How Fluid Pseudoplasticity and Elasticity Affect Propeller Flows in Biogas Fermenters

Mixing in biogas fermenters is complex due to the non-Newtonian rheology of biogenic substrates, which exhibit both pseudoplasticity and elasticity. It is yet unclear how these non-Newtonian properties affect propeller flows and the mixing behavior in fermenters. Therefore, propeller flows in Newtonian as well as shear-thinning inelastic and elastic fluids are compared numerically and validated against particle image velocity (PIV) data. Elastic normal stresses lead to an increase of pumping rates in the laminar regime and a suppression of the formation of a propeller jet in the transitional regime. Thus, flow rates are severely overestimated by the inelastic, shear-thinning model in this regime. The results indicate that elasticity is critical for an accurate modeling of flows of biogenic substrates.