A comprehensive non-equilibrium thermodynamic analysis applied to a vapor–liquid two-phase flow of a pure fluid

In this study, a comprehensive thermodynamic 1-D analysis applied to a steady state two-phase flow of a pure fluid is proposed taking into account the non-equilibrium state between the phases and the capillary work. Different equations explicitly expressing the first and the second law of thermodynamics are presented. The role of the specific heat capacities at constant vapor quality is emphasized. In addition, it is shown that the capillary work can always be neglected in the calculation of the variation of the vapor quality whereas the flashing effect must be taken into account under certain conditions. Furthermore, the role of the vapor enthalpy variation is highlighted in the definition of the elementary variation of the vapor quality at a non-equilibrium state between the phases (dryout conditions). Finally, from an entropy analysis, the mean two-phase flow temperature under non-equilibrium conditions applicable in the Newton law has been proposed.

► We study two-phase flow with phase change from non-equilibrium thermodynamics.
► We show that capillary work is always negligible in the calculation of vapor quality.
► We show that flashing effect can be significant in the calculation of vapor quality.
► Sensible heat must be included in the calculation of vapor quality at non-equilibrium.
► Mean two-phase flow temperature under non-equilibrium conditions is defined.