15.1.5 Defining Cell Zone and Boundary Conditions for Species

You will need to specify the inlet mass fraction for all species in your simulation. In addition, for pressure outlets you will set species mass fractions to be used in case of backflow. At walls, ANSYS FLUENT will apply a zero-gradient (zero-flux) boundary condition for all species by default, although you can change each specie boundary condition to a specified value. If you have surface reactions defined (see Section  15.2), you can choose to enable wall-surface reactions and select the chemical mechanism. For fluid zones, you also have the option of specifying a reaction mechanism. Input of cell zone and boundary conditions is described in Chapter  7.

Non-Reflecting Boundary Conditions (NRBCs) are not compatible with species transport models. They are mainly used to solve ideal-gas single-species flow. For information about NRBCs, see section Section  7.4.

Note that you will explicitly set mass fractions only for the first $N-1$ species. The solver will compute the mass fraction of the last specie by subtracting the total of the specified mass fractions from 1. If you want to explicitly specify the mass fraction of the last specie, you must reorder the species in the list (in the Create/Edit Materials dialog box), as described in Section  15.1.3.

Diffusion at Inlets with the Pressure-Based Solver

For the pressure-based solver in ANSYS FLUENT, the net transport of species at inlets consists of both convection and diffusion components. The convection component is fixed by the specified inlet specie mass or mole fraction, whereas the diffusion component depends on the gradient of the computed specie concentration field (which is not known a priori). At very small convective inlet velocities, for example when modeling perforated combustion liners with an inlet, substantial mass can be gained or lost through the inlet due to diffusion. For this reason, inlet diffusion is disabled by default, but can be enabled with the Inlet Diffusion option in the Species Model dialog box.

Models Species Edit...