Abstract Summary (Max 250 words)
Dual-shaft, turbine–anchor (DSTA) stirred tanks are widely used for mixing complex rheological fluids due to their versatility across a broad range of rheology. Different impeller types can be installed at varying heights and speeds to serve specific functions, such as pumping, shearing/aggregate break-up or dead-zone elimination. Despite widespread industrial use, limited literature exists on DSTA mixer design and performance. In this study, transitional-regime mixing was investigated in a 19 L flat-bottomed DSTA equipped with a top-driven anchor and an eccentric Rushton turbine (RT) or down-pumping pitched blade turbine (PBTd). Acid–base decolourisation, particle image velocimetry (PIV) and torque measurements were used to assess mixing time (θ₉₅), flow patterns, velocity fields and power consumption. Complementary CFD simulations were performed using STAR-CCM+ to support interpretation of the experimental results. The results show that the effect of eccentric impeller type (RT or PBTd) on mixing time depends on the anchor rotation speed, NANC. At maximum NANC, mixing times are independent of impeller type, whereas at lower NANC—including fixed anchor (NANC = 0 rpm)—the PBTd achieves faster mixing (for equivalent total specific power, ̄ε). Correlations were also developed to quantify changes in anchor power draw due to impeller interactions as functions of the Reynolds number ratio, ReECC/ReANC—yielding R2 values of 0.76 and 0.97 for co- and counter-rotation modes, respectively.
