Evaporation and Condensation in ANSYS Fluent: Complete Phase Change Simulation Guide

Evaporation and Condensation in ANSYS Fluent: Complete Phase Change Simulation Guide

Phase change is a very common physical phenomenon in engineering. It happens when a substance changes its state, like water turning into steam. In Computational Fluid Dynamics (CFD), simulating these changes accurately is critical for many industries, from power plants to air conditioning systems. If you need ready-to-use project files, check our Mass Transfer CFD Simulation category. These tutorials cover all types of mass transfer. This guide focuses on Evaporation and Condensation in ANSYS Fluent.

Contents

What is Phase Change? Phase change means a material moves between solid, liquid, and gas states. The main types are:

  • Evaporation: Liquid turns into vapor (gas) when you add heat.
  • Condensation: Vapor turns into liquid when you remove heat.
  • Sublimation: Solid turns directly into gas.
  • Melting/Freezing: Solid turns to liquid, or liquid to solid.

Figure 1: The main phase change mechanisms: Evaporation, Condensation, Melting, and Freezing.

Many engineering problems involve mass transfer between phases. For example: Boiling in a kettle or nuclear reactor (Evaporation). Water droplets forming on a cold window (Condensation). Fuel sprays in engines (Evaporation). In ANSYS Fluent, you must use specific models to simulate these processes correctly.

Phase Change vs. Cavitation vs. Wet Steam

It is easy to confuse different phase change types. In ANSYS Fluent, they are modeled differently:

  1. Evaporation/Condensation: Driven by temperature changes (heat transfer). This is the focus of this blog.
  2. Cavitation: Driven by pressure changes (like in pumps). If you are interested in this, check our detailed guide on Cavitation comprehensive blog.
  3. Wet Steam: For high-speed steam flows (like turbines).

In this comprehensive guide, we will cover:

  • The Fundamentals of Phase Change in CFD.
  • How the Lee Model works in ANSYS Fluent.
  • How to set up Saturation Temperature and Latent Heat.
  • Step-by-step setup for Evaporation and Condensation.
  • Tips for solving Flashing problems.

We will also mention helpful Mass Transfer CFD Simulation Tutorials to help you practice. By the end of this post, you will know exactly how to simulate evaporation and condensation in ANSYS Fluent confidently.

Fundamentals of Evaporation and Condensation

Before setting up the simulation in ANSYS Fluent, it is important to understand the physics behind it. The Physics of Phase Change Evaporation and condensation are driven by the difference between the fluid’s temperature and its Saturation Temperature ( T_{sat} ).

  • If Liquid Temperature > T_{sat}  → Evaporation (Liquid becomes Vapor).
  • If Vapor Temperature < T_{sat}   → Condensation (Vapor becomes Liquid).

In CFD, we do not just “see” phase change. We must calculate it using mathematical equations. ANSYS Fluent uses the Conservation Equations (Eulerian or Mixture model) to track fluid flow. To simulate phase change, it adds a special term called the Mass Transfer Source Term to these equations.

Figure 2: The Eulerian Conservation Equations in ANSYS Fluent showing the mass transfer source term () in the continuity equation

Basically, the solver removes mass from one phase (like water) and adds it to the other phase (like steam) based on the mass transfer rate. When water turns into steam, it absorbs energy. This energy is called Latent Heat. In ANSYS Fluent, you do not define “Latent Heat” directly as a single number. Instead, you use Standard State Enthalpy:

  • Vapor Enthalpy must be higher than Liquid Enthalpy.
  • The solver calculates Latent Heat automatically:

L = H_{vapor} - H_{liquid}

Figure 3: How Latent Heat is calculated in ANSYS Fluent using the Standard State Enthalpy difference between vapor and liquid phases

Phase Change Models in ANSYS Fluent

In ANSYS Fluent, simulating evaporation and condensation requires choosing the right mathematical model. The software offers specific models to handle the mass transfer between liquid and vapor phases. When you open the Phase Interaction panel, you can define how mass moves from one phase to another. For evaporation and condensation, ANSYS Fluent provides three main approaches:

Figure 4: The Phase Interaction Panel in ANSYS Fluent. Here you can select different mass transfer mechanisms like Evaporation-Condensation or cavitation.

