Effect Of Impeller Type And Scale-up On Spatial Distribution Of Shear Rate In A Stirred Tank

As an important hydrodynamic parameter in industrial mixing,shear rate influences the apparent viscosity of a non-Newtonian fluid,the vitality of cells and enzymes as well as the size and structure of aggregates.The requirements on shear rate characteristics produced by impellers are dependent on the diversified mixing systems and mixing goals.In recent years,flow field analysis and in situ characterization techniques are often applied in the studies on shear rate.But there is a lack of research on the comparison of the shear rate distributions produced by the impellers with different flow patterns in the stirred tank at the constant power consumption per unit volume(Pv).Meanwhile,scale-up of reactors is a necessary process for applying laboratory research results to industry,but the differences between large-scale and small-scale ones are inevitable due to the influence of structural dimensions,mixing effect etc.The constant Pv rule is one of the most common methods for scale-up of stirred tanks,which are helpful to maintain a similar dispersed phase size and reduce the enzyme inactivation rate after scale-up.The local hydrodynamic characterizations including shear rate may play a vital role during scale-up for shear-sensitive systems,while the change trends of shear rate in a larger stirred tank are less concerned.Therefore,the spatial distribution characteristics of shear rate for different impellers,the average shear rates and their changes after scale-up are of great significance to better understanding of the mixing process and guiding the application of impellers in industrial occasions.In the present work,the particle image velocimetry(PIV)technique and computational fluid dynamics(CFD)tools were used to study the effect of impeller types and scale-up on the average shear rate(γavg)and the spatial distribution of shear rate in the stirred tanks under the condition of constant Pv.Firstly,by PIV experiments,the shear rate distributions of five impellers,i.e.,Rushton disk turbine(RDT),45° Pitched blade turbine pumping down(PBTD),45°Pitched blade turbine pumping up(PBTU),Centripetal turbine(CT)and Elephant ear impeller(EE),at the constant Pv were measured and compared in the stirred tank.The experimental results show that the general distribution characteristics of shear rate are closely related to flow patterns.The shear coefficient(Ks,imp)at the impeller outlet is linearly related to the impeller flow number(Nq)in general and decreases with the increase of Nq at the constant Pv.Besides,not only the average shear rate in the whole tank but also the local value in vicinity of impeller increases linearly with the impeller speed.Then,with the assistance of CFD method,the influence of fluid rheological characteristics and scale-up at the constant Pv on the shear rate distribution were also explored.It is found that the pseudoplasticity of the fluid has little effect on the overall flow field structure in the stirred tank operated with a CT impeller,while the circulation of the non-Newtonian fluid in the upper space of the stirred tank is weakened Meanwhile,the shear rate values of the non-Newtonian fluid near the impeller are lower than that of the Newtonian fluid and the degree of difference is affected by the concentration of the non-Newtonian fluid.The average shear rate of the whole tank decreases gradually with the increase of the impeller tip velocity(Utip)based on the constant Pv and geometric similarity.While the yavg for RDT decreases faster than that of other impellers with Utip.The shear rate value corresponding to the maximum probability of the shear rate distribution decreases after scale-up.Finally,the dual-impeller combination and the novel multi-blade combined impeller(MBC)were adopted to adjust shear rate distribution in a larger stirred tank at the constant Pv.It is shown that the MBC impeller is able to more effectively improve the distribution uniformity of shear rate in the stirred tank.