24.2.7 Specifying Heterogeneous Reactions

You can use ANSYS FLUENT to define multiple heterogeneous reactions and stoichiometry using the Phase Interaction dialog box (e.g., Figure  24.2.5).

Phases Interaction...

1.   In the Phases task page (Figure  24.2.2), click the Interaction... button to open the Phase Interaction dialog box.

Figure 24.2.5: The Phase Interaction Dialog Box for Heterogeneous Reactions

2.   Click the Reactions tab in the Phase Interaction dialog box.

3.   Set the total number of reactions (volumetric reactions, wall surface reactions, and particle surface reactions) in the Total Number of Heterogeneous Reactions field. (Use the arrows to change the value, or type in the value and press <Enter>.)

4.   Enable the Heterogeneous Stiff Chemistry Solver option if your inter-phase reaction mechanism contains numerically stiff reactions. This option can improve convergence and is available for transient Eulerian multiphase simulations. When this option is enabled, ANSYS FLUENT uses a fractional step algorithm where the flow is advanced without reaction sources for a time-step, and then the chemistry is integrated point-by-point for the same time-step. The stiff chemistry scheme solves all species in all phases coupled. Note that it is possible to include homogeneous (intra-phase) reactions along with the heterogeneous reactions in the (Phase Interaction dialog box (instead of in the reaction mechanism in the Create/Edit Materials dialog box), and these reactions will be solved with the stiff solver. The stiff ODE solver tolerances can be set using the following text command:

solve $\rightarrow$ set $\rightarrow$ heterogeneous-stiff-chemistry

5.   Specify the Reaction Name of each reaction that you want to define.

6.   Set the ID of each reaction you want to define. (Again, if you type in the value be sure to press <Enter>.)

7.   For each reaction, specify how many reactants and products are involved in the reaction by increasing the value of the Number of Reactants and the Number of Products. Select each reactant or product in the Reaction tab and then set its stoichiometric coefficient in the Stoich. Coefficient field. (The stoichiometric coefficient is the constant $\nu'_{i,r}$ or $\nu''_{i,r}$ in this equation in the separate Theory Guide.)

8.   For each reaction, indicate the Phase and Species and the stoichiometric coefficient for each of your reactants and products.

9.   For each reaction, indicate an applicable user-defined function using the Reaction Rate Function drop-down list.

The heterogeneous reaction rates can only be specified using a user-defined function. A UDF is available for an Arrhenius-type reaction with rate exponents that are equivalent to the stoichiometric coefficients.

For more information, see the separate UDF Manual.

ANSYS FLUENT assumes that the reactants are mixed thoroughly before reacting together, thus the heat and momentum transfer is based on this assumption. This assumption can be deactivated using a text command. For more information, contact your ANSYS FLUENT support engineer.