Power Consumption of a Rushton Turbine Mixing Viscous Newtonian and Shear-thinning Fluids: Comparison between Experimental and Numerical Results

A fluid dynamics software package (Fluent) was used to calculate the local velocities in a standard tank fitted with a Rushton turbine. We defined the local average shear rate γ˙¯ as a user variable. The power was computed from the values of γ˙ with a user subroutine. The influence of the rheological behaviour of the fluid was investigated: the shape factor K_p for a Newtonian fluid, and in the case of a shear-thinning fluid Metzner and Otto′s concept was used to determine the effective shear rate γ˙_e · γ˙_e verified the classical relation: γ˙_e = K_s·N. The influence of position and stirring speed on the local average shear rate γ˙¯ was also studied. As expected, higher values were obtained near the turbine. At a fixed position, γ˙¯ was found to be proportional to the stirring speed N. The power values obtained by numerical procedures were compared to experimental results: the Fluent software yielded results in excellent agreement with experimental findings. The calculation of local parameters such as the local average shear rate γ˙¯ as well as global characteristic shape factors demonstrate the validity of Fluent software in the laminar range.