In many industrial processes, different fluids or materials flow together. This is called multiphase flow. To understand these physical behaviors, we use specific tools in CFD. One of the most common and efficient choices is the Fluent mixture model.
What is the Mixture Model? It is a simplified approach in ANSYS Fluent. It is designed for situations where different phases are mixed and move together, but at different velocities. Think of it like dust particles flowing with air or small bubbles rising in water. The model works by treating the different phases as if they are interpenetrating. This means it assumes the phases can share the same space at the same time. It solves a single set of equations for the combined mixture instead of separate equations for each phase. This approach makes simulation much faster and less demanding on your computer.
The Mixture Model is best used when you have dispersed phases that follow the main flow closely. This is ideal in the chemical industry for designing bubble columns. Using a mixture model fluent simulation allows us to predict the gas holdup and mixing efficiency. This helps in scaling up the reactor from a small lab design to a large industrial size, saving significant cost and time. To see how these industrial problems are solved, you can explore various multiphase CFD simulation tutorials.
Figure 1: Some examples of Mixture Multiphase simulation provided by CFDLAND
When to Use the Mixture Approach
Choosing the correct solver is a vital step in your flow analysis. The Mixture model offers a great balance between calculation speed and physical accuracy for well-mixed flows. Because it solves one set of equations for the mixture rather than tracking every single phase separately, it is computationally cheaper than the full Eulerian multiphase model. You should choose this method when you have a dispersed flow (like bubbly liquids or light slurries) and want a reliable result without high CPU costs.
To see a direct, side-by-side comparison of all available solvers (including VOF and DPM) and learn when to use each one, please read our complete guide on choosing the right multiphase model.
Understanding the Mixture Model Fundamentals
To use this method correctly, you need to understand its core ideas. The model is built on a few smart simplifications that make it both fast and effective. The most important idea is the concept of interpenetrating continua. This means the model assumes that different phases, like gas bubbles and water, can exist in the same place at the same time. Think of a wet sponge: the solid sponge material and the liquid water occupy the same volume together. The mixture multiphase CFD approach treats a flow of dusty air in the same way. In every small computational cell, it calculates a single value for pressure and velocity for the mixture as a whole.
Figure 2: Illustration of interpenetrating continua: both gas (red) and liquid (blue) phases are treated as being mixed and occupying the same computational cell in the Mixture Model
This works so well because it uses an assumption called “local equilibrium”. This means that the dispersed phase (the bubbles or particles) responds to the main fluid’s motion almost instantly. Imagine a light feather caught in a gust of wind; it immediately travels at the wind’s speed. This assumption holds true when the particle relaxation time is very short, typically between 0.001 and 0.01 seconds.
So, how do the phases interact? Instead of calculating highly detailed forces between phases, the model focuses on how fast the phases move relative to each other. This is called the slip velocity. The model first calculates the average velocity of the mixture, u_m, which is a weighted average of the phase velocities.
Mixture Velocity = (share of phase 1 * its velocity) + (share of phase 2 * its velocity)
Then, it uses an algebraic formula to find the slip velocity (u_pq). This is simply the difference between the speed of the secondary phase (u_p) and the primary phase (u_q). By solving for this relative motion algebraically, the model avoids solving full momentum equations for each phase, saving a huge amount of time.
Figure 3: Real-world flow regimes. The Mixture Model is ideal for the well-mixed Bubbly Flow (left), where the local equilibrium assumption is valid. It is less suitable for the Churn Turbulent Flow (right), where phase interaction is more complex
The Mathematics Behind the Mixture Model
Now let’s look at the simple math it uses. You do not need to be a math expert to understand this. The software uses three main equations to describe the flow of the entire mixture, rather than individual phases.
The first rule is the Mixture Mass Conservation. It simply states that mass is conserved. The amount of the mixture flowing into a small space equals the amount flowing out, unless the overall density changes.
