By Multilayer Rigid-flexible Impeller

Mixture of non-Newtonian fluid involves many fields,such as food,medicine,chemical processing and wastewater treatment,and the energy consumption is high.Strengthening the mixing process will save energy and reduce emissions.However,non-Newtonian fluid have certain rheological properties and high viscosity,when mixing with a traditional impeller,momentum is difficult to transfer,and a stable flow field interface is easily formed outside the area swept by the impeller,stir dead zone is difficult to effectively eliminate,which is not conducive to effective mixing of fluid.Therefore,it is necessary to establish a method to enhance the efficient mixing of non-Newtonian fluid.In this thesis,the single-phase and solid-liquid mixtures of non-Newtonian fluid were studied by methods of experiment and simulation.The effects of different impeller designs on LLE and dimensionless mixing rate was investigated in single-phase fluid.The flow field structure,cave effect,helicity distribution,vorticity distribution and velocity distribution were studied by FLUENT simulation.The effects of impeller type,CMC concentration,solid density and total solid content on chaotic characteristics and mixing time were investigated in solid-liquid two-phase mixing.Meanwhile,the characteristics of cloud map of solid distribution and distribution characteristics of point-line solid holdup were analyzed with simulation.The conclusions are as follows:1.Single-phase mixed system（1）The experimental results showed that the chaotic mixing performance was better when the impeller combination mode was RF-（PBTD+PBTD+DT）,the Angle of the impeller arrangement was 60^?,and the length installation ratio of the flexible parts was1.2 and 0.8.The multilayer rigid-flexible impeller could generate multi-frequency disturbance,increase the complexity of flow field,improve the degree of chaos and mixing rate.At the speed was greater than 88 r/min,the LLE of the multilayer rigid-flexible impeller system was greater than 0,at the speed was greater than 125 r/min,the LLE of the multilayer rigid impeller system was greater than 0,the speed required for the multilayer rigid-flexible impeller system to enter the chaotic state was reduced by 29.6%compared with that of the rigid impeller system.The relationship between mixing time（T_m）and mixing energy per unit volume（W_v）was obtained,the multilayer rigid impeller system was T_m=-2.532×10^-4 W_v+2478.2027,and the multilayer rigid-flexible impeller system was T_m=-2.146×10^-4 W_v+1764.2106.（2）The numerical simulation results showed that the multilayer rigid-flexible impeller could change the flow direction of the fluid in the stirred tank,strengthen the fluid convection,disperse the vortex ring structure of the flow field,expand the chaotic mixing domain and reduce the dead zone.Meanwhile,the speed difference in the sweeping area of the flexible components of the multilayer rigid-flexible impeller system was relatively large,so that the high value areas of vorticity and helicity were more than that of the multilayer rigid impeller system.In the area with high vorticity and helicity,the fluid moves along the rotating boundary with higher intensity and the vorticity was larger,the fluid instability was enhanced,thereby enhancing the mixing process.For the pressure distribution,the axial pressure in the stirred reactor of the multilayer rigid-flexible impeller system fluctuates obviously,and the average pressure of the multilayer rigid-flexible impeller system was 327.03 Pa,and the average pressure of the multilayer rigid impeller system was 94.16 Pa,and the difference between the two was 232.87 Pa.2.Solid-liquid mixed system（1）The experimental results showed that the LLE of the multilayer rigid-flexible impeller system was larger than that of the multilayer rigid impeller,and the mixing time was smaller than that of the multilayer rigid impeller.At the P_v of 10600 W·m^-3,the LLE of the multilayer rigid-flexible impeller system was 1.54 times that of the multilayer rigid impeller,which improved the solid-liquid chaotic performance.The mixing time of multilayer rigid impeller and multilayer rigid-flexible impeller had a good linear relationship with the logarithm of power per unit volume and the mixing time decreased with the increased of power.Under the same power consumption,the solid distribution depth of the multilayer rigid-flexible impeller system was greater than that of the multilayer rigid impeller system,and the entrainment performance was higher,the drag force and fluid turbulent force were larger,and the solid-liquid dispersion was enhanced.LLE of system achieved the maximum value at the P_v of 10600 W·m^-3,the CMC concentration of 0.8%,1.0%,and 1.2%correspond to 0.1553,0.1432,and 0.1212,the lower the viscosity of the liquid phase,the higher the degree of chaos and the greater the solid phase velocity,which was more conducive to solid phase dispersion.（2）The numerical simulation results showed that the multilayer rigid-flexible impeller had a larger axial velocity,which increased the suction performance of the impeller,and the difference of solid holdup at each monitoring point was small,so it had better uniformity.In the axial monitoring line,the average velocity of solid phase in the stirred reactor with multilayer rigid-flexible impeller was 1.9 times that of multilayer rigid impeller,the overall turbulence degree of the flow field was increased and the solid-liquid mixing process was enhanced.In summary,the multilayer rigid-flexible impeller can improve the complexity and turbulence degree of the flow field in the stirred reactor,disperse the vortex ring structure of the flow field,strengthen the fluid convection,expand the chaotic mixing domain and reduce the dead zone.Meanwhile,the multilayer rigid-flexible impeller system has higher vorticity and helicity,which increases vorticity and fluid instability,and improves chaotic mixing performance.Therefore,the multilayer rigid-flexible impeller can enhance the chaotic mixing process of non-Newtonian fluid.The above research can provide a theoretical basis for industrial design and experimental optimization.