Discussion on the stability of heated channels with different fluids at supercritical pressure

The paper extends previous work on the stability of heated channels with fluids at supercritical pressure as predicted by available models. A set of dimensionless numbers proposed to predict the threshold of instabilities is further discussed to highlight their capabilities and possible improvements. In particular, it is shown that the choice made for the reference value of the derivative of specific volume with respect to specific enthalpy is justified as an extension of classical formulations adopted for boiling channels. Moreover, the degree of universality to be expected by the use of these dimensionless numbers while using different fluids is clarified; in this aim four different fluids are considered: water, carbon dioxide, ammonia and refrigerant R23. In order to provide a clear perspective of the usefulness of the proposed dimensionless numbers for dealing with different fluids, linear stability maps generated by a previously developed in-house code, making use of balance equations in dimensionless form, are then compared with the results obtained by computations performed in dimensional terms. In this aim, both an in-house code and RELAP5 are used. The reference considered system is a long circular channel with uniform heating and no singular pressure drops, already addressed in previous analyses, here assumed both in vertical upward and in horizontal flow conditions. The comparison of the predictions obtained for the different fluids allows to ascertain the level of applicability of the dimensionless numbers and, as an interesting by-product, confirms the possibility to encounter static instabilities also in systems at supercritical pressure.