As discussed in Section 15.1.1, if you use a mixture material from the database, most mixture and species properties will already be defined. You may follow the procedures in this section to check the current properties, modify some of the properties, or set all properties for a brand-new mixture material that you are defining from scratch.
Remember that you will need to define properties for the mixture material and also for its constituent species. It is important that you define the mixture properties before setting any properties for the constituent species, since the species property inputs may depend on the methods you use to define the properties of the mixture. The recommended sequence for property inputs is as follows:
These steps, all of which are performed in the Create/Edit Materials dialog box, are described in detail in this section.
Defining the Species in the Mixture
If you are using a mixture material from the database, the species in the mixture will already be defined for you. If you are creating your own material or modifying the species in an existing material, you will need to define them yourself.
In the Create/Edit Materials dialog box (Figure 15.1.2), check that the Material Type is set to mixture and your mixture is selected in the Fluent Mixture Materials list. Click the Edit... button to the right of Mixture Species to open the Species dialog box (Figure 15.1.3).
Overview of the Species Dialog Box
In the Species dialog box, the Selected Species list shows all of the fluid-phase species in the mixture. If you are modeling wall or particle surface reactions, the Selected Solid Species list will show all of the bulk solid species in the mixture. Solid species are species that are deposit to, or etch from, wall boundaries or discrete-phase particles (e.g., Si(s)) and do not exist as fluid-phase species. If you are modeling wall surface reactions with site balancing, where species adsorb onto the wall surface, react, and then desorb off the surface, the Selected Site Species list will show all of the site species in the mixture.
The use of solid and site species with wall surface reactions is described in Section 15.2. See Section 15.3 for information about particle surface reactions.
| The order of the species
Selected Species list is very important.
ANSYS FLUENT considers the last specie in the list to be the bulk specie.
You should therefore be careful to retain the most abundant specie (by mass) as the last specie when you add species to or delete species from a mixture material.
The Available Materials list shows materials that are available but not in the mixture. Generally, you will see air in this list, since air is always available by default.
Adding Species to the Mixture
If you are creating a mixture from scratch or starting from an existing mixture and adding some missing species, you will first need to load the desired species from the database (or create them, if they are not present in the database) so that they will be available to the solver. The procedure for adding species is listed below. (You will need to close the Species dialog box before you begin, since it is a "modal'' dialog box that will not allow you to do anything else when it is open.)
| If you do not see the specie you are looking for in the database, you can create a new fluid material for that specie, following the instructions in Section
8.1.2, and then continue with step 2, below.
| Adding a specie to the list will alter the order
of the species. You should be sure that the last specie in the list is the bulk specie,
and you should check all cell and boundary zone conditions, under-relaxation factors, and other solution parameters that you have set, as described in detail below.
Removing Species from the Mixture
To remove a specie from the mixture, simply select it in the Selected Species list (or the Selected Site Species or Selected Solid Species list) and click the Remove button below the list. The specie will be removed from the list and added to the Available Materials list.
| Removing a specie from the list will alter the order of the species. You should be sure that the last specie in the list is the bulk species, and you should check any cell zone or boundary conditions, under-relaxation factors, or other solution parameters that you have set, as described in detail below.
If you find that the last specie in the Selected Species list is not the most abundant specie (as it should be), you will need to rearrange the species to obtain the proper order.
The Naming and Ordering of Species
As discussed above, you should retain the most abundant specie as the last one in the Selected Species list when you add or remove species. Additional considerations you should be aware of when adding and deleting species are presented here.
There are three characteristics of a specie that identify it to the solver: name, chemical formula, and position in the list of species in the Species dialog box. Changing these characteristics will have the following effects:
If your ANSYS FLUENT model involves chemical reactions, you can next define the reactions in which the defined species participate. This will be necessary only if you are creating a mixture material from scratch, you have modified the species, or you want to redefine the reactions for some other reason.
Depending on which turbulence-chemistry interaction model you selected in the Species Model dialog box (see Section 15.1.2), the appropriate reaction model will be displayed in the Reaction drop-down list in the Edit Material dialog box. If you are using the laminar finite-rate or Eddy-Dissipation Concept model, the reaction model will be finite-rate; if you are using the eddy-dissipation model, the reaction model will be eddy-dissipation; if you are using the finite-rate/eddy-dissipation model, the reaction model will be finite-rate/eddy-dissipation.
Inputs for Reaction Definition
To define the reactions, click the Edit... button to the right of Reaction. The Reactions dialog box (Figure 15.1.4) will open.
The steps for defining reactions are as follows:
Note that if your model includes discrete-phase combusting particles, you should include the particulate surface reaction(s) (e.g., char burnout, multiple char oxidation) in the number of reactions only if you plan to use the multiple surface reactions model for surface combustion.
There are two general classes of reactions that can be handled by the Reactions dialog box, so it is important that the parameters for each reaction are entered correctly. The classes of reactions are as follows:
where , , , , , , , and .
Figure 15.1.4 shows the coefficient inputs for the combustion of methane. (See also the methane-air mixture material in the Database Materials dialog box.)
Note that, in certain cases, you may wish to model a reaction where product species affect the forward rate. For such cases, set the product rate exponent ( ) to the desired value. An example of such a reaction is the gas-shift reaction (see the carbon-monoxide-air mixture material in the Database Materials dialog box), in which the presence of water has an effect on the reaction rate:
In the gas-shift reaction, the rate expression may be defined as:
where , , , , , , , and .
where , , , , , and .
