Seven questions about supercritical fluids - towards a new fluid state diagram

摘要

In this paper, we discuss properties of supercritical and real fluids, following the overarching question: 'What is a supercritical fluid?'. It seems there is little common ground when researchers in our field discuss these matters as no systematic assessment of this material is available. This paper follows an exploratory approach, in which we analyze whether common terminology and assumptions have a solid footing in the underlying physics. We use molecular dynamics (MD) simulations and fluid reference data to compare physical properties of fluids with respect to the critical isobar and isotherm, and find that there is no contradiction between a fluid being supercritical and an ideal gas; that there is no difference between a liquid and a transcritical fluid; that there are different thermodynamic states in the supercritical domain which may be uniquely identified as either liquid or gaseous. This suggests a revised state diagram, in which low-temperature liquid states and higher temperature gaseous states are divided by the coexistence-line (subcritical) and pseudoboiling-line (supercritical). As a corollary, we investigate whether this implies the existence of a supercritical latent heat of vaporization and show that for pressures smaller than three times the critical pressure, any isobaric heating process from a liquid to an ideal gas state requires approximately the same amount of energy, regardless of pressure. Finally, we use 1D flamelet data and large-eddy-simulation results to demonstrate that these pure fluid considerations are relevant for injection and mixing in combustion chambers.