Combustion CFD Simulation

What is Combustion?

Combustion, or burning is a high-temperature, exothermic redox chemical reaction between a fuel (such as hydrocarbons or hydrogen) and an oxidizing agent (typically atmospheric oxygen). This reaction releases heat and often light, making it a key process in energy production. In most cases, combustion produces gaseous products like carbon dioxide (CO₂) and water vapor (H₂O), often visible as smoke or flames. While not all combustion results in fire, the presence of a flame indicates that the reacting substances have vaporized.

Figure 1- The internal combustion on car engine

Combustion necessitates three essential elements: a fuel source, an oxidizing agent, and an initial source of heat. The fuel serves as the substance that undergoes combustion. Oxygen, acting as the oxidizer, combines with the fuel to release energy in the form of heat and light. Initially, a spark, flame, or other heat source is required to initiate the combustion process. Once ignited, the heat generated by the reaction sustains the combustion, allowing it to continue without the need for continuous external ignition. These three components—fuel, oxidizer, and heat—must be present in the correct proportions for combustion to occur effectively.

Figure 2- The fire triangle

This process powers vehicles, aircraft, rockets, heating systems, and power plants, and is a central topic in combustion science and technology. However, extensive use of hydrocarbon combustion contributes significantly to greenhouse gas emissions and climate change. That’s why combustion science is increasingly important. Scientists and engineers aim to make combustion more efficient, cleaner, and cost-effective by using better fuels, reducing emissions, and minimizing harmful byproducts.

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 Equations for Combustion

Combustion involves chemical reactions that convert fuel and oxygen into heat, light, and exhaust gases—and understanding these reactions starts with examining their chemical equations, which help us quantify energy release, evaluate fuel efficiency, and design cleaner, more effective combustion systems.

Figure 4- Overall formulation of chemical equations for combustion

1. One of the most basic combustion reactions is the burning of hydrogen. In this reaction, two hydrogen molecules react with one oxygen molecule to produce water vapor. The reaction gives off energy because oxygen molecules consist of two oxygen atoms joined by a double bond, and breaking that bond releases heat energy.

2H₂ + O₂ → 2H₂O + 286 kJ/mol of heat

1. The simplest hydrocarbon involved in combustion is methane (CH₄). Methane generates more heat per mole than hydrogen because it contains four single bonds between the carbon and hydrogen atoms. Its combustion reaction is:

CH₄ + 2O₂ → CO₂ + 2H₂O + 890 kJ/mol of heat

1. Propane (C₃H₈), a larger hydrocarbon, has two carbon–carbon bonds and eight carbon–hydrogen bonds. Its combustion reaction is:

C₃H₈ + 5O₂ → 3CO₂ + 4H₂O + 2,220 kJ/mol of heat

1. Gasoline is made of various hydrocarbons, but the main component is octane (C₈H₁₈), which has eight carbon atoms and eighteen hydrogen atoms. That results in seven carbon–carbon bonds and 18 carbon–hydrogen bonds:

2C₈H₁₈ + 25O₂ → 16CO₂ + 18H₂O + 5,483 kJ/mol of heat

A stoichiometric combustion reaction is a theoretically perfect reaction where fuel and oxygen are present in exact proportions. This leads to complete combustion, maximum heat release, and the highest possible efficiency.

Factors Affecting Combustion Efficiency

Several key factors influence how efficiently combustion occurs. By understanding and optimizing these elements, engineers can enhance the thermodynamic performance of combustion systems:

1. Fuel Composition: The chemical structure and makeup of the fuel greatly affect both the energy output and the types of emissions produced. The nature of molecular bonds determines ignition energy and heat release, while non-hydrocarbon elements influence emission types. Ongoing research focuses on improving fuel chemistry with alternatives like biofuels, synthetic gases, and renewable jet fuels.

