CFD-PBM simulation of droplets size distribution in rotor-stator mixing devices

Abstract

Rotor-Stator mixing devices (RS) have found widespread application in mixing, dispersion and emulsification processes to acquire the desirable droplet size distribution (DSD) which is critical to the function and quality of products. Thus the precise control of DSD through the rational design and optimization of process or formulation is highly demanded. Computational fluid dynamics (CFD) becomes increasingly important to simulate the complex turbulence flow in RS devices which has significant impact on the final DSD. CFD can also be integrated with population balance equations (PBE) to predict the DSD as long as the droplet breakage and coalescence rates could be accurately modeled by the kernel functions. However, the underlying physics of droplet breakage and coalescence is complex and far from being well understood. We simulated the liquid–liquid two-phase flow and DSD for a weak coalescence emulsification system in a Megatron RS mixer with the CFD-PBM approach, then tentatively proposed a novel approach for correcting the breakage kernels based on the Energy-Minimization Multi-Scale (EMMS) concept. This method features the multi-scale resolution of energy dissipation, and utilizes the so-called meso-scale energy dissipation to derive a correction factor for the breakage rate for PBE. The results show that the new model can greatly improve the CFD-PBM simulation, and the DSD predicted is in good agreement with experiments, demonstrating the rationality and potential of this new approach.