Reacting Model Reference

You select the Reacting model for physics continua in which reactions occur in the bulk of the fluid.

表 1. Reacting Model Reference
Theory	See Reacting Flow.
Provided By	[physics continuum] > Models > Reaction Regime
Example Node Path	Continua > Physics 1 > Models > Reacting
Requires	Material: Multi-Component Gas or Multi-Component Liquid
Activates	Physics Models	Reacting Flow Models
	Model Controls (child nodes)	Reactions	Contains details of the reactions in the chemical mechanism. You can define these reactions manually, or they appear upon importing a chemical mechanism. See Importing Species and Reactions or Defining Chemical Reactions Manually.
		[Reaction]
		Reactants
		Products
		Properties
		Reacting System Properties

[Reaction] > Properties

The Properties node contains further sub-nodes, Complex Reaction Properties, and Reaction Coefficient. When Third Bodies is activated under Arrhenius Coefficients, a Third Body Efficiencies sub node is also available. If pressure-dependent reactions are defined in the imported chemical mechanism, another node also appears, named after the specific method that is used to specify the pressure-dependent parameters—either, Lindemann Fall-Off Formulation, Troe Fall-Off Formulation, or SRI Fall-Off Formulation. The values for Lindemann, Troe, or SRI, are set automatically from the imported chemical mechanism. However, if necessary, it is possible to set these values manually.

Complex Reaction Properties

Pressure Dependent: Allows you to specify that this reaction uses a pressure-dependent Lindemann mechanism. When activated, the Lindemann Fall-Off Formulation node appears.

Reaction Coefficient

The Arrhenius reaction parameters are used to calculate chemical reaction rates from finite-rate kinetics.


Method	Corresponding Sub-Nodes
Arrhenius Coefficients	Pre-exponent Defines the pre-exponential factor, $A$ , in Eqn. (3365).

Arrhenius Coefficients: Properties

Temperature Exponent, Beta: Sets the temperature exponent, $β$ , for this reaction in Eqn. (3365).
Activation Energy, Ea: Sets the activation energy, $E_{a}$ , for this reaction in Eqn. (3365).
Reversible: When activated, the reaction is specified as reversible. In a reversible reaction, you specify the forward rate coefficients and select how the backward rate is calculated using the Properties > Reverse Reaction Coefficient node. When reversible reactions are read from a Chemkin mechanism, this property is activated and the reaction properties are set automatically. If you define a reversible reaction, make sure that you specify values for the enthalpy, entropy, and specific heat for each component of the mixture. For the specific heat, you choose either the thermodynamic polynomial or polynomial in T option.
Third Bodies: By default, each species in the mixture is assumed to contribute with equal efficiency as a third body. Hence all species are given a third body efficiency of unity. If one or more species contribute more or less as a third body, you can activate this property and then specify the efficiency coefficient of that species under the Third Body Efficiencies node. If the imported mechanism contains details of third body efficiencies, this property becomes activated and the values are imported automatically.

The Arrhenius Coefficients node also provides the Pre-exponent sub-node that defines the pre-exponential factor,

A

, in Eqn. (3365).

Reverse Reaction Coefficient

Available when Reversible is activated under the Reaction Coefficient > Arrhenius Coefficients node.

Method	Corresponding Sub-Nodes
Reverse Coefficient By Equilibrium Constant	Reverse Coefficient By Equilibrium Constant Uses the Equilibrium Constant to calculate the backward reaction rate coefficient. This calculation requires that enthalpy, entropy, and specific heat are defined in the reaction definition.
Arrhenius Coefficients For Reverse Reaction	Arrhenius Coefficients For Reverse Reaction Temperature Exponent, Beta Sets the temperature exponent, $β$ , for the backward reaction in Eqn. (3365). Activation Energy, Ea Sets the activation energy, $E_{a}$ , for the backward reaction in Eqn. (3365). Arrhenius Coefficients For Reverse Reaction > Pre-exponent Defines the pre-exponential factor, $A$ , for the backward reaction in Eqn. (3365).

Method

Corresponding Sub-Nodes

Reverse Coefficient By Equilibrium Constant

Reverse Coefficient By Equilibrium Constant: Uses the Equilibrium Constant to calculate the backward reaction rate coefficient. This calculation requires that enthalpy, entropy, and specific heat are defined in the reaction definition.

Arrhenius Coefficients For Reverse Reaction

Arrhenius Coefficients For Reverse Reaction

Temperature Exponent, Beta: Sets the temperature exponent, $β$ , for the backward reaction in Eqn. (3365).
Activation Energy, Ea: Sets the activation energy, $E_{a}$ , for the backward reaction in Eqn. (3365).

Arrhenius Coefficients For Reverse Reaction > Pre-exponent: Defines the pre-exponential factor, $A$ , for the backward reaction in Eqn. (3365).

Third Body Efficiencies

If third body efficiencies are imported as part of the chemical mechanism, details of the third body species and their efficiency coefficients are displayed under this node. If necessary, you can define third body efficiencies manually by right-clicking this node and selecting the option, Add Third Body Coefficient > [species (continuum)]. Each species that you include appears as a sub-node for which you can specify the third body efficiency coefficient,

α_{i j}

, in Eqn. (3372).

Lindemann Fall-Off Formulation

Specifies values for the Arrhenius rate parameters for the high-pressure limits (

A_{\infty}

β_{\infty}

, and

E_{\infty}

in Eqn. (3378)) and low-pressure limits (

A_{0}

β_{0}

, and

E_{0}

in Eqn. (3377)), to account for the pressure-dependence of the rate constants,

A_{j}

β_{j}

, and

E_{j}

, respectively, in Eqn. (3365). Terminology within the reaction mechanism determines if the values represent high or low pressure limits.

