Separator: Two-Phase Flow

Separator Two-Phase Flow, CFD Simulation ANSYS Fluent Training

Description

This project simulates the interaction between air and water flow within a separator chamber using ANSYS Fluent. The model represents a cylindrical separator chamber initially filled with water, with water entering through a vertical pipe at the top and exiting through a bottom pipe, while airflow enters and exits through horizontal tubes positioned on the chamber's lateral surface — the inlet lower on the side and the outlet higher up. The inlet air carries a mass flow rate of 0.001 kg/s, while the inlet water carries 0.1 kg/s.

Since water is denser than air, it naturally settles toward the lower portion of the chamber while air occupies the space above the water surface. This two-phase arrangement was captured using the VOF multiphase model, with water as the primary phase and air as the secondary phase — an appropriate choice given the water forms a distinct free-surface layer rather than mixing with the surrounding air. Surface tension between the two phases was defined at 0.072 N/m to accurately capture this interface behavior.

Geometry & Mesh

The 3D geometry was designed in Design Modeler, representing a cylindrical chamber with vertical inlet and outlet pipes for water flow and horizontal inlet and outlet pipes for air flow. The domain was meshed in ANSYS Meshing using an unstructured grid totaling 972,786 elements.

Methodology

Several assumptions were applied: a pressure-based solver was used, the simulation was run under steady-state conditions, and gravitational effects were included at -9.81 m/s² along the vertical axis.

Key simulation settings included:

• Viscous model: Standard k-epsilon with standard wall functions

• Multiphase model: VOF with two Eulerian phases (water and air), implicit formulation, and dispersed interface modeling

• Boundary conditions: Separate mass flow inlets for gas (0.001 kg/s air, 0 kg/s water) and liquid (0.1 kg/s water, 0 kg/s air); pressure outlets for both gas and liquid streams at 0 Pa gauge pressure; stationary walls throughout

• Solution methods: Coupled pressure-velocity coupling, PRESTO! for pressure discretization, and first-order upwind schemes for momentum, turbulent kinetic energy, turbulent dissipation rate, and volume fraction

• Initialization: Standard method, with the domain initialized to 0 Pa gauge pressure, zero velocity, an air volume fraction of 0, and a water volume fraction of 1

Conclusion

Results include 2D and 3D contours of mixture pressure, mixture velocity, and volume fraction for both the water and air phases. The results confirm the expected phase separation behavior: airflow moves upward due to its lower density, while water flow moves downward due to its greater density — demonstrating that the separator successfully divides the two phases as intended.