The coupling between flow and temperature field simulation is always necessary when the simplified estimation of the heat transfer conditions is no longer sufficient for the quality of the results. Since the common CFD solvers for thermal observations can represent not only the fluid volume but also the structural volume, an integrated multiphysics simulation is very well and easily possible.
The reasons that speak in favour of a separate approach to solving the multidisciplinary task, however, are individually different. For example, if the structural model is used later for other analyses and is therefore required anyway, double meshing of the structural components makes little sense. The FEM solvers also have better possibilities to describe the structures in a simplified way and thus with fewer elements, for example a shell model of thin walled components instead of solid elements.
If the heat transport between fluid and structure is primarily caused by forced convection, the feedback of the component temperatures to the flow results is usually low. In this case, the link can also be unilateral. With free convection, however, the interaction is much more significant and a complete bilateral coupling is often necessary.
The data transferred from the flow simulation to the temperature simulation are the local fluid temperatures and the associated heat transfer coefficient. In the case of a complete coupling, the local wall or surface temperatures flow back to the flow analysis.