Solid-liquid flow at dilute concentrations in an axially stirred vessel investigated using particle image velocimetry

Particle image velocimetry (PIV) has been used to quantify velocities and turbulence levels of the continuous and dispersed phases in a vessel agitated by a 45 pitched-blade turbine (PBT). To be able to measure velocities of both the continuous and dispersed phases in suspensions with a concentration of solids up to 1.5% by volume, a technique using matching of refractive indices in combination with image analysis to separate the phases was applied. It is found that the measurement technique works well and that 600 recordings are sufficient to estimate mean and RMS velocities. The mean velocities of both phases decrease while the RMS velocities increase as the solid content increases. The decrease in velocity is to a large extent a result of the impeller discharge being diverted towards the vertical tank wall at a distance from the tank bottom that increases with increasing volume fraction.