Study On Flow Field And Mixing Performance In Pseudoplastic Fluid Stirred With Impeller Of Perturbed-Bladed Turbine

Mixing of viscous non-Newtonian fluids are commomly encountered operation in many process industries, which is one of the difficulties for stirring technology. Pseudoplastic fluids possesing a yield stress are most common class of non-Newtonian fluids, exhibiting the shear-thinning viscosity property. Mixing of such fluids result in the formation of a well-mixed region (so called cavern) around the impeller surrounded by stagnant and/or slow-moving fluids elsewhere in the mixing vessel. These poor mixing conditions have been implicated in lower mass tranfer rates. Moreover, Mixing is often obliged to be conducted under low Reynolds number conditions due to the high viscosity fluids and shear-sensitiveness of materials. These low Reynolds number flow conditions may bring the isolated mixing regions (IMRs), which comprise two troidal vortices formed respectively above and below an impeller, in stirred tank. These regions exchange materials with the bulk of the flow only through diffusion, which is slow relative to convective processes. Therefore, the flow feature, cavern development and efficiently chaotic mixing performance of pseudoplastic fluids in a stirred tank equipped with the6PBT impeller are investigated by numerical simulation method in this paper. The research results promote the understanding of the mixing mechanism of pseudoplastic fluids, lay a theoretical basis and provide valuable instructions for the design of the efficient agitation equipments.The rheological measurements of xanthan gum solutions (a typical kind of pseudoplastic fluid possessing a yield stress) were carryed out by rheometer for analyzing the relationship between apparent viscosity and shear rate and the influence of mass concentration and temperature on the rheological parameters. It is found that apparent viscosity of the solutions decreases sharply as shear rate increases, which exhibits a good shear-thinning rheology, and the solutions rheology is well fitted to the Herschel-Bulkley rheological model. The flow behavior index of solutions decreases as mass concentration increases, but it almost independent of temperature variations. The resulting rheological data can provide a basis for numerical simulation later. The computed data of power consumption agree well with the measured data in experiment of stirring glycerine, and the numerical results of the flow macro structure and velocity distribution by k-ε turbulent model also agree well with PIV experimental data in water, which validated the CFD model developed in laminar flow and turbulent flow regime. These results show that CFD calcalations can pick up the features of flow fields and can be effectively used to predict the flow and mixing characteristics of fluids.Based on laminar viscosity model, cavern development during agitation of pseudoplastic fluids with the impeller of perturbed six-bent-bladed turbine (6PBT) and impeller of six-bent-bladed turbine (6BT) was studied by numerical simulation. The cavern boumdary velocity was properly determined and the cavern dimensions of shape, diameter and height to diameter ratio versus Reynolds number were analysed, in the meantime, the course of cavern development was also described. The results show that the cylindrical cavern development can be found as Reynolds number increases, which shows a good agreement with the representation of the cylindrical model (Elson s model). The slope from a log-log plot of Dc/D verse NpRey, as well as the variations of cavern height to diameter ratio, have nothing to do with the rheological behavior index of fluids all together. However, the rate constant of axial fluid expansion describing cavern height once the cavern touches the vessel base and wall,β, is not alike,β=0.55for6BT impeller,β=0.79for6PBT impeller, respectively, so6PBT impeller is more beneficial to eliminate the cavern effect in stirring pseudoplastic fluids.The power consumption and pressure distribution of Newtonian and pseudoplastic fluids in stirred tank with6PBT and6BT impeller were calculated by the numerical model established. The efficient agitation mechanism of6PBT impeller was made a thorough discussion, and the complete correlations of kp and ks to impeller geometry were established. The results show that, in the laminar flow regime, all the power consumption data obtained on the basis of apparent viscosity concept for various shear thinning fluids are in agreement with those for Newtonian fluids, the critical Reynolds number of Newtonian and pseudoplastic fluids in laminar flow regime is10and30, respectively. In addition, there is no correlation between ks and rheological behavior index, n, on the whole, and6PBT impeller can lead to a saved energy as rotating speed increases. The stronger swirling motion may emerge in stirred tank with6PBT impeller under the action of Coriolis force, which is exceedingly advantageous to promotion of mixing efficiency. Moreover, the power consumption and average shear rate in stirring pseudoplastic fluids may be accurately predicted by the correlations of kp and ks, comparatively.The flow fields and mixing characteristics of pseudoplastic fluids in stirred tank with6PBT impeller,6BT impeller and dual-impeller combination were numerically investigated by standard k-ε turbulent model, respectively. The properties of the chaotic flow induced by6PBT impeller were made a thorough inquiry, and the influences of the rotating speed with6PBT impeller on shear rate, flow number and pumping capacity of fluids were also made a detailed discussion. Moreover, the influences of the different layer clearances with dual-impeller combination on the flow structure were analyzed, the proper layer clearance at different speed was determined in the meantime. The results show that, compared with6BT impeller,6PBT impeller can destroy the isolated mixing regions with its dissymmetrical structure of flow field and induce the chaotic flow in whole vessel. The tracer mixing process is close dependent on the flow field structure,6PBT impeller has a major superiority on mixing rate and mixing effiency to6BT impeller, in which, concerning the mixing energy per unit volume,6PBT impeller merely accountes for a52%proportion of6BT impeller. Besides, the dual-impeller combination of6PBT impeller as lower impeller and impeller of three-pitched-bladed turbine as upper one is discovered to have a clear superiority on mixing process of pseudoplastic fluids. At the end, this kind of dual-impeller combination is applied to the medium-sized trial-production stirring process of FCC catalyst gum formation on-the-spot. The wearability of catalyst particle is greatly improved with the increase of particle wear index from2.6to1.9, and the product reaches the high quality. It is concluded that the dual-impeller combination can meet the productive demands in agitation of catalyst gum formation, so is worth application and dissemination.