Combustion is a process that powers our world. It happens in a car engine, a power plant, or an industrial furnace. In simple words, combustion is a controlled way of burning something. A fuel, like natural gas, reacts with an oxidizer, like air. This chemical process happens very fast and creates a lot of heat and light. But engineers always have big questions. How can we make engines use less fuel? How can we make the burning process cleaner and create less pollution like NOx or soot? Building and testing real engines for every new idea is very expensive and takes a lot of time.
This is where combustion CFD simulation helps us. It is like a digital lab on a computer. We use computational fluid dynamics (CFD) to build a virtual engine or combustion chamber. This combustion modeling lets us see inside the flame without building anything. It allows us to study the reacting flow and understand it better. With a CFD combustion analysis, we can answer important questions. Is all the fuel burning? How hot does it get? Are we making bad gases? This helps us design better, cleaner, and more efficient systems.
This guide will teach you the basics of combustion CFD, from the simple physics to the models used in software like ANSYS Fluent. To see how these powerful simulations work in real projects, you can explore our detailed tutorials on Combustion CFD Simulations.
Figure 1: showcase of advanced combustion CFD simulation applications, including ramjets, RDCs, and industrial reformers, available in the CFDLAND tutorial shop.
The Physics Behind Combustion Simulation
To do a combustion analysis on a computer, we must first teach the software the basic rules of fire. The simulation needs to understand what makes a flame, how fast it burns, and the physics that control the process. This knowledge is the foundation for any good reacting flow simulation.
The Combustion Triangle in CFD Context
You may know about the fire triangle. It says you need three things for a fire: Fuel, Oxidizer, and Heat. In a combustion CFD simulation, we must define these three parts for our digital model.
- Fuel: This is what burns. In our simulation, we tell the software what the fuel is, such as methane for a furnace, hydrogen for clean energy, or liquid diesel for an engine. The type of fuel is very important for the simulation.
- Oxidizer: This is what the fuel reacts with to burn. In most cases, the oxidizer is oxygen from the air. Oxygen plays a very important role in combustion. Without enough oxygen, the burning process cannot happen, or it will be incomplete. Incomplete combustion produces harmful gases like carbon monoxide. The amount of available oxygen directly controls the speed and efficiency of the flame. Our study on the “Oxygen Effect on Combustion CFD Simulation” shows how changing the oxygen level can greatly impact the flame temperature and the amount of pollution created.
- Heat (Ignition Source): A fire needs a spark or heat to start. A simulation is the same. Combustion will not begin in the simulation unless we provide a starting heat source. We often do this by setting a small area of our model to a very high temperature, which acts like a virtual match.
Figure 2: three essential parts of combustion—Fuel, Oxidizer, and Heat—must all be defined in a CFD simulation to start and sustain a flame.
Chemical Kinetics Fundamentals
Chemical kinetics is about how fast chemical reactions happen. For a fire, it tells us the speed of the burning process. This speed is not always the same. It changes with temperature and pressure.
- Reaction Mechanisms: This is like the recipe for the fire. A simple recipe is called a global mechanism. For example: Methane + Air → Heat + Water + CO2. A more complex recipe is a detailed mechanism, which includes all the small, quick chemical steps in between.
- Arrhenius Rate: This is the rule that defines the speed of the reaction. It says that reactions get much faster when the temperature goes up. The activation energy is the minimum energy needed to start the reaction. We use the Arrhenius rate to model this effect in our finite rate chemistry simulations.
Main Governing Equations
The software uses core physics equations to predict what the flame will do. They may look complex, but their purpose is simple. They answer the most important questions about the combustion process.
- Species Transport Equation: This equation tracks all the different chemicals in the simulation. It answers the questions: Where is the fuel? Where is the oxygen? And where are the products like carbon dioxide being created? The species transport model is essential for all reacting flow simulations.
The first part shows how the amount of a chemical () changes over time.
The second part shows how the chemical is moved around by the flow of the gas (this is convection).
The third part shows how the chemical spreads out from high concentration to low concentration (this is diffusion).
The last part () is the most important for combustion. It is the source term, which shows how the chemical is created or destroyed by the chemical reactions.
- Energy Equation: This equation calculates the temperature everywhere in our model. It tracks the heat release rate from the chemical reactions and figures out how this heat moves. This helps us predict the final flame temperature.
The left side of the equation tracks how energy () changes and moves with the flow.
The first part on the right shows how heat moves through the material (this is conduction).
The second part on the right () is the heat source. For combustion, this comes from the energy released by the chemical reactions, which makes the temperature go up.
- Turbulence-Chemistry Interaction: In almost all industrial systems, the flow of gas is turbulent, not smooth. Think of how wind makes a candle flame dance and mix. The way this turbulent mixing affects the speed of the chemical reaction is called turbulence-chemistry interaction. This is a very important effect that a good combustion model must include to get accurate results, as it often controls the overall burning rate.
