The Reacting Flow Coupling Feature
Use the Reacting Flow () multiphysics coupling to simulate mass transport and reactions in a gas or liquid mixture where the fluid flow can be dependent on the mixture composition.
Domain Level Synchronization
The Reacting Flow coupling synchronizes the features from a Single-Phase Flow, or Brinkman Equations, interface and a Transport of Concentrated Species interface. When added, the density in the Single-Phase Flow interface is automatically synchronized to the one defined by the Transport of Concentrated Species interface. Conversely, the velocity field used by the latter interface is synchronized to the one computed in the former interface.
The Stefan Velocity
The Reacting Flow coupling feature automatically couples mass transfer on boundaries and applies a corresponding velocity contribution for the flow. Prescribing a net mass boundary flux in the Transport of Concentrated Species interface, either using a Flux or Mass Fraction feature, the Reacting Flow feature computes The Stefan Velocity and applies this in Wall features using the same selection.
Mass Transfer to Other Phases in Porous Media
When coupled to the Brinkman Equations interface, the Reacting Flow node automatically computes the net mass source or sink in a Reactions (when Mass transfer to other phases is enabled) node in the Transport of Concentrated Species interface and adds the corresponding source/sink to the momentum equations of the Fluid and Matrix Properties domains.
Turbulent Mass Transfer
When a turbulence model is used, the Reacting Flow coupling applies turbulence modeling for the mass transport in the following manners:
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Turbulent mass transport is added, defined from the turbulent viscosity and a turbulent Schmidt number (for more information, see Turbulent Mass Transport Models).
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Mass transport wall functions. When a turbulence model using wall functions is used for the fluid flow, the Reacting Flow coupling automatically adds wall functions for the mass transport on the same boundaries (for more information, see Mass Transport Wall Functions).
Settings
The Label is the default multiphysics coupling feature name.
The Name is used primarily as a scope prefix for variables defined by the coupling node. Refer to such variables in expressions using the pattern <name>.<variable_name>. In order to distinguish between variables belonging to different coupling nodes or physics interfaces, the name string must be unique. Only letters, numbers, and underscores (_) are permitted in the Name field. The first character must be a letter.
The default Name (for the first multiphysics coupling feature in the model) is rf1.
Domain Selection
The Reacting Flow coupling is automatically defined on the intersection of the selections for the coupled interfaces.
The Selection list displays the domains where the coupling feature is active.
Coupled Interfaces
This section defines the physics involved in the multiphysics coupling. The Fluid flow and Species transport lists include all applicable physics interfaces.
The default values depend on how this coupling node is created.
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If it is added from the Physics ribbon (Windows users), Physics contextual toolbar (Mac and Linux users), or context menu (all users), then the first physics interface of each type in the component is selected as the default.
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If it is added automatically when a multiphysics interface is chosen in the Model Wizard or Add Physics window, then the two participating physics interfaces are selected.
You can also select None from either list to uncouple the node from a physics interface. If the physics interface is removed from the Model Builder, for example Laminar Flow is deleted, then the Species transport list defaults to None as there is nothing to couple to.
Click the Go to Source buttons () to move to the main physics interface node for the selected physics interface.
Click the Show or Hide Physics Properties Settings button () to toggle the display of physics properties settings affecting the coupling feature. When a turbulence model is used, turbulent mass transfer is automatically accounted for (see the settings in the Turbulence section below). Using Reacting Flow, the mass transfer treatment at walls follows that applied for the fluid flow. Therefore the Wall treatment setting is also displayed when using a turbulence model. For more information on turbulent mass transfer at walls, see the section Mass Transport Wall Functions in the CFD Module User’s Guide.
If a physics interface is deleted and then added to the model again, then in order to reestablish the coupling, you need to choose the physics interface again from the Fluid flow or Species transport lists. This is applicable to all multiphysics coupling nodes that would normally default to the once present interface. See Multiphysics Modeling Approaches in the COMSOL Multiphysics Reference Manual.
Turbulence
When the fluid flow interface uses a turbulence model, select an option from the Mass transport turbulence model list — Kays-Crawford, High Schmidt Number, or User-defined turbulent Schmidt number.
For User-defined turbulent Schmidt number, enter a Turbulent Schmidt number ScT (dimensionless).
The turbulent mass transfer added to the mass fraction equations is defined as
where μT is the turbulent viscosity defined by the flow interface, and the turbulent Schmidt number, ScT, depends on the Mass transport turbulence model used.
Note, since the Reacting Flow coupling feature adds the turbulent mass transport, it should not be combined with a Turbulent Mixing feature (subfeature to Transport Properties in the Transport of Concentrated Species interface).