Combustion Simulation in ANSYS Fluent: A Complete CFD Guide

Combustion is a process that powers our world. It happens in a car engine, a power plant, or a 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. It 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 soot? Building and testing real engines for every new idea is very expensive. It also takes a lot of time.

This is where a simulation lab helps us. We use combustion CFD to build a virtual engine or chamber. This combustion simulation lets us see inside the flame without building anything real. 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 shows you the basics of Ansys combustion, from the simple physics to the software settings. To see how these powerful tools work in real projects, you can check our Combustion CFD Simulation Tutorials.

Figure 1: Showcase of advanced combustion simulation applications, including ramjets, RDCs, and industrial reformers.

The Physics Behind Combustion Simulation

To run a combustion simulation on a computer, we must first teach the software the basic rules of fire. The solver needs to understand what makes a flame, how fast it burns, and the physics that control the process. This knowledge builds the foundation for any accurate reacting flow study.

The Combustion Triangle in CFD Context

You probably know the fire triangle. It states that you need three things for a fire: fuel, an oxidizer, and heat. In a CFD combustion setup, we must carefully define these three parts for our model.

• Fuel: This is what burns. We tell the software the fuel type, like methane for a furnace or liquid diesel for an engine. Choosing the correct fuel is very important for the final results.
• Oxidizer: This is what the fuel reacts with to burn. Usually, the oxidizer is oxygen from the air. Oxygen plays a major role in the reaction. Without enough oxygen, the burning process stops or becomes incomplete. Incomplete burning produces harmful gases like carbon monoxide. The amount of available oxygen directly controls the speed and heat of the flame. Our tutorial on the Oxygen Effect on Combustion CFD Simulation shows how changing the oxygen level can greatly alter the flame temperature and pollution levels.
• Heat (Ignition Source): A fire needs a spark or heat to start. A combustion simulation is exactly the same. The reaction will not begin 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.

The Chemistry of Combustion

Combustion is a type of redox reaction, where electrons transfer between substances. These reactions are key to processes like photosynthesis, respiration, rusting, and burning. Here are some key terms related to combustion:

• Oxidizing agent: Accepts electrons and causes other substances to lose electrons.
• Reducing agent: Donates electrons and causes other substances to gain them.
• Fuel: A reducing agent, often hydrocarbons like gasoline or natural gas. Some metals or reactive elements can also be fuels.
• Emissions: Byproducts of combustion. While heat and light are the main goals, controlling emissions is a big focus of combustion research.
• Hydrocarbon: Molecules made of hydrogen and carbon, often found in fossil fuels like coal and oil.
• Carbon oxides: CO and CO₂ — the most common gases released from burning carbon-based fuels.
• Nitrogen oxides (NOx): Harmful gases formed during combustion in air; major pollutants.
• Flame: The visible part of combustion, where heat excites electrons, releasing light.
• Catalyst: Speeds up reactions, lowers the required temperature, and helps reduce emissions.
• Pyrolysis: Heat-driven breakdown of fuels into gases before combustion, without needing oxygen.

Figure 3: some key terms used to describe the chemistry of combustion

Chemical Kinetics Fundamentals

Chemical kinetics explains how fast chemical reactions happen. For a fire, it tells us the speed of the burning process. This speed is never constant because it changes with temperature and pressure.

• Reaction Mechanisms: This acts 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. This detailed version includes all the small, quick chemical steps in between the start and end of the fire.
• Arrhenius Rate: This is the rule that defines the reaction speed. It states that reactions happen 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 temperature effect in our chemistry setups.

• Chemical Equations: This is the chemical recipe for the fire. In a simulation, we write these as chemical equations to quantify the heat release. For example, the basic combustion of methane (natural gas) is written as: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat For larger hydrocarbons, the recipes get bigger. Propane (C₃H₈) or the octane in gasoline (C₈H₁₈) require much more oxygen to burn completely. Detailed mechanisms in CFD track all the tiny, intermediate steps of these equations to accurately predict the final temperature.

Figure 4:  Overall formulation of chemical equations for combustion

Main Governing Equations

The software uses core physics equations to predict what the flame will do. They answer the most important questions about the combustion process.

Species Transport Equation:

This equation tracks all the different chemicals. It finds where the fuel and oxygen are, and where products like carbon dioxide form.

• The first part shows how the mass fraction of a chemical (Yi) changes over time.
• The second part shows how the gas flow moves the chemical through convection.
• The third part shows how the chemical spreads out through diffusion.
• The last part (Ri) is the most important for combustion. It is the source term, showing how chemical reactions create or destroy the chemical.

\frac{\partial(\rho Y_i)}{\partial t} + \nabla \cdot (\rho \vec{v} Y_i) = - \nabla \cdot \vec{J}_i + R_i

Energy Equation:

This equation calculates the temperature everywhere in our model. It tracks the heat released by the chemical reactions and figures out how this heat moves.

