Transitional Flow in a Rushton Turbine Stirred Tank

The way in which the single phase flow of Newtonian liquids in the vicinity of the impeller in a Rushton turbine stirred tank goes through a laminar – turbulent transition has been studied in detail experimentally (with Particle Image Velocimetry, PIV) as well as computationally. For Reynolds numbers equal to or higher than 6,000, the average velocities and velocity fluctuation levels scale well with the impeller tip speed, i.e. show Reynolds independent behavior. Surprising flow structures were measured – and confirmed through independent experimental repetitions – at Reynolds numbers around 1,300. Upon reducing the Reynolds number from values in the fully turbulent regime, the trailing vortex system behind the impeller blades weakens with the upper vortex weakening much stronger than the lower vortex. Simulations with a variety of methods (direct numerical simulations, transitional turbulence modeling) and software implementations (ANSYS-Fluent commercial software, lattice-Boltzmann in-house software) have only partial success in representing the experimentally observed laminar – turbulent transition.