An experimental characterization of downwards gas–liquid annular flow by laser-induced fluorescence: Flow regimes and film statistics

•Downwards annular flow was characterized experimentally over a range of conditions.•Spatially resolved LIF measurements were used to visualize the flows quantitatively.•Flow behavior and gas entrainment into the liquid film were observed and analyzed.•Statistical data on liquid film thickness, wave events and gas entrainment are reported.•Four flow regimes were identified, including a new ‘disturbance’ wave regime. Downwards co-current gas–liquid annular flows were studied experimentally and characterized. An advanced optical laser-based measurement technique, namely Planar Laser-Induced Fluorescence (PLIF), was used for the visualization of the annular flow over a range of liquid Reynolds numbers ReL=306–1532 and gas Reynolds numbers ReG=0–84600. Four distinct flow regimes, namely the ‘dual-wave’, ‘thick ripple’, ‘disturbance wave’ and ‘regular wave’ regimes, have been identified based on qualitative information and a consequent quantitative analysis that provided information on the film thickness, interface and wave statistics, and gas entrainment into the liquid film. The mean film thickness data are generally in good agreement with previous studies. Evidence suggests that the turbulent gas phase affects strongly the shape of the interface, and that the mechanism for gas entrainment into the liquid film is strongly reliant on the existence of large-amplitude waves.