15.1.1 Overview of User Inputs for Modeling Species Transport and Reactions

The basic steps for setting up a problem involving species transport and reactions are listed below, and the details about performing each step are presented in Sections  15.1.2- 15.1.5. Additional information about setting up and solving the problem is provided in Sections  15.1.6- 15.1.8.

1.   Enable species transport and volumetric reactions, and specify the mixture material. See Section  15.1.2. (The mixture material concept is explained below.)

2.   If you are also modeling wall or particle surface reactions, enable wall surface and/or particle surface reactions as well. See Sections  15.2 and 15.3 for details.

3.   Check and/or define the properties of the mixture. (See Section  15.1.3.) Mixture properties include the following:

• species in the mixture

• reactions

• other physical properties (e.g., viscosity, specific heat)

4.   Check and/or set the properties of the individual species in the mixture. (See Section  15.1.3.)

5.   Set specie cell zone and boundary conditions. (See Section  15.1.5.)

In many cases, you will not need to modify any physical properties because the solver gets species properties, reactions, etc. from the materials database when you choose the mixture material. Some properties, however, may not be defined in the database. You will be warned when you choose your material if any required properties need to be set, and you can then assign appropriate values for these properties. You may also want to check the database values of other properties to be sure that they are correct for your particular application. For details about modifying an existing mixture material or creating a new one from scratch, see Section  15.1.3. Modifications to the mixture material can include the following:

• Addition or removal of species

• Changing the chemical reactions

• Modifying other material properties for the mixture

• Modifying material properties for the mixture's constituent species

If you are solving a reacting flow, you will usually want to define the mixture's specific heat as a function of composition, and the specific heat of each specie as a function of temperature. You may want to do the same for other properties as well. By default, most specie specific heats in the database are piecewise-polynomial functions of temperature, but you may choose to specify a different temperature-dependent function if you know of one that is more suitable for your problem.

Mixture Materials

The concept of mixture materials has been implemented in ANSYS FLUENT to facilitate the setup of species transport and reacting flow. A mixture material may be thought of as a set of species and a list of rules governing their interaction. The mixture material carries with it the following information:

• A list of the constituent species, referred to as "fluid'' materials

• A list of mixing laws dictating how mixture properties (density, viscosity, specific heat, etc.) are to be derived from the properties of individual species if composition-dependent properties are desired

• A direct specification of mixture properties if composition-independent properties are desired

• Diffusion coefficients for individual species in the mixture

• Other material properties (e.g., absorption and scattering coefficients) that are not associated with individual species

• A set of reactions, including a reaction type (finite-rate, eddy-dissipation, etc.) and stoichiometry and rate constants

Both mixture materials and fluid materials are stored in the ANSYS FLUENT materials database. Many common mixture materials are included (e.g., methane-air, propane-air). Generally, one/two-step reaction mechanisms and many physical properties of the mixture and its constituent species are defined in the database. When you indicate which mixture material you want to use, the appropriate mixture material, fluid materials, and properties are loaded into the solver. If any necessary information about the selected material (or the constituent fluid materials) is missing, the solver will inform you that you need to specify it. In addition, you may choose to modify any of the predefined properties. See Section  8.1.2 for information about the sources of ANSYS FLUENT's database property data.

For example, if you plan to model combustion of a methane-air mixture, you do not need to explicitly specify the species involved in the reaction or the reaction itself. You will simply select methane-air as the mixture material to be used, and the relevant species (CH $_4$, O $_2$, CO $_2$, H $_2$O, and N $_2$) and reaction data will be loaded into the solver from the database. You can then check the species, reactions, and other properties and define any properties that are missing and/or modify any properties for which you wish to use different values or functions. You will generally want to define a composition- and temperature-dependent specific heat, and you may want to define additional properties as functions of temperature and/or composition.

The use of mixture materials gives you the flexibility to use one of the many predefined mixtures, modify one of these mixtures, or create your own mixture material. Customization of mixture materials is performed in the Create/Edit Materials dialog box, as described in Section  15.1.3.