Figure 5- Common types of fuels

1. Fuel-to-Oxygen Ratio: Efficient combustion relies on the correct proportion of fuel and oxygen. A balanced mix ensures complete combustion while avoiding undesired side reactions or excess emissions.
2. Temperature: The rate and completeness of combustion reactions depend heavily on temperature. Insufficient heat can lead to incomplete combustion, while excessive temperatures may trigger the formation of pollutants like NOx.
3. Pressure: Elevated gas pressure accelerates combustion reactions and increases heat output. This is why many combustion systems, such as engines and turbines, include a compression stage.
4. Mixing Quality: Effective combustion requires thorough mixing of fuel and oxidizer. The way these substances interact—affected by turbulence, gas speed, and flame dynamics—plays a crucial role in ensuring efficient reactions.
5. Flame Shape and Stability: The flame’s form and consistency are central to combustion efficiency, as the reaction primarily happens at the flame front. Proper heat transfer within the flame and between it and surrounding air helps maintain optimal combustion conditions.

Balancing these factors presents significant challenges for designers. For example, combustion kinetics are influenced by a complex interaction of variables such as fuel-air ratio, mixing intensity, temperature, and pressure, all of which must be carefully managed to ensure optimal performance and minimal emissions.

Types of Combustion

While the underlying principle of all combustion reactions remains the same — a chemical reaction between fuel and an oxidizer — the behavior, efficiency, heat output, and emissions vary based on how combustion occurs. Understanding the types of combustion is crucial in combustion science and technology, especially for optimizing systems like combustion chambers, combustion engines, and environmental applications.

Complete Combustion

Complete combustion occurs when a fuel burns in the presence of a sufficient amount of oxygen, resulting in the production of carbon dioxide (CO₂), water vapor (H₂O), and significant heat. This reaction is highly desirable because it maximizes energy output and minimizes pollutants such as soot or carbon monoxide.

Fuel+O2→CO2+H2O+heat

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

Incomplete Combustion

When there is an insufficient oxygen supply, incomplete combustion occurs. This reaction produces carbon monoxide (CO), soot, and sometimes unburned hydrocarbons. These byproducts not only reduce energy efficiency but also pose serious health and environmental risks.

Combustion reaction example: CH₄ + O₂ → CO + C + H₂O + less heat

Figure 6- Complete combustion vs. incomplete combustion

Spontaneous Combustion

Spontaneous combustion is a rare form where certain substances ignite without an external flame or spark. It occurs due to slow oxidation within the material that gradually builds up enough heat to surpass the ignition temperature, for instance, oily rags, phosphorus, and compost piles with bacterial fermentation This type is especially important in combustion safety analysis.

Figure 7- Spontaneous combustion can lead to dangerous fires, especially in environments

Overall, Spontaneous combustion is a critical phenomenon that can lead to dangerous fires, especially in environments where highly flammable materials are present. Understanding this process, the substances prone to it, and the regulations in place can help prevent severe accidents.

Rapid Combustion

Rapid combustion is the most familiar form — it occurs when a fuel ignites quickly in the presence of a spark or flame and releases light and heat. This is typical of flame-producing reactions such as in stoves, candles, and car engines. Such Combustion examples, used in: power generation, cooking and heating, and combustion chamber design for propulsion systems.

Figure 8- Example of rapid combustion is used in stoves for cooking.

Explosive Combustion

When the reaction speed of combustion is extremely high and causes a rapid expansion of gases, it’s known as explosive combustion. It’s often associated with detonation, leading to a shockwave, such as fireworks, gasoline explosions, and air-fuel mixture detonations in misfiring engines. Moreover, Hydrogen combustion in confined spaces can also become explosive under the right conditions.

Figure 9- Bomb explosion id kind of explosive combustion

Smoldering or Slow Combustion

Also called surface combustion, this is a low-temperature, flameless form of combustion where solid fuels oxidize slowly. It generates less heat and no visible flame. Examples are burning of charcoal, cigarette burning, and decay of organic matter. This type is often studied in fire safety and combustion theory and modelling for fire-prone environments.

