Flow And Mixing Characteristics In Dynamic Mixer

Dynamic mixers for polymer blending have been widely used in chemical fiber,plastic and rubber industries.However,only few works can be found in the literature about dynamic mixers over the last decades.Due to complex geometry and transient flow characteristics,it is not an easy task to obtain the flow and mixing characteristics in a dynamic mixer by visualization technologies.The design and industrial scale-up of the dynamic mixers still use the semi-theoretical and semi-empirical methods,so the researches on the capture of these complex flow phenomena and the mixing characteristics are still insufficient.One of the main reasons is the lack of ideal visualization technologies to measure complex flow and concentration distribution.Firstly,a laminar lid-driven cavity flow with Newtonian fluids was constructed to represent the fundamental characteristics of an industrial dynamic mixer.The flow patterns and dynamic mixing process from layering to blending in the cavity were measured by using PIV and PLIF experiments,respectively.The refractive indices of the two miscible liquids involved were carefully matched to allow for unhindered optical access.The mixing process was predicted by using CFD simulations including models for species transport.The simulated flow and mixing results are in good agreement with the experimental data.The effects of density difference and viscosity of the two miscible fluids on the mixing process were evaluated.Minor variations in the densities of the fluids have significant influence on the mixing process in terms of the coefficient of variation as a function of time.The dimensionless group Ar/Re(Archimedes number over Reynolds number)is proposed to characterize the mixing process in the cavity.The mixing performance in terms of the COV of fluid concentration becomes better with the decrease of Ar/Re.At constant Ar/Re in the range of 1.19?Ar/Re?119,we can obtain approximately the same mixing process by changing the fluids properties,geometric and operation parameters.After that,a lid-driven cavity flow with non-Newtonian fluids was also investigated.The flow patterns and mixing process in the cavity were measured by using PIV and PLIF experiments,respectively.The refractive indices of the two miscible liquids involved were carefully matched.The mixing process was predicted by using CFD simulations including models for species transport.The simulated flow and mixing results are in good agreement with the experimental data.The difference between the flow fields in the lid-driven cavity with nonNewtonian and Newtonian fluids was investigated.The effects of density difference and viscosity of the two miscible fluids on the mixing process were evaluated.The dimensionless group Ar/Re was also applied to characterize the mixing process of two non-Newtonian fluids.Then,the flow fields under laminar conditions in two typical regions of a cavity transfer dynamic mixer consisting of an inner rotor and outer stator were visualized by refractive index(RI)matched PIV experiments.The RI of the working liquids and the transparent solid parts of rotor and stator were matched to allow for unobstructed optical access.The flow fields were predicted by using CFD simulations including models for species transport.Various flow patterns in the dynamic mixer are discussed.The simulated flow fields in the two investigated regions agree well with the experimental data.The effect of gap width between rotor and stator(?)on the scalar mixing was evaluated,and smaller ? will achieve better mixing because of the enhanced shearing and higher energy consumption.At constant Ar/Re value,we obtain approximately the same mixing performance and concentration distribution.At the same time,better mixing performance can be achieved with smaller Ar/Re.The influence of operating parameters on the mixing effect of the dynamic mixer with Newtonian fluids is investigated.The mixing effect is proportional to the rotor speed and inversely proportional to the flow rate.The dimensionless a is proposed to find the best operating parameters for the mixing in dynamic mixer with Newtonian fluids.At last,the flow and mixing of non-Newtonian fluids in the dynamic mixer was predicted by using CFD simulations including models for species transport.The similarities and differences of the simulated flow fields in the two investigated regions between the non-Newtonian and Newtonian systems are studied.Various flow patterns of non-Newtonian fluids in the dynamic mixer are discussed.The effect of gap width between rotor and stator(?)on the scalar mixing of non-Newtonian fluids was evaluated,and smaller ? will achieve better mixing because of the enhanced shearing and higher energy consumption.The dimensionless group Ar/Re was also applied to characterize the mixing process of two non-Newtonian fluids in complex dynamic mixer.At constant Ar/Re value,we obtain approximately the same mixing performance and concentration distribution.At the same time,better mixing performance can be achieved with smaller Ar/Re.Finally,the influence of operating parameters on the mixing effect of the dynamic mixer with the non-Newtonian fluids is investigated.The mixing effect is proportional to the rotor speed and inversely proportional to the flow rate.The dimensionless a is proposed to find the best operating parameters for the mixing in dynamic mixer with non-Newtonian fluids.