Combustion Modeling in OpenFOAM

Advantages of Combustion Model in OpenFoam

Aspect	OpenFOAM (Open-Source)	Commercial Solvers (Fluent, STAR-CCM+, etc.)
Cost	Free, open-source → no license fees	Expensive license costs (lakhs/year in India)
Customiz Posts ation	Full access to source code → can modify/add new combustion models, chemistry, or turbulence closures	Limited customization; extensions need UDFs (Fluent) or scripting (STAR), but not core model changes
Combustion Models	Wide choice: finite-rate, EDM, EDC, flamelet, partially premixed, thickened flame (LES), soot, radiation	Also many models, but some advanced ones (like research flamelets or custom kinetics) require extra licenses
Flexibility	Can integrate external chemistry packages (e.g., Cantera, CHEMKIN, OpenSMOKE++) into OpenFOAM	Integration possible, but more restricted and often requires paid add-ons
Transparency	Open equations & numerics → easier to validate, debug, and publish research	Proprietary → black-box algorithms, difficult to check or modify numerical methods
Scalability	Excellent parallelization for HPC clusters; widely used in academia & research	Scales well too, but parallel license costs can be high
Community & Research	Huge academic community developing combustion models (fireFoam, XiFoam, sprayFoam) → cutting-edge research	Industry-focused; strong support but slower adoption of new academic models
Learning Curve	Steeper — requires knowledge of coding (C++), setup via dictionaries	Easier GUI, faster setup for engineers in industry
Validation & Industrial Use	Best for R&D, new model development, customized combustion physics	Better for production/industrial validation, certified support, ready-made workflows

Combustion CFD Models in OpenFOAM

• Computational Fluid Dynamics (CFD) plays a crucial role in understanding and optimizing combustion processes across various industries, including power generation, automotive engines, and industrial furnaces.
• OpenFOAM, a widely used open-source CFD software, provides multiple combustion models to simulate different types of reacting flows. This article explores the key combustion models available in OpenFOAM and their applications.

 

Combustion Model	Description / Approach	Applications	Advantages	Limitations
Finite-Rate Chemistry	Directly solves detailed chemical kinetics using Arrhenius laws	Ignition studies, laminar flames, flame structure analysis	High accuracy, captures kinetics	Very expensive, stiff equations, impractical for large domains
Eddy Dissipation Model (EDM)	Assumes reaction rate limited by turbulence mixing (mixing-controlled)	Boilers, furnaces, gas burners	Simple, fast, stable	Ignores detailed chemical kinetics
Eddy Dissipation Concept (EDC)	Combines EDM with finite-rate kinetics in fine turbulent structures	Industrial burners, IC engines	Balance of speed & accuracy	Higher cost than EDM
Partially Premixed Model	Uses mixture fraction + tabulated flamelets to model both premixed & diffusion flames	Gas turbines, dual-mode flames	Efficient, handles mixed regimes	Requires flamelet libraries
Flamelet Model	Pre-computed laminar flamelets mapped into turbulent flow	Diffusion flames, gas turbines	Captures detailed chemistry at lower cost	Limited by flamelet database accuracy
Thickened Flame Model (TFM)	Thickens flame zone artificially to resolve it on coarse LES mesh	LES of premixed flames, research	Suitable for LES, handles turbulence–flame	Needs empirical tuning
Laminar Model	Assumes laminar burning, solves full chemistry	Small lab flames, validation	Accurate for laminar flows	Not valid for turbulence
Turbulent Flame Speed Models (e.g., in XiFoam)	Models flame wrinkling & turbulent flame speed	Premixed flames, SI engines	Captures flame propagation	Limited to premixed regimes

• In OpenFOAM, the Finite Volume Method (FVM)-based Object-Oriented Approach to Modeling (FOAM) provides a robust framework for implementing various combustion models. OpenFOAM supports a range of combustion models, categorized as follows:

