A Thermal Impedance Model for Partially Coupled Interfaces in FEM with Application to First Wall Protection

This work presents a thermal impedance-based model for the analysis of heat transfer across partially coupled interfaces within a Finite Element Method (FEM) framework. The approach reformulates the interfacial behavior using an effective thermal transmission coefficient, replacing traditional reflection-based descriptions and providing a more physically consistent representation of energy transfer between dissimilar media. The total heat flux is expressed as a function of transient thermal impedance and the fraction of energy transmitted through the interface, enabling a direct and interpretable formulation suitable for numerical implementation. An additional corrective term is introduced to account for non-ideal interfacial effects, such as surface roughness, contact pressure variations, phonon coupling, and local deviations from thermal equilibrium. This term is treated as a calibration parameter, allowing integration of experimental data or higher-fidelity simulations. The model is particularly relevant for scenarios where interfacial thermal resistance plays a dominant role, including systems involving vacuum gaps or rarefied media. Special attention is given to applications in first wall protection under high-energy particle fluxes, where accurate prediction of heat transfer is critical for material integrity and system performance. A validation strategy is proposed based on FEM implementation, including reference cases, non-equilibrium regimes, and comparison with experimental data. Possible extensions of the model include temperature-dependent thermal impedance, radiative heat transfer contributions, and spatial or temporal variation of the transmission coefficient. The proposed formulation provides improved internal consistency, clearer physical interpretation of parameters, and practical applicability for advanced thermal analysis in complex systems.