Eddy Break-Up Model Reference

The reaction rates that are used in the transport equations are calculated as functions of the mean species concentrations, turbulence characteristics and, depending on the specific model that is used, temperature. A mean enthalpy equation is solved in addition to the species transport equations. The mean temperature and density are then calculated knowing the mean enthalpy and species concentrations.

Four variants are available for this model, selectable from the Reaction Control property in the Properties window.

Table 1. Eddy Break-Up Combustion Models
Theory	See Eddy Break-Up
Provided By	[physics continuum] > Models > [Non-Premixed] Combustion Models
Example Node Path	Continua > Physics 1 > Models > Eddy Break-Up
Requires	Material: either Multi-Component Gas or Multi-Component Liquid Reaction Regime: Reacting Reactive Flow: Reacting Species Transport
Properties	Key properties are: Reaction Control, Store Reaction Rates, Begin, Source Enabled Trigger, and Source Term Limiting. See EBU properties.
Specific Right-Click Actions	See Right-Click Actions.
Activates	Model Controls (child nodes)	Reactions. See Reactions.
	Region Settings	Active Regions Option. See Region Settings.
	Monitors	FuelMassFr: Fuel Mass Fraction.
	Other Continuum Nodes	The Ignitors node provides the right-click option to create a Fixed Temperature Ignitor. See Ignitors.
	Field Functions	Chemistry Heat Release Rate, Residence Time. See EBU Field Functions

EBU Model Properties

Reaction Control

Selects the variant of EBU combustion model. Apart from the Kinetics Only variant, these models are not compatible with the Spalart-Allmaras and Spalart-Allmaras-DES turbulence models.

Standard EBU
Selects Standard EBU—assumes that the reaction rate is dictated solely by the turbulent mixing time scale.
Hybrid
Selects Hybrid—reaction rate is the minimum of the rate predicted by turbulent mixing and the rate predicted by finite-rate chemical kinetics.
Combined Time Scale
Selects Combined Time Scale—reaction rate is given by the sum of the rates predicted by turbulent mixing and by chemical kinetics.
Kinetics Only
Selects Kinetics Only—assumes that the reaction rate is dictated by finite-rate chemical kinetics. This option is very similar to the Complex Chemistry option, however, without the stiff solver for solving several stiff reactions simultaneously. Therefore, it is suggested that you use the Complex Chemistry model instead of EBU with Kinetics Only.

Store Reaction Rates

Activates the capability to post-process reaction rates, forward and backward (where applicable).

Activated
Stores reaction rates during the simulation run and makes them available among scalar field functions.

After the simulation run, each reaction rate is available in the scalar field function list under the item Reaction Rate Forward or Reaction Rate Backward.

If you set the Reaction Control property of the EBU model to Standard EBU, you can only visualize forward reaction rates.
Deactivated
Does not store reaction rates (default).

Begin: Specifies the iteration or time-step after which the reactions are activated. During previous iterations or time-steps, reactions are deactivated. This feature is useful when modeling significant flow as you can deactivate reactions until a flow solution is found.

Source Term Limiting

For the EBU combustion model (particularly with the Kinetics Only option), the specified reaction rate can be large enough to drive the reactant concentration below zero. To keep all reactant concentrations in a physically correct range, activate this limit on the final reaction rate that is applied to the species transport equation.

For most applications, leave this option deactivated. Use it only when a reasonable convergence cannot be obtained and the species concentrations clearly show unphysical values.

Activated
Limits the final reaction rate that is applied to the species transport equation Eqn. (3462). Provides the field function, Residence Time (chemistry time step).
Deactivated
Does not limit the final reaction rate (default).

Right-Click Actions

Import Chemistry Definition (Chemkin format): Activates a standard Open dialog that allows you to import a chemical mechanism file, thermodynamic data file, and optionally, a transport properties file.
Delete Chemistry Definition: Removes the reactions—and optionally, the species—that are used for the chemistry definition.

Reactions

Right-Click Actions

New Reaction: Creates a Reaction [n] sub-node.; See Reaction [n] Reference.

[reaction]

The defined reaction Type can only be Eddy Break-Up.

Reactants: Defines the reactant components of the Eddy Break-Up reaction.

Products: Defines the product components of the the Eddy Break-Up reaction.

Properties > Eddy Break-Up Coefficient

Available with the Standard EBU, Hybrid, and Combined Time Scale EBU combustion models.

Defines the reaction rate method for the Eddy Break-Up reaction.

Use Products for the Rate: When activated, it uses products Eqn. (3455) for computing the reaction rate.

EBU Rate Cofficient, A: Specifies the coefficient $A_{e b u}$ in Eqn. (3454) for EBU reaction rate.

EBU Rate Cofficient, B: Specifies the coefficient $B_{e b u}$ in Eqn. (3455) for EBU reaction rate.

Properties > Reaction Coefficient

Available with the Hybrid, Combined Time Scale, and Kinetics Only EBU combustion models.

Defines the reaction rate method for EBU reaction. Following are the methods that can be applied:


Method	Corresponding Method Node/Sub-Node
Arrhenius Coefficients The Arrhenius reaction parameters are used to calculate chemical reaction rates from finite-rate kinetics.	Arrhenius Coefficients Pre-exponent Defines the pre-exponential factor, $A$ , in Eqn. (3365).
User Reaction Coefficient The user reaction coefficient allows you to specify custom reaction coefficients.	User Reaction Coefficient Specifies the custom reaction coefficients. After combining with species concentration contributions, the units for the corresponding reaction rates are Quantity/Length³-Time. See Eqn. (3364).

Region Settings

Applies any region:

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

EBU Field Functions

Chemistry Heat Release Rate: The chemistry heat release rate is $\dot{h}$ in Eqn. (3366).
Residence Time: For steady state simulations, represents the time duration, $t_{r}$ in Eqn. (3462), over which the chemistry is calculated (transient/unsteady simulations use the time step as the duration for calculating the chemistry). This field function is available when the property, Source Term Limiting, is activated.