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An analysis of steam-void fraction in two-phase flow is carried out, utilizing the principle that in a steady-state thermodynamic process the rate of entropy production is minimum. The two-phase flow is idealized in the analysis to be a truly steady-state process. The effects of liquid entrainment and wall friction on the void fraction and slip ratio are evaluated. It is found that the slip-ratio in an idealized two-phase flow with zero wall friction and zero entrainment equals (ρf/ρg)1/3. Data from a number of experiments are found to be bracketed between this result and the result obtained by assuming complete entrainment (slip ratio = 1).
S.M. Zivi (1964) studied this question.