# Combustion

Source: https://help.sim-flow.com/documentation/panels/combustion
Last updated: 2026-10-14

## Introduction

![img 595905a7454ad](https://help.sim-flow.com/documentation/panels/images/combustion/img_595905a7454ad.png)

The **Combustion** panel defines models related to species transport and combustion.

## Reaction Mechanism

In the **Mechanism** tab, the reaction mechanism is defined. To define species transport only, the list of **Species** is required. To model combustion, except for **Species**, also **Reactions** are required. If you are interested in modeling combustion, you also have to specify **Reactions**. You can define species and reactions manually or import an entire mechanism in CHEMKIN format.

### Importing Mechanism

![img 5959062e9d904](https://help.sim-flow.com/documentation/panels/images/combustion/img_5959062e9d904.png)

The reaction mechanism in CHEMKIN format can be imported by clicking the **Import** button.

![img 5959063957866](https://help.sim-flow.com/documentation/panels/images/combustion/img_5959063957866.png)

After clicking the **Import** button, the **CHEMKIN Import** window will be displayed. In this window, two files are expected. The first file should contain the **CHEMKIN Mechanism** definition. The second file should contain the **Thermodynamic Database** with properties for each of the species used in the mechanism. The **Thermodynamic Database** file is optional. If it is not specified, properties for each of the species will be taken from the built-in database.

### Adding Species From Database

![img 59590672e8066](https://help.sim-flow.com/documentation/panels/images/combustion/img_59590672e8066.png)

If a prepared mechanism in the CHEMKIN format is not available, the species can be added from the built-in database. In the **Add Specie** field, the name of the specie should be given. While typing, all species whose names start with the text given will be displayed in the list below. By clicking the specie listed, it will be added to the model.

### Species

![img 595906a4ec165](https://help.sim-flow.com/documentation/panels/images/combustion/img_595906a4ec165.png)

In the **Species** list, all currently used species are displayed. There are two markers that can be attached to species in the mechanism:

- **Inert Specie** - 1
- **Fuel Specie** - 2

The concentration of a specie marked as **Inert** will not be calculated from a transport equation. Instead, it will be set to satisfy the constraint that the sum of concentrations of all species should be equal to 1.

Marking one of the species as **Fuel** is required by certain combustion models.

#### Editing Specie Properties

![img 595906eba3e48](https://help.sim-flow.com/documentation/panels/images/combustion/img_595906eba3e48.png)

After selecting a specie in the **Species** list and clicking the ![edit properties](https://help.sim-flow.com/documentation/panels/images/combustion/edit-properties.png) **Edit Species Properties**, a window will be displayed. In this window, the default [Janaf](https://help.sim-flow.com/documentation/panels/thermo#_thermodynamics_models) and [Sutherland](https://help.sim-flow.com/documentation/panels/thermo#_thermodynamics_models) coefficients of this specie can be changed.

### Reactions

![img 595907225b039](https://help.sim-flow.com/documentation/panels/images/combustion/img_595907225b039.png)

The **Reactions** panel lists all reactions, whether imported or manually created.

## Combustion Modeling

![img 595907628b20d](https://help.sim-flow.com/documentation/panels/images/combustion/img_595907628b20d.png)

The **Combustion** tab defines a combustion model. Optionally, the **Enable Combustion** option temporarily enables and disables combustion calculations. Disabling combustion can be beneficial during the initial stages of the simulation when the flow field is far from the final solution and calculating combustion would potentially increase the convergence.

The following inputs are available in the **Combustion Model**:

- **Infinitely Fast Chemistry**
- **Diffusion**
- **Eddy Dissipation Model**
- **Laminar**
- **Eddy Dissipation Concept**
- **Partially Stirred Reactor**

### Infinitely Fast Chemistry Model

![img 595907a348ae6](https://help.sim-flow.com/documentation/panels/images/combustion/img_595907a348ae6.png)

**Infinitely Fast Chemistry** is a simple combustion model that assumes that fuel, oxidizer and products meet simultaneously and there is sufficient time to achieve equilibrium.

**Inputs:**

- **C** - constant that controls the distribution of the heat release over time
- **Semi-Implicit** - controls whether the reaction rate is computed explicitly or in a semi-implicit manner

### Diffusion Combustion Model

![img 595907ebeac90](https://help.sim-flow.com/documentation/panels/images/combustion/img_595907ebeac90.png)

**Diffusion** assumes that fuel and air are mixing while burning as the fire propagates and increases in size. The model is also based on a single-step combustion reaction, similarly to the **Infinitely Fast Chemistry** model.

**Inputs:**

- **C** - constant that controls the distribution of the heat release over time
- **Semi-Implicit** - controls whether the reaction rate is computed explicitly or in a semi-implicit manner

### Eddy Dissipation Model (EDM)

![combustion model edm](https://help.sim-flow.com/documentation/panels/images/combustion/combustion-model-edm.png)

**Eddy Dissipation Model** computes the rate of reaction under the assumption that chemical kinetics are fast compared to the rate at which reactants are mixed by turbulent eddies. The reaction rate is determined by the smallest of turbulent and diffusion time scales. The turbulent time scale is calculated from:

\(\tau_{turb} = \frac{1}{C_{EDC}}\frac{k}{\epsilon}\)

**Inputs:**

- **\(C_{EDM}\)** - model constant
- **\(C_{d}\)** - diffusivity constant
- **Semi-Implicit** - controls whether the reaction rate is computed explicitly or in a semi-implicit manner

### Laminar Model

![img 5959093a3b937](https://help.sim-flow.com/documentation/panels/images/combustion/img_5959093a3b937.png)

The **Laminar** model computes reaction rates directly from local thermodynamic conditions (specie concentration, temperature, pressure). It assumes no influence of turbulence on the reaction rate.

