Two-phase air-water flow through a large diameter vertical 180° return bend

An experimental study of churn-annular flow behaviour and mean film fraction of an air-water mixture flowing through a vertical 180 degrees return bend using an electrical conductance technique is reported. Measurements were made of film fraction using probes placed before, within the bend (45 degrees, 90 degrees, 135 degrees) and after the bend. The bend, made of transparent acrylic resin, has a diameter of 127 mm and a curvature ratio (R/D) of 3. The superficial velocities of air ranged from 3.5 to 16.1 m/s and those for water from 0.02 to 0.2 m/s. Flow patterns were identified using the characteristic signatures of Probability Density Function (PDF) plots of the time series of mean film fraction. The average film fraction is identified to be higher in the straight pipes than in the bends. The study also identified that at low gas superficial velocity, the film fraction for the riser was generally greater than for the downcomer. For low liquid and higher gas flow rates, film break down occurs at the 45 degrees bend due to gravity drainage. The condition for which the liquid goes to the outside or inside of the bend are identified based on a modified form of Froude number based on published material. A comparison between the present work and that of Hills (1973) based on the mean film fraction showed same tendency. (C) 2012 Elsevier Ltd. All rights reserved.