Author(s): Stefan Felder, Benjamin Hohermuth, Michael Pfister, Robert Boes
Keywords: Air–water flow properties; Conductivity probes; Fiber-optical probes; High-velocity air-water flows; Phase-detection intrusive probes;
Abstract: Air entrainment is a common feature of high-velocity free-surface flows. Air-water flow properties inside the flows are typically measured with phase-detection intrusive probes comprising fibre-optical and conductivity probes. Despite their common use, the most-suited phase-detection probe system for high-velocity flows is unknown. Novel comparative analyses of phase-detection probes were conducted in high-velocity air-water flows on a stepped spillway and in a tunnel chute. Experiments were performed with a side-by-side double-tip conductivity probe and several double-tip fibre-optical probes with different inline positioning of the probe tips as well as a fibre-optical side-by-side probe. Experiments were conducted for a range of flow conditions and flow velocities up to 15 m/s using the same acquisition system and post-processing software. While void fraction and interfacial velocities were in close agreement for the side-by-side probes in the tunnel chute and for the two probes used in the stepped spillway configuration, the comparison indicated that the probe tip geometry, especially the orientation of the probe tip to the main flow, can significantly affect the results. Several air-water flow properties showed differences including the bubble count rate, chord times and cluster properties linked with different tip sizes. The results indicate that the side-by-side phase-detection probe tip design has advantages in high-velocity air-water flows because the inline positioning affects the trailing tip data and the calculation of the interfacial velocity. The experiments highlight the need for a sturdy probe design for measurements in high-velocity flows, favoring the conductivity probe for such flow conditions and potential future employment in prototype-scale air-water flows.