Experimental Investigation And Numerical Simulation Of Flow Characteristics In Vessels Stirred By Disc Turbines

Stirred vessels equipped with disc turbines are widely used in many industry processes, such as petrochemical, biochemical, pharmaceutical and food industry. So the hydrodynamics of the flow is very important for process design and optimization. In this thesis, different flow patterns in vessels stirred by multiple impellers were investigated by using experimental and computational methods.The experiments were carried out in a perspex vessel of 0.476m in diameter. The depth of tap water is equal to the diameter of the vessel. The phase resolved PIV technique with high resolution was used. The trailing vortex, mean velocity, root mean square (rms) velocity and turbulent kinetic energy (TKE) of different flow patterns were investigated. Various flow styles of Rushton impeller were obtained by varying the impeller numbers, the diameter and the clearance of impeller. Double loop flow pattern was adopted for curved surface disc turbines. The experiments show that a counter rotating vortex pair was generated under all the flow patterns with all the disc turbines, but the locations of the axis of the trailing vortex are different. In the phase averaged results, the region with high TKE is located near the impeller tip. In the phase resolved results, this region is centered between two trailing vortices, and is transported to the bulk of the tank by the movement of vortices. In the single loop flow pattern, the normalized velocity and TKE are almost same for different Reynolds number when the geometrical parameters are identical, however, there are some difference for the scale similar stirred vessels. The flow pattern is tightly related to the diameter and the clearance of the impeller. The double to single loop transition can be observed when the impeller clearance is reduced, but the flow structure can vary inversely when the impeller diameter is increased. Both the trailing vortex and the TKE are influenced by the shape of the impellers, and when impeller is streamline, the trailing vortices damp and the levels of TKE decrease quickly.User defined subroutines were embedded in the commercial CFD software FLUENT, and were used to process the data of large eddy simulation (LES). The double and the single loop flow patterns in a Rushton impeller stirred vessel were investigated by using LES, and were compared with the PIV experiments and the simulation results of k-e model. The effects of subgrid stress model and spatial resolution were discussed. The LES results show that the complex flow field can be represented by LES. The velocity of double loop flow pattern can be well predicted by k-e model, but the TKE is underestimated in comparison with the PIV and LES. As for the single loop flow pattern, both the velocity and the TKE are erroneously predicted by k-e model, however, the results obtained by LES are in agreement with PIV data. LES with different SGS models does not lead to significant changes in the flow field predictions, and DSL model yields a more accurate result than SSL and DKE models with high spatial resolution.The parallel, merging and diverging flows in a dual Rushton impeller stirred tank were investigated by using LES, and the effect of boundary conditions and data processing methods were discussed. The results show that the complex flow can be well predicted by LES in comparison with the k-e model. The boundary conditions should be consistent with the physical models. The turbulence data should be processed by using phase averaged and phase resolved methods, in order to avoid the pseudo-turbulence caused by the impeller rotation.The pressure coefficient and velocity of the curved surface disc turbines were investigated by using realizable k-e model. The effect of the impeller shape on the flow structure was discussed, and a modified impeller on the basis of half elliptical blades disc turbine (HEDT) was proposed. The results show that both the power number and pressure coefficient of Rushton impeller predicted by realizable k-e model are in agreement with the experimental results. The low pressure zones of the half circular blades disc turbine (CD) impeller is obviously less than the Rushton impeller. Among the four curved surface disc turbines, the low pressure zones increase as following: parabolic blades turbine A(PT-A), PT-B, HEDT and CD. The separation points move to the tail region as the impeller is more streamline. The low pressure zones are transported with the movement of the trailing vortices. The horizontal slots on the blades of HEDT impeller can reduce the low pressure regions and weaken the tip vortices.