Experimental investigations of turbulent fragmenting stresses in a rotor stator mixer. Part 1. Estimation of turbulent stresses and comparison to breakup visualizations

Artikel i vetenskaplig tidskrift, 2017

Despite large industrial relevance, the relation between rotor-stator geometry, hydrodynamics and drop breakup is poorly understood, partly since no methods for measuring the fragmenting stresses acting on drops have been established. This study attempts to bridge this gap by developing, applying and evaluating two approaches for estimating local turbulent stresses based on particle image velocimetry data: namely one traditional but indirect approach based on the dissipation rate of turbulent kinetic energy, and another more direct approach based on the spatial turbulent spectrum that has proven useful in other high-intensity emulsification processing. The approaches are evaluated in terms of validity of underlying assumptions, how they compare to breakup visualizations in the same geometry and with regard to the reliability of primary measurables. Results show three consistent regions of high stress in the rotor-stator region: in a plume extending into the stator-hole from the trailing edge, in the shear layers of the jet exiting the hole and in the macroscopic flow structure formed after the rotor blocks a stator hole. Following, a drop travelling along an average velocity flow field, the measurement predict disrupting stresses exceeding the stabilizing stress at the stator hole exit, at approximately the same position where drop breakup is observed in breakup visualizations. Both methods are therefore able to predict the most likely breakup positions. It is also concluded that both methods have limitations, and that average stress alone cannot describe all aspects of the fragmentation process in rotor-stator mixers.