Author(s): Benjamin Hohermuth
Linked Author(s): Benjamin Hohermuth
Keywords: Air demand, bottom outlet, cavitation, hydraulic structures, two-phase flow
Abstract: Bottom outlets are key safety devices of high-head dams to control the reservoir water level. The high-speed free-surface flow downstream of the bottom outlet gate leads to considerable air entrainment and air transport. To avoid negative pressures at the bottom outlet and consequently prevent problems with gate vibrations, cavitation and flow chocking, a sufficient amount of air has to be supplied through an aeration chamber. Existing approaches to predict the air demand of bottom outlets show a large scatter and the knowledge on the transition from free-surface to fully pressurized flow is still scarce. Consequently, current design guidelines do not allow a coherent design. The goal of the present study is to improve the existing design guidelines by means of a systematic physical model investigation. The energy heads in the model are significantly larger than in previous studies and thus closer to prototype conditions. Furthermore, the effect of the aeration system, the tunnel length and the energy head on the air demand are quantified. The results show that the Froude number at the vena contracta has the largest effect on air demand. Short tunnels and large loss coefficients of the aeration system result in lower air demand values. Moderate tunnel fillings cause maximal air demand while both small and large fillings reduce the air demand. A new empirical relation for the air demand is presented considering all investigated parameters. The minimal negative pressures in the tunnel are calculated by applying the Bernoulli equation to the aeration system. The air pressure development along the tunnel is shown and the influence of the air flow from the downstream tunnel end is discussed
Year: 2017