Figure 10- Smoldering combustion: (Left) smoldering embers and ash residue, (Right) Cross-section of a polyurethane slab 125 mm in diameter smoldered in microgravity conditions.

Flameless Combustion

In flameless combustion, the fuel-air mixture burns uniformly throughout the reaction zone without visible flames. It occurs under specific conditions — typically at lower temperatures and within porous materials or controlled reactors.

Benefits:

• Lower NOₓ emissions
• Higher efficiency
• Uniform heat distribution

Industrial relevance:

• Power plants
• Waste incineration
• Combustion CFD simulation models to reduce hot spots and pollutant formation

Figure 11- Contours of temperature for flame (a) and flameless (b) conditions with the relative experimental images

Why Classifying Combustion Types Matters

Classifying combustion types is not just an academic exercise — it directly informs how we design combustion systems, predict emissions, and simulate combustion in platforms like ANSYS Fluent. Engineers and scientists must consider flame characteristics, emissions, heat transfer, and chemical behavior under various conditions. By understanding the different types of combustion, we can better apply combustion in safe, sustainable, and efficient ways — from clean energy production to advanced propulsion systems.

ANSYS Fluent Combustion Simulation

ANSYS Fluent combustion simulation enables engineers to accurately model and analyze complex combustion processes in a wide range of applications—from engines and turbines to furnaces and burners. With advanced models for reacting flows, flame dynamics, and pollutant formation, Fluent helps optimize performance, improve efficiency, and reduce emissions. Overall, there are different Species Models in ANSYS Fluent, used for defining and configuring combustion and chemical reaction simulations.

Figure 12- The Species Model dialog box allows you to set parameters related to the calculation of species transport and combustion.

Species Transport Model

ANSYS FLUENT can model the mixing and transport of chemical species by solving conservation equations describing convection, diffusion, and reaction sources for each component specie. Multiple simultaneous chemical reactions can be modeled, with reactions occurring in the bulk phase (volumetric reactions) and/or on wall or particle surfaces, and in the porous region. Species transport modeling capabilities, both with and without reactions.

Figure 13- Species transport model in ANSYS Fluent

In the settings (Fig.13), the user selects a mixture material (e.g., a predefined or custom gas mixture) and can import detailed reaction mechanisms using the CHEMKIN Mechanism option. The number of species in the system is also specified, which is critical for defining the complexity of the simulation.

For chemical reactions, the user can enable volumetric reactions (for reactions occurring in the bulk flow) or surface reactions. The Turbulence-Chemistry Interaction (TCI) section offers options like “Finite-Rate/No TCI,” which assumes no turbulence impact on chemical reactions, or “Finite-Rate/Eddy-Dissipation,” which considers turbulence effects on reaction rates. Additional settings include enabling thermal diffusion, energy diffusion, and selecting appropriate thermodynamic databases for accurate material properties. This model is ideal for non-combustive or custom-reactive flow simulations.

Non-Premixed Combustion Model

In Non-Premixed Combustion, fuel and oxidizer enter the reaction zone in distinct streams. This is in contrast to premixed systems, in which reactants are mixed at the molecular level before burning. Examples of non-premixed combustion include pulverized coal furnaces, diesel internal-combustion engines and pool fires.

Figure 14- Non-premixed combustion model in ANSYS Fluent

Fig.14 shows the settings for the non-premixed combustion model in ANSYS Fluent. This model is used when the fuel and oxidizer enter separately and mix only inside the combustion zone. In this model, users can select different state relation options to define how the chemical reactions are treated. For example, chemical equilibrium assumes that the reactions happen instantly and reach equilibrium, while options like steady diffusion flamelet provide a more detailed representation of flame structure for turbulent combustion. The energy treatment can be set to adiabatic, assuming no heat loss, or non-adiabatic, which accounts for heat transfer with the surroundings. Stream options allow the addition of secondary or empirical fuel streams for more complex flow scenarios.

