Once you have defined the reactions, you will next create a sub-task for each species transport equation (Equation 19–5). A transport equation is needed for at least each of the species involved in a chemical reaction (except as noted below). Species not involved in any reactions can also be given a transport equation.
Follow the steps below for each species transport equation:
Define a sub-task for the species transport equation.
Create a sub-task
Select the Transport of species sub-task type.
Transport of species
Important: To save CPU time, you can specify a simple Closure equation rather than a transport equation for one of the species. See Defining a Closure Equation for details. A Transport of species sub-task must be defined for all other species.
Select the species for which you want to solve a transport equation. This will give the sub-task the name of "Transport of species", where species is the species you have selected.
Define the domain on which you want this equation to apply.
Domain of the sub-task
When applicable, it is assumed that the computational domain for a species adjusts to the domain of the velocity field, so this menu item will not be available. As a result, a change in the domain for the flow problem also induces a change in the domain for the transport equation.
If there is no flow problem, then the domain of the transport equation is not a priori limited. All transport equations, however, must share a common domain defined here. If the domain of any of your transport equations is changed, then the domain of all other transport equations will be changed accordingly.
Define the details of the transport equation.
Material data
Specify the density of the species.
Density
Specify the average species concentration, if necessary.
Average concentration
If a flow problem has already been defined, the concentration is taken from the model for the concentration of the flow, which can depend on other variables, including species.
If no flow problem exists, you will need to specify the concentration here for both steady-state and transient problems. Here, zero values for the product mass fractions must be specified, along with homogeneous values for the reactant species.
These values will be used as initial guesses for the numerical scheme in a steady-state problem, or as initial values for a time-dependent simulation.
Note: If the product species appear in the reaction rate expression, no reaction will take place if the product species have a zero concentration. In this case, you should either modify the reaction mechanism or set the average concentration of the product species to a nonzero value.
(optional) Specify a constant volumetric source for the transport equation (
in Equation 19–5).
Concentration source per unit volume
This value will be in addition to the source term arising from the chemical reactions.
Specify a nonzero value for the species diffusivity.
Diffusivity
Transport of species by diffusion is often negligible as compared to that by advection. For numerical reasons, however, it is recommended that the diffusivity be a strictly positive number in steady-state simulations.
Consider the advection term in the genreal transport equation.
Disable advection term
By default, the advection term in the general transport equationn is enabled. (General Transport Equation)
Define the boundary conditions for the species transport equation. Boundary conditions can be specified in terms of concentration (essential boundary condition) or in terms of concentration flux (natural boundary condition). The concentration variables for the transport equation are mass fractions.
Concentration boundary conditions
Select the boundary for which you want to set concentration conditions.
Click Modify.
Select the boundary condition type you want to impose. For each boundary, there are multiple possible conditions. By default, Ansys Polydata assumes a zero mass fraction on all boundaries.
Click Mass fraction imposed to specify a constant mass fraction or a linear function.
Mass fraction imposed
Here, you can select a Constant or a Linear function of coordinates of the form X=A+BX+CY+DZ.
Click Flux density imposed to specify a mass fraction flux.
Flux density imposed
Click Insulated boundary to specify an insulated boundary.
Insulated boundary
Click Source of connected condition to specify the "source" boundary in a pair of non-conformal boundaries that need to be connected.
Source of the connected condition
Non-conformal boundary conditions are described in Non-Conformal Boundaries. The inputs for non-conformal concentration boundary conditions are the same as those for flow boundary conditions, as described in Connecting Non-Conformal Boundaries, except for the numerical parameters. The first two numerical parameters (element dilatation and amplitude of volume generation) are the same as for flow boundary conditions, but the stabilization factor for flow conditions is replaced by a smoothing factor for concentration conditions. The smoothing factor controls the species diffusion tangential to the surface of the connected boundaries. Ideally, there should be no species diffusion within the connected boundaries, but setting this parameter to zero can lead to numerical problems (for example, zero pivot) or instabilities. It is therefore recommended to set the smoothing factor to a very small nonzero value.
Important: Changing the value of the smoothing factor from the default value is not recommended, except on the advice of your support engineer.
Click Mass fraction profile (CSV) to specify a mass fraction distribution outside of Ansys Polydata (for example, using a spreadsheet such as Excel), and then read it in to define the conditions on a boundary section.
Mass fraction profile (CSV)
The format for the mass fraction distribution data must be a CSV (comma separated variables) file, which is a common format for tabulated data that can be read into spreadsheet programs such as Excel. The profile file should contain a list of data points (
, 
, 
) and a list of concentration scalars
(
). For more information about setting up a
file with the correct format, you can follow the
instructions in Using the CSV File to Initialize Solution Variables for creating a
template CSV file.
Repeat these steps for each species for which you want to define a transport equation.
An important point regarding the behavior of species near boundaries is the following. Suppose that the fluid sticks at the wall boundaries. Along such boundaries, residence time is arbitrarily large (for steady-state problems); the species do not move, so they eventually change from reactants to products.
Species near the wall boundaries, however, do move, and gradually change from reactants to products as the chemical reaction proceeds. Therefore, steep concentration gradients in the direction normal to the wall boundaries are generated. The closer to the entry section, the steeper the boundary layer. The natural remedy to this situation is to assume that the fluid slips at the wall boundaries. If slip is assumed, species characteristics are all evolving in a similar manner. You can select slip conditions at the wall using the Flow boundary conditions menu item.