CFD study of homogenisation with dual Rushton turbines - comparison with experimental results, Part II: Using the multiple reference frame

Experimental transient responses of six conductivity probes distributed throughout a stirred tank have been used to determine the terminal mixing times, theta(95), in the turbulent flow regime. These transients were simulated using a fully predictive mode of the structured Fluent CFD code based on a sliding mesh method(1). Here, a new procedure based on a multiple reference frame (MRF) method with grid refinement is employed. Both k-epsilon and the standard RNG k-epsilon models with the MRF substantially eliminated the wrongly predicted large areas of tangential circulation in a direction opposite to the impeller rotation reported elsewhere. The simulated distribution of the tracer in time using the new approach is also more realistic and the transient responses are shorter, though the computed mixing times were still about two times longer than the measured values. These computations by k-epsilon and RNG k-epsilon models with MRF and mesh refinements for momentum transfer and calculating concentration fields in a single, stationary frame of reference represent an improvement in comparison with those predictions obtained without it.