Interfacial structure of gas-liquid annular and stratified flows in downward inclined pipes

•Results on 3D measurements of the instantaneous film thickness of annular and stratified flows in downward vertical and inclined gas liquid pipe flow are reported.•Measurements were performed using a non-intrusive multilayer conductance liquid film sensor.•Time-dependent data were accumulated simultaneously over the entire pipe circumference and along the pipe axis.•Statistical analysis of the interfacial waves showed co-existence of unique interfacial structures, including co-existence of different wave types. An experimental study on the interfacial structure of annular and stratified flows in downward vertical and inclined pipes was conducted using a non-intrusive multilayer conductance liquid film sensor. The sensor performs simultaneous measurements of the film thickness in the flow direction and across the entire pipe circumference, allowing for a three-dimensional reconstruction of the wavy interface. Pipe inclination from vertical imposes an asymmetric film distribution and an interfacial structure that is highly dependent on both the azimuthal and inclination angles. A statistical analysis of the interfacial waves was performed as a function of pipe inclination and azimuthal angle, showing several unique structures, including the simultaneous co-existence of three superposed wave types, i.e. small ripples, larger disturbance waves and rarely even larger waves. Additional parameters examined in this study include film azimuthal coverage, mean film and substrate thickness and wave height, frequency and velocity. Since the velocity of disturbance waves is independent of the azimuthal location, the shape of the wave front as a function of azimuthal angle was investigated. This study provides new insight into the complex interfacial wavy structure of separated flow in downward inclined pipes at relatively low gas flow rates. [Display omitted]