CFD investigation of hydrodynamics and mixing performance in an anaerobic reactor: effects of impeller configuration, feeding position, and fluid rheology

Mixing efficiency is critical for mass and heat transfer, microbial activity, and process stability in anaerobic reactors. In this study, a three-dimensional CFD model was developed to investigate the coupled effects of impeller configuration, rotational speed, feeding position, and fluid viscosity on hydrodynamics and mixing performance in an anaerobic reactor. The results showed that increasing rotational speed enhanced momentum transfer from the impeller to the bulk fluid, producing a more uniform velocity field and a larger effective mixing region. Tracer simulations indicated that feeding position strongly influenced mixing performance because the initial tracer pathway depended on its hydrodynamic connectivity with the impeller-driven circulation loop. In addition, higher viscosity increased flow resistance, weakened bulk circulation, and prolonged mixing time. Overall, mixing behavior was governed by the combined effects of impeller design, operating conditions, feeding location, and fluid rheology. Quantitative comparison further showed that mixing performance was not determined by local velocity magnitude alone, but by the coordinated distribution of radial and axial transport within the reactor. These findings provide mechanistic insight into flow–mixing interactions and offer guidance for reactor design, feeding position optimization, and energy-efficient operation of anaerobic digestion systems.