Dimensionless Numbers for Analysing Fluid-dynamic Stability and Heat Transfer at Supercritical Pressures

The paper shortly summarises the developments carried out in past and recent times at the University of Pisa to establish suitable dimensionless numbers to be applied in the analysis of flow stability and heat transfer with different fluids at supercritical pressures. Developments in this field were stimulated by the repeated involvement in different coordinated research projects (CRPs) promoted by the International Atomic Energy Agency (IAEA) and also in actions financed by the European Commission in past decades. The first ideas about the most appropriate modelling of the dynamic behaviour of a continuously expanding fluids in flow ducts, as those at supercritical pressure flowing along heated pipes, were borrowed from the formalism adopted for boiling channels. While the clear similarities between the dynamic behaviour of boiling channels and heated pipes with supercritical fluids stimulated an early translation of two-phase flow concepts and tools into corresponding ones for supercritical pressure, dealing with heat transfer phenomena requested a more complex process. This involved the achievement of improved capabilities in CFD modelling of heat transfer phenomena and the conception of an elaborated fluid-to-fluid similarity theory. The overall rationale developed on this basis is here summarised and future lines for research are proposed.