• The left side tracks how energy (E) changes and moves with the flow.
• The first part on the right shows how heat moves through conduction.
• The second part on the right (Sh) is the heat source. For combustion, this comes from the energy released by the reactions, which makes the temperature rise.

\frac{\partial(\rho E)}{\partial t} + \nabla \cdot (\vec{v}(\rho E + p)) = \nabla \cdot (k_{eff} \nabla T) + S_h

Turbulence-Chemistry Interaction:

 In most industrial systems, the gas flow is turbulent. Think of how wind makes a candle flame dance. This turbulent mixing greatly affects the speed of the chemical reaction. A good combustion simulation must include this interaction to predict accurate burning rates.

Figure 5: CFD solvers use fundamental governing equations to track chemical species, calculate temperature, and model the important effect of turbulent mixing.

What Is Combustion? Complete and Incomplete Combustion

Before we pick a software setting, we must answer a basic question: what is combustion? The simplest combustion definition is that it is a high-temperature chemical process where a fuel reacts with an oxidizer to release heat and light. This reaction powers everything from car engines to industrial furnaces. However, the products of this fire depend heavily on how much oxygen is available. We divide this into two main types.

Complete Combustion

Complete combustion happens when a fuel burns with plenty of oxygen. Because there is a rich supply of air, the fuel breaks down entirely. This perfect reaction produces only carbon dioxide (CO2) and water vapor (H2O), along with a large amount of heat. Engineers usually want to achieve complete combustion because it gives the maximum energy output and creates no harmful soot.

Fuel+O2→CO2+H2O+heat

For example, the combustion of methane (a hydrocarbon) in oxygen is a classic complete combustion reaction.

Incomplete Combustion

On the other hand, incomplete combustion happens when there is not enough oxygen to burn all the fuel. The fire still burns, but the reaction cannot finish properly. Instead of just clean water and carbon dioxide, incomplete combustion produces dangerous byproducts. It creates toxic carbon monoxide (CO) gas, unburned carbon particles known as soot, and less heat. This dirty burning lowers the energy efficiency of the system and harms the environment. Preventing these bad emissions is a major reason why engineers use combustion simulation tools to test and improve their designs.

Combustion reaction example:

CH₄ + O₂ → CO + C + H₂O + less heat

Figure 6: A combustion reaction diagram. Complete combustion produces CO2 and H2O. Incomplete combustion produces CO and soot. 

Factors Affecting Combustion Efficiency

To achieve complete combustion and avoid toxic soot, engineers use combustion CFD to balance several critical factors:

• Fuel-to-Oxygen Ratio: The most important rule. You must provide the exact amount of air required by the chemical equation.
• Mixing Quality: How well the turbulence blends the fuel and oxidizer together before they burn.
• Temperature & Pressure: High pressure in an engine cylinder speeds up the reaction, while keeping the temperature stable prevents the formation of NOx pollutants.
• Fuel Composition: Different fuels (like hydrogen vs. heavy diesel) require different ignition energies and produce different emissions.

Figure 7: Fuel composition has direct affect on efficiency

Choosing How to Model the Reaction

Now that we understand the basic physics, we need to choose the right tool for our simulation. In CFD software, there are different mathematical approaches to solve the reacting flow equations. Each approach acts as a special tool designed for a specific type of fire. Choosing the correct approach is the most important step for getting an accurate result.

To get the best outcome in your Ansys fluent combustion modeling, you should match the software settings to the real-world physics of your flame. We will look at the main groups of these tools below. If you want a step-by-step comparison of these specific settings, please read our guide on advanced combustion models in ANSYS Fluent. Below, we will look at the main approaches.

Species Transport Model

The species transport setup is the most direct way to simulate a chemical reaction. As we learned earlier, it solves a transport equation for every chemical we want to track. It directly calculates where the fuel, oxidizer, and products are. It also tracks how they are created or destroyed by the reactions. This approach is very useful when the reaction speed is the most important factor. If the fire burns slowly, or if you need to study a detailed chemical recipe with just a few species, this is the right choice.

Eddy Dissipation Approach (EDM/EDC)

In most industrial systems, like a gas turbine or a large furnace, the gas 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 itself. Instead, it is how quickly the turbulent wind can mix the fuel and air together. This is called mixing-controlled combustion.

• EDM Basics: The Eddy Dissipation setup is the perfect tool for this situation. It assumes that the chemistry is very fast. Therefore, the overall reaction rate is controlled by the turbulence mixing time. It is a popular choice for many industrial CFD combustion studies.
• EDC for Detailed Chemistry: A more advanced version is the Eddy Dissipation Concept (EDC). This concept is also for mixing-controlled fires, but it allows you to use a detailed chemical recipe. It is very powerful for predicting minor gases that form during the burn.

