Liquid Film Thickness in Vertical Circular Pipes Under Flooding Conditions at the Top End

In our previous study, we measured the void fraction α, pressure gradient dP/dz, and countercurrent flow limitation in a vertical circular pipe (diameter D = 20 mm) under flooding conditions at the square top end and working fluids of air and water to obtain the wall friction factor f_w and the interfacial friction factor f_i based on the annular flow model. The thickness of the falling liquid film δ obtained from the measured α was relatively well expressed by the correlation for the free-falling film, and the obtained f_w was well expressed by the Fanning friction factor f for a circular pipe. Measurements of α in vertical pipes under flooding conditions are few. In this study, therefore, we evaluated α and δ from the measured dP/dz under flooding at the square top end reported by Bharathan et al. with D = 50.8 mm and air-water and by Ilyukhin et al. with D = 20 mm and working fluids of steam and water at pressures of P = 0.6 to 4.1 MPa. As a result, we found that δ obtained from the measured dP/dz and the correlation of f_w = f were well correlated in terms of the liquid Reynolds number Re_L. The obtained δ was well expressed by the Nusselt’s correlation for the free-falling film in the region of laminar flows, but the obtained δ was larger than the Feind’s correlation for the free-falling film in the region of turbulent flows due to the interfacial friction. We also discussed effects of the diameter and fluid properties on the interfacial friction factor f_i.