Experimental And CFD Study On Flow Characteristics Of Fluid In A Draft-Tube Stirred Tank

Mixing power is important for appraising performance of impeller and the effect of mixing in a stirred tank. In this work, the whole power draw and power numbers in both water and Non-Newtonian fluids (CMC-water solution, CMC-syrup solution, and HEC-syrup solution) were investigated in a draft tube stirred tank of0.476m diameter with different liquid height, stirred by a WH-5impeller. The results indicate that in the transitional region(Re,15-809), the power number decreases as Re is increasing for Non-Newtonian fluids, and the higher viscosity, the larger decrease.Mixing time of three Non-Newtonian fluids was measured using acid-base method. It is demonstrated that such a combination of draft tube stirred tank and WH-5impeller in this work can be use to mix viscous materials quickly and uniformly. The results are important for both optimum design and industrial production of stirred tank with a draft tube.Flow field is investigated by using Five-hole Pitot Tube Probe. The effect of draft-tube on flow field was studied in detail by analyzing the fluid velocity data. It was shown that the flow field distribution in the vessel equipped with draft tube is relatively stable. There is a large scale axial circumfluence in the tank, and the existence of draft tube is in favor of high efficiency mixing. Through the measurement and analysis of flow field, mixing principle in this reactor is acquired, which can be the basis of optimization.The power draw and flow field were simulated using FLUENT, and the results were in good agreement with the experimental data. It can be found from the comparison that the laminar model is fit for the simulation in laminar and low Re of transitional region, while RNG k-e model for high Re of transitional region, and standard k-e model for fully turbulent region. Chosen impeller speed and Pm were chosen as two norms, the comparison of velocity components and flow fields in laminar, transitional, and turbulent flow regions was carried out as well.In this study, it is found that there exist ill region under the impeller, and the extent increases as the Re does. In addition, there are a few same regions near the draft tube. In this work, researcher gave two recommendations for eliminating such regions. The results are important to the optimum design of industrial stirred tank with draft tube.