CFD Modeling Of Flow Characteristics In Multistage Rotor-stator Mixers

Multistage rotor-stator mixers (RS) are widely utilized in dispersion process suchas foam generation and emulsification to generate high shear rate, featuring narrowgaps between rotors and stators in the axial direction and close clearance betweenrotors and the vessel wall in the radial direction. Study on the hydrodynamic behaviorin such devices can provide guidance in optimizing the existing mixers or developingand designing novel mixers with high performance, which is very important to intenseenergy and environmental problems. In this work, the hydrodynamic behaviors inmultistage rotor-stator mixers are systematically investigated using CFD simulations.Three dimensional CFD model of multistage rotor-stator mixers are developed toinvestigate the local flow field such as velocity profile, shear rate, flow type,Kolmogorov length, turbulent dissipation rate. The relationship between thoseparameters and rotational speed as well as fluid properties is thoroughly investigatedand is critical for process design and optimization.Power consumption is characterized for Newtonian fluids and shear-thinningpower law fluids in the laminar and turbulent regimes. The radial clearance ratio aswell as axial gap between rotors and stators is found to have a non-negligible effect onthe proportional constant between the power number and Reynolds number as well asshear rate. For non-Newtonian fluids, the Metzner-Otto concept is reported to besuitable for many other types of mixers to estimate the effective shear rate. CFDsimulations indicate that this concept is still suitable for the multistage RS systems. Anew correlation for the power number is proposed, considering the radial clearanceand axial gap, and can serve as a unified correlation for Newtonian and shear-thinningpower law fluids in the laminar regime in multistage RS systems.Population balance model (PBM) is used to describe the bubble size distributionin a multistage rotor-stator system, and a bubble breakup model considering viscousand inertial breakup mechanism is proposed. A CFD-PBM coupling model isproposed to combine the advantages of CFD to predict the entire flow field and ofPBM to predict the local bubble size distribution. The proposed model is successful in predicting the bubble size distribution, showing the good capability to predict thehydrodynamic behaviors in multistage rotor-stator mixers.