Research On Micromixing Characteristics Of Coaxial Mixers

Mixing is widely used in process industries,such as chemical engineering,pharmaceutical,food,metallurgy,and papermaking industries.The mixing process includes the macromixing and micromixing.Macromixing is the process of macroscopic fluid flow carrying fluid elements in the mixer.Micromixing refers to the process of material homogenization from Kolmogorov scale to the molecular scale after the turbulent diffusion.The micromixing performance of mixer has a great impact on the fast complicated reactions.While it also influences the conversion rate,the distribution of the final product and the quality of the final product.Many researchers have studied on micromixing.Most of them adopted consecutive competing reactions and most of the impellers they used were Rushton and hydrofoil impellers.However,the viscosity was relatively low.As for the widely used large blade impeller mixer and coaxial mixer,there is less attention being paid.So,it is necessary to study the micromixing characteristics of the coaxial mixer in high viscous systems.The experimental research and numerical simulation approach were applied to investigate the micromixing performance of the coaxial mixer in high viscosity system.The iodide-iodate parallel competing reaction was adopted in the experiment to investigate the micromixing performance of three types of coaxial mixers which have different inner impellers.The effect of various types of inner impellers,the power consumption of impellers,feeding location,the diameter of the inner impellers,rotation mode and system viscosity on micromixing characteristics of the coaxial mixer were investigated.Furthermore,the mathematical model for the micromixing of coaxial mixer was established.The influence of feeding positions in the blade region and the distance between the inner impeller and the bottom on the micromixing performance was studied.The micromixing performances of three types of coaxial impeller which have different double layer inner impellers were compared,thus,the dominant impeller type was selected based on the simulation results.Meanwhile,the simulation results were applied to elucidate the mechanism of micromixing.The mechanism was elucidated from the aspects of local energy dissipation rate and turbulent kinetic energy.It could be concluded that mixrominxing performance is advantaged when the inner impeller is PBTD-6+Rushton and the feeding location is around the lower inner impeller.Finally,a prediction model of the segregation index that can characterize the micromixing perfomence of the dominate impeller type was established,which guide for the design and selection of mixers in the industry.