Effect of viscosity on liquid-liquid dispersions within milli-scale symmetric confined impinging jets*

This work presents the formation of liquid-liquid dispersions in a symmetric confined impinging jets (CIJs) contactor, where two immiscible liquids enter from opposing directions into a mixing chamber. Both the inlet channels of the liquids and the outlet channels have dimensions in the micro or mm scale. The configuration leads to rapid mixing or emulsification of the liquids. CIJs find numerous applications such as extraction, rapid precipitation, and flash synthesis. They are favoured for the enhanced mass transfer and process intensification. However, the hydrodynamics at the impinging zone and the emulsification for symmetric CIJs are still not well explored.

The dispersion formation is studied both experimentally, with high-speed imaging, and numerically with computational fluid dynamics (CFD) simulations. Experimentally, the droplet size distribution was measured for different fluids. Water (viscosity of 1.38 mPa∙s) was used as the continuous phase, while kerosene (viscosity: 2.04 mPa∙s) and silicone oils (viscosities: 10 mPa∙s, 50 mPa∙s) were used as the dispersed phase. The results show that when kerosene is dispersed, the drop sizes follow a log-normal distributions and are significantly influenced by the energy dissipation rate in the impinging zone. With the more viscous liquids, the distributions shift to larger sizes, while bimodal distributions are observed. This is attributed to different Rayleigh-Plateau wavelengths and the contribution from satellite drops. Specific interfacial areas and the Sauter mean diameter are obtained at different operating conditions. Compared with asymmetric CIJs with only one outlet, it is found that the emulsions formed in symmetric CIJs have smaller droplet sizes and larger specific interfacial area at higher energy dissipation rates, suggesting that symmetric CIJs intensify the processes. The CFD simulations utilise a transition from Volume of Fluid to Discrete Phase Model to capture the mechanism of droplet formation from the initial continuous liquid films. The predicted drop sizes are found to agree well with the experimental measurements.