Behaviour of rarified steam at very high temperature an orientation-averaged interaction potential approach towards its accurate description

We propose an orientation-averaged intermolecular potential model for the accurate computation of relevant thermodynamic and transport properties of steam at conditions typically encountered in extreme environments such as rocky exoplanetary atmospheres, aircraft and rocket engines, high temperature steam reforming and electrochemical reactors, as well as advanced ultra-supercritical steam power generators. We assess the reliability of the potential model to describe accurately the temperature dependence of relevant thermodynamic properties of rarified steam up to and beyond 3000K. Moreover, we discuss some fundamental issues including (i) the conditions at which an orientation-averaged intermolecular potential for a molecular fluid becomes an accurate representation of the fluid behaviour in extreme environments, (ii) the temperature range of validity of the approximations underlying an orientation averaging approach and, (iii) the methodology of force-field parameterisation to obtain an optimised representation, as well as the concomitant modelling of the thermodynamics and transport properties of rarified fluids. Finally, we illustrate how the simultaneous accurate description of the temperature dependence of the fluid’s collision integrals and second virial coefficient is crucial to the successful modelling of rarified steam in extreme environments.