Design And Improvement Of Agitators Using PIV And CFD Methods

Stirred tank design is confined to the use of empirical correlations due to unavailable and uncomplete theoretical methods. Computational fluid dynamics (CFD) methods can simulate three-dimension flow field in a stirred tank, and the flow field for different configurations changing with impeller type, impeller size and operating conditions can be easily got. However, the credibility of CFD simulations remains to be improved due to the inaccuracy of turbulence models and constitutive equations and the dependency of simulated results on grids. The particle image velocimetry (PIV) technique can provide microscopic flow information, such as instantaneous velocity, shear field, turbulent kinetic energy dissipation rate field. Further macroeconomic information, such as circulation flow number, average shear rate, stirring power, mixing time can be deduced. However, PIV experiment requires transparency of fluid and heavy workload to get flow information of full tank.Verified by PIV experiments, CFD simulations will be performed to obtain microscopic and macroscopic flow information, such as flow field, shear rate field, stirring power, circulation flow, mixing efficiency. The design and improvement of impeller shape and internals will be conducted to achieve the design of high efficiency impellers and wide viscosity-range impellers.(1) Based on the analysis of microscopic and macroscopic flow information, the relationship between blade geometry of axial agitators and flow efficiency is revealed. It can provide guidance for the design and improvement of high efficiency axial flow impeller. The simulated mean axial velocity, mean radial velocity, and turbulent kinetic energy by standard κ-ε turbulent model were validated by PIV data. This shows that the standard κ-ε turbulent model predicts mean velocity well, but underestimates the turbulent kinetic energy near the blades. The effect of blade shape on local dynamics was investigated from microscopic perspective. The simulation results demonstrated that the value of the specific energy dissipation rate in the leading edge of the blades especially around the blade tip is much higher than others. Therefore, one of the blade improvement methods is to cut off leading straight corner and form an arc shape to reduce local specific energy dissipation rate. Besides, the change of the blade shape by increasing the area of trailing corner for the m-PBT increases the magnitudes of radial velocity and axial velocity in the impeller vicinity.(2) The effect of internal diamond baffles on the flow patterns generated by double helical-ribbon (DHR) impeller was investigated through PIV experiments and CFD simulations which are based on SST k-co model coupled with the sliding mesh method. In laminar flow regime, the diamond baffles has little effect on the flow pattern. In transitional flow regime, for the unbaffled configuration, two vortexes zones exit, one occurs around the helical ribbons and the other vortex exists below the ribbon. After inserting the diamond baffles, the two vortexes are replaced by a global circulation. The internal diamond baffles similar to the traditional baffles against wall can weaken tangential flow, enhance axial and radial flow. Besides, the internal baffles enlarge the versatile capacity of double helical ribbon impeller to cover wide flow regimes from laminar regime to transitional regime. Therefore, the reasonable design of internals is an effective way to improve mixing process.(3) Broken helical-ribbon (BHR) impeller which can mix efficiently in a wide range of Re has been developed, and it has higher mixing efficiency than DHR impeller. In laminar flow regime, the outer and the inner blades exhibit opposite flow. The circulation loops created by upper and lower blades merge to form a global circulation which is similar with that induced by DHR. In transitional flow regime, the fluid in the center of the tank or near the wall flows upward, merges together at liquid surface and then flows downward. A more homogeneous shear distribution is formed by the BHR impeller in comparison to the DHR impeller. The diameter of inner blades closer to the hub has little effect on hydrodynamics. Clearance between impellers is critical to achieve global circulation. The mixing energy per unit mass for the improved BHR impeller is20%less than that for DHR impeller.