Under certain circumstances, phase transitions can occur in porous media consisting of a porous solid saturated with liquids and gases, for example, due to a freezing process, the liquid or parts of the liquid can turn into ice, which is then connected with the porous solid, or due to a drying process, the liquid or parts of the liquid are converted to vapor, which is then a component of the gas phase. Although some special proboems of phase transitions in porous media have already been treated, a general theory on the basis of thermodynamics is still to be explored. The present paper is concerned with the development of thermodynamic restrictions for the constitutive relations of an elastic, compressible porous solid, filled with two compressible fluids, whereby it is assumed that the three phases have different temperatures. The investigations reveal that the mass changes are essentially, among others, connected to the differences of the chemical potentials and the energy transitions to the differences of the reciprocal of the temperatures, which is well-known in classical thermodynamics of gases.

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