CFD analysis for mixing performance of different types of household biodigesters

Household biodigesters are self-mixing anaerobic digesters used mostly in rural areas of developing countries as a reliable source of clean cooking energy. For an efficient anaerobic digestion process, the mixing of slurry inside the digester is regarded as one of the most important parameters. In this study, the mixing of slurry in three different designs of household digesters, namely the fixed-dome digester (GGC 2047 model), plug-flow digester and prefabricated plastic digester, are investigated and compared using computational fluid dynamics. A 3D transient simulation is performed using a multiphase volume of fluid (VOF) model in Ansys® Fluent release 16.0. The rheological properties of the feedstock are considered identical for all three digesters. The volume of the plug-flow and prefabricated plastic digesters is designed to be 1 m3 while the volume of the GGC 2047 digester was 6 m3 as the standard size of the household digester. The regions inside the digester where the velocity of slurry is <0.02 m/s are regarded as dead zones and the obtained results were analysed and compared using velocity patterns and dead-zone formation. It is found that the prefabricated plastic digester model has a relatively higher percentage of dead volume (74.6%) and the plug-flow digester has the lowest percentage (54%) of dead volume among digesters that were compared in this study. The study will serve as the basis for designers and researchers to improve the design of household digesters for better mixing performances.