| Stirred vessels equipped with disc turbines are widely used in many industry processes,such as petrochemical,biochemical,pharmaceutical and food industry etc,so the hydrodynamics of the flow is very important for process design and optimization.In this paper,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 1.4 times than the diameter of the vessel. Three pairs of impellers were used in the experiments with diameters of D1=0.33T,D2=0.40T and D3=0.50T respectively.The multi-block and 360°ensemble-averaged approaches were used in the PIV measurement.Three typical flow patterns,named merging flow,parallel flow and diverging flow, were obtained by changing the clearance of the lower impeller above the tank base(C1)and the spacing between the impellers(C2)from radial and axial angle-resolved velocity distributions.The results show that the parallel flow is generated in the value of C2≥0.40T,C2≥0.38T and C2≥0.32T on the condition that C1 is set to D constantly,and the merging flow in the value of C2≤0.38T,C2≤0.36T and C2≤0.27T in the D1,D2 and D3 systems, respectively.When C2 is equal to D constantly,the diverging flow occurs in the value of C1≤0.15T in all three systems.The flow number NQ of the three pairs of impellers were calculated in parallel flow at the same Re.Its value of 0.757,0.842 and 0.894 for the lower impeller is increaseing larger than 0.798, 0.800 and 0.820 for the upper impeller with impeller diameters increasing. Trailing vortices generated by the lower impeller in diverging flow were shown by the 10°angle-resolved velocity measurements.And it was found that the highest turbulence kinetic energy(k/Vtip2=0.12-0.15,and even greater than 0.15)was in the region of the boundary between two vortices. |
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