Step 4: Materials

Materials

  In this step, you will modify the default setting for the mixture by enabling the gas law. By default, the mixture material uses constant properties. Retain this constant property assumption for now, allowing only the mixture density to vary with temperature and composition. The influence of variable property inputs on the combustion prediction will be examined in a later part of the tutorial.

1.   Revise the properties for the mixture materials.

Materials Mixture Create/Edit...

  The Create/Edit Materials dialog box will display the mixture material ( methane-air) that was selected in the Species Model dialog box. The properties for this mixture material have been copied from the ANSYS FLUENT database and will be modified in the following steps.

(a)   Click the Edit... button to the right of the Mixture Species drop-down list to open the Species dialog box.

  You can add or remove species from the mixture material as necessary using the Species dialog box.

i.   Retain the default selections from the Selected Species selection list.

  The species that make up the methane-air mixture are predefined and require no modification.

ii.   Click OK to close the Species dialog box.

(b)   Click the Edit... button to the right of the Reaction drop-down list to open the Reactions dialog box.

  The eddy-dissipation reaction model ignores chemical kinetics (i.e., the Arrhenius rate) and uses only the parameters in the Mixing Rate group box in the Reactions dialog box. The Arrhenius Rate group box will therefore be inactive. The values for Rate Exponent and Arrhenius Rate parameters are included in the database and are employed when the alternate finite-rate/eddy-dissipation model is used.

i.   Retain the default values in the Mixing Rate group box.

ii.   Click OK to close the Reactions dialog box.

(c)   Retain the selection of incompressible-ideal-gas from the Density drop-down list.

(d)   Select constant from the Cp drop-down list and enter 1000  ${\rm J/kg-K}$ for the specific heat value.

  Scroll down to find the Cp drop-down list and number-entry box.

(e)   Click Change/Create to accept the material property settings.

(f)   Close the Create/Edit Materials dialog box.

  The initial calculation will be performed assuming that all properties except density are constant. The use of constant transport properties (viscosity, thermal conductivity, and mass diffusivity coefficients) is acceptable because the flow is fully turbulent. The molecular transport properties will play a minor role compared to turbulent transport. The assumption of constant specific heat, however, has a strong effect on the combustion solution. You will change this property definition in Step 7: Solution with Varying Heat Capacity.