Figure 3: CFD solvers use fundamental governing equations to track chemical species, calculate temperature, and model the important effect of turbulent mixing.
Essential Combustion Models in CFD
Now that we understand the basic physics, we need to choose the right tool for our simulation. In ANSYS Fluent and other CFD software, there are different combustion models. Each one is a special tool designed for a specific type of fire. Choosing the right model is the most important step for getting an accurate combustion analysis. We will look at the main groups of models.
Species Transport Model
The Species Transport model is the most direct way to simulate a chemical reaction. As we learned in the last section, it solves the species transport equation for every chemical we want to track. It directly calculates where the fuel, oxidizer, and products are and how they are created or destroyed by the chemical reactions. This model is very useful when the chemical reaction speed (chemical kinetics) is the most important factor. If the reaction is relatively slow, or if you need to study a detailed chemical mechanism with just a few species, this is the right choice.
- Finite-Rate vs. Fast Chemistry: This model uses a finite-rate chemistry approach, where it calculates the reaction speed using the Arrhenius rate. This is different from assuming fast chemistry, where the reaction happens instantly as soon as fuel and oxidizer meet. This model is best when the chemistry time scale is more important than the mixing time scale.
Eddy Dissipation Models
In most industrial combustion systems, like a gas turbine or a large furnace, the flow is very turbulent. In these cases, the fuel and air react almost instantly when they meet. The real limit on the burning speed is not the chemical reaction, but how quickly the turbulent eddies can mix the fuel and air together. This is called mixing-controlled combustion.
- EDM Basics: The Eddy Dissipation Model (EDM) is the perfect tool for this situation. It assumes that the chemistry is very fast, so the overall reaction rate is controlled by the turbulence mixing time. It is a robust and widely used model for many industrial reacting flow simulations.
- EDC for Detailed Chemistry: A more advanced version is the Eddy Dissipation Concept (EDC) model. The EDC model is also for mixing-controlled combustion, but it allows you to use a detailed chemical mechanism. It is very powerful for predicting minor species that are important for pollutant formation, like NOx.
These models are very useful for many industrial problems. For example, in a complex heat recirculating combustor, the EDM helps us understand how multiple fuel injectors create a stable, efficient flame. Similarly, for unsteady problems like pulse combustion, the Eddy Dissipation model can predict the fast-paced burning inside the chamber.
Figure 4: The Eddy Dissipation Concept (EDC) model links the combustion rate to turbulent mixing, a key interaction in high-speed industrial reacting flows.
Mixture Fraction Approach
This is a very clever and efficient approach for non-premixed combustion, where fuel and air enter the combustion chamber separately and mix as they burn. Instead of solving a transport equation for every single species, this method solves a transport equation for just one variable: the mixture fraction (f).
The mixture fraction is a number that tells us the local mixture of fuel and air. A value of f=0 means there is only pure oxidizer (air), and a value of f=1 means there is only pure fuel. The software uses this one variable to look up all other properties, like temperature and species concentrations, from a pre-calculated table.
- PDF and Flamelet Models: This approach includes advanced methods to create the lookup tables. Probability Density Function (PDF) methods account for the effect of turbulence on the mixture fraction. Flamelet models are a very popular and efficient way to create the tables. They solve the detailed combustion chemistry once and map the results to the mixture fraction. This approach is excellent for simulating diffusion flames, like those in diesel engines or industrial burners.
The Flamelet model is very powerful and efficient, making it a great choice for advanced problems like studying combustion acoustics, where we need to predict both the flame and the noise it produces. It is also used in simulations involving liquid fuels, such as modeling non-premixed combustion with fuel droplets, where the mixture fraction approach correctly models the burning of the fuel vapor around the droplets.
Figure 5: The Mixture Fraction approach simplifies non-premixed combustion analysis by tracking a single variable to determine the local fuel-air ratio and flame properties, as we did in these two great examples
Types of Combustion Systems
Not all fires are the same. The way fuel and air are mixed before they burn changes everything about the flame. In combustion CFD simulation, we must understand these differences to choose the right model and set up the problem correctly. Generally, flames can be put into three main groups based on how the fuel and oxidizer are mixed.
Figure 6: Combustion available types in Fluent
Non-Premixed Combustion
This is the most common type of flame you see in everyday life. In non-premixed combustion, the fuel and the oxidizer (usually air) are not mixed together before they enter the combustion chamber. Instead, they enter separately and mix as they burn. A simple candle flame is a perfect example: the wax vapor (fuel) rises and mixes with the air around it to create a flame.
