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The wavy structure of liquid film in downward annular gas-liquid flow with and without liquid entrainment is examined for a wide range of gas velocities using high-speed laser-induced fluorescence technique. It is shown that the wavy structure is always represented by two types of waves, long-lived and short-lived waves with the latter always generated at the back slopes of the long-lived waves. Main regularities of short-lived wave generation and evolution are also described.

Annular flow represents the combined flow of liquid film along the channel wall and high-velocity gas stream along the center of the channel. At high gas and liquid flow rates liquid droplets are entrained from the film surface into the core of the gas stream. Entrainment contributes significantly to integral heat and mass transfer, and its modeling entails high practical interest. It is now acknowledged that two types of waves coexist on the film …