Understanding Combustion Modeling in OpenFOAM

• Combustion modeling in OpenFOAM is based on solving transport equations for species concentration, energy, and turbulence.
• The choice of the combustion model depends on factors such as fuel type, turbulence-chemistry interaction, and computational resources. The major categories of combustion models include:
• Non-Premixed (Diffusion) Combustion Models
• Premixed Combustion Models
• Partially Premixed Combustion Models
• Finite-Rate Chemistry Models

 

 

Non-Premixed (Diffusion) Combustion Models

Non-premixed combustion occurs when fuel and oxidizer mix at the reaction zone, commonly seen in diesel engines and industrial furnaces. OpenFOAM provides the following models:

• Eddy Dissipation Model (EDM):
  • Assumes that turbulence dominates the reaction rate, making it computationally efficient but less accurate for detailed chemistry.
• Flamelet Model: Uses precomputed flamelet libraries to model the combustion process without solving complex chemistry.
• Conditional Moment Closure (CMC): A more advanced approach for handling turbulence-chemistry interactions in diffusion flames.

Premixed Combustion Models

Premixed combustion occurs when fuel and oxidizer mix before ignition, commonly found in spark-ignition engines and gas turbines. OpenFOAM includes:

• Progress Variable Approach: Uses a transported progress variable to track flame propagation.
• G-Equation Model: A level-set method that tracks the turbulent flame front.
• Tabulated Flamelet Model: Uses precomputed flamelet data for premixed combustion.

Partially Premixed Combustion Models

Partially premixed combustion occurs when fuel and oxidizer are mixed to some extent before combustion. OpenFOAM provides:

• Partially Stirred Reactor (PaSR) Model: Combines the Eddy Dissipation Model with finite-rate chemistry for improved accuracy.
• Flamelet Generated Manifold (FGM) Model: Uses precomputed tabulated flame structures to capture combustion dynamics.

Finite-Rate ChemCreate Calculatoristry Models

For detailed combustion chemistry, OpenFOAM supports:

• Arrhenius Rate Model: Uses detailed reaction kinetics and solves transport equations for each species.
• Eulerian Multi-Phase Combustion Model: Handles combustion in multiphase flows, including solid and liquid fuels.

Specialized Combustion Models

OpenFOAM also includes models for specific applications, such as:

• Solid Fuel Combustion: Simulates biomass and coal combustion, including devolatilization and char oxidation.
• Diesel Spray Combustion: Uses spray models for fuel injection and combustion in diesel engines.

Choosing the Right Combustion Model

The selection of a combustion model depends on several factors:

Criteria	Recommended Model
High computational efficiency	Eddy Dissipation Model (EDM)
Detailed chemistry	Finite-Rate Chemistry Models
Premixed flames	G-Equation Model, Progress Variable
Non-premixed flames	Flamelet Model, CMC
Partially premixed flames	PaSR, FGM
Solid fuel combustion	Solid Fuel Combustion Model

Category	Solver	Combustion Model Options	Best Suited For	Advantages	Limitations
General Reacting Flow	reactingFoam	Finite-rate, EDM, EDC	Gaseous combustion, furnaces, boilers	Flexible, includes turbulence & heat transfer	Can be computationally expensive
Compressible Reacting Flow	rhoReactingFoam	Finite-rate, EDM, EDC	High-speed flows, shock-influenced combustion	Accounts for compressibility & density changes	More complex & expensive
Premixed Combustion	XiFoam	Turbulent flame speed, wrinkling	Gas burners, spark-ignition engines	Good for flame propagation & premixed flames	Not suitable for diffusion flames
Spray Combustion	reactingParcelFoam (new) / dieselFoam (old)	Liquid fuel injection, evaporation, finite-rate	Diesel/gasoline IC engines, spray burners	Models atomization, evaporation & ignition	Requires detailed spray & fuel data
Fire & Large-Scale Flames	fireFoam	Buoyant turbulent diffusion flame models, soot, radiation	Fire safety, accidental fire, enclosure fires	Includes radiation (P1, fvDOM), soot	Large domain → very high computational cost
Partially Premixed Combustion	Used in reactingFoam with mixture-fraction models	Flamelet, partially premixed	Gas turbines, burners with dual modes	Efficient for complex premixed + diffusion flames	Needs flamelet library / tabulation
LES Combustion	Any LES-compatible solver (reactingFoam, XiFoam)	Thickened Flame Model (TFM), flamelets	Research, LES of flames, turbulence-chemistry	Resolves turbulence-chemistry interactions	Requires fine mesh & LES expertise

