What is default material of boundary walls if heat transfer takes place

Dear Developer!

  1. What is the default material of the boundary walls in the heat transfer case?
  2. Where I can change the properties of boundary walls (material type, thermal conductivity, other coefficients, etc)?

There is no “default material” for the walls. Instead, the boundaries are described by certain differential equations, with coefficients you can specify:

You can set these per-phase - use the Fluid and Solids subtabs to set the properties for each phase.