In our first guide, A Guide to Combustion Simulation in ANSYS Fluent: From Theory to Practice, we explored the fundamental “what” and “why” of combustion modeling. We learned about the basic physics, the different types of flames, and the key concepts behind the essential combustion models. That guide gave you the theoretical foundation you need to understand the complex world of reacting flow.
Now, we move from theory to application. This second guide is a practical journey into the “how.” We will focus on the specific steps and essential settings needed to implement these models within the ANSYS Fluent interface. We will not repeat the basic theory, but instead build upon it, showing you how to translate your understanding into a successful combustion CFD simulation. This is your hands-on guide to setting up, solving, and analyzing reacting flows.
The very first and most important step in any Fluent combustion setup is choosing the correct modeling approach. This decision is made in the main Species Model dialog box, which acts as the control center for your entire simulation. Your choice here determines the equations that will be solved and the specific parameters you will need to define later. The main options you will be presented with are:
- Species Transport
- Non-Premixed Combustion
- Premixed Combustion
- Partially Premixed Combustion
- Composition PDF Transport
Each of these paths is designed for different types of combustion problems, and in the following sections, we will walk through the practical setup for the most widely used methods. We will cover everything from defining your mixture material and importing CHEMKIN mechanisms to configuring the chemistry solver and understanding turbulence-chemistry interactions.
While this guide will cover the setup process, you can see these models applied to solve complex, real-world engineering challenges in our comprehensive Combustion Tutorials.
Figure 1: The main Species Model dialog box in ANSYS Fluent, where the user selects the fundamental approach for a combustion CFD simulation.
Setting Up Species Transport and Reactions
In our first guide, we learned that the Species Transport model is the most direct way to simulate a chemical reaction. It tracks every chemical involved and is the best choice when the speed of the chemical reactions, known as the finite-rate chemistry, controls the flame. Now, we will learn how to set this model up in ANSYS Fluent.
Activating the Model and Defining Your Mixture
The first step is to activate the model by selecting Species Transport in the Species Model dialog box. Once activated, you must define your Mixture Material. This is a crucial step where you tell Fluent the “recipe” for your combustion, including all the chemicals (species) and the reactions between them.
For simple reactions, you might use a pre-defined mixture from Fluent’s database, like methane-air. However, for most detailed industrial and academic simulations, you will need to import a specialized chemical mechanism. This is often done by using the Import CHEMKIN Mechanism option. For example, in our advanced tutorials, we simulate complex scenarios like MILD combustion using a CHEMCKIN mechanism and Methane-Air combustion using the detailed GRI mechanism, which involve 177 chemical reactions. Importing a CHEMKIN file is the standard way to handle such complex chemistry.
Figure 2: visual representation of a complex CHEMKIN mechanism used in the Species Transport model to accurately simulate methane-air combustion with all intermediate reactions.
Types of Reactions: Volumetric vs. Surface
After defining the mixture, you need to tell Fluent where the reactions happen. There are two main types:
- Volumetric Reactions: These are the most common. The reactions occur in the fluid volume, like the main flame in a combustion chamber. You must enable this for almost all combustion simulations. The speed of these reactions is calculated using the Arrhenius rate equation:
This equation tells the solver that as the temperature (T) increases, the reaction rate (k) increases very quickly.
- Wall Surface Reactions: These reactions happen only on a surface or wall. This is used for special cases like catalytic converters or reformers. For very specific or novel surface kinetics, a User-Defined Function (UDF) is often required to correctly model the reaction rate. For example, in our tutorial on Methanol Steam Reforming, a User-Defined Function (UDF) defines the specific reaction kinetics happening on the catalyst wall to produce hydrogen.
Figure 3: Wall reaction CFD study using ANSYS Fluent. In many industrial reactors, the chemical reaction happens on the surface of a catalyst on the reactor walls. To model this correctly, a custom program called a Steam reforming UDF (User-Defined Function) was used.
Choosing the Right Chemistry Solver
Solving hundreds of chemical reactions is computationally very difficult, especially when some reactions are thousands of times faster than others. This is called “stiff” chemistry. Fluent provides powerful tools to handle this under the Chemistry Solver option:
- Stiff Chemistry Solver: This is Fluent’s built-in, highly efficient solver for stiff chemical systems. For most cases involving detailed chemistry, this is the recommended choice as it balances speed and accuracy very well.
- CHEMKIN-CFD Solver: This option uses the specialized Ansys CHEMKIN solver, the industry standard for large and very stiff chemistry mechanisms. If your simulation involves a very detailed mechanism (hundreds of species), this solver is often required for a stable solution.
