Numerical Simulation of the mixing process of four completely miscible phases of two different biogas digester

This work aims to identify the mixing performance of a biogas digester with four fluid phases, which are completely separated at the beginning. Based on an Euler-Euler model the mixing performance is calculated with numerical methods. This report is based mainly on the presentation of the project AD-CDF (financed by FFG). The stirring in biogas fermenters is an important factor of the performance of biogas production. The process itself runs in four stages. Bacteria are applied in all of these stages, each requiring a different pH value and nutrients. Thus, efficient stirring in biogas fermenters is crucial for a high yield of biogas. Two established plants were chosen for the simulation. The first plant was built up in Reidling by the company AAT, the second plant is located in Strem and was built by the company Thöni. Both companies, as well as the operators of the two plants, were project partners of AD-CFD. A propeller stirrer and a vertical paddle stirrer were set up in Reidling, while in Strem two horizontal paddle stirrers were installed. The geometry of these plants was transformed into a numerical model; subsequently the mixing performance of the fermenter set-up was calculated. Fluent 6.3.26TM was chosen as solver. The standard deviation of the phase composition in each cell of the numerical grid was calculated to identify the goodness of mixture and an average value were defined. The chronological sequence of this average value was tracked; the best technical accomplishable goodness of mixture is achieved as soon as it reached its minimum