Flow and mixing characteristics in a stirred tank equipped with a swirl-type impeller (SWINGSTIR®)

• A swirl-type impeller (SWINGSTIR®) was studied using experiments and CFD simulations. • Mixing performance decreased with increasing blade width due to bottom recirculation. • Larger rotation diameter enhanced turbulence intensity and improved axial mixing. • Nonlinear interaction between blade width and rotation diameter affected mixing. • Optimal conditions were identified using orbit circularity and area balance parameters. The performance of mixing in stirred vessels is strongly influenced by impeller geometry. In particular, swirl-type impellers such as SWINGSTIR® exhibit unique flow behavior due to their non-rotational, oscillatory motion. However, the effects of impeller blade width ( d I ) and rotation diameter ( d R ) on mixing and flow characteristics remain insufficiently understood. This study investigates the influence of d I and d R on mixing performance through a combination of decolorization experiments, ultrasonic velocity profiler (UVP) measurements, and computational fluid dynamics (CFD) simulations using the moving particle semi-implicit (MPS) method. A series of configurations with varying geometries were tested to identify optimal mixing conditions. Mixing performance decreased with increasing blade width d I , likely due to enhanced recirculating flow near the vessel bottom that inhibited axial mixing. Conversely, increasing d R improved turbulence intensity and enhanced mixing efficiency. The trajectory of the impeller tip—shaped by both d I and d R —played a critical role: larger d I caused orbit distortion, while larger d R created a cavity near the vessel center. Quantitative evaluation using orbit circularity and area balance parameters showed that mixing was most effective when the impeller orbit was nearly circular and the spatial distribution of flow regions was balanced. These results highlight the importance of geometric interaction in impeller design and provide guidance for optimizing mixing performance in swirl-type systems.