Experimental Study on Air Bubble Diffusion in Developing Region of Self-Aerated Open Channel Flow with Pressure Outlet

Free surface self-aeration is frequently observed in steep rivers, spillways and high-speed flow in open channel connecting with a pressure outlet in bottom outlet tunnels. The air entrainment induces a drastic change in the fluid characteristics. For some distance, the flow is developing, and there is a flux region for air into the water. When the air bubbles are transported to their maximum depth in the water, the flow is considered fully aerated, but continues to entrain more air and thus is still developing. In this study, the air bubble diffusion of developing region is analyzed experimentally in self-aerated open channel flow. Experiments are performed along a flume with pressure outlet. The air concentration distribution, chord length and amount of air bubbles are recorded by using a tip conductivity probe on the centerline of the flow. A new parameter chord fraction R was developed to analyze the air bubbles characteristics in self-aerated flows. The results demonstrate that in the air-water developing regions, the air concentration increased along the way and the development of self-aeration region is slow and does not diffuse to the channel bottom during the present work. The probability of air bubble chord lengths is the largest for chord lengths between 0.05mm and 1mm. The rate of chord length accumulation of air bubbles is less than that of amount of air bubbles for a certain measuring point, which indicates that large air bubble is the main part of air entrainment for the concentration in self-aerated flows. This trend decreases with the increase of water depth.