Dear Makis,
Thank you for your kind response. The problem I am attempting to address is analogous to the Heat Exchanger (Simplified) tutorial, with the distinction that, in my case, the intermediary fluid is also a solid. I have employed the basic type of Conjugate Heat Transfer (CHT) coupling at the interfaces. The issue I am encountering is that the system fails to converge for various combinations of thermal conductivities. I have initiated a discussion on this matter in a separate thread, which can be accessed via the following link: Convergence Issues in Multi-Coupling Scheme.
To streamline our discourse, I suggest we consolidate our conversation into a single thread.