16.4 Defining the Stream Compositions

In ANSYS FLUENT, you will input only the boundary species (i.e., the fuel, oxidizer, and if necessary, secondary stream species). The intermediate and product species will be determined automatically.

ANSYS FLUENT provides you with an initial list of common boundary species ( ch4, h2, jet-a<g>, n2 and o2). If your fuel and/or oxidizer is composed of different species, you can add them to the boundary Species list. All boundary species must exist in the chemical database and you must enter their names in the same format used in the database, otherwise an error message will be issued.

After defining the boundary species that will be considered in the reaction system, you must define their mole or mass fractions at the fuel and oxidizer inlets and at the secondary inlet, if one exists. (If you choose to define the fuel or secondary stream composition empirically, you will instead enter the parameters described at the end of this section.) For the example shown in this figure in the separate Theory Guide , for example, the fuel inlet consists of 60% CH $_4$, 20% CO, 10% CO $_2$, and 10% C $_3$H $_8$.

Finally, the inlet stream temperatures of your reacting system are required for construction of the look-up table and computation of the equilibrium chemistry model.

For the equilibrium chemistry model, the species names are entered using the Boundary tab in the Species Model dialog box (Figure  16.4.1). If you are generating a steady or unsteady laminar flamelet, the list of boundary species will be automatically filled as all the species in the CHEMKIN mechanism, and you will be unable to change these.

Figure 16.4.1: The Species Model Dialog Box ( Boundary Tab)

The steps for adding new species and defining their compositions is as follows:

1.   (equilibrium chemistry model only) If your fuel, oxidizer, or secondary streams are composed of species other than the default species list, type the chemical formula (e.g., so2 or SO2 for SO $_2$) under Boundary Species and click Add. The species will be added to the Species list. Continue in this manner until all of the boundary species you want to include are shown in the Species list.

To remove a species from the list, type the chemical formula under Boundary Species and click Remove. To print a list of all species in the thermodynamic database file ( thermo.db) in the console window, click List Available Species.

2.   Under Species Unit, specify whether you want to enter the Mass Fraction or Mole Fraction. Mass Fraction is the default.

3.   For each relevant species in the Species list, specify its mass or mole fraction for each stream ( Fuel, Oxid, or Second as appropriate) by entering values in the table. Note that if you change from Mass Fraction to Mole Fraction (or vice versa), all values will be automatically converted if they sum to 0 or 1, so be sure that you are entering either all mass fractions or all mole fractions as appropriate. If the values do not sum to 0 or 1, an error will be issued.

4.   Under Temperature, specify the following inputs:

Fuel   is the temperature of the fuel inlet in your model. In adiabatic simulations, this input (together with the oxidizer inlet temperature) determines the inlet stream temperatures that will be used by ANSYS FLUENT. In non-adiabatic systems, this input should match the inlet thermal boundary condition that you will use in ANSYS FLUENT (although you will enter this boundary condition again in the ANSYS FLUENT session). If your ANSYS FLUENT model will use liquid fuel or coal combustion, define the inlet fuel temperature as the temperature at which vaporization/devolatilization begins (i.e., the Vaporization Temperature specified for the discrete-phase material--see Section  23.5). For such non-adiabatic systems, the inlet temperature will be used only to adjust the look-up table grid (e.g., the discrete enthalpy values for which the look-up table is computed). Note that if you have more than one fuel inlet, and these inlets are not at the same temperature, you must define your system as non-adiabatic. In this case, you should enter the fuel inlet temperature as the value at the dominant fuel inlet.

Oxid   is the temperature of the oxidizer inlet in your model. The issues raised in the discussion of the input of the fuel inlet temperature (directly above) pertain to this input as well.

Second   is the temperature of the secondary stream inlet in your model. (This item will appear only when you have defined a secondary inlet.) The issues raised in the discussion of the input of the fuel inlet temperature (directly above) pertain to this input as well.