A technique for measuring ensemble-averaged, three-component liquid velocity fields in two-phase, gas–liquid, intermittent pipe flows

Gas–liquid intermittent flows can be found in many engineering applications, nevertheless a detailed knowledge of this flow pattern is still not fully available. In the present work, an experimental study was conducted with the objective of developing a measurement procedure capable of providing ensemble-averaged three-component velocity fields in the liquid phase of a gas–liquid, intermittent, horizontal flow in a pipe. To this end, a high-frequency stereoscopic particle image velocimetry system (SPIV) was employed, combined with the laser induced fluorescence (LIF) technique to separate the light scattered by the liquid–gas interfaces from that emitted by the fluorescent tracer particles. A set of photogates was used to trigger the SPIV system, allowing for the measurement of velocity fields in the liquid plug, downstream of the elongated bubble, and in the liquid film, upstream of the elongated bubble nose position. The triggered measurements allowed the determination of ensemble-averaged three-component velocity fields at different positions in relation to the bubble nose, obtained from the replication of a sufficiently large number of bubble passage events. Contours of the liquid flow streamwise vorticity component in cross-stream planes upstream and downstream of the bubble nose tip were also obtained from the SPIV measurements. The photogate system was also employed to measure the bubble velocity. This information was used to transform time-based into space-based velocity field data. This allowed the construction of a three-dimensional representation of the ensemble-averaged structure of the gas bubble nose and the associated vortical structures induced in the liquid flow. The three-component velocity information obtained revealed the influence of the gas bubble motion on the liquid flow in the plug and liquid film regions.