Turbulence-Chemistry Interaction (TCI) Models
This is the perfect place to discuss Turbulence-Chemistry Interaction (TCI) models. After you activate volumetric reactions, you must tell Fluent how the turbulence in the flow will affect the chemical reaction rates. This choice is made in the Turbulence-Chemistry Interaction section of the dialog box. In Blog 1, we discussed the theory; here is how you select them in Fluent:
- Finite-Rate (No TCI): This option only calculates the Arrhenius rate and completely ignores the effects of turbulence. It is best for laminar flames or when the chemical reactions are much slower than the turbulent mixing.
- Eddy-Dissipation (EDM): This model assumes the chemistry is infinitely fast and the reaction is controlled only by how fast turbulence can mix the fuel and air.
- Finite-Rate/Eddy-Dissipation: This is a very robust and common choice. It calculates both the Arrhenius chemical rate and the turbulent mixing rate, and then uses whichever is slower as the limiting factor.
- Eddy-Dissipation Concept (EDC): This is a more advanced model that allows you to use a detailed chemical mechanism (like from a CHEMKIN file) and models its interaction with the fine-scale turbulent eddies. The EDC model is very powerful for accurately predicting pollutant formation, like NOx.
Figure 4: Decrease of a methane mass fraction influence by combustion gas mixture
Figure 5: Temperature profiles depending on the distance (in axis of tube
Setting Up Non-Premixed Combustion
In Blog 1, we learned that non-premixed combustion is used when fuel and oxidizer enter the chamber separately and mix as they burn. This is very common in industrial furnaces, diesel engines, and gas turbines. To simulate this efficiently, ANSYS Fluent uses the powerful Mixture Fraction approach. This approach is often much faster and more stable than the Species Transport model for these types of flames. This section will show you how to set up this model for your simulation.
The Mixture Fraction: One Variable to Rule Them All
Instead of solving a transport equation for every single chemical species (like CH₄, O₂, CO₂, etc.), this approach solves a transport equation for just one or two variables: the mixture fraction (f). The mixture fraction is a clever way to track the local blend of fuel and oxidizer at any point in the domain.
- A value of f = 0 means there is only pure oxidizer (e.g., air).
- A value of f = 1 means there is only pure fuel.
- Values between 0 and 1 represent a mixture of the two.
The main idea is that all other important properties, like temperature, density, and the mass fraction of species like CO₂ and H₂O, can be determined just by knowing the value of the mixture fraction. This drastically simplifies the problem.
Generating the PDF Table: A Pre-Calculated Chemistry Library
Fluent doesn’t calculate the detailed chemistry in every cell during the main simulation. Instead, it uses a pre-calculated lookup table, called the PDF (Probability Density Function) table. Before you start the main CFD simulation, you generate this table. The PDF table is a comprehensive library that contains all the thermochemical data (species concentrations, temperature, density, etc.) mapped to the mixture fraction.
During the simulation, Fluent calculates the mixture fraction in each cell and then simply looks up the corresponding values in the PDF table. This makes the simulation much faster and more stable than solving transport equations for dozens of species. The “PDF” part of the name is important because the table also accounts for the effects of turbulence on the mixture fraction, which automatically handles the turbulence-chemistry interaction for this model.
Figure 6: A sample plot of the mean temperature distribution within the chamber, calculated by the PDF flamelet model.
Setting Up the Model in Fluent
The setup process involves three main steps inside the Species Model dialog box after you select Non-Premixed Combustion.
Figure 7: Key setup tabs for the Non-Premixed Combustion model, where you define the chemistry source (e.g., Flamelet) and the composition of the fuel and oxidizer streams.
Step 1: Choose Your Chemistry Source (Chemistry Tab)
This is where you tell Fluent how to create the PDF table. You have two main choices under the State Relation dropdown list:
- Chemical Equilibrium: This is the simplest and fastest method. It assumes that the chemical reactions are infinitely fast and always in perfect equilibrium. This is a good starting point for many simulations but may not be accurate for flames with slow chemistry, like those forming NOx.
- Steady Diffusion Flamelet: This is a more advanced and accurate method. It solves the detailed chemical reactions for a simple one-dimensional flame to create a flamelet library. This library then becomes the source for your PDF table. You will need to Create Flamelet by importing a CHEMKIN mechanism, just as we did in the Species Transport model. This is the recommended approach for most industrial applications where accuracy is important. The flamelet model is powerful enough for advanced studies, such as our tutorial on Combustion Acoustics using a Non-Premixed Flamelet Model, where it accurately predicts the flame structure needed for noise analysis.
Figure 8: Combustion Acoustics, Using Broadband Noise Model and Non-Premixed Flamelet Model
Step 2: Define Your Fuel and Air Streams (Boundary Tab)
Next, you must tell Fluent exactly what is in your fuel stream and what is in your oxidizer stream. In the Boundary tab, you will define the species and temperatures for each stream.
