Effect of fluid viscosities on the liquid-liquid slug flow and pressure drop in a rectangular microreactor.

• The effects of both μ c and μ d on slug flow are investigated. • The slug flow occurrence is predicted based on composite dimensionless numbers. • For squeezing regime, the droplet scaling law is different from the classical theory. • The film thickness increased more rapidly with Ca D than in gas–liquid slug flow. • Correlations were proposed to reflect the effect of recirculations in both phases on the pressure drop. The hydrodynamics of liquid–liquid slug flow with various fluid systems were investigated, aiming at revealing the viscous effects of both phases. The operation range, droplet formation, film thickness and pressure drop of slug flow were studied and discussed. The results reveal the importance of inertia and viscous forces of both phases in the transition of flow patterns. Accordingly, a universal flow map based on composite terms of Ca c Re c 0.5 and Ca d Re d 0.5 was adopted, which worked well for a wide viscosity range. For droplet formation in the present system, both the squeezing and shearing regimes deviated from the classical ideal models. Under higher capillary number, the film was accumulated at the corners and lateral planes, leading to distinct droplet shapes from literature. For the two-phase pressure drop, new empirical correlations were proposed to describe the recirculation inside both the droplets and slugs. The findings can provide better guidance for optimized reactor design. [ABSTRACT FROM AUTHOR]