Study On The Cavern And Mixing Performance Of The Pseudoplastic Fluid Stirred With Dual-impeller Combination

Pseudoplastic fluid is a common non-Newtonian fluid.The most essential difference between pseudoplastic fluid and Newtonian fluid is the particularity of fluid viscosity.The shear stress of the pseudoplastic fluid needs to reach its own initial yield stress before it can enter the flow state,and under the action of different shear stresses,its viscosity value also changes.It is precisely because of this complex rheology,pseudoplastic fluid with rheological index n?0.3 in laminar flow state has a large shear stress near the blade,and then the viscosity of the fluid is lower which has a better flow state.But in the area far away from the stirring blade,the viscosity of the fluid is still large and the flow is slower.Even part of the fluid did not overcome the initial yield stress and was in a stagnant state.The special phenomenon of pseudoplastic fluid in the stirred tank is called as cavern.The occurrence of the cavern indicates that the fluid flow in the stirred tank is terrible,which seriously affects the mixing quality and mixing efficiency.It is of great significance to study the expand law of cavern and eliminate it for efficient stirring of pseudoplastic fluids.Therefore,this paper takes the xanthan gum solution as the research object,and establishes the Herschel-Bulkley rheological model of the xanthan gum solution by measuring the rheological parameters.Based on it,numerical simulation of the cavern characteristics of the pseudoplastic fluid stirred with dual-impeller combination in the stirred tank was carried out.The results showed that at the lower rotation speeds,the cavern of the dual-impeller is shown as upper and lower parts which independent of each other.The fluid between the two impellers is not enough to overcome the initial yield stress and enter the flow state.As the rotation speed increased,the cavern of the dual-impeller begins to connect which showing a gourd shape,and finally form a unity.At this time,the effect of the dual-impeller is similar to a larger impeller.During the expansion of the cavern from separation to unity,the height to diameter ratio of a single cavern tends to increased which showed that there is an interaction relationship between the caverns of the dual-impeller.After that,the cavern of dual-impeller expands with a stable height to diameter ratio.Simulation results of the cavern were in good agreement with that Elson's model from a log-log plot of D_c/D versus N_pRe_y.The linear fitting slopes of 6PBT-6PBT and6PBT-6BT are 0.29 and 0.25,respectively,which indicated that the Elson model can be used to predict the evolution of the cavern of dual-impeller.The volume ratio of the cavern showed that the 6PBT-6BT has more advantages in eliminating cavern than6PBT-6PBT.According to the corresponding apparent viscosity distribution,the critical speed of the cavern was completely eliminated can be determined based on the cavern volume ratio reaching 90%.Based on the laminar flow calculation model of the xanthan gum solution,the calculation results of mixing process at different speeds were analyzed with a tracer of water.It is showed that,the mixing time numbers(t_m)of 6PBT-6BT is smaller while the 6PBT-6PBT had higher dimensionless number of C₃.It showed that the axial pumping capacity of the 6PBT-6BT is stronger and the 6PBT-6PBT has a stronger shearing capacity.At the same speed,the 6PBT-6BT both had a higher mixing rate and mixing efficiency than 6PBT-6PBT,which indicating that the 6PBT-6BT dual-impeller combination has more advantages in pseudoplastic fluid mixing process.