The hydrodynamic characteristics of downwards co-current two-phase (gas-liquid) flows inside a vertical tube (ID = 32 mm) have been investigated experimentally. Advanced optical techniques, namely Laser Induced Fluorescence and Particle Tracking Velocimetry, were utilised for the characterisation of these flows over a wide range of gas and liquid superficial velocities (U G = 0 -34 m·s -1 and U L = 0.034 -0.182 m·s -1 ), corresponding to Reynolds numbers Re G = 0 -84,600 and Re L = 1,230

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... A flow regime map, which contains a previously unreported flow regime, is constructed based on the flow observations. The quantitative analysis of the liquid films allows the generation of film thickness, wave frequency, bubble size, bubble frequency and velocity profile data. It was found that the different observed flow regimes possess a characteristic combination of the investigated quantitative parameters. A model, based on modified mixing-length theory, was used to predict the liquid film velocity profiles and good agreement was found with the experimental results.