The most common and flexible option is the Lee Model. This model calculates the rate of evaporation or condensation based on how far the temperature is from the saturation temperature ( T_{sat}  ). It uses a tunable coefficient, often called the evaporation-condensation frequency, to control how quickly the phase change occurs. The Lee model is very versatile and is used in a wide range of applications. For example, it is used to simulate the complex cycle of Condensation and Evaporation in a Pulsating Heat Pipe. It can also effectively model the process in a Single Slope Solar Still, where water evaporates from heat and condenses on a cooler surface. Furthermore, it can even be adapted for problems like simulating an Isolated Bubble in Nucleate Boiling, showing its power in detailed boiling analysis.

Evaporation and Condensation in ANSYS Fluent: Complete Phase Change Simulation Guide Evaporation and Condensation in ANSYS Fluent: Complete Phase Change Simulation Guide

Figure 5: Practical applications of Lee model performed by CFDLAND experts

Another option is the Thermal Phase Change Model. This model works differently because it bases the mass transfer rate on the total heat balance at the liquid-vapor interface. It assumes that the heat transfer at the surface is the only thing driving the phase change. This makes it a very physical model for problems where heat transfer is the dominant factor.

Figure 6: Thermal phase change Panel in Fluent

For more detailed simulations, ANSYS Fluent offers the Two-Resistance Model. This advanced model considers the heat transfer resistance on both the liquid side and the vapor side of the interface. It allows you to define separate heat transfer coefficients for each phase, which provides a more accurate prediction in complex multiphysics problems. In the following sections, we will look more closely at how the Lee Model and the Thermal Phase Change Model work, as they are the most frequently used models in engineering simulations.

 

The Lee Model

The Lee Model is the most widely used phase change model in ANSYS Fluent for evaporation and condensation. It is a simplified model that is easy to set up and provides stable results for many applications. The main idea of the Lee Model is that the rate of mass transfer is directly proportional to the temperature difference between a phase and the saturation temperature ( T_{sat} ). When the liquid is hotter than , it evaporates. When the vapor is cooler than , it condenses. The governing equations of the Lee Model in ANSYS Fluent:

\frac{\partial(\rho_v \alpha)}{\partial t} + \nabla(\rho_v v \alpha) = \dot{m}_{l \to v} - \dot{m}_{v \to l}

\dot{m}_{l \to v} = \lambda_c \alpha_l \rho_l \frac{(T_l - T_{sat})}{T_{sat}}

\dot{m}_{v \to l} = \lambda_c \alpha_v \rho_v \frac{(T_v - T_{sat})}{T_{sat}}

\lambda_c = \frac{6}{d} \beta \sqrt{\frac{M}{2\pi R T_{sat}}} L \left(\frac{\rho_l}{\rho_l - \rho_g}\right)

This process is controlled by a tunable coefficient, , often called the evaporation-condensation frequency. This coefficient controls how quickly the phase change occurs.

  • A large coefficient means the phase change happens very fast to bring the temperature back to saturation.
  • A small coefficient means the phase change is slower.

The default value for this coefficient in ANSYS Fluent is 0.1. However, this is just a starting point. For accurate results, you often need to tune this value by comparing your simulation to experimental data. The recommended range for tuning is typically between 0.001 and 100. A good strategy is to start with the default and adjust it based on your results.

Figure 7: The setup panel for the Evaporation-Condensation Model in Fluent, where you can define the saturation temperature and tune the coefficients.

The main limitation of the Lee Model is that the coefficient is not based on first principles; it is a tuning parameter. This means it might not capture complex physics without proper validation. However, for most industrial CFD simulations, it provides a robust and effective way to model evaporation and condensation.

Conclusion

Simulating evaporation and condensation in ANSYS Fluent is a powerful way to solve complex engineering problems. In this guide, we covered the most important steps for a successful CFD simulation.

We started with the basic physics, explaining how saturation temperature and latent heat drive the phase change process. We then introduced the different models available in Fluent and focused on the Lee Model, which is the most common choice for these types of simulations. You learned how its equations work and how to use the tunable coefficient to get accurate results.

Are you ready to apply what you have learned? Explore our wide range of Mass Transfer CFD Simulation Tutorials to practice. If you have a specific project in mind and need expert help, our team is ready to assist. You can Order a Project from CFDLAND and get professional results.

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