CFD study of homogenisation with dual Rushton turbines - Comparison with experimental results

The transient responses of six conductivity probes distributed throughout a stirred tank of diameter T (=0.72 m) and an aspect ratio of 2, equipped with dual-Rushton turbines (D=T/2), were measured. Three impeller speeds (75, 100 and 150 rpm) were used and the terminal mixing times, theta(95), were also determined. These experiments were also modelled using the structured Fluent CFD code based on the finite volume method with a sliding mesh option. The k-epsilon and RNG k-epsilon model both predicted similar local axial and radial mean velocities. However, both also predicted large areas of tangential circulation in a direction opposite to the impeller rotation and all the local turbulence quantities and the tangential component of mean velocity were considerably different. Stability problems with the RNG k-epsilon model meant that it could not be used for computing transient responses. On the other hand, many features of the experimental transient responses and those from the k-epsilon model were similar though the computed theta(95) were about two to three times longer than the measured values. It is suggested this difference arises because the mass exchange between the four distinct axial-radial circulation loops was highly underpredicted by CFD.