# Non-equilibrium theories of rarefied gases: internal variables and extended thermodynamics

## TL;DR

This paper compares two advanced non-equilibrium theories for rarefied gases—RET and NET-IV—highlighting their physical principles, modeling capabilities, and how they can be integrated or related through entropy considerations.

## Contribution

It provides a comparative analysis of RET and NET-IV theories, extending classical thermodynamics to better model rarefied gases and exploring their compatibility.

## Key findings

- RET and NET-IV can be related through generalized entropy currents
- Theories extend classical laws to describe rarefied gases more accurately
- Compatibility between RET and NET-IV can be achieved through structural analysis

## Abstract

Limits of classical constitutive laws such as Fourier and Navier-Stokes equations are discovered since decades. However, the proper extensions -- generalizations of these are not unique. They differ in the underlying physical principles and in modeling capabilities. In this paper, two different theories are discussed and compared to each other, namely the kinetic theory-based Rational Extended Thermodynamics (RET) and non-equilibrium thermodynamics with internal variables (NET-IV). First, the paper starts with the case of rigid heat conductors summarizing the result achieved so far. Then a typical example of compressible bodies is shown by presenting the first generalization for rarefied gases, called Meixner's theory. It is further extended using generalized entropy current in the framework of NET-IV. It is shown how its structure is related to RET and how the compatibility between them can be acquired.