The flow fields in a dual Rushton impeller stirred tank with diameter of 0.48 m(T)were measured by using Particle Image Velocimetry(PIV).Three different size impellers were used in the experiments wlth diameters of D=33T.0.40T and 0.507,respectively.The multi-block and 360°ensemble-averaged approaches were used to measure the radial and axial angle-resolved velocity distributions.Three typical flow patterns,named,merging flow,parallel flow and diverging flow,were obtained by changing the clearance of the bottom impeller above the tank base(C1)and the spacing between the two impellers(C2).The results show that while C1 is equal to D,the parallel flow occurs as C2≥0.40T,C2≥0.38T and C2≥0.32T and the merging flow occurs as C2≤0.38T,C2≤o.36T and C2≤0.27T for the impellers with diameter of D=0.33T,0.40T and 0.50T,respectively.When C2 is equal to D,the diverging flow occurs in the value of C1≤0.15T for all three impellers.Thc flow numbers of these impellers were calculated for the parallel flow.Trailing vortices generated by the lower impeller for the diverging flow were shown bv the 10°angle-resolved velocity measurements.The peak Value of turbulence kinetic energy(k/V2tip=0.12-0.15 or above)appears along the center of the impeller discharging stream.