**Inputs:**

- **Integrate Reaction Rate** - specifies whether to use instantaneous or integrated reaction rates

### Eddy Dissipation Concept (EDC) turbulent combustion model

![combustion model edc](https://help.sim-flow.com/documentation/panels/images/combustion/combustion-model-edc.png)

This model considers that the reaction occurs in the regions of the flow where the dissipation of turbulence kinetic energy takes place (fine structures). The mass fraction of the fine structures and the mean residence time are provided by an energy cascade model.

There are many versions and developments of the EDC model, 4 of which are currently supported in this implementation: v1981, v1996, v2005 and v2016. The model variant is selected using the **Version** option, e.g. v2016.

### Partially Stirred Reactor Model (PaSR)

![img 5959097e6caf1](https://help.sim-flow.com/documentation/panels/images/combustion/img_5959097e6caf1.png)

The **Partially Stirred Reactor** model calculates the reaction rate based on both turbulence and chemistry time scales. It additionally allows for scaling the turbulent mixing time scale.

**Inputs:**

- **\(C_{mix}\)** - coefficient for scaling turbulent time scale
- **Turbulent Reaction** - include turbulent effects
- **Integrate Reaction Rate** - specifies whether to use instantaneous or integrated reaction rates

## Chemistry

![img 595909b4d3e7d](https://help.sim-flow.com/documentation/panels/images/combustion/img_595909b4d3e7d.png)

In the **Chemistry** tab, you can define how chemical reactions are calculated. You have various options to control the computational process:

- **Enable Chemistry** - enables or disables chemistry computation
- **\(\Delta t_0 [s]\)** - initial time step for chemistry integration
- **\(T_{react} [K]\)** - temperature below which chemistry is not computed
- **Chemistry Solver** - method for integrating reaction rates
- **Enable Reduction** - enables reaction mechanism reduction to reduce computational cost
- **Enable Tabulation** - enables tabulation of already computed reactions

### Chemistry Solver

![img 595909fc2d5c1](https://help.sim-flow.com/documentation/panels/images/combustion/img_595909fc2d5c1.png)

The **Chemistry Solver** option allows you to define which technique should be used to integrate chemistry. Available chemistry solvers:

- **ODE**
- **Implicit Euler**

#### ODE Solver

![img 59590a493cf4d](https://help.sim-flow.com/documentation/panels/images/combustion/img_59590a493cf4d.png)

The **ODE** solver uses an ordinary differential equation (ODE) solver to integrate chemistry.

**Inputs:**

- **Solver** - ODE solver for chemistry integration

**Available ODE solvers:**

- **SEULEUX**
- **RODAS** - \(3^{rd}\) and \(4^{th}\) order
- **Rosenbrock** - \(2^{nd}\), \(3^{rd}\) and \(4^{th}\) order
- **Cash-Karp Runge-Kutta**
- **Dormand-Prince Runge-Kutta**
- **Runge-Kutta-Fehlberg**
- **Euler**
- **Euler Semi-Implicit**
- **Trapezoid**

#### Implicit Euler

![img 59590a56e4c5f](https://help.sim-flow.com/documentation/panels/images/combustion/img_59590a56e4c5f.png)

The **Implicit Euler** uses the implicit Euler technique to integrate chemistry.

**Inputs:**

- **\(C_{\tau,chem}\)** - chemistry time scale
- **Equilibrium Limiter** - enables equilibrium rate limiter

### Mechanism Reduction

![img 59590ae1f2a2e](https://help.sim-flow.com/documentation/panels/images/combustion/img_59590ae1f2a2e.png)

The mechanism reduction option uses, instead of a full reaction mechanism, reduced mechanisms that are valid for the local thermochemical conditions.

**Inputs:**

- **Method** - mechanism reduction method
- **Tolerance** - tolerance for the mechanism reduction algorithm

**Mechanism Reduction Methods:**

- **DAC** - dynamic adaptive chemistry
- **DRG** - directed relation graph
- **DRGEP** - directed relation graph with error propagation
- **PFA** - path flux analysis
- **EFA** - element flux analysis

### Chemistry Tabulation

![img 59590b187e3b2](https://help.sim-flow.com/documentation/panels/images/combustion/img_59590b187e3b2.png)

The chemistry tabulation technique intends to reuse computationally demanding results, e.g. the integration of large and stiff ODE systems, by storing those results and all the data needed to retrieve them.

**Inputs:**

- **Method** - tabulation method, currently only in-situ adaptive tabulation (ISAT) is available
- **Tolerance** - tolerance for retrieving data from the table instead of computing them directly
- **Species** - scale factor for species
- **Temperature** - scale factor for temperature
- **Pressure** - scale factor for pressure
- **Time Step** - scale factor for time step