Author(s): Simone Pagliara; Benjamin Hohermuth; Robert Boes
Keywords: Ir-water flow properties; Air bubbles; Dual-tip phase-detection probe; Conductivity probe; Image processing
Abstract: High-velocity air-water flows are frequently observed at hydraulic structures, e.g., spillways and low-level outlets, and they are characterized by a strong interaction between the two phases and by void fractions ranging from a few percent to up to 100% at the free surface. Intrusive phase-detection probes represent the most widely used instrumentation in hydraulic engineering to measure air-water flow properties, such as void fraction, interfacial velocity, and particle chord lengths. Such probes are especially effective for applications characterized by high levels of aeration where optical and acoustic techniques would fail. Herein, an experimental setup capable of producing air bubbles moving vertically at selected velocities is presented. The setup consists of (i) a vertical, acrylic glass pipe with variable water flow rates, (ii) an air-injection system with variable air flow rates, and (iii) a stereo-camera setup, comprising a beam splitter and a high-speed camera. The beam splitter setup allows simultaneously obtaining perpendicular stereo images of the bubbles, enabling three-dimensional information on bubble velocities and shape to be recorded. The bubble properties reconstructed from the high-speed images were compared to interfacial velocities and chord times obtained from a dual-tip phase-detection conductivity probe. In conclusion, the results from this study help to quantify the measurement uncertainty of commonly used air-water flow instrumentation. The details on the three-dimensional features of the bubbles may also enable to further improve the design of phase-detection probes in future studies.