Reacting Species Transport Workflow

Use this workflow for simulating multi-component liquid reactions or gas-phase turbulent flames which involve kinetic phenomena such as flame ignition, slowly forming pollutants, flame quenching, and extinction.

The steps in this workflow are intended to follow on from the initial steps in the Reacting Flow General Workflow.

Note	If you are modeling Reacting Channels, follow the Reacting Channels Workflow—even when using a reacting species transport model. See Reacting Channels Workflow.

For the physics continuum or phase that represents the reacting flow, select the following models—in addition to the models that are previously selected, with Auto-Select recommended models activated:


Group Box	Model
Reacting Flow Models	Reacting Species Transport
Reacting Species Models	For multi-component gas turbulent flames with a large mechanism: Complex Chemistry For multi-component liquid-liquid reactions limited by finite-rate kinetics: Complex Chemistry For multi-component liquid or gas reactions with a simple mechanism: Eddy Break-Up For multi-component liquid or gas reactions which are slow: Eddy Break-Up with the Kinetics Only property activated. For multi-component liquid reactions with a simple mechanism in which the chemical reactions are fast (molecular mixing time-scales are long): Eddy Contact Micromixing
Turbulence Chemistry Interactions (for Complex Chemistry)	For turbulent flames with premixed or partially-premixed flame fronts: Turbulent Flame Speed Closure For steady-state diffusion flames: Eddy Dissipation Concept For premixed, partially-premixed, and unsteady flames, or slow reactions: Laminar Flame Concept

Select any Optional models.
For example:
- NOx Emission or Soot Emissions—to model the formation of these pollutants.
- Fluid Film—to model intraphase or interphase reactions such as in trickle-bed reactors, or selective catalytic reduction (SCR) reactions in urea film. See Modeling Fluid Film.
- The Radiation model is useful for modeling applications in which radiative heat transfer is important, as is the case for most combustion systems. The soot emission models influence the Participating Media Radiation (DOM) and Gray Thermal Radiation models by contributing to the absorption coefficient of the continuous phase (the absorption coefficient describing both absorption and emission).
- VOF—Useful for modeling multi-phase reacting flow applications where two phases co-exist with a distinct interface, such as glass furnaces.
- Gravity—Use when gravity forces significantly influence the solution, for example in fire simulations.
If using Emissions models or Surface Chemistry, follow the specific workflow before continuing with the following steps.
See Emissions Workflow, or Surface Chemistry Workflow.
Define the parameters of the Reacting Species physics/phase model:

Note It is possible to specify several combustion model constants as parameters. See Global Parameters.
- For the Complex Chemistry model, define the properties of the Complex Chemistry model and its sub-nodes.
  1. Select the Models > Complex Chemistry node and set the required Properties.
    The CVODE solver calculates reaction rates and solves complex chemistry problems in Simcenter STAR-CCM+.
  2. Select the Complex Chemistry > Chemistry Acceleration node and, if required, activate the necessary acceleration options:
    - Clustering (activated by default)
    - Dynamic Mechanism Reduction
    - ISAT (not compatible with Dynamic Mechanism Reduction and not advised with Clustering)
    Then define the acceleration option Properties, see Chemistry Acceleration Properties.
  3. To relax species to their equilibrium compositions over a set time-scale, select the Complex Chemistry > Approximation Options node and activate Relax to Chemical Equilibrium. Select the Approximation Options > Relax to Chemical Equilibrium node and set the Properties.
  For more information, see Complex Chemistry Model Reference.
- For the Eddy Break-Up (EBU) or Eddy Contact Micromixing (ECM) model, define the EBU or ECM properties.
  - The EBU property, Reaction Control, is set to Hybrid by default—which is appropriate for most simulations. Hybrid specifies the reaction rate as the smaller of the two rates that are predicted by turbulent mixing and finite-rate chemical kinetics. However, if the rate is determined solely by the turbulent mixing scale, select Standard EBU instead.
  See Eddy Break-Up Model Reference or Eddy Contact Micromixing Model Reference.
Import or create the mechanism which represents the reacting flow reactions.
- For Complex Chemistry, import the complex chemistry definition using the Models > Complex Chemistry node. It is possible to create the mechanism using the Models > Reacting > Reactions node—however, this approach is impractical, as most Complex Chemistry definitions are too large. You can find chemical mechanisms that are developed by academia on university websites worldwide.
- For EBU or ECM, create or import reactions using the Models > Reacting > Reactions node.
See Importing Species and Reactions.
Specify the properties of the turbulence-chemistry interactions models.
- When using the Turbulent Flame Speed Closure (TFC) model, set the turbulent flame speed method (see Turbulent Flame Speed) and define the properties of the TFC model.
  The TFC model simulates thin flames which can cause rapid density changes associated with larger normal stresses. Normal stresses are by default neglected in turbulence kinetic energy, which causes unphysically high levels of turbulence which results in fuel-consumption rates that are too fast. Therefore, it is also recommended to activate the Normal Stress Term property for the turbulence model (see Normal Stress Term).
- When using the Eddy Dissipation Concept model, or the Laminar Flame Concept model, you can adjust the flame position by changing the Multi-Component Gas > Material Properties > Turbulent Schmidt Number.
Set the parameters of any other [continuum] > Models or [phase] > Models, as required.
Return to the Reacting Flow General Workflow.