Numerical Simulation Of Liquid Mixing Characteristics In Unbaffled Stirred Tank

Mixing widely exists in many industries such as petrochemical,biopharmaceutical,food processing,metallurgy and mining,etc.Research on the mixing process in a stirred tank can not only provide theoretical guidance for optimizing the operating conditions of the mixing process,but also provide an important basis for the design and scale up of the stirred reactor.Researchers have developed a variety of experimental methods for measuring the mixing process in stirred tank.But almost all have their own limitations and most of these technologies are limited to laboratory scale.For example,planar laser induced fluorescence technology(PLIF)cannot be used for the measurement of opaque systems.In recent years,with the rapid improvement of computer performance,many researchers have used CFD technology to numerically simulate the liquid phase mixing process under various conditions,but some of the simulated data deviate greatly from the experimental results.And some reports of the mixing process is not detailed enough.Therefore,the purpose of this article is to use CFD technology to conduct a detailed and accurate study about the mixing process of single-phase miscible liquid in a stirred tank to evaluate the ability of CFD technology to study the mixing of fluids in a stirred tank.In this paper,the standard k-ε turbulence model and the species transport model in FLUENT are used to numerically simulate the liquid mixing process in unbaffled stirred tank.The impellers are the 30° four-pitched-blade turbine(PBT)and the six-straight-blade rushton turbine(RT).The Reynolds number is 2083-12500.And some data in the PBT impeller are compared with the PLIF experiment.At the same time,the effects of monitoring position,rotation speed,impeller clearance,feed position of tracer on mixing time and rotation speed on mixing energy per unit volume were studied.The results show that:(1)The diffusion process of the tracer in the tank exhibits a stratified phenomenon,the mixing at the central of main circulation flow region is slow.And the change process of tracer concentration of the two impellers is close when the impeller clearance is H/3.However,the concentration homogenization speed of the RT impeller is significantly faster than that of the PBT impeller at H/2.The entire mixing process is closely related to the flow field characteristics.(2)The mixing time of the local monitoring surfaces FA-FD is relatively close as a whole,and it is generally more than twice that of the monitoring surface cross-section.This is because that the "false mixing" state is formed on the cross-section.So it is more objective to use the average of the former as the actual mixing time.(3)The dimensionless mixing time increases with the increase of the rotation speed.When the impeller clearance is H/2,the average relative error of the simulation results and the PLIF experimental value of PBT impeller is 7.8%.And they are basically consistent.When the impeller clearance is H/3,the dimensionless mixing time at high rotation speed will suddenly decrease.In the unbaffled stirred tank,the RT impeller has a shorter mixing time than the PBT impeller.(4)In the unbaffled stirred tank,the lower the rotation speed,the higher the mixing energy efficiency.When the impeller clearance is H/3,the mixing energy efficiency of the two impellers is also higher at each speed.When the impeller clearance is H/2,the mixing energy efficiency of the two impellers is higher at low rotation speed.The mixing energy efficiency of the RT impeller is significantly lower than that of the PBT paddle by 49.4%at high rotation speed.(5)The mixing time of the four cases decreases with the increase of work per unit volume.The mixing time of the two types of impellers is more affected by the input power when the impeller clearance is H/3 than that of H/2.When the logarithm of the input power is less than 1.3,the mixing energy efficiency of the RT impeller is the highest when the impeller clearance is H/2.(6)The mixing time is more short and close to each other when feeding on the main circulation flow paths.If there is a vortex at the feed position,the mixing time will be prolonged.When feeding at the blade end of the RT impeller,although the initial rapid mixing is close to the mixing-well state,the final true mixing-well state is very slowly to achieve.So the mixing time of which is much larger than other feeding points.