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The objective of this study was to reduce the uncertainties of correlations for flow characteristics in vertical pipes under flooding at the top end. The void fraction α, pressure gradient dP/dz, and countercurrent flow limitation (CCFL) were previously measured with diameter D = 40 mm and working fluid of air and water. The wall friction and interfacial friction factors (f w  and f i ) were obtained based on the annular flow model, and CCFL and f w  were evaluated in detail. Hence, attention was turned to detailed evaluations of α and f i . Liquid film thickness δ and interfacial friction factor f i  for smooth film (SF) due to flooding at the top end were obtained using the previously derived f w  correlation and existing dP/dz data with D = 20 to 50.8 mm and pressure P = 0.1 to 4.1 MPa, and empirical correlations for δ and f i  were derived. The δ term was well expressed by a function of the liquid Reynolds number Re L , and the uncertainty of the δ correlation was ±0.0062 for α = 0.87 to 0.98. f i  was expressed by a function of δ/L (where L is the Laplace length) or the Kutateladze parameter K G *, the dimensionless diameter D* (=D/L), and the density ratio of the gas and liquid phases ρ G /ρ L . The applicability of the derived correlations to conditions of D = 300 mm and P = 7 MPa was evaluated, and the f i  correlation was modified based on f i  values computed with the δ correlation. The drift-flux parameters for SF were also considered.

Void Fraction and Interfacial Friction in Vertical Circular Pipes with the Square Top End Under Flooding Conditions