Velocity profiles and shear strain rate variability in the USP Dissolution Testing Apparatus 2 at different impeller agitation speeds

The fluid velocity profiles at different locations inside a standard USP Dissolution Testing Apparatus 2 were experimentally obtained via Laser Doppler Velocimetry (LDV) at three impeller agitations speeds, namely 50 rpm, 75 rpm and 100 rpm. The experimental results were compared with the predictions obtained with Computational Fluid Dynamics (CFD) where the κ-ω model with low Reynolds number correction was used to account for turbulence effects. In general, good agreement was found between the experimental LDV velocity measurements and the CFD simulation predictions. The non-dimensional tangential, axial and radial velocity profiles (scaled with the impeller tip speed) and the flow pattern were found to be nearly independent of the agitation speed in most regions of the vessel, implying that increasing the agitation speed generally produced a corresponding increase in the local values of the velocity. However, the velocity profiles and flow pattern in the inner core region just below the impeller, where the dissolving tablet is usually located, were found to be much less sensitive to agitation speed. In this region, the axial and radial velocities were especially low and were not significantly affected by agitation increases. This inner core region at the center of the vessel bottom persisted irrespective of agitation intensity. The CFD predictions also indicated that increasing the agitation speed resulted in a higher shear strain rate distribution along the vessel bottom, although the strain rate was always very low at the center of the vessel bottom, even when the agitation speed was increased.