Experimental Study of Laminar Flow Characteristics in a Stirred Vessel Using Simultaneous PLIF–PIV Measurements

Isolated mixing regions (IMRs) commonly persist during steady laminar stirring and limit the attainable mixing degree. Planar laser-induced fluorescence (PLIF) and particle image velocimetry (PIV) were combined for simultaneous measurements in this study. Experiments were conducted with pure glycerol and microbubbles as PIV tracers over Re = 1.76–10.58. Relatively stable IMRs were identified by PLIF throughout the investigated Re range. As Re increased, the mean specific kinetic energy in the measurement plane rose from 2.84 × 10−6 m2/s2 to 1.77 × 10−3 m2/s2, whereas the steady-state mixing degree remained near 70%. PIV results showed that the velocity components obtained from boundary and center projections agreed well in magnitude and trend. At all investigated Re values, the center-radial projected velocities at the IMR boundary points remained close to zero. Increasing rotational speed primarily intensified fluid motion within each region and tangential motion near the IMR boundary, without generating persistent, coherent radial exchange. Restricted boundary-normal motion is therefore an important reason for IMR persistence and the limited improvement in steady-state mixing degree. These findings provide quantitative experimental evidence for understanding steady IMRs and enhancing laminar mixing.