Multiphase flow is a type of fluid flow that has two or more different materials moving together. These materials, or phases, do not mix at the smallest level, like sugar in water. Instead, you can see a clear difference between them. This kind of flow is everywhere in nature and in many machines. For example, you can see it in power plants with cooling towers, in the oil and gas industry, and even in systems that clean our water. These are just a few examples of important multiphase flow applications.
Because these flows are often very complex, it is difficult to study them with simple experiments. This is why we use CFD, or computational fluid dynamics, to get a better understanding of how they work. With a good CFD multiphase modeling approach, we can see how the different phases interact inside a machine or process. This helps engineers design better and more efficient products. For instance, using ANSYS Fluent multiphase tools allows us to simulate these complicated flows on a computer. You can explore many different projects and tutorials in our Multiphase CFD Simulations section to learn more.
Figure 1: From industrial cooling towers to natural phenomena, multiphase flows are everywhere, making CFD simulation a vital analysis tool.
Understanding Different Types of Multiphase Flows
To understand a multiphase flow, you just need to ask two simple questions. The answers will help you organize almost any multiphase problem you can think of.
- First Question: What is it made of? We look at the states of matter: gas, liquid, or solid. This gives us the main families of multiphase flow. The most common are gas-liquid flows (like bubbles in soda), liquid-solid flows (like muddy water in a river), and gas-solid flows (like smoke from a fire). Identifying which family your problem belongs to is the first easy step.
Figure 2: Visualizing the primary multiphase flow families: gas-solid, gas-liquid, and liquid-solid
- Second Question: How are the phases mixed? After you know what the phases are, you look at their arrangement. There are two main arrangements. The first is a dispersed flow, where small amounts of one phase are spread out, or dispersed, inside another continuous phase. Think of raindrops (liquid) falling through the air (gas), which is a perfect example of a dispersed gas-liquid flow. The second arrangement is a separated flow, where the phases are not mixed and have a clear, large boundary between them. Think of oil sitting on top of water. This is also called a free surface flow. Knowing the answers to these two questions is the most important step in choosing the right multiphase model in ANSYS Fluent.
Figure 3: The key difference in multiphase modeling: a dispersed flow versus a separated flow, crucial for choosing the right CFD approach.
Figure 4: This flowchart simplifies model selection by classifying flows based on phase state and arrangement, leading to the correct CFD model in ANSYS Fluent.
Key Concepts You Need to Know
To correctly set up a multiphase CFD simulation, you must first understand the language used to describe it. These key concepts are directly linked to the governing equations that the ANSYS Fluent solver uses to predict the flow’s behavior.
Volume Fraction (α)
The most basic idea in multiphase flow is the volume fraction. It simply tells us how much of a certain space is filled by each phase. It is a value between 0 and 1. For a phase ‘q’, the formula is:
α_q = V_q / V
Here, V_q is the volume of phase ‘q’ and V is the total volume of the small area you are looking at. A very important rule in the governing equations is that the sum of all volume fractions must equal one:
Σ α_q = 1
This means that every part of the space in the simulation is accounted for, which is a fundamental law for the solver.
Dilute vs. Dense Flows
Next, we must know if our flow is dilute or dense. In a dilute flow, the particles, droplets, or bubbles are far apart. The main continuous fluid controls their movement through forces like drag. But in a dense flow, the particles are very close together. This means they will hit each other often.
This difference is critical because for dense flows, the governing equations of motion must include extra terms to account for the forces of these particle-particle collisions. Forgetting to model these collision forces will give you the wrong answer in a dense flow simulation, like in a fluidized bed.
Figure 5: A comparison of dilute flow (left), governed by fluid drag, and dense flow (right), where particle collisions dominate the physics.
Phase Coupling
Phase coupling describes how the different phases affect each other’s movement. This directly changes the momentum equations in your CFD multiphase modeling.
- One-Way Coupling: The main continuous fluid pushes the particles, but the particles are too small or too few to affect the fluid. The fluid’s momentum equation is solved without any effect from the particles.
- Two-Way Coupling: The fluid and particles affect each other. The fluid pushes the particles, and the particles push back on the fluid. To model this, a “source term” representing the force from the particles is added to the fluid’s momentum equation.
- Four-Way Coupling: This is for dense flows. It includes two-way coupling, but also adds forces from particle-particle collisions.
Choosing the correct level of coupling ensures that the governing equations you are solving accurately represent the physics of your problem.
Figure 6: As particle volume fraction increases, the physical coupling becomes more complex, requiring more advanced CFD models.
Particle Response Time (τp) and Stokes Number (St)
These two values are extremely important for understanding dispersed flows. The Particle Response Time (τp) tells us how quickly a particle can react to a change in the fluid’s speed. For a small spherical particle, the formula is:
τp = (ρp * dp²) / (18 * μq)
Here, ρp is the particle’s density, dp is the particle’s diameter, and μq is the viscosity (or thickness) of the continuous fluid. Heavy, large particles in a thin fluid have a long response time.
The Stokes Number (St) is even more useful. It compares the particle’s response time to the time it takes for the fluid to change, τf.
