In many industries, especially oil and gas production, a mixture of oil, water, and gas is a common product. Separating these three fluids is a critical first step. It is essential for protecting the environment, meeting product quality standards, and making operations efficient. The most common tool for this job is the three-phase separator, a large vessel that uses gravity to do the hard work. A 3-phase separator CFD simulation is the best way for engineers to see what is happening inside and to design better, more efficient units. This report shows a detailed Three-phase Oil-water-air Separator CFD analysis using ANSYS Fluent to visualize and understand the separation process, guided by the excellent methods in the reference paper [1].

• Reference [1]: Le, Thuy Thi, et al. “Three-phase Eulerian computational fluid dynamics of air–water–oil separator under off-shore operation.” Journal of Petroleum Science and Engineering171 (2018): 731-747.

Figure 1: A schematic showing the basic principle of a three-phase separator CFD model.

Simulation Process: Modeling the Oil-Water-Air Separator CFD Simulation

The first step was to build the 3D model of the multiphase separator cfd geometry. Because the design is symmetrical, we only needed to model one half of it, which saves a lot of computer time. As seen in Figure 2, the model includes an inlet pipe, a cap over the inlet to prevent splashing, and a “bucket” that helps to calm the flow. Then, a mesh containing 200,664 tetrahedral cells was created.

Inside ANSYS Fluent, the powerful Eulerian multiphase model was used. This model is perfect for a 3-phase separator Fluent simulation because it can track three separate fluids (air, oil, and water) as they move and interact. We also included important forces like drag and surface tension, which affect how the fluids separate. At the air outlet on top, a porous zone was added to act like a filter, ensuring only clean air leaves the separator.

Figure 2: The geometry and mesh for the Oil-Water-Air Separator CFD simulation, showing the symmetric half-model.

Post-processing: CFD Analysis of Separation Mechanism and Performance

The simulation results provide a clear and fully substantiated story of how the separator works, starting the moment the fluid mixture enters. The flow first hits the inlet cap, which slows it down and spreads it out. This is a critical design feature that prevents the high-speed inlet from churning the tank and making separation difficult. Once the mixture is in the main vessel, gravity immediately gets to work. Air, being much lighter than the liquids, naturally and quickly rises to the top due to buoyancy. The air volume fraction contour in Figure 3 proves this perfectly, showing a distinct layer of air (blue) collecting at the top of the separator, ready to exit through the outlet.

While the air separates quickly, separating the oil and water is a slower and more delicate process because their densities are much closer. This is where the bucket design becomes very important. The bucket creates a calm, quiet zone in the main body of the separator. This gives the liquids enough “residence time” for gravity to slowly pull the heavier water to the bottom and allow the lighter oil to float on top. The oil volume fraction contour in Figure 3 substantiates this, showing a clear, well-defined layer of oil (yellow-green) forming between the top air layer and the bottom water layer. The most significant achievement of this Three-phase Oil-water-air Separator Fluent simulation is that it shows how each design feature contributes to the final goal. It visualizes the inlet cap reducing turbulence, the air separating quickly, and the bucket providing the calm conditions needed for the slow oil-water stratification. This provides a complete engineering validation of the separator’s design.

Figure 3: Volume fraction of air (left) and oil (right) from the 3-phase separator CFD simulation, showing effective separation into distinct layers.