(203e) Geometrical Optimization of Side-Entry Propeller Designs for the Homogenization Process in Biogas Fermenters

Mixing in biogas fermenters is of high importance for the stability of the process: Local gradients in temperature, concentration as well as stratification effects, i.e. swimming and sinking layers, have to be minimized. All this has to be achieved at very low power inputs to sustain an economic process, which is especially difficult considering the distinct non-Newtonian rheology of most biogenic substrates.

The most common mixing technology in biogas industry is the mechanical agitation with one or multiple side-entry propellers. To deal with the above-described complex agitation problem, a large number of different propeller designs has been developed in industry. This complicates the choice of an efficient agitator for a plant operator since very limited data are available for a specific propeller geometry.

The abundance of propeller geometries results from the application of varying design principles. Oftentimes propellers are developed using rules for marine propulsion, e.g. These designs lead to an inefficient mixing process, though, as is shown in this presentation by decolorization experiments conducted in a pilot-scale fermenter (V_R = 724 L, s. Fig. 1).

Based on these mixing experiments, fundamental rules are developed in this talk for an efficient propeller design. These rules are then applied in CFD studies, in which various geometrical propeller parameters are tested for their effect on efficiency. Using 3D printed models, these numerically derived relationships are then also validated experimentally in mixing and PIV experiments in pilot-scale.

It can be shown that the efficiency of a propeller for side-entry mixing applications is described by the constant c_Eff = F_Ax ρ^-1 n^-1 d^-1 P^-0.5, where F_Ax is axial thrust, ρ density, n stirring rate, and P power input. The number of propeller blades is of minor importance for the efficiency of a propeller geometry (s. Fig. 2 left; higher c_Eff denote more efficient designs). A pitch angle of α ≈ 40° is most efficient; this angle should be applied at the blade tips to achieve a high pumping rate to power input ratio (s. Fig, 2 right; a blade twist of 20° fulfills this criterion). Compared to flat blades, the use of a suitable hydrofoil design can increase efficiency by more than 20%.