Analysis of fluiddynamic and heat transfer phenomena with supercritical fluids

The objective of this work is obtaining a better understanding of the heat transfer phenomena occurring when dealing with fluids at supercritical pressures, with the aim of paving the way for the development of the Generation IV Supercritical Water Cooled Reactor (SCWR) nuclear power plant. At the beginning of the present work, no reliable technique for predicting heat transfer phenomena in these conditions was available, including both CFD and heat transfer correlations. The phenomena occurring in heat transfer to supercritical fluids are in fact much more complex than the ones occurring in fluids in standard conditions. In particular, this is due to the strong variations of the thermodynamic properties occurring in the vicinity of the so called “pseudo-critical temperature”, which marks the single-phase transition from the liquid-like to the gas-like conditions. In addition, buoyancy phenomena imply both impairments and improvements of heat transfer conditions depending on the flow direction and thermal conditions. In fact, in upward flows, the buoyancy forces may imply a relaminarization of the flow inducing a heat transfer deterioration phenomenon. Further along the heated length the same phenomena may induce new velocity distributions (M-shaped) which result in a recovery of the turbulence conditions and, as a consequence, in a new heat transfer improvement. In downward flow cases, instead, buoyancy forces always have a positive effect since they increase the shear stresses in the vicinity of the wall and, as a consequence, improve the heat transfer conditions. Heat transfer deterioration and heat transfer recovery occurring in upward flows are the hardest conditions to deal with and a better prediction of these phenomena adopting CFD analysis is the topic of the present research. The RANS techniques adopted in the study do not require large computational effort and allow studying even complicated geometries; on the other hand, they are less accurate and reliable than LES and DNS. As a consequence, some particular phenomena may be neglected or modelled in a too simple way for dealing with supercritical fluids, making the results inaccurate. Different paths were considered in the present research project in order to find out which could be the lacking ingredient in the CFD models that are now providing us with better results.