Investigation and optimization of the mixing in a biogas digester with a laboratory experiment and an artificial model substrate

The flow and mixing dynamic of biogas digester substrates are challenging to be described. Rheology has to be considered as a non-Newtonian fluid, whose viscosity changes with the strain rate. The Ostwald-de Waele power law is a well applicable mathematical relationship, which useful describes the complexity of the flow behavior, permits mathematical predictions and experimental correlations. A home-made laboratory scale-down digester has been developed at the German Technische Hochschule Ingolstadt (THI) to investigate in scientific laboratory the fluid dynamics. The real substrate has been replaced with an artificial substrate to facilitate the investigation. The fluid velocity has been observed using Particle Image Velocimetry (PIV) and Acoustic Doppler Velocimetry (ADV). Three different configurations for the mixers have been developed and investigated, using propellers with different dimensions and a paddle-mixer system. The fluid dynamics have been computational modelled with numerical methods based on the Ostwald-de Waele power law. Specific algorithms have been developed using the commercial software tool StarCCM+. The simulation outputs have been evaluated and interpreted in relation with the experimental data to validate the mixing concepts. The success of our work confirms the good correlation of numerical simulations and laboratory experimental models. It contributes to a better understanding of the mixing process in real biogas digesters, what supports the competitiveness of biogas plants in future