Irreversible thermodynamics in a radiating fluid

In this paper we try to find out what dissipative processes take place in a radiating fluid with a boundary surface, as a consequence of the radiation-matter interaction. We state a kinetic model based on Boltzmann-like equations for atoms and photons. We assume that the medium is in a hydrodynamic regime, while the radiation field is in a nonequilibrium state. The kinetic equations are consistently solved with the entropy balance equation, in agreement with the second principle of thermodynamics, in order to obtain the constitutive relations. These constitutive relations imply a generalization of the photohydrodynamic equations, because they take into account the transport of energy and momentum connected with the thermalization regime of the radiation field near the boundary surface. In addition, we analyze the phenomenological approach to the problem by means of the Gibbs formula