Coherent Flame Model Reference

In this model, a flame area density and a progress variable on the grid are tracked through transport equations. The mean species concentrations come from the underlying combustion model—Chemical Equilibrium, Steady Laminar Flamelet, or Flamelet Generated Manifold. A mean enthalpy transport equation is also solved. The mean temperature, density, and viscosity are then calculated knowing the mean enthalpy and species concentrations.

Table 1. Coherent Flame Model Reference
Theory	See Flame Propagation.
Provided By	When using the FGM model: [physics continuum] > Models > Progress Variable Source When using the Chemical Equilibrium or SLF model: [physics continuum] > Models > Flame Propagation
Example Node Path	Continua > Physics 1 > Models > Coherent Flame Model (CFM)
Requires	Material: Multi-Component Gas Reaction Regime: Reacting Reacting Flow Models: Flamelet then either: Flamelet Models: Flamelet Generated Manifold (FGM) Flamelet Models: Chemical Equilibrium or Steady Laminar Flamelet Flame Type: Partially-Premixed Flame
Properties	Key properties are the coefficients $α$ , $β$ , $a$ , and $B$ . See Coherent Flame Model (CFM) Properties.
Activates	Model Controls (child nodes)	Laminar Flame Speed
	Initial Conditions	Flame Area Density Progress Variable See Initial Conditions.
	Boundary Inputs	Flame Area Density Progress Variable See Boundary Settings.
	Region Settings	Active Reactions Option. See Region Settings.
	Other Continuum Nodes	The Ignitors node provides the right-click option to create a Flame Area Density Ignitor. See Ignitors.
	Solvers	When using the FGM model see FGM Combustion. When using the Chemical Equilibrium or SLF model, the CFM Combustion Solver is activated. See CFM Combustion Solver Properties.
	Monitors	Fad: Flame Area Density ProgVarCFM: Progress Variable CFM
	Field Functions	Flame Area Density Progress Variable Progress Variable Variance Unnormalized Progress Variable CFM Unnormalized Progress Variable Variance See Field Functions.

Coherent Flame Model (CFM) Properties

Convection

Sets the discretization scheme that Simcenter STAR-CCM+ uses for computing the convection flux on a cell face in appropriate transport equations. More information is given in the related topic for the Convection Term:

1st-Order: First-order upwind scheme. This scheme scales the transported quantity by the upstream or downstream mass flowrate depending on flow direction. Only use when a higher-order scheme fails to give convergence, or in order to obtain an initial solution before switching to a higher-order scheme.
2nd-Order: Second-order upwind scheme. This scheme introduces linear interpolation of cell values on either side of the upstream or downstream face. Using this scheme can lead to poorer convergence properties, but gives accuracy as good as or better than the first-order scheme.

Secondary Gradients

There are two sources of secondary gradients in Simcenter STAR-CCM+ flow solvers:

boundary secondary gradients for diffusion
interior secondary gradients at cell faces

Use this property to control which gradients are included in the solver. On gives both gradients while Off excludes them. Interior Only and Boundaries Only select the corresponding gradients.

Flow Boundary Diffusion

When activated, diffusion is calculated across the flow boundary for all combustion scalars (for example, mixture fraction, mixture fraction variance, and progress variable).

The following properties are also available when using the Chemical Equilibrium model or the Steady Laminar Flamelet model.

CFM Constant, Alpha: Coefficient $α$ , from Eqn. (3562).
CFM Constant, Beta: Coefficient $β$ , from Eqn. (3562).
CFM Constant, A: Coefficient $a$ , from Eqn. (3562).
CFM Constant, B: Coefficient $B$ , from Eqn. (3569).

Laminar Flame Speed

Laminar Flame Speed

Provides options for controlling the unstrained laminar flame speed. Unavailable when the Turbulent Flame-Speed Closure (TFC) Source Option property is set to User Defined Source.

Method	Corresponding Method Node
Flamelet Table Laminar Flame Speed Available when the FGM model is used and the Reactor Type is set to 1D Premixed Freely Propagating.	Uses the laminar flame speed that is stored in the flamelet table generated by the FGM Table Generator. See FGM Table. Activates the Flamelet Table Laminar Flame Speed node.
Gulder Laminar Flame Speed	Uses the Gülder laminar flame speed correlation Eqn. (3579). Activates the Gulder Laminar Flame Speed node which allows you to select a fuel using the Fuel Name property.
Metghalchi Laminar Flame Speed	Uses the Metghalchi laminar flame speed correlation Eqn. (3573). Activates the Metghalchi Laminar Flame Speed node which allows you to select a fuel using the Fuel Name property.
Universal Laminar Flame Speed	Simcenter STAR-CCM+ identifies the best laminar flame speed correlation for each individual fuel in a mixture of fuels and then uses the Hirasawa method to calculate the laminar flame speed of the blended mixture of fuels that are specified Eqn. (3571). Hydrocarbons, alcohols, hydrogen and ammonia are considered as fuels. Activates the Universal Laminar Flame Speed node.
User Defined Laminar Flame Speed	Allows you to specify the unstrained laminar flame speed. Activates the User Defined Laminar Flame Speed node.

Initial Conditions

Flame Area Density: The initial flame area per unit volume. $Σ$ in Eqn. (3563).
Progress Variable: You specify an initial progress variable between 0.0 (unburnt) and 1.0 (fully burnt).

Boundary Settings

Flow Boundaries (except outlet)

Flame Area Density: The flame area per unit volume. $Σ$ in Eqn. (3563).
Progress Variable: You specify a progress variable between 0.0 (unburnt) and 1.0 (fully burnt).

Wall Boundary

Wall Combustion Scalar: Selects the scalars for the wall combustion calculation.; See Wall Combustion Scalar Option.

Region Settings

Applies to any region:

Active Reactions Option: Activates or deactivates chemical reactions in this region.

CFM Combustion Solver Properties

Under-Relaxation Factor: In order to promote convergence, this property is used to under-relax changes of the solution during the iterative process. If residuals show solution divergence and do not decrease, reduce the under-relaxation factor for the relevant solvers. The default value is 0.9.
Solver Frozen: When On, the solver does not update any quantity during an iteration. It is Off by default. This is a debugging option that can result in non-recoverable errors and wrong solutions due to missing storage. See Finite Volume Solvers Reference for details.
Reconstruction Frozen: When activated, Simcenter STAR-CCM+ does not update reconstruction gradients with each iteration, but rather uses gradients from the last iteration in which they were updated. Activate Temporary Storage Retained in conjunction with this property. This property is deactivated by default.
Reconstruction Zeroed: When On, the solver sets reconstruction gradients to zero at the next iteration. This action means that face values used for upwinding (Eqn. (905)) and for computing cell gradients (Eqn. (917) and Eqn. (918)) become first-order estimates. This property is Off by default. If you turn this property Off after having it On, the solver recomputes the gradients on the next iteration.
Temporary Storage Retained: When activated, Simcenter STAR-CCM+ retains additional field data that the solver generates during an iteration. The particular data retained depends on the solver, and becomes available as field functions during subsequent iterations. Deactivated by default.

Field Functions

Flame Area Density: $Σ$ in Eqn. (3563).
Progress Variable: $c$ in Eqn. (3331).
Progress Variable Variance: $c_{var}$ in Eqn. (3332).
Unnormalized Progress Variable CFM: $y$ in Eqn. (3329).
Unnormalized Progress Variable Variance: $y_{var}$ in Eqn. (3330).