PIV measurements of mean flow and turbulence modulation in dilute solid-liquid stirred tanks

This work is aimed at investigating the turbulent hydrodynamics of solid-liquid stirred tanks with particular attention to the effects of the dispersed phase on the turbulence levels of the continuous phase. The experimental data are collected by a two-phase Particle Image Velocimetry (PIV) technique in a fully baffled flat-bottomed cylindrical vessel stirred by a standard Rushton turbine. The effect of particle size on the liquid turbulence levels was investigated by adopting two different glass particles mean sizes (dp=774 μm and dp=115 μm); the particle contents was increased stepwise from zero (single phase system) up to 0.2 vol. % with the bigger particles and up to 0.1 % with the smaller one, the difference being due to the different optical behaviour of the two suspensions. Overall, moderate dampening of liquid turbulent fluctuations was found with smaller particles, while turbulence enhancement was observed with the bigger ones. From preliminary interpretation of the results, based on the ratio between the particle diameter and the integral length scale of the continuous phase, our data agree with the Gore and Crowe (1989) findings relevant to pipe flow and free jets.