Effects of Air Vent Size and Location Design on Air Supply Efficiency in Flood Discharge Tunnel Operations

Low-level outlets are essential flood discharge structures for high-head dams. High-speed free-surface flows releasing into tunnels lead to substantial air discharge, and the air supply is important for gate vibration mitigation, cavitation protection by aeration, and slug flow transition phenomena. An additional challenge is the excessive wind noise due to low air supply efficiency, threatening staff working conditions in flood discharge operations. However, the current understanding of the effects of air vent sizes and locations on the air supply efficiency is still limited. In the present study, based on a series of experimental tests, air vent designs of long flood discharge tunnels, including a single vent and two combined working vents, are analyzed with respect to the air discharge variation. For air vent designs with a small vent size and an installation location downstream, the air discharge into the tunnel is low, and the air supply efficiency is low even for two combined working vents. Equations for the air supply efficiency are proposed, which enables quantifying the ventilation performance, and related vent parameters are recommended for favorable air ventilation designs. The present investigation provides a reference for the flood discharge tunnel design and ventilation operation assessment in hydraulic engineering.

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