(87c) Using Thrust As an Indirect Measure for Mixing Efficiency of Submerged Propeller Agitators in Non-Newtonian and Viscoelastic Media

Using thrust as an indirect measure for mixing efficiency of submerged propeller agitators in non-Newtonian and viscoelastic media

Markus Kolano, Manuel Brehmer, Matthias Kraume, TU Berlin, Chair of Chemical and Process Engineering, Ackerstraße 76, 13355 Berlin, Germany

In Germany, most biogas fermenters are mixed using one or multiple submerged propeller agitators. The composition of fermented substrates is highly variable and changes in rheological behavior can lead to an insufficient mixing performance. Thus, operational parameters like stirring rates as well as geometrical orientation of the agitators have to be continuously adjusted to the rheological properties. Evaluating the efficiency of these mixing systems is difficult, since used substrates are opaque and optical measurement techniques are not applicable. Indirect measures are therefore needed.

In this talk, we evaluate whether thrust can be used to determine the mixing efficiency of aforementioned systems. Using the particle image velocimetry method in combination with thrust measurements via strain gauges as well as mixing times determined by decolorization, relationships between flow numbers, axial forces and mixing times are experimentally derived for various single and multiple propeller mixing systems in laboratory scale. For that, transparent model fluids with varying (viscoelastic) flow properties are used, allowing for the derivation of rheological dependencies. The experimental data also serves as validation basis for numerical simulations. In these, viscoelastic flow properties of the model fluids as well as bio substrates, which are determined using a novel relative measurement technique, are used.

The general flow behavior of the propeller jets can be categorized by the elasticity number (Fig. 1, left). While in the elastically dominated regime an axial flow profile is produced, the interplay of inertial and elastic forces causes distinct radial flow components in the transition regime. In that regime, the axial flow component of the propeller jet can be well described by thrust (Fig. 1, right). Since mixing times are predominantly dependent on axial flow in the here analyzed mixing systems, we can show thrust can finally be used to estimate mixing times as well.

Figure 1: Propeller jet angle in dependence of elasticity number (left); axial flow and thrust in the transition regime (right)