These setups are very useful for many industrial problems. For example, in a complex heat recirculating combustor, this approach helps us understand how multiple fuel injectors create a stable flame. Similarly, for unsteady problems like pulse combustion, it can predict the fast-paced burning inside the chamber.

Figure 8: 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 fast method for non-premixed fires, where fuel and air enter the chamber separately. Instead of tracking every single chemical, this method solves an equation for just one variable: the mixture fraction. The mixture fraction is a simple number that tells us the local ratio of fuel and air. A value of zero means there is only pure air. A value of one means there is only pure fuel. The software uses this one number to look up all other properties, like temperature and gas types, from a pre-calculated table.

• PDF and Flamelet Methods: This approach includes advanced ways to create those lookup tables. Probability Density Function (PDF) methods account for the effect of turbulence on the mixture. Flamelet methods are also a very popular way to build the tables. They solve the complex chemistry once and save the results. This approach is excellent for simulating diffusion flames, like those in diesel engines.

The Flamelet method is very powerful. It makes a great choice for advanced problems like studying combustion acoustics, where we need to predict both the flame and the noise it makes. It is also used in simulations involving liquid fuels, such as modeling non-premixed combustion with fuel droplets, where this method correctly models the burning fuel vapor.

Figure 9: 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 mix before they burn changes the flame entirely. In a combustion simulation, we must understand these differences to choose the right model. Generally, flames fall into three main groups based on this mixing.

Figure 10: Combustion available types in Fluent.

Non-Premixed Combustion

This is the most common flame you see every day. The fuel and air do not mix before entering the chamber. Instead, they enter separately and mix as they burn. A candle flame is a perfect example. The wax vapor rises and mixes with the air to create a flame. We often call this a diffusion flame. These flames are very stable and robust. Engineers use them heavily in diesel engines and industrial furnaces.

Premixed Combustion

In this system, the fuel and oxidizer mix completely before they reach the flame. This perfect mixture then flows into the burning zone. Think of a modern car engine, where gasoline vapor mixes with air before the spark plug fires. When this mixture burns, it creates a thin flame front that travels through the unburned gas. Premixed systems are highly efficient and produce very low emissions. However, they can be less stable. They carry risks like flashback, where the flame travels backward into the pipe.

Figure 11: A comparison of flame characteristics between diffusion and premixed flames.

Partially Premixed Combustion

As the name suggests, this is a combination of the other two types. Some fuel and air are premixed, but another separate stream is injected directly into the flame. This combines the best of both worlds. It provides the stability of a diffusion flame and the high efficiency of a premixed flame. Modern combustor designs, like low-emission gas turbines, use this approach.

Figure 12: An example of a partially premixed flame.

Special and Rare Combustion Behaviors

While most CFD simulations focus on premixed or diffusion flames, engineers also study special or extreme burning behaviors:

• Spontaneous Combustion: This happens when a material (like oily rags or compost) slowly oxidizes and builds up internal heat until it ignites without any external spark.
• Smoldering (Slow Combustion): A low-temperature, flameless burn on the surface of a solid, like glowing charcoal or burning wood embers.
• Explosive & Rapid Combustion: Extremely high-speed reactions that cause a rapid expansion of gases and shockwaves. This is seen in engine detonations or fireworks.
• Flameless Combustion: A highly advanced industrial setup where the fuel and air mix and burn uniformly without a visible flame. This is used in modern power plants because it provides highly uniform heat and drastically lowers NOx emissions.

Figure 13- a) Spontaneous combustion can lead to dangerous fires b) Bomb explosion is kind of explosive combustion  C) Smoldering combustion: (Left) smoldering embers and ash residue, (Right) Cross-section of a polyurethane slab smoldered in microgravity conditions. d) Contours of temperature for flame (a) and flameless (b) conditions with the relative experimental images

World of Applications: Combustion in Action

The rules and models we have discussed are not just theory. Engineers use them to design the most advanced technologies in the world today.

Figure 14: Diverse Applications of Combustion in Daily Life and Industry

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 15:Designing aerospace propulsion, with high-altitude rocket engines CFD simulation for space and air-breathing ramjets engines CFD simulation  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. For instance, you can check our performed rotating detonation combustor CFD Simulation.
• 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 16: 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 get the steam methane reforming CFD simulation files.

 

Conclusion and Your Next Steps

We have traveled a long way in this guide. We started with the basic physics of a flame. We then explored the essential tools used in software like ANSYS Fluent. We saw how to choose the right approach for premixed, non-premixed, or partially premixed systems. Finally, we looked at amazing applications, from rockets to clean energy processes.

You now have a solid understanding of combustion simulation. This knowledge is just the first step. The true power of a combustion CFD analysis is that it gives you the exact data you need to design better, cleaner systems. If you need professional help for a complex design, our Order a CFD Project service provides expert modeling and analysis. You can also book our CFD Consultation Service to get one-on-one guidance from our engineering team.