- Boundary Species: You will specify the chemical formula for your Fuel species (e.g., ch4 for methane) and your Oxid (oxidizer) species (e.g., o2 and n2).
- Temperature: You must set the inlet Temperature for the fuel and oxidizer streams, for example, 300K for both.
- Specify Species in: Here, you define the composition of your streams using either Mass Fraction or Mole Fraction. For air, you would specify a mass fraction of approximately 0.23 for o2 and 0.77 for n2.
Step 3: The Importance of Operating Pressure
A critical setting in the Chemistry tab is the Operating Pressure. The pressure of the system has a huge impact on combustion. Higher pressures increase the density of the gas, which makes molecules closer together. This causes chemical reactions to happen much faster. In systems like internal combustion engines or gas turbines, the pressure is very high, and setting this value correctly is essential for getting an accurate result. If you are modeling a system where pressure changes significantly, you should also enable the Compressibility Effects option.
After the PDF table is generated, you are ready to set your boundary conditions (specifying where fuel and air enter) and solve the main CFD simulation.
Premixed, Partially Premixed, and Advanced Models
We have covered the practical setup for the two most common combustion modeling approaches: Species Transport and Non-Premixed Combustion. However, ANSYS Fluent offers other advanced models for specific flame types. In this section, we will briefly introduce these models and then focus on a critical part of any simulation.
Premixed Combustion
As we discussed in Blog 1, premixed combustion occurs when fuel and oxidizer are perfectly mixed before they burn. This is common in lean-premixed gas turbines and gasoline engines. For these systems, Fluent has a dedicated Premixed Combustion model.
This model does not track a mixture fraction because the fuel and air are already mixed. Instead, it tracks the progress of the flame using a progress variable.
- A value of c = 0 means the mixture is unburnt.
- A value of c = 1 means the mixture is completely burnt.
The model solves a transport equation for this progress variable to predict where the flame front is located. When you select Premixed Combustion in the Species Model dialog box, you will need to define properties of the unburnt mixture, such as the Laminar Flame Speed, which controls how fast the flame propagates.
Partially Premixed Combustion
This is a hybrid model that combines the features of both non-premixed and premixed systems. It is designed for complex flames where some parts are premixed and others burn as diffusion flames. This is common in modern direct-injection engines. For example, in our tutorial on NOx Formation in Methane Combustion Using Partially Premixed Combustion, this model is used to simulate a flame with excess air. It accurately captures the flame structure and, most importantly, identifies the high-temperature regions where thermal NOx is produced. This level of detail is critical for designing cleaner combustion systems.
Figure 9: NOx concentration predicted using the Partially Premixed model, which is essential for analyzing and reducing pollutant emissions in combustors.
The model is also essential for simulating complex, real-world hardware. In gas turbines, managing the extreme heat is just as important as the combustion itself. Our tutorial on a Gas Turbine Combustor with Effusive Cooling Holes shows how cool air is injected through the combustor walls to protect them. The combustion process inside this chamber is partially premixed, and this model is needed to correctly simulate the interaction between the hot, reacting flow and the protective film of cooling air.
Figure 10: Gas Turbine Combustor simulation, demonstrating a professional application of the Partially Premixed Combustion model to analyze the interaction between the flame and effusive cooling.
Composition PDF Transport
For the highest level of accuracy, Fluent offers the Composition PDF Transport model. This is the most computationally expensive and advanced combustion model available. In the Non-Premixed model, we used a PDF table that was based on an assumed shape for the Probability Density Function. The Composition PDF Transport model is different: it solves a full transport equation for the PDF itself. This means it doesn’t make assumptions about the turbulence-chemistry interaction and can capture complex phenomena like local flame extinction and re-ignition with very high fidelity. Because of its computational cost, this model is typically used for academic research or for detailed validation of simpler models, rather than for routine industrial design.
Conclusion
We have traveled from the fundamental theory of combustion in our first guide to the practical, step-by-step world of setting up simulations in ANSYS Fluent. We began with the Species Transport model, the most direct approach for finite-rate chemistry, and saw how to import detailed CHEMKIN mechanisms and select the appropriate Turbulence-Chemistry Interaction model. We then moved to the highly efficient Non-Premixed Combustion model, demystifying the mixture fraction concept and the process of generating PDF tables from flamelets. Finally, we explored the specialized models for Premixed and Partially Premixed systems, showing how they are used for advanced applications like predicting NOx emissions and simulating complex gas turbine combustors.
The best way to master these skills is by applying them. All the examples mentioned here, and many more covering a wide range of industrial applications, can be found in our comprehensive combustion CFD tutorials.
If your project is more complex, or you require professional assistance for your academic or industrial work, our Order Project service offers expert consultation and simulation services. We are here to help you achieve your goals effectively.