Model settings include specifying the operating pressure and the fuel stream’s rich flammability limit, which defines the minimum fuel concentration needed to sustain combustion. There is also a coal calculator for coal combustion simulations. Additionally, PDF options like inlet diffusion and compressibility effects help refine the modeling of turbulence-chemistry interactions. The thermodynamic database file contains essential species properties needed for accurate combustion simulation. Overall, this panel provides detailed controls to balance simulation accuracy and computational cost for non-premixed combustion cases.

Partially Premixed Combustion

Ansys Fluent provides a Partially Premixed Combustion model that is based on the non-premixed combustion model. In other words, this is a combination of both premixed and non-premixed combustion, and used when the fuel and oxidizer are partially mixed before combustion occurs. This mode is common in turbulent flames or practical combustion systems.

Figure 15- Partially premixed combustion model in ANSYS Fluent

As shown in Fig.15, ANSYS Fluent has three types of partially premixed models, namely Chemical Equilibrium, Steady Diffusion Flamelet, and Flamelet Generated Manifold. The Chemical Equilibrium and Steady Diffusion Flamelet models assume the premixed flame front is infinitely thin, with unburnt reactants ahead and burnt products behind it. The composition of the burnt products is modeled either by assuming chemical equilibrium or by using steady laminar diffusion flamelets, which represent the flame structure more realistically. The Flamelet Generated Manifold (FGM) model assumes that the thermochemical states in a turbulent flame resemble those in a laminar flame. It parameterizes these states by mixture fraction and reaction progress, where reaction progress increases continuously from unburnt reactants to burnt products across a flame with finite thickness. This approach allows more detailed and accurate modeling of partially premixed flames, especially in turbulent conditions.

In the premixed combustion model settings, you can choose between two state relation equations: C Equation and G Equation. The C Equation is a simpler formulation used for modeling premixed flames by solving a single transport equation for a progress variable, which represents the extent of reaction. The G Equation is more advanced and solves a transport equation for the flame front location, allowing better capture of flame dynamics and propagation in complex flows.

Composition PDF Transport Model

ANSYS Fluent provides a Composition PDF Transport model for modeling finite-rate chemistry effects in turbulent flames. The composition PDF transport model, should be used when you are interested in simulating finite-rate chemical kinetic effects in turbulent reacting flows. With an appropriate chemical mechanism, kinetically-controlled species such as CO and NOx, as well as flame extinction and ignition, can be predicted. PDF transport simulations are computationally expensive, and it is recommended that you start your modeling with small meshes, and preferably in 2D.

Figure 16- Composition pdf transport model in ANSYS Fluent

ANSYS Fluent has two different discretizations of the composition PDF transport equation, namely Lagrangian and Eulerian (Fig.16). The Lagrangian method is strictly more accurate than the Eulerian method, but requires significantly longer run time to converge. The Lagrangian approach tracks individual fluid particles, while the Eulerian method solves the PDF on a fixed grid. Reaction types can be selected as Volumetric, for typical chemical reactions, or Electrochemical, for reactions involving electric charge transfer. The Chemistry Solver dropdown provides options such as the Stiff Chemistry Solver to handle complex, stiff chemical kinetics, and users can adjust solver settings via the Integration Parameters button.

In the Mixing tab, users select the mixing model that defines how species mix within the turbulent flow. The Modified Curl model is the default, but alternatives like IEM (Interaction by Exchange with the Mean) and EMST (Euclidean Minimum Spanning Tree) are also available. The Mixing Constant controls the mixing rate. Additional features include importing detailed chemical mechanisms using CHEMKIN files and specifying the thermodynamic database file path for species properties. There is also an option for Liquid Micro-Mixing to account for mixing effects in multiphase flows. Boundary species can be configured to define species at domain boundaries, providing comprehensive control over the combustion simulation.

Combustion applications

Combustion serves a wide range of functions across various applications. In most cases, the heat generated by combustion is either used to drive additional chemical reactions—such as in cooking—or to expand gases that perform mechanical work, like in internal combustion engines. Before the invention of electric lighting, combustion was the sole means of producing artificial light. Today, many traditional heat and pressure-based combustion systems are being gradually replaced by electrical alternatives.

