Study of formation and development of disturbance waves in annular gas–liquid flow

• Spatiotemporal evolution of waves near tangential slot inlet was studied with LIF.
• Disturbance waves are formed by coalescence of small-scale Kelvin–Helmholtz waves.
• Formation of ephemeral waves and transition to entrainment occur in similar manner.
• Initial acceleration of individual disturbance waves is defined by gas velocity.
• Energy transfer from high to low frequencies is observed with downstream evolution.

Wavy structure of downward annular gas–liquid flow with liquid entrainment in 15 mm pipe was studied using high-speed laser-induced fluorescence technique. Measurements were performed near the inlet, which was organized as a tangential slot. Spatiotemporal records of film thickness were obtained over the first 100 mm of the pipe length in order to investigate formation and initial stages of development of the disturbance waves. It was shown that for high enough gas and liquid flow rates disturbance waves appear and start to dominate in the wavy structure of liquid film within the area of interrogation. Disturbance waves were found to be formed due to coalescence of small high-frequency waves appearing at the inlet. Similar mechanisms of formation of large waves with fast ripples on them were observed far downstream for waves near transition to entrainment and for ephemeral waves in flow regimes with entrainment. Significant individual acceleration of disturbance waves at the initial stage of their development was observed. Spectral analysis has shown strong energy transfer from high to low frequencies, which is in agreement to the proposed mechanism of waves formation.