Troe Fall-Off Formulation

Specifies values for

α

T^{* * *}

T^{*}

, and also optionally

T^{* *}

, in Eqn. (3380), which are used to calculate

F_{c e n t}

F_{c e n t}

is then used to calculate the blending factor,

F

, in Eqn. (3379).

SRI Fall-Off Formulation

Specifies values for

a

b

c

, and also optionally

d

and

e

, in Eqn. (3381), which are used to calculate the blending factor,

F

, in Eqn. (3379).

Reacting System Properties

Species Reaction Source Jacobian

When using user-coded reaction rates, it is strongly recommended to use the Numerical option for the Jacobian calculation. However, if you require the additional computational speed of an analytical Jacobian, Simcenter STAR-CCM+ does allow the option of a user-code analytical Jacobian.


Method	Corresponding Sub-Nodes
Analytical An analytical Jacobian is used in the CVODE solver.	Analytical When the Species Reaction Sources method is set to Internal, Simcenter STAR-CCM+ calculates the Jacobian analytically. When the Species Reaction Sources method is set to User-Defined, you specify a user function that defines the analytical Jacobian. See Working With User Functions.
Numerical Jacobian The Jacobian is calculated numerically.	Numerical

A Jacobian is a square matrix of derivatives of the reaction rate with respect to the species specific mole fractions (mass fraction divided by molecular weight), and temperature. See Eqn. (3425) and Eqn. (3426).

Species Reaction Sources


Method	Corresponding Sub-Nodes
Internal The Simcenter STAR-CCM+ CVODE solver provides the reaction rate source terms for reacting species.	None.
User-Defined Provides the User-Defined Species Sources Specification node which allows you to specify whether you are modifying the internally calculated reaction rate source terms, defining entirely new reaction rate source terms, or modifying the rates of single reactions.	User-Defined Allows you to specify a previously imported user function that defines the species reaction rate source terms. See Working With User Functions. Field Functions Specifies the scalar field function to be used in corresponding user function for modifying the calculated reaction rates or defining reaction rate source terms. The following text is an example of how to access field functions in the user code: `struct UserAccessibleData udata = (struct UserAccessibleData )data; if (udata->_fieldFunctionsData._nFF > 1) { double ff0 = udata->_fieldFunctionsData._ffVal[0]; double ff1 = udata->_fieldFunctionsData._ffVal[1]; ... }` If you are using clustering in your simulation, these field functions will appear automatically under the Clustering > Components node.

User-Defined Reactions Density

Available when the User Defined EOS (Equation of State) model is selected with the Complex Chemistry model, and the density of the multi-component gas is specified as a field function.

User Function: Specifies the density of the multi-component gas as user code, for the calculation of the species source terms.; The output of your code must be identical to the field function you created for the density of the multi-component gas.

User-Defined Reactions Density Jacobian

Available when the User Defined EOS (Equation of State) model is selected with the Complex Chemistry model, and the density of the multi-component gas is specified as a field function.

User Function: Specifies the Jacobian of the density of the multi-component gas as user code, for species source term calculations.; The user code for the density Jacobian must return $\frac{d ρ}{d t}$ and $\frac{d ρ}{d z_{i}}$ . $z_{i} = \frac{Y_{i}}{W_{i}}$ is the specific mole fraction where $Y_{i}$ is the mass fraction and $W_{i}$ is the molecular weight of the i'th species.

The following text is an example of user-coded functions that return the density and density derivatives for an ideal gas:

#include "UserAccessibleData.h"

void rho(double *rho, double T,  unsigned int nSpe, double *z, void *data)
{
  double const Ru = 8314.4621;
  struct UserAccessibleData *udata = (struct UserAccessibleData *)data
  double Rgas = 0.0;
  for( unsigned int i=0; i<nSpe; i++)
  {
    Rgas += z[i];
  }

  Rgas *= Ru;
  *rho = udata->_pressure / ( Rgas * T);  
}

void rhoJac(double *drhodT, double *drhodzi, double *rho, double T,  unsigned int nSpe, double *z, void *data)
{
  struct UserAccessibleData *udata = (struct UserAccessibleData *)data;

  double Rgas = 0.0;
  double const Ru = 8314.4621;
  for( unsigned int i=0; i<nSpe; i++)
  {
    Rgas += z[i];
    
  }

  Rgas *= Ru;
  *rho = udata->_pressure / ( Rgas * T );  

  *drhodT = -*rho/T;
  *drhodzi = -*rho/Rgas * Ru;
}

#include "uclib.h"

void uclib()
{
  ucfunc(rho, "UserDensityForChemistry", "rho");
  ucfunc(rhoJac, "UserDensityJacobianForChemistry", "rhoJac");
}

User-Defined Species Sources Specification

Available when the Species Reaction Sources method is set to User-Defined.


Method	Corresponding Sub-Nodes
Calculate Species Sources	Calculate Species Sources Specifies that entirely new reaction rate source terms are to be defined with a previously imported user function. In this case, the internal reaction rate sources are not calculated. Displays the following property: Internal Reaction Energy Source When activated, the source term of the ODE energy equation is calculated internally, assuming a constant pressure reactor. See Eqn. (3422). When deactivated, the ODE energy source term must be calculated and stored in the (N+1)'th element of the reaction rate $\dot{ω}$ vector.
Modify Internal Species Sources	Modify Internal Species Sources Specifies that the existing reaction rate source terms are to be modified with a previously imported user function —typically by multiplying the existing values with a constant.
Modify Internal Reaction Rates	Modify Internal Reaction Rates Specifies that existing rates of individual reactions are to be modified. Forward and reverse reaction rates are provided. Internal reaction rates can be modified with a previously imported user function.

You use the Species Reaction Sources > User-Defined node to select the appropriate user function. For more information on how to create user functions, see Working With User Functions.