Molecular extended thermodynamics of rarefied polyatomic gases and wave velocities for increasing number of moments

Molecular extended thermodynamics of rarefied polyatomic gases is characterized by two hierarchies of equations for moments of a suitable distribution function in which the internal degrees of freedom of a molecule is taken into account. On the basis of physical relevance the truncation orders of the two hierarchies are proven to be not independent on each other, and the closure procedures based on the maximum entropy principle (MEP) and on the entropy principle (EP) are proven to be equivalent. The characteristic velocities of the emerging hyperbolic system of differential equations are compared to those obtained for monatomic gases and the lower bound estimate for the maximum equilibrium characteristic velocity established for monatomic gases (characterized by only one hierarchy for moments with truncation order of moments N) by Boillat and Ruggeri (1997)λ(N)E,maxc0⩾65(N−12),(c0=53kmT) is proven to hold also for rarefied polyatomic gases independently from the degrees of freedom of a molecule. •Molecular extended thermodynamics of rarefied polyatomic gases is studied.•The relation between two hierarchies of equations for moments is derived.•The equivalence of maximum entropy principle and entropy principle is proven.•The characteristic velocities are compared to those of monatomic gases.•The lower bound of the maximum characteristic velocity is estimated.