CFD study of homogenization with dual Rushton turbines - Comparison with experimental results part I: Initial studies

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 models 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 in the version of the Fluent(TM) CFD code available for this work meant that transient conductivity responses could not be computed using this method. On the other hand, the k - epsilon model gave stable solution and many features of the experimental transient responses and those from the model were similar. However, 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.