Simulation of a Rushton Turbine Mixing Yield Stress Fluids: Application of the Metzner-Otto Concept

The mixing of yield stress fluids is investigated numerically in the laminar regime using a fluid dynamics software package (Fluent). First, we investigate the case of the Couette system where theorical values of the shape factors K_p and K_s can be found. These values lead to a theorical expression of the power P which was successfully compared to numerical values. Next, we studied the case of the Rushton turbine. We studied the influence of position and stirring speed on the local average shear rate γ˙. As expected, we obtained higher values near the turbine. At a fixed position, γ˙ was found proportional to the stirring speed N. We verified that the Metzner-Otto concept remains valid for fluid exhibiting a yield stress and that the Metzner-Otto constant ( K_s ) varies slightly. Comparison between numerical power and those obtained using an average value of Metzner-Otto constant ( K_s = 8.5 ) led to an excellent agreement.