When theories and experiments meet: Rarefied gases as a benchmark of non-equilibrium thermodynamic models

The role of thermodynamics in deriving constitutive equations is unique, and various approaches have been developed in the last decades. In the present paper, the frameworks of Extended Irre-versible Thermodynamics (EIT) and Non-Equilibrium Thermodynamics with Internal Variables (NET-IV) are discussed and compared to each other on the basis of a particular problem of rarefied gases. In this comparison, both theoretical and experimental aspects are taken into account. Eventually, an experiment by Meyer and Sessler covering a wide range of pressures and frequencies is investigated. Here, concentrating on the scaling properties and the density dependence of parameters, the change of speed of sound in terms of frequency and pressure is recovered using NET-IV, and this fitting is compared to the results of Lebon and Cloot using EIT. NET-IV and EIT are seen to be more flexible than Rational Extended Thermodynamics (RET) concerning the functional form of the transport coefficients, which gives them some advantages over RET in some ranges of values of gas density.