Explanation of Multiphase Volume of Fluid (VOF) and Regime Transitions

In many fluid systems, the flow is multiphase, which means that there are particles in the fluid or at least two different phases of the fluid, like gas and liquid or two different substances. Multiphase flow can be seen in oil and water mixed together. It is also called multiphase if there is combustion or phase change in the system.

Multiphase CFD modelling can be done in a number of different ways. The right method is picked based on the problem’s specifics and the needs of the case. Check out the different multiphase CFD options that cfdland.com has to offer by going to that website. Using ANSYS Fluent, they have finished many projects in this field for a wide range of industries.

There are many multiphase simulation methods, which can be divided into two general categories. The first category is the Eulerian-Eulerian methods, in which the Navier-Stokes equations are solved for all materials in the domain. The second category is the Eulerian-Lagrangian method. In this method, the Navier-Stokes equations are solved for the fluid, while F = ma equation is applied to the solid particles. One of the methods within the Eulerian-Eulerian subset is the Volume of Fluid (VOF) method.

Volume of Fluid (VOF)

VOF is one of the multiphase flow simulation methods in which the boundary between phases is clearly defined. The radius of curvature of the boundary between the two phases must be much larger than the size of the mesh. In this method, the two phases cannot mix together. VOF is often used to solve problems involving free surface flows, like water waves and sloshing in tanks. It is also often used to solve problems involving droplet dynamics, like in fuel injection and inkjet printing.

Regime Transitions

In multiphase flows, regime transitions refer to changes in fluid flow patterns. VOF is a suitable simulation option in many such cases. Some of the problems associated with regime transitions are:

  • Slug flow: In this type of multiphase flow, one phase includes slugs or bubbles, which are usually in the gas phase and move within a fluid. This phenomenon is often observed in a pipeline or channel.
  • Annular flow: One fluid makes a thin film on the inside of a pipe or other structure like this, and another fluid goes through the middle. This flow pattern is often seen in systems with gas and liquid. In these systems, the gas fills the pipe’s core, and the liquid covers the walls in a continuous film.
  • Stratified flow: In this type of multiphase system, the materials are layered on top of each other due to density differences. The lower the layer, the denser it is. If you try to mix water and oil together and then let them sit for a while, you will eventually see a layered structure.
  • Dispersed flow: In this multiphase system, the dispersed phase includes droplets or bubbles that are dispersed in the continuous phase. The mixture of water and oil or gas and water causes the formation of this type of multiphase flow.

What was stated above was only a part of the types of multiphase flow, where the state of each system can change between different regimes. The VOF method, as mentioned in the examples, is a good option for simulating these regime transitions.

In ANSYS Fluent, in the Multiphase model window, it is possible to select the VOF and set its details

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Conclusion

Multiphase flows are observed in various fields of engineering and industry. One method to study such flows is multiphase CFD simulation, which is divided into various approaches; here, we will examine the VOF method. We have seen that this method is a suitable option for many regime transitions. To explore projects that can be conducted in the field of multiphase CFD using ANSYS Fluent software, visit CFDLAND.com. You can confidently order your CFD simulation projects to them, trusting in the expertise of their professionals.

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