Stirring is one of the most important tasks in chemical, pulp, polymer and food industries. It determines process parameters like power consumption, mixing time, heat transfer etc. Usually dimensionless numbers like Newton and Reynold numbers are used to characterize the systems, where the latter one is strongly dependent on viscosity. Stirring usually creates a distribution of shear rates in a tank. For Newtonian fluids, where the viscosity does not depend on the shear rate, the calculation of the Reynolds number is straightforward when using a constant viscosity.

However, in the case of non-Newtonian fluid behavior the shear field within a stirred tank corresponds to a viscosity field. In the laminar flow region the concept of effective viscosity by Metzner and Otto is well established. In the transition region between laminar and turbulent flow, the existing concepts use three or even more empirical parameters to determine the specific power input. The effective shear stress concept is a general but straightforward approach to calculate the power input for shear thinning fluids over the whole flow region by introducing only one shear coefficient as an empirical parameter. The Metzner-Otto relation is obtained as a limiting case for the laminar region. The shear coefficient of the new approach is related to the Metzner-Otto constant and peculiarities of the Metzner-Otto concept can be explained. The concept is validated for eight different stirrer systems. Mixing time, maximum shear rate and heat transfer can also be calculated using this approach. The presented new concept is also applicable for other apparatuses, e.g. static mixers and pipes.