## 14.1.5 Defining Boundary Conditions for Species

You will need to specify the inlet mass fraction for each species in your simulation. In addition, for pressure outlets you will set species mass fractions to be used in case of backflow at the exit. At walls, FLUENT will apply a zero-gradient (zero-flux) boundary condition for all species unless you have defined a surface reaction at that wall (see Section  14.2), you have defined a reaction mechanism at that wall (see Section  14.1.4), or you choose to specify species mass fractions at the wall. For fluid zones, you also have the option of specifying a reaction mechanism. Input of 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-specie flow. For information about NRBCs, see section Section  7.23.

 Note that you will explicitly set mass fractions only for the first species. The solver will compute the mass fraction of the last species by subtracting the total of the specified mass fractions from 1. If you want to explicitly specify the mass fraction of the last species, you must reorder the species in the list (in the Materials panel), as described in Section  14.1.4.

Diffusion at Inlets with the Pressure-Based Solver

For the pressure-based solver in FLUENT the net transport of species at inlets consists of both convection and diffusion components. The convection component is fixed by the inlet species concentration, whereas the diffusion component depends on the gradient of the computed species concentration field (which is not known a priori). In some cases, you may wish to include only the convective transport of species through the inlets of your domain. You can do this by disabling inlet species diffusion. By default, FLUENT includes the diffusion flux of species at inlets. To turn off inlet diffusion, use either the define/models/species/inlet-diffusion? text command, or the Species Model panel. Generally, you would want to disable the Inlet Diffusion option if at one of the inlets the convective flux is very small, resulting in mass loss by diffusion through that inlet.

Define Models Species Transport & Reaction...

Once Species Transport is selected, you can then disable the Inlet Diffusion option.

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