St =particle response time/ flow system response time= τp / τf
This number tells us how the phases will behave:
- If St is much less than 1 (St << 1): The particles are very light and respond instantly. They will follow the fluid’s path perfectly. In this case, the governing equations can be simplified, as you might not need to solve separate velocity equations for each phase.
- If St is much greater than 1 (St >> 1): The particles are heavy and cannot respond to the fluid. They will continue on their own path. In this situation, you must solve separate momentum equations for the fluid and the particles because their velocities will be very different.
Figure 7: The Stokes number’s effect: low St particles follow fluid streamlines, while high St particles continue on their own path due to inertia.
Three Main CFD Approaches for Multiphase Modeling
There is no single model for every multiphase flow problem. The best model depends on how the phases are mixed. A separated flow like a wave is very different from a dispersed flow like dust in the air. For this reason, ANSYS Fluent gives us a few main approaches to choose from. Let’s look at the three most important ones.
Eulerian-Lagrangian Approach
This approach is best for dilute dispersed flows. This is when you have a small number of particles, droplets, or bubbles moving inside a main fluid. In ANSYS Fluent, this model is called the Discrete Phase Model (DPM). The idea is simple:
- We treat the main fluid as one continuous thing and solve the normal fluid flow equations for it.
- We then track each individual particle as it travels through that fluid.
The main governing rule for this model is Newton’s second law of motion, which is solved for every single particle to find its path. This approach is perfect for problems like spray drying, or coal combustion. It works best when the volume of particles is less than 10% of the total volume. If you want to see how this model is used in real projects, you can find many DPM CFD simulation tutorials that show you step-by-step examples.
Figure 8: ANSYS Fluent’s DPM is ideal for tracking individual particle paths in dilute flows, as seen in these spray and cyclone examples.
Eulerian-Eulerian Approach
This is the most powerful and general multiphase CFD model. It is also called the multi-fluid model. It does not track each particle one by one. Instead, it thinks of both phases as fluids that are mixed and flowing together everywhere.
The main idea is that a complete set of flow equations is solved for each phase separately. It calculates the volume fraction for each phase in every part of the simulation. Because it treats every phase as a fluid, it can be used for almost any kind of multiphase problem, from dilute to very dense flows. It is the best choice for complex problems like bubbly flows or fluidized beds where the phases interact a lot.
Volume of Fluid (VOF) Approach
The Volume of Fluid (VOF) model is a special tool for separated flows. You should use this model when you have two fluids that do not mix, like oil and water. The main goal of the VOF model is to find the exact location of the line, or interface, between them. It is perfect for free-surface flows.
The VOF model is different because it solves only one set of flow equations for the whole mixture. It finds the interface by using the volume fraction. In each small area of the simulation, it checks if the area is full of the first fluid, full of the second fluid, or a mix. The areas with a mix are where the interface is. If you want to learn everything about this method, you can read a comprehensive guide on the Volume of Fluid (VOF) model, which explains all the details for free.
Figure 9: The VOF model precisely tracks the sharp interface between different fluids in free-surface and separated flow simulations.
Choosing the Right Model for Your Application
Now that we know the three main approaches, how do we choose the right one for our multiphase CFD simulation? The answer is simple: you must look at your flow. Is it a dispersed flow with particles, or is it a separated flow with a clear interface?
To make it easy, here is a table that shows which model is best for different kinds of problems. This will help you select the correct tool in ANSYS Fluent.
| Model Name | Best For (Flow Type) | How It Works (Key Idea) | Common Examples |
| Eulerian-Lagrangian (DPM) | Dilute, dispersed flows where the particles are less than 10% of the volume. | It solves for the main fluid and then tracks the path of every single particle or droplet. | Spray drying, dust moving in a cyclone, rain droplets, very light bubbly flow. |
| Eulerian-Eulerian | Almost any kind of flow, but it is especially powerful for dense, dispersed flows. | It treats all phases as fluids that are mixed and flowing together everywhere. | Dense bubbly flows, fluidized beds, slurry transport, and sediment moving in water. |
| Volume of Fluid (VOF) | Separated flows where two fluids do not mix and have a clear line between them. | It solves one set of equations and carefully tracks the exact position of the interface. | Dam breaks, waves on the ocean, liquid sloshing in a tank, filling a container. |
As you can see, the most important step is to first understand your flow regime. Once you know if your flow is dispersed or separated, dilute or dense, choosing the right multiphase modeling approach becomes much easier.
Conclusion
We have learned a lot about multiphase flows. We started by seeing that these flows are everywhere, from power plants to the environment. Then, we learned about the important ideas and the different models available in ANSYS Fluent. Here are the most important highlights to remember:
- First, always look at your flow. The most important step is to know if your flow is dispersed (like smoke) or separated (like oil and water). This one choice will guide you to the correct model.
- For dilute dispersed flows with a small number of particles, the DPM model is the best choice because it tracks the path of each particle.
- For dense or complex flows where the phases are very mixed, the Eulerian model is the most powerful tool because it treats both phases as fluids.
- For separated flows where you need to see the clear line between two fluids, the VOF model is the perfect choice because its main job is to track that interface.
By understanding your flow regime first, you can confidently choose the right tool for your multiphase CFD simulation. Now you have the basic knowledge to start exploring these powerful models and gain a deeper understanding of your own engineering problems.