Combustion Model Libraries in OpenFOAM

• In OpenFOAM, combustion models are implemented through various libraries that provide the necessary solvers, turbulence-chemistry interaction models, and thermophysical properties.
• The combustion models are primarily found within the following OpenFOAM libraries:

These libraries define different combustion models, categorized based on the type of reaction and mixing process.

Model	Approach	Applications	Pros	Cons
Finite-Rate Chemistry	Solves detailed chemical kinetics (Arrhenius laws)	Ignition, flame structure	High accuracy for kinetics	Expensive, stiff equations
Eddy Dissipation Model (EDM)	Reaction rate controlled by turbulence mixing	Boilers, furnaces, diffusion flames	Simple, fast	Ignores detailed kinetics
EDC (Eddy Dissipation Concept)	Combines finite-rate + turbulence interaction	Industrial flames, IC engines	Balance of cost & accuracy	More costly than EDM
Flamelet Models	Pre-tabulated chemistry (mixture fraction)	Turbulent diffusion flames, gas turbines	Handles detailed chemistry efficiently	Needs flamelet library prep
Thickened Flame Model (TFM)	Thickens flame zone for LES	LES premixed flames	Works on coarse LES meshes	Extra modeling assumptions

Combustion Model Base Library (combustionModels)

• Located in: src/thermophysicalModels/combustionModels
• Contains the base class for different combustion models.
• Provides key functionalities for solving reaction kinetics and species transport.

Turbulent Combustion Models (turbulentCombustionModels)

• Located in: src/thermophysicalModels/combustionModels/turbulentCombustionModels
• Includes models for turbulence-chemistry interaction.
• Examples:
  • Eddy Dissipation Model (EDM)
  • Partially Stirred Reactor (PaSR)
  • Flamelet Model
  • Conditional Moment Closure (CMC)

Finite-Rate Chemistry Models (laminarChemistryModels)

• Located in: src/thermophysicalModels/laminar
• Handles detailed reaction mechanisms.
• Uses Arrhenius rate equations to compute reaction rates.

Flame Propagation Models (XiCombustionModels)

• Located in: src/thermophysicalModels/combustionModels/XiCombustionModels
• Used for premixed and partially premixed flames.
• Implements G-Equation and Progress Variable Models.

Reacting Flow Solvers (reactingFoam and fireFoam)

• reactingFoam: General-purpose solver for combustion applications.
• fireFoam: Specialized for fire simulations and large eddy simulation (LES) combustion modeling.

Chemistry Libraries in OpenFOAM

These support detailed chemical kinetics and species transport.

a. chemistryModel

• Located in: src/thermophysicalModels/chemistryModel
• Handles finite-rate chemistry calculations.
• Supports mechanisms from Chemkin and Cantera.

b. reactingMixture

• Located in: src/thermophysicalModels/specie/mixtures/reactingMixture
• Defines thermophysical properties of reactive species.

c. solidThermo

• Located in: src/thermophysicalModels/solidThermo
• Used for solid fuel combustion modeling (biomass, coal).

---

Choosing the OpenFOAM Library

Combustion Type	Library to Use
Non-premixed (diffusion)	turbulentCombustionModels
Premixed flames	XiCombustionModels
Finite-rate chemistry	chemistryModel, laminarChemistryModels
Spray combustion	reactingFoam
Solid fuel combustion	solidThermo

Conclusion

• OpenFOAM offers a diverse set of combustion models, allowing researchers and engineers to simulate complex reacting flows with varying levels of detail.
• Whether you need a simple turbulence-controlled model or a detailed finite-rate chemistry simulation, OpenFOAM provides flexible and open-source solutions for combustion CFD.
• Understanding these models and selecting the right one for your application is key to accurate and efficient simulations.