Figure 17- Diverse Applications of Combustion in Daily Life and Industry

• Lighting: Since prehistoric times, humans have relied on combustion and flame for illumination. While modern lighting has mostly shifted to electricity, combustion is still used in oil lamps, propane torches, and candles, especially in areas without electrical access or for ambiance.
• Heating and Cooking: One of the earliest uses of combustion was for warmth and food preparation. From open wood fires to modern natural gas ovens, combustion reaction is central to domestic life. Today, many heating systems still use fuels combusted in combustion chambers or combustion tubes.
• Natural Fires: In nature, combustion appears in the form of forest fires and wildfires, often sparked by lightning. These natural phenomena play a critical ecological role by clearing dead biomass and renewing habitats.
• Internal Combustion Engines (ICEs): Internal combustion remains at the heart of vehicles and machinery. In ICEs, fuel burns inside a piston, and the resulting pressure is converted into mechanical energy. Engineers often use advanced simulation tools like ANSYS Fluent and Ansys Forte to model the complex combustion chamber behavior, optimize performance, and reduce emissions.
• Turbomachinery and Power Generation: Combustion chambers are central to gas turbines used in aircraft engines, power generation, and pumping systems. These systems convert the energy from expanding combustion gases into mechanical and electrical energy through high-speed turbines. Advanced combustion theory and modelling is essential to improve turbine performance.
• Rotating Detonation Engines (RDEs): RDEs represent an innovative form of propulsion. Instead of compressing air-fuel mixtures with pistons, they utilize a supersonic detonation wave traveling in a circular chamber. This cutting-edge technology falls under advanced combustion science and technology.
• Rocket Propulsion: In rockets, combustion is contained on one side of the combustion chamber and uncontained on the other—producing thrust. Rockets may use liquid fuel combustion (with liquid oxygen and hydrogen) or solid propellants, which involve a self-contained oxidizer and fuel blend. Hybrid models often use a solid hydrocarbon with a liquid oxidizer like nitrous oxide.
• Industrial Burners: Many industries depend on combustion for thermal energy. Industrial burners are used for melting, distillation, drying, and steam generation. The heat produced is vital for manufacturing processes and can even be used in combustion tubes for heating systems that transfer energy across distances.

Simulation of Combustion by ANSYS Fluent

ANSYS Fluent is a powerful tool in combustion science and technology, widely used for combustion CFD simulation across industries. It models combustion and flame behavior using methods like finite-rate chemistry and eddy dissipation, making it ideal for simulating combustion reactions in internal combustion engines, combustion chambers, and combustion tubes. Fluent supports complex cases such as hydrogen combustion CFD and gas turbine combustion chambers, offering accurate predictions for fuel-air mixing, ignition, and emissions.

Figure 18- Ramjet Combustion CFD: Simulating Solid Fuel Engines with ANSYS Fluent

Additionally, Fluent’s multiphase CFD simulation capabilities play a crucial role in combustion scenarios, allowing engineers to model interactions between multiple phases, such as fuel droplets or solid particles, which are common in combustion processes. This functionality enables accurate representation of phase behavior, including droplet evaporation and particle dispersion, contributing to more insightful analyses and optimized designs of combustion systems. With Fluent, engineers can efficiently simulate and optimize combustion systems, leading to improved efficiency, reduced emissions, and enhanced design performance.

CFDLAND’s Expertise in Combustion Modeling Using ANSYS Fluent

Looking to harness the power of advanced combustion modeling? At CFDLAND, we specialize in combustion CFD simulations using ANSYS Fluent, delivering high-quality, accurate results for a wide range of engineering applications.

From internal combustion engines to rocket engine design, our expertise covers all aspects of combustion chamber CFD simulation. Explore our CFD SHOP to find a wide selection of ready-to-use combustion projects, or easily outsource your custom project to our expert team for fast, reliable results.