Aeration and two-phase flow characteristics of low-level outlets

Abstract

Low-level outlets are key safety elements of high-head dams. Their main purposes are the regulation of the reservoir water level for maintenance works or during floods, and the fast reservoir drawdown in case of imminent danger or structural damage of the dam. Furthermore, low-level outlets allow for a controlled first impounding of the reservoir. Additional purposes include sediment flushing to limit reservoir sedimentation and the release of environmental flows. The high-speed water jet ejecting form the gate creates an air-water flow in the outlet tunnel leading to considerable air transport. Therefore, the air supply vent is a key element for safe low-level outlet operation. Despite their importance, design recommendations for low-level outlets are scarce and reliable information on key parameters such as air demand is not available. This study focuses on the effect of air vent design and tunnel length on air demand and overall low-level outlet performance. Physical model tests were conducted in a Froude-scaled model at energy heads up to 30 m. The effect of air vent size and loss coefficient as well as tunnel length and slope on air demand was analyzed in the model. Measurements with phase-detection intrusive probes allowed a detailed study of air-water flow properties at close to prototype conditions. Additionally, the capabilities and limitations of three numerical models to simulate air-water flows in low-level outlets were examined. The model tests showed that a small air vent size or a large air vent loss coefficient can trigger slug flow, especially for long tunnels. A newly developed flow pattern map allows to avoid these potentially harmful slug flow conditions. For free-surface flow a new design equation was developed considering that the air demand increases with increasing contraction Froude number, air vent size and tunnel length as well as decreasing air vent loss coefficient and tunnel slope. Newly collected and existing prototype data show a significantly higher air demand than the model equation, possibly due to effects of tunnel profile transition, roughness and scale effects. The new design equation was adapted to the prototype data to account for these effects. The basic air-water flow properties such as void fraction and interfacial velocity exhibit the same self-similar behavior as in open-channel flow. Their development along the tunnel can be described with new semi-empirical equations. A test series on scale effects revealed that scale effects regarding void fraction and interfacial velocity are negligible for sufficiently large Reynolds numbers. However, the size of air-water entities decreases up to the highest tested Reynolds numbers indicating scale effects. The numerical simulation of turbulent air entrainment in low-level outlets with a mixture model was successful for 10 m, but not 30 m of head. The implementation of wall-like turbulence treatment at the interface is needed for a realistic simulation of air flow above the free surface. A combination of both methods requires further numerical model development for which the detailed data of this study provides a good basis. The results of this study improve the process understanding of air-water flows in low-level outlets and contribute to their safe design.