Turbulent solid-liquid mixing: challenges moving from dilute to dense suspensions

Turbulent solid-liquid mixing in stirred tanks is far more complex than turbulent single-phase mixing due to the fluid-particle and particle-particle interactions. Depending on the local solid volume fraction, in the stirred tanks the dominant mechanisms can vary significantly, going from regions where the dispersed phase distribution mainly depends on the turbulent liquid flow, which is unaffected by the particles, to regions where the particles modify the liquid phase flow and the interactions between particles cannot be neglected. The aim of this contribution is reflecting on the main aspects that have been comprehensively understood and others that require further investigations for the reliable prediction of the many industrial operations involving turbulent solid-liquid mixing in stirred tanks. The talk will overview selected aspects of the solid suspension and the solid distribution in stirred tanks, based on experimental local variables, such as liquid and solid mean velocities and turbulent fluctuations, solid concentration distribution, liquid mixing time, in a wide range of conditions, covering both very dilute and dense suspensions. The modelling main uncertainties particularly in the formulation of the fluid-particle and particle-particle interactions forces will be discussed based on both experimental and computed results. Current challenges and future developments of both experimental and modelling methods will be addressed.