Research On Mixing Mechanism And Transfer Characteristics Excited By Stirring In A Twin Cam Mixer

The mixing that distributes material or heat evenly by means of mechanical stirring is widely used in a variety of industries.As a key process in the industrial process,mixing is common in various applications.The mixing quality directly affects the product performance,so the mixing device as an important factor to determine the mixing quality has been widely studied.As the simplified mixing device,a twin cam mixer（TCM）shares many geometric characteristics in common with the cross-section of industrial mixers.At present,the mixing mechanism of TCM has not been systematically studied,and the mass and heat transfer mechanism still needs to be further studied.Therefore,TCM is taken as the research object to study the characteristics of fluid deformation,chaotic advection,mass transfer and heat transfer in the mixer.The influence of the operating parameters（rotation mode,rotation speed and speed fluctuation）and structural parameters（height-base ratio and eccentricity）on mixing characteristics is analyzed.The specific research content is as follows:（1）The fluid deformation in TCM is revealed.Based on the theory on the kinematics of the fluid deformation,the characterization methods for the fluid deformation are proposed.A visualization experiment platform for the flow field in the TCM is established,and Particle Image Velocimetry（PIV）is employed to analyze the characteristics of the fluid movement and stretch in TCM and verify the simulation model.The fluid deformation is evaluated by introducing the mixing index,maximum linear stretch rate and Lagrangian coherent structure（LCS）.The results show that,under the two rotating modes,the maximum linear stretch rate of the region where the mixing index is greater than 0.7 in the middle of the mixer is relatively higher,about 1 s^-1,so the stretching efficiency in the middle of the mixer is higher.However,the fluid transmission is different in the middle of the mixer under the two rotating modes.There is a hyperbolic fixed point in this region under co-rotating mode,while there are several lobes along vertical direction under counter-rotating mode.By changing the operating and structural parameters,the stretching in the mixer can be enhanced,and the more complex and global fluid deformation structures can be obtained.（2）The chaotic advection in TCM is revealed.Based on the theory of chaotic advection,the chaotic advection in TCM is studied by the tracer experiment and discrete phase simulation.Based on the geometry characteristics of TCM,the time series Poincarésection and the novel method for obtaining Lyapunov exponent are proposed to evaluate the chaotic advection.The results show that stretching,folding and bifurcation in the flow field of the mixer,and there are periodic structures in the time series Poincarésection,which proves that there is horseshoe mapping chaotic advection in TCM.In addition,changing the operating and structural parameters can promote particle separation and enhance chaotic advection.When the amount of disturbance is 4,the average Lyapunov exponent is up to 13.17.（3）The mass transfer characteristics and strengthening methods of TCM is studied.Based on the mixing mechanism revealed above and the field synergy theory of convective mass transfer,the mechanism of mass transfer in TCM is revealed by species transport simulation.The influence of operating and structural parameters on the mass transfer performance of the mixer is studied.The results show that the mass transfer mechanism in TCM mainly includes the macroscopic convection between the left and right cavity,chaotic advection and molecular diffusion.The mass transfer can be divided into three stages:convection mass transfer,transition and diffusion mass transfer.By optimizing operating parameters or structural parameters,the convective mass transfer can be enhanced,and the mass transfer efficiency can be increased by more than 50%.In addition,the prediction models of mass transfer efficiency with operating and structural characteristic numbers being independent variables are established,which provides a clear way to enhance the mass transfer of the mixer.（4）The heat transfer characteristics and strengthening methods of TCM is studied.Based on the mixing mechanism revealed above and the field synergy theory of convective heat transfer the heat transfer mechanism of TCM in two heating modes is revealed by non-isothermal simulation.The influence of operating and structural parameters on the heat transfer performance of the mixer is studied.The results show that when the wall of the mixing cavity is heat source,the heat transfer mainly depends on conduction;when the surface of the cam is heat source,the heat transfer is dominated by convection first,and then gradually by conduction.The latter has higher heat transfer efficiency.Optimizing operating parameters or structural parameters can enhance the convective heat transfer significantly,and increase the heat transfer efficiency more than100%.In addition,the prediction models of heat transfer efficiency based on operating and structural characteristic numbers are established under the two heating modes,which clarifies the method to enhance the heat transfer of the mixer.The method proposed in this study provides a new route to characterize the performance of the mixer.The research results have important guiding significance for the design and optimization of corresponding industrial mixers,as well as the formulation of mixing process.