See step 6 below for information about how to enable reversible reactions.
| It is important to note that if you have selected the British units system, the Arrhenius factor should still be input in SI units. This is because
ANSYS FLUENT applies no conversion factor to your input of
(the conversion factor is 1.0) when you work in British units, as the correct conversion factor depends on your inputs for
| It is not necessary to include the third-body efficiencies. You should not enable the
Third-Body Efficiencies option unless you have accurate data for these parameters.
Under Reaction Parameters, select the appropriate Reaction Type ( lindemann, troe, or sri). See this section in the separate Theory Guide for details about the three methods. Next, you must specify if the Bath Gas Concentration ( in this equation in the separate Theory Guide) is to be defined as the concentration of the mixture, or as the concentration of one of the mixture's constituent species, by selecting the appropriate item in the drop-down list.
The parameters you specified under Arrhenius Rate in the Reactions dialog box represent the high-pressure Arrhenius parameters. You can, however, specify values for the following parameters under Low Pressure Arrhenius Rate:
If you selected troe for the Reaction Type, you can specify values for Alpha, T1, T2, and T3 ( , , , and in this equation in the separate Theory Guide) under Troe parameters. If you selected sri for the Reaction Type, you can specify values for a, b, c, d, and e ( , , , , and in this equation in the separate Theory Guide) under SRI parameters.
In the Coverage Dependent Reaction dialog box, all the site species of the reaction will be present with a default value of 0 for all the parameters, corresponding to no surface coverage modification. Enter the relevant values of the parameters , , and (as defined in this equation in the separate Theory Guide) for all the species for which the reaction has coverage dependence.
A is the constant in the turbulent mixing rate ( this equation and this equation in the separate Theory Guide) when it is applied to a specie that appears as a reactant in this reaction. The default setting of 4.0 is based on the empirically derived values given by Magnussen et al. [ 47].
B is the constant in the turbulent mixing rate ( this equation in the separate Theory Guide) when it is applied to a specie that appears as a product in this reaction. The default setting of 0.5 is based on the empirically derived values given by Magnussen et al. [ 47].
Defining Species and Reactions for Fuel Mixtures
Quite often, combustion systems will include fuel that is not easily described as a pure specie (such as CH or C H ). Complex hydrocarbons, including fuel oil or even wood chips, may be difficult to define in terms of such pure species. However, if you have available the heating value and the ultimate analysis (elemental composition) of the fuel, you can define an equivalent fuel specie and an equivalent heat of formation for this fuel. Consider, for example, a fuel known to contain 50% C, 6% H, and 44% O by weight. Dividing by atomic weights, you can arrive at a "fuel'' specie with the molecular formula C H O . You can start from a similar, existing specie or create a specie from scratch, and assign it a molecular weight of 100.04 kg/kgmol (4.17 12 + 6 1 + 2.75 16). The chemical reaction would be considered to be
You will need to set the appropriate stoichiometric coefficients for this reaction.
The heat of formation (or standard-state enthalpy) for the fuel specie can be calculated from the known heating value since
where is the standard-state enthalpy on a molar basis. Note the sign convention in Equation 15.1-1: is negative when the reaction is exothermic.
Defining Zone-Based Reaction Mechanisms
If your ANSYS FLUENT model involves reactions that are confined to a specific area of the domain, you can define "reaction mechanisms" to enable different reactions selectively in different geometrical zones. You can create reaction mechanisms by selecting reactions from those defined in the Reactions dialog box and grouping them. You can then assign a particular mechanism to a particular zone.
Inputs for Reaction Mechanism Definition
To define a reaction mechanism, click the Edit... button to the right of Mechanism. The Reaction Mechanisms dialog box (Figure 15.1.8) will open.
The steps for defining a reaction mechanism are as follows:
Click the Define... button. This will open the Site Parameters dialog box (Figure 15.1.9), where you will define the parameters of the site specie.
Under Site Species, select the appropriate species from the drop-down list(s) and specify the fractional Initial Site Coverage for each specie. For steady-state calculations, it is recommended (though not strictly required) that the initial values of Initial Site Coverage sum to unity. For transient calculations, it is required that these values sum to unity.
Click Apply in the Site Parameters dialog box to store the new values.
Defining Physical Properties for the Mixture
When your ANSYS FLUENT model includes chemical species, the following physical properties must be defined, either by you or by the database, for the mixture material:
Detailed descriptions of these property inputs are provided in Chapter 8.
| Remember to click the
Change/Create button when you are done setting the properties of the mixture material. The properties that appear for each of the constituent species will depend on your settings for the properties of the mixture material. If, for example, you specify a composition-dependent viscosity for the mixture, you will need to define viscosity for each specie.
Defining Physical Properties for the Species in the Mixture
For each of the fluid materials in the mixture, you (or the database) must define the following physical properties:
Detailed descriptions of these property inputs are provided in Chapter 8.
| Global reaction mechanisms with one or two steps inevitably neglect the intermediate species. In high-temperature flames, neglecting these dissociated species may cause the temperature to be overpredicted. A more realistic temperature field can be obtained by increasing the specific heat capacity for each specie. Rose and Cooper [
66] have created a set of specific heat polynomials as a function of temperature.
The specific heat capacity for each specie is calculated as
The modified polynomial coefficients (J/kg-K) from [ 60] are provided in Tables 15.1.1 and 15.1.2.
|2.16182e 02||6.81428e 01||1.56841e 01||1.7372e 01|
|1.48638e 04||7.08589e 03||5.39904e 04||6.9e 04|
|4.48421e 08||4.71368e 06||3.01061e 07||---|
|---||8.51317e 10||5.05048e 11||---|
|5.46776e 04||3.64357e 03||5.58304e 04|
|2.38224e 07||2.86327e 06||1.20247e 06|
|1.89204e 10||7.59578e 10||1.14741e 09|