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

Background

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.

Method

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.

Significant findings

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.