Study On Cavern Model And Its Development Law Of Pseudoplastic Fluid Stirred With Perturbed Turbine Impeller

Pseudoplastic fluid with obvious yield stress is a kind of non-Newtonian fluid.In addition to higher viscosity,they also exhibit complex rheological properties such as shear thinning behavior and initial yield stress.The agitation of such fluids often forms an intensive mixing region(called as cavern)of relatively rapid motion close to the impeller and a poorly-mixed zone(called as stagnant or near-stagnant region)of very slow motion in the remainder of the vessel,which will greatly affect the mixing efficiency and may be accompanied by the production of by-products.Therefore,the cavern-effect and its development law of pseudoplastic fluids in a stirred tank equipped with 6PBT impeller were studied by the method of numerical simulation combined with experiment in this paper.The research results will provide a theoretical reference for the design of the efficient agitation equipments.Firstly,the rheometer was used to test the rheological parameters of xanthan gum solutions with different concentrations.The results show that the relationship between shear stress and shear rate is in accordance with Herschel-Bulkley model,and the parameters of the model are obtained by the nonlinear fitting method,which provides data support for the establishment of the subsequent viscosity model.In order to verify the established laminar flow model,the simulation results and experimental data are compared with the two aspects of power consumption and flow field structure.It is found that the CFD simulation results agree well with experimental data,which indicates that results obtained by the laminar flow model established in this paper are reliable.By studying the variation of the apparent viscosity of the xanthan gum solution along the radial direction in the stirred tank,a new method for determining the cavern boundary is proposed,namely the yield viscosity method,that is,the fluid viscosity at the cavern boundary should be 25% yield viscosity.Studies furtherly show that the yield viscosity method is not affected by the speed and impeller type,and its prediction accuracy of the cavern boundary is higher.It can better reflect the development of the boundary,and it maintains higher accuracy at high speed,and has obvious advantages compared with the velocity method.Based on the axial force cavern model,a heart-shaped cavern model was established to predict the cavern in the flow field,and the prediction result of four cavern models was compared with that obtained by CFD simulation,respectively.It is found that at the lower Reynolds number,the boundary curve determined by the heart-shaped model is closest to the CFD result.The cylinder model ignores the special changes in the top transition section and bottom of the cavern.At the meantime,the cavern obtained by the spherical model is too large,while the torus model has a significantly smaller prediction of cavern.As the Reynolds number is increasing gradually,the heart-shaped model always maintains the prediction accuracy of the cavern boundary,and the boundary curve fits well with the CFD simulation results.However,the spherical model's prediction of the cavern is always too large and it prematurely estimates the contact time between the cavern boundary and the wall surface with the increasing Reynolds number.And the cylinder model's prediction of the cavern is gradually smaller,while the circular model is more appropriate to describe the core area of the cavern as its prediction of cavern is too small.