FLUENT UDFs for Non-Equilibrium Heat Transfer in Porous Media (Two-Energy Model)

FLUENT UDF manual might not explicitly mention using two temperatures together in a single User Defined Function (UDF). However, there are ways to achieve the non-equilibrium heat transfer with a two-energy model using UDFs. Here's how you can approach it:

1. Define UDFs for Source Terms:

Create separate UDFs for the source terms in both the solid and fluid energy equations. In these UDFs, you can access the following information:

• Cell-centered Variables: You can use C_CENTROID(c,t) to get the current time step values for various variables at the cell centroid, including fluid temperature (C_T(c,t)) and other relevant properties.
• Custom User Defined Memory (C_UDM): This allows you to store and access data from previous time steps. You can define separate UDFMs for solid and fluid temperatures from the previous time step (e.g., C_UDM(c,T_SOLID_PREV) and C_UDM(c,T_FLUID_PREV)) and update them within the UDFs.

2. Calculation Steps within UDFs:

Within your UDFs, follow these steps:

• Calculate Coupling Term: Calculate the heat transfer term G(Ts-Tf) for the fluid source term and G(Tf-Ts) for the solid source term. You can access the current fluid temperature C_T(c,t) and store the previously saved solid temperature from UDM (C_UDM(c,T_SOLID_PREV)) for the fluid source term calculation. Similarly, use the current solid temperature from UDM (C_UDM(c,T_FLUID_PREV)) for the fluid temperature in the solid source term calculation.
• Update UDM: After calculating the source term, update the UDM for the current time step's solid and fluid temperatures:
  • C_UDM(c,T_SOLID_CURRENT) = C_T(c,t) (for fluid UDF)
  • C_UDM(c,T_FLUID_CURRENT) = C_T(c,t) (for solid UDF)

3. Linking UDFs in FLUENT:

In FLUENT, define source terms in the energy equations for both solid and fluid phases. Link the corresponding UDFs you created (one for the fluid source term and another for the solid source term) to these source terms.

4. Initialization:

For the first time step, you might need to provide initial values for the UDM of both solid and fluid temperatures. You can set them to the initial temperature condition or any other appropriate value.

Additional Notes:

• Make sure your UDFs are compiled correctly and loaded into FLUENT.
• Double-check the UDF syntax and logic to ensure proper calculation and update of the UDM variables.
• Consider using DEFINE_PROFILE for defining the heat transfer coefficient G if it's a function of other variables.

By implementing these steps, you should be able to access and utilize both solid and fluid temperatures from the previous time step within your UDFs for calculating the source terms in the two-energy model for non-equilibrium heat transfer in porous media.

Further Resources:

• FLUENT User Defined Functions (UDFs) Manual: https://www.ansys.com/training-center/course-catalog/fluids/ansys-fluent-using-user-defined-functions-udfs (Search for "UDF" or "User Defined Functions")
• Examples of UDFs for Porous Media: You might find online resources or tutorials showcasing UDFs for specific porous media applications. These can provide a starting point and inspiration for your UDF development.

Remember, UDF development can be complex. If you encounter difficulties, consider seeking help from experienced FLUENT users or CFD professionals.