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14.1.2 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  14.1.3- 14.1.5. Additional information about setting up and solving the problem is provided in Sections  14.1.6- 14.1.8.

1.   Enable species transport and volumetric reactions, and specify the mixture material. See Section  14.1.3. (The mixture material concept is explained below.)

2.   If you are also modeling wall or particle surface reactions, turn on wall surface and/or particle surface reactions as well. See Sections  14.2 and 14.3 for details.

3.   Check and/or define the properties of the mixture. (See Section  14.1.4.) 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  14.1.4.)

5.   Set species boundary conditions. (See Section  14.1.5.)

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  14.1.4. Modifications to the mixture material can include the following:

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 species as a function of temperature. You may want to do the same for other properties as well. By default, constant properties are used, but for the properties of some species, there is a piecewise-polynomial function of temperature that exists in the database and is available for your use. You may also 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 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:

Both mixture materials and fluid materials are stored in the 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 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 Materials panel, as described in Section  14.1.4.

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