On the mechanism of final heat transfer restoration at the transition to gas-like fluid at supercritical pressure: A description by CFD analyses

Highlights: • Progresses in the prediction of heat transfer to supercritical pressure carbon dioxide by a CFD model are presented. • "Final heat transfer restoration" phenomenon is investigated trough CFD codes. • CFD results may help understanding the causes of the addressed phenomenon. - Abstract: In this paper, while presenting progresses in the prediction of heat transfer to supercritical pressure carbon dioxide by a CFD model, a particular focus is assigned to an interesting phenomenon highlighted by the considered experimental database. This phenomenon is observed in several operating conditions when deteriorated heat transfer starts because of flow laminarisation, as a consequence of buoyancy effects at sufficiently low inlet temperature, while it is suddenly suppressed when reaching higher enthalpy along the pipe or when the inlet temperature is increased. The observed behaviour has a clear root in the density differences between the fluid at the wall and in bulk, which are firstly responsible for the onset of deterioration and then become no more sufficient to sustain it. While experimental data can only show the deteriorated heat transfer to appear and disappear, in terms of values of increasing and decreasing temperatures at the wall, a recently upgraded turbulence model showed an interesting ability to reproduce these wall temperature data, at the same time providing a clear picture of the conditions justifying the observed behaviour. In this regard, the good predictions of wall temperature, as the only measurable data, in some of the addressed conditions give confidence that the model is helpful to explain the reasons for the observed behaviour, shading light on the mechanisms producing this interesting phenomenon, so clearly shown in the considered experimental data set.