Discover how model phase transitions in multiphase CFD using Ansys Fluent to dynamically select DPM, VOF, or EWF— ensuring accurate, efficient simulation of complex liquid–gas interactions in industrial applications.
Challenge
Model transitions in multiphase CFD address the challenges of computational cost, model validity, and physical coupling between dispersed and continuous phases. Fully resolving thin liquid films with the Volume of Fluid (VOF) method is computationally expensive, while the Eulerian Wall Film (EWF) model becomes invalid when the film grows too thick. The Discrete Phase Model (DPM) also fails to represent film coalescence once droplets merge. The interaction between droplets and liquid films is common industrial applications such as spray cooling, condensation in heat exchangers, engine wall wetting, chemical coating processes, pipe and duct mist flows, and HVAC humidification systems.
Engineering Solution
By enabling transitions between DPM, VOF, and EWF, the Ansys Fluent solver automatically applies the most suitable model for local conditions—tracking droplets with DPM, resolving thicker films with VOF, and modeling thin wall films with EWF. This ensures efficient, accurate simulation of droplet impingement, film growth, and accumulation, capturing realistic physics while keeping computational demands manageable.
Model transitions in CFD enable accurate and efficient simulation of complex multiphase phenomena such as droplet impact, film formation, and accumulation. They allow the solver to dynamically switch between DPM, VOF, and EWF frameworks depending on local conditions.
Model transitions balance accuracy and computational cost:
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DPM efficiently tracks dispersed droplets.
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VOF resolves thicker, continuous films.
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EWF models very thin wall films as 2D surface flows.
Without transitions, a single model cannot efficiently capture both dispersed and continuous phases.
When dispersed droplets (DPM) coalesce or impact a surface, they may form a continuous liquid film. Once local accumulation exceeds a threshold, the solver converts droplets to a resolved VOF phase.
This transition captures the onset of a continuous film with realistic surface tension and flow dynamics.
VOF accurately represents thick liquid films but becomes costly when the film is very thin. When the film thickness drops below a defined limit, it is transferred to the Eulerian Wall Film (EWF) model.
EWF treats the film as a thin, wall-bound layer governed by surface transport equations, reducing computational effort. If the film thickens again, it can revert to VOF.
We demonstrate the potential of model transitions in a U-shaped pipe with air carrying fine water droplets. The VOF model is implemented using two phases: air and water. The DPM and EWF phases are defined using the same material as the secondary VOF phase. We also need to define the criteria for transitioning from one model to another, as shown in the screenshots below:
After setting up the model and running the simulation in transient model, with DPM enabled and time steps of 0.0001 s, we observe important trends:
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Droplets (DPM) impact walls as it turns, and form a thin film (EWF), with a size up to 0.5mm (shown in red).
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As the film grows thicker in the bottom of the U-shaped tube, it transitions to a resolved VOF region.
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Droplets continue to deposit and can be absorbed by both EWF on the walls and growing VOF films accumulating at the bottom.
Using animations with scenes to post-process the results from the runs, we can observe the dynamics of the formation of a water film around the walls (using a contour on the walls of the pipe), and the thicker VOF film at the bottom of the pipe (colored in magenta), with the DPM particles interacting with both.
This hybrid modeling approach captures the full sequence — droplet impingement → film growth → liquid accumulation — with physical fidelity and reduced computational cost. Other designs, such as mist separators can also be modeled with this approach, as shown in detail in Ansys Learning Hub. Additionally, heat transfer and phase change can be integrated depending on modeling requirements.
A video describing the key steps in setting up the model and post-processing is shown below:
In this next video, we demonstrate a way to extract information from DPM particles in a transient simulation, such as their total mass in the domain, or their average velocity, using user-defined functions (UDFs) and report definitions:
Benefits
The dynamic model transitions enabled in Ansys Fluent deliver significant advantages for industrial multiphase flows involving droplets, films, and liquid accumulation:
- Predictive Insights: Simulations reveal how changing flow or temperature affects film growth and uniformity.
- Reduced Experimentation: Model transitions reduce trial-and-error by highlighting sensitivity to key process variables.
- Process Optimization: Enables rapid identification of optimal conditions for uniform films and effective drainage.
- Scalability: Automated transitions scale seamlessly from lab setups to full-scale industrial systems.
These transitions allow for precise, efficient multiphase simulations—improving designs and validating processes with reduced resource use.
Ozen Engineering Expertise
Ozen Engineering Inc. leverages its extensive consulting expertise in CFD, FEA, optics, photonics, and electromagnetic simulations to achieve exceptional results across various engineering projects, addressing complex challenges.
We offer support, mentoring, and consulting services to enhance the performance and reliability. Trust our proven track record to accelerate projects, optimize performance, and deliver high-quality, cost-effective results. For more information, please visit https://ozeninc.com.
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