Slug flow is a special pattern that happens when tiny gas bubbles and liquid drops move together through very small tubes called capillary microreactors. These tubes, often thinner than a hair, make the bubbles and liquid line up one after another, like cars on a train. This special two-phase flow is amazing because it helps chemicals mix and react much better than in big tanks. The movement creates small swirls inside the liquid that stir everything up very well. Because the tubes are so small, chemicals can mix very quickly. Scientists use this method to create tiny chemical factories on a small chip, which are sometimes called “lab-on-a-chip” devices. This numerical study uses Computational Fluid Dynamics (CFD) to understand this process, with validation from a key research paper [1].

• Reference [1]: Kashid, Madhvanand N., et al. “Computational modelling of slug flow in a capillary microreactor.” Journal of Computational and Applied Mathematics2 (2007): 487-497.

Figure 1: A diagram illustrating various flow types in microchannels, including the targeted Slug Flow CFD regime.

Simulation Process: Fluent Setup, Modeling Two-Phase Flow with the VOF Method

The capillary microreactor we studied has a very small diameter between 0.5-1 mm and is attached to a Y-shaped inlet. To prepare for the simulation, we created a 2D computational domain and filled it with 17,331 structured quadrilateral cells using ANSYS Meshing. To model the two different fluids (gas and liquid) that do not mix, we used the Volume of Fluid (VOF) methodology. This method is excellent for tracking the interface between two fluids. Because the formation of slugs happens over time, we used a Transient (unsteady) solver to watch the flow develop step-by-step.

Figure 2: Schematic of the computational domain used for the Capillary Microreactor CFD simulation, based on the reference paper [1].

Post-processing: CFD Analysis, Visualizing Slug Formation and Flow Dynamics

The air volume fraction contour below shows the precise interaction of water and air bubbles inside our microreactor. In the contour, we can clearly see a stable slug flow pattern forming. The air bubbles (shown in red, representing an air volume fraction up to 0.57 or 57%) are neatly separated by sections of water (shown in blue). We successfully created uniform slugs that are about 3-4 times longer than the channel’s diameter, which is a key goal for good microreactor design. At the Y-shaped outlet on the right, a clear separation occurs: the air bubbles tend to follow the upper path, while the water mainly flows along the lower path. This demonstrates how two-phase flow physics can be used to passively sort fluids.

Figure 3: Air volume fraction distribution from the Slug Flow In Capillary Microreactor Fluent simulation, showing slug formation and phase separation.

The velocity contour provides more detail on the fluid motion that drives the mixing. The flow speed ranges from 0 m/s at the walls (due to the no-slip condition) to a maximum of 0.014 m/s in the center of the channel. This difference in speed between the center and the walls is what creates the internal circulation inside each liquid slug, which is responsible for the excellent mixing performance of these devices. The velocity pattern also confirms that the flow is stable and well-behaved throughout the microchannel. The most important achievement of this simulation is the accurate prediction of stable, uniform slug formation and the resulting phase separation at the outlet. This result validates our CFD model as a reliable tool for designing and optimizing lab-on-a-chip devices and capillary microreactors for chemical processing and analysis.

Figure 4: Velocity field from the Capillary Microreactor Fluent analysis, highlighting the flow profile that creates internal mixing within the slugs.