Preliminary experimental investigation on Air entrainment capacity by offset chute aerator

Abstract

Aerators built in chute spillways and spillway tunnels are regarded as the most effective and economical means to prevent cavitation damages during high-speed flood discharge. This paper presents some preliminary results of an experimental study on the air entrainment and air-water flow properties of the aerated flow over simple offset chute aerators. The air-water flow was measured using a conductivity phase-detection probe, facilitated with high-speed video observations. Although the measurements were performed across the flow from chute bottom to free surface, the main focus was the bottom aeration below the clear-water core, as it affected directly the air concentration next to the chute bed surface. The air entrainment capacity of the offset aerator was characterised with the effective air entrainment rate given by the air concentration and velocity distributions. The traditional prediction of the air entrainment discharge from the offset air cavity below the jet was based on the cavity length and jet velocity. Herein the method was refined by consideration of an effective cavity length starting from the onset location of bubble entrainment as well as the air detrainment due to the jet impact on the chute bottom. In addition to the cavity length and the jet Froude number, the effects of the jet impact angle were also noted, as suggested by previous researches.