This type of flame is often called a diffusion flame because the mixing happens through diffusion. These flames are usually very stable and robust. They are used in many industrial applications like diesel engines, industrial furnaces, and some gas turbines. The Mixture Fraction and Flamelet models that we discussed in the last section are specifically designed for these types of flames and are very efficient.
Premixed Combustion
In premixed combustion, the fuel and oxidizer are completely mixed at a molecular level before they get to the flame. This perfect mixture then flows into the combustion zone where it burns. Think of the fuel system in a modern gasoline car engine, where gasoline vapor is mixed with air before the spark plug ignites it.
When this mixture burns, it creates a very thin flame front that travels through the unburned gas. Premixed systems can be very efficient and can produce very low emissions, especially when they run with extra air (lean-premixed combustion). This is why they are used in high-efficiency gas turbines and modern car engines. However, they can be less stable than non-premixed flames and can have risks like flashback, where the flame travels backward into the mixing system.
Figure 7: A comparison of flame characteristics, showing the distinct structures of a stable non-premixed (diffusion) flame versus a propagating premixed flame front
Partially Premixed Combustion
As the name says, partially premixed combustion is a combination of the other two types. In these systems, some of the fuel and air are premixed, but there is also a separate stream of fuel or air that is injected into the flame. This approach combines the benefits of both systems. It has the good stability of a non-premixed flame and the high efficiency and low emissions of a premixed flame. This makes it a very attractive option for modern combustor design, such as in direct-injection gasoline engines and advanced, low-emission gas turbines. Simulating these flames is more complex because the CFD model must be able to handle both premixed and non-premixed burning at the same time.
Figure 8: An example of a partially premixed flame, which combines features of both premixed and non-premixed systems to enhance stability and control emissions.
World of Applications: Combustion in Action
The principles of combustion and the CFD models we have discussed are not just theory. They are used to design and analyze some of the most important and advanced technologies today. From making essential chemicals to exploring space, combustion is a key process that engineers study and optimize every day.
Propulsion and Aerospace
Propulsion is one of the biggest applications of combustion. This includes everything from the internal combustion engines in cars to powerful rocket engines.
- High-Altitude Rocket Engines: These engines are essential for space exploration and putting satellites in orbit. They must work perfectly in the vacuum of space where there is no air. CFD helps engineers design nozzles and combustion chambers that are efficient at very high altitudes.
- Ramjet Engines: For flight at very high speeds (supersonic), engineers use special air-breathing engines called ramjets. A ramjet has no moving parts and uses its high speed to compress air for combustion. They are used in advanced missiles and aircraft.
Figure 9: Combustion CFD simulation is critical for designing aerospace propulsion, from high-altitude rocket engines for space to air-breathing ramjets for supersonic flight.
Advanced and Future Engine Concepts
Engineers are always looking for more efficient ways to use combustion. This has led to new ideas that use detonation instead of the slower burning process found in normal engines.
- Rotating Detonation Combustors (RDC): This is a next-generation engine concept. Instead of a steady flame, an RDC uses a continuous detonation wave—a supersonic explosion—that travels in a circle inside a channel. This process can be much more efficient than traditional jet engines.
- Detonation Wave Propagation: To design engines like the RDC, it is critical to understand the fundamental physics of how these supersonic flames move. CFD simulations of detonation wave propagation help researchers see how the wave behaves, which is key to making these advanced engines work.
Figure 10: Rotating Detonation Combustors (RDCs) are an advanced concept that uses a continuous, rotating detonation wave to achieve higher efficiency.
Industrial and Chemical Processes
Combustion is not only for making things move. It is also essential for industrial chemical processes that create valuable products.
- Steam Methane Reforming (SMR): This is a very important industrial process. It is the main method for producing hydrogen from natural gas (methane). It uses high temperatures from controlled combustion to drive a chemical reaction between steam and methane. The hydrogen produced is a vital chemical for many industries and is also a clean fuel.
You can find detailed examples and simulations of these advanced applications in our combustion CFD tutorials.
Conclusion and Your Next Steps
We have traveled a long way in this guide. We started with the basic physics of what happens in a fire—the “what is combustion” question. We then moved from theory to practice, exploring the essential combustion models used in software like ANSYS Fluent. We saw how engineers choose the right model, whether it is for a premixed, non-premixed, or partially-premixed system. Finally, we looked at the amazing applications of this technology, from powerful rocket engines to important industrial processes.
You now have a solid understanding of the fundamentals of combustion CFD simulation. This knowledge is the first step. The true power of CFD is that it gives us a huge amount of information to design better, cleaner, and more efficient systems. But theory is just the beginning. The next step is to use this knowledge in practice.
If you require professional assistance for your academic or industrial work, our Order Project service offers expert consultation and simulation for complex CFD projects. We are here to help you achieve your goals effectively. Thank you for following this guide. The world of combustion simulation is complex, but it is also a very rewarding field for solving real-world engineering challenges.