\frac{\partial \rho_m}{\partial t} + \nabla(\rho_m \vec{u}_m) = 0
Next is the Mixture Momentum Equation. It calculates the velocity of the mixture based on all the forces acting on it, like pressure, gravity, and friction.
\frac{\partial (\rho \vec{u}_m)}{\partial t} + \nabla \cdot (\rho_m \vec{u}_m \vec{u}<em>m) = -\nabla p + \nabla [\mu</em>{eff} (\nabla \vec{u}_m + \nabla \vec{u}<em>m^T)] + \rho_m \vec{g} + \vec{F} + \nabla \sum</em>{k=1}^{n} \alpha_k \rho_k \vec{u}_k’ \vec{u}_k’
Change in Mixture’s Motion = – (Pressure Force) + (Friction Force) + (Gravity) + (Other Forces)
This single equation solves for one velocity for the whole domain. This is the main reason why the model is so fast.
The third key part is the Secondary Phase Fraction Equation. This formula keeps track of how much of each phase is present in every part of the flow. As the mixture moves, this equation updates the volume fractions of the secondary phases correctly.
Finally, the model handles phase differences using slip velocity and drift velocity. In reality, light bubbles in water rise faster than the water. The drift velocity is the speed of a secondary phase compared to the center-of-mass velocity of the mixture. The software assumes the slip is caused by the particle’s acceleration and its relaxation time (τ_p).
u_pq ≈ τ_p * a_p
By using this simple formula instead of a full, separate momentum equation, the solver can account for phases moving at different speeds while remaining very efficient.
Setting Up the Mixture Model in ANSYS Fluent
Setting up a mixture multiphase fluent example is a logical process. First, you open the Multiphase Model window. In the Models tab, you select Mixture. Right away, you will see a box called Model Parameters. The most important setting here is to enable Slip Velocity. This step is essential because it tells the software to calculate the velocity difference between your phases. You also set the total number of phases here (which the software interface labels as Eulerian Phases, even though we are using the Mixture method). Usually, this is set to 2 for simple problems.
Figure 4: The main Multiphase Model window in ANSYS Fluent. Key steps are: (1) Select ‘Mixture’, (2) Enable ‘Slip Velocity’, and (3) Set the ‘Number of Eulerian Phases’
Next, click on the Phases tab. This is where you choose your primary and secondary phases. For each secondary phase, you have to define its properties. The main property is the diameter of the particles or bubbles.
After defining the phases, you move to the Phase Interaction tab. Here, you configure your drag models. For a simple flow with small, round particles, the Schiller-Naumann model is a good starting point. For other cases, like the bubble column, the Grace drag law is a better choice. Getting the drag and slip velocity correct is essential for accurate results, especially in complex simulations like a two-phase ejector considering slip velocity. Following these steps will allow you to set up a wide variety of multiphase problems, from simple bubbly flows to advanced heat transfer simulations like two-phase nanofluid forced convection or designing a nanofluid minichannel heat sink. Getting the drag and slip velocity correct is necessary for accurate results. Following these steps will allow you to set up a wide variety of problems, from basic two-phase flow regimes to advanced heat transfer simulations.
Figure 5: 3 VALIDATION examples showing the correct employment of Mixture multiphase model in ANSYS Fluent
Conclusion
The Mixture Model is a highly efficient tool for your simulation toolbox. Its main advantage is offering a perfect balance: it is much faster than running a fully separated Eulerian solver, yet it still captures the true physical effect of slip velocity between phases. This makes the approach ideal for industrial problems where phases are mixed but move at slightly different speeds, such as sedimentation and flows with light particle loading. As we have seen, setting up the model is a clear, step-by-step process. By correctly defining your phases, interactions, and drag laws, you can get accurate results without high computational costs.
Do you need help configuring your phase interactions or calculating slip velocities? You can easily order a CFD simulation project or book a CFD consultation service with our engineering team to ensure your industrial setup is perfectly optimized.