Experimental flow field studies on MIG and INTERMIG impellers under varying fluid viscosity and baffle number using stereo particle image velocimetry

MIG (multi-stage impulse counterflow) and INTERMIG (interference multi-stage impulse counterflow) impellers are considered highly effective for various mixing tasks and are widely used in industrial processes. However, experimental data on their flow characteristics remain scarce, as detailed design information is not publicly available and their special geometry, with high impeller-to-tank ratios and staggered multi-stage arrangement, poses significant challenges for optical measurements. This study presents two novel experimental particle image velocimetry (PIV) approaches that overcome these limitations. Partially transparent impellers allow shadow-free, full-field data acquisition, while a glycerol-based refractive-index-matching fluid ensures distortion-free optical access at varying fluid viscosities. Using these techniques, flow characteristics of MIG and INTERMIG impellers are systematically investigated for the first time in one-, two-, and three-stage configurations, for both pumping directions, varying baffle numbers, and different fluid viscosities. The results reveal pronounced configuration-dependent hydrodynamics, including variations in velocity distribution, stage interaction, surface aeration, bottom and wall velocities, and the relative contributions of axial, radial, and tangential velocity components. This study provides deeper insight into MIG and INTERMIG flow behavior and establishes a comprehensive experimental basis for more precise, configuration-specific guidelines, which enable process-tailored impeller design and are essential to fully exploit their versatility.