Air-water flows at hydraulic structures: Experimental investigations of interfacial characteristics and air-water mass transfer

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Air-water mass transfer in multiphase flows plays a key role in designing hydraulic structures. Air entrainment processes typically occur in spillways, drop shafts, stilling basins or in the casing of hydraulic impulse turbines. Current knowledge of aerated flows mainly relies on experimental model investigations. For laboratory experiments, dependent scale effects have to be taken into account by ensuring similarity between model and nature, implying geometrical, kinematic and dynamic similarity. In aerated flows, consistent values of Froude-, Reynolds- and Weber-number in model and prototype cannot be fulfilled in geometrical similar models. Experiments results show that some parameters, such as bubble sizes and turbulent scales, are likely to be affected by scale effects, even in 1:2 or 1:3 scale models. Experiments in prototype scale (1:1) are barely available yet. Consequently, future research has to focus on new field measurements, performed in situ at full-scale (Chanson 2013: Hydraulics of aerated flows: qui pro quo? Journal of Hydraulic Research, 51(3), 223-243). The mass transfer of a volatile gas into a liquid mainly depends on interfacial area, diffusion coefficient and the gradient of the gas concentration. In this context, the integration of the mass transfer equation represents an analytical approach determining mass transfer. Preliminary investigations were conducted on a stepped chute with small inclination at the University of Queensland. Direct comparisons between dissolved oxygen measurements and numerical integration of the mass transfer equation showed good agreement between the two methods (Tombees 2005: Air-water mass transfer on a stepped waterway, Journal of Environmental Engineering, 1377-1386). These promising results, the urgent need of large-scale measurements and recent progresses in researching aerated flows, e.g. in signal processing and comparative analyses of different experiments, give reason to conduct new investigations in this field of research. Within the scope of the proposed project, detailed multiphase flow and oxygenation measurements on a stepped chute with steep inclination (1V:0.8H) will be conducted in laboratory and prototype scale. The planned in-situ measurements will contribute to close the existing research gap arising from missing large-scale investigations and the collected data will be used to quantify scaling effects, especially with respect to dissolved oxygen and aeration efficiency. Furthermore, experimental results of air-water flow properties are used for numerical integration of the mass transfer equation and results will be compared with measurements of the oxygen transfer rate. With the simultaneous measurement of oxygen transfer and air-water flow properties, substantial knowledge concerning the description of aerated flows is gained.