Study and characterization of gas-liquid slug flow in an annular duct, using high speed video camera, wire-mesh sensor and PIV

An experimental study is presented on air-water two-phase flow in a 10.5-m-long annular duct with an external diameter of 155 mm and an inner diameter of 60 mm. Particle image velocimetry (PIV) is applied to obtain instantaneous velocity measurements of the flow field. The annular duct inclination is of 5° from the horizontal. A CCD camera (2448 pixel × 2050 pixel, 5 Mpixel,12-bit) was positioned in the test section to record the seeding particles. The illumination was provided by a double pulsed PIV laser (Nd:YAG, frequency doubled to 532 nm) with a measured pulse intensity of 70 mJ/pulse. It was used at 15 Hz (resulting in the independence of the velocity samples). Based on the instantaneous local velocities, Probability Density Functions (PDF) and mean velocities are calculated. Two-phase flow arranged in the slug-flow pattern is observed, at superficial velocities of jw = 0.154 m/s and ja = 0.044 m/s. 3000 samples per case are processed using cross-correlation procedure, for the PIV analysis. A home-made Annular Wire-Mesh Sensor (AWMS) was applied to obtain time-signal-measured void-fraction data as a function of the electrical permittivity. The average bubble velocity is estimated by two techniques, (i) High-Speed Video Recording and (ii) Particle Image Velocimetry (PIV) together with the AWMS. A comparison of the two techniques is presented. A new technique based on AWMS for the measurement of bubble-passage frequency, bubble length and slug length is proposed. It was observed: (i) deceleration of the water phase beneath the bubble as it passes, shown by velocity profiles at different bubble locations, (ii) an increase in bubble velocity as air superficial velocity is increased and (iii) the complexity of the flow pattern, shown in details by AWMS cross-sectional images. The new experimental results are of great value for comparison with CFD models and for the development of more refined pressure-drop prediction tools in two-phase annular-duct flows.