Study on free-surface aeration with high-velocity air-water flows in open-channels

Sammanfattning: China’s biggest source of the renewable energy is hydropower, with a share of 85 %, in 2013, of the total renewable power generation. For the tenth sequential year, China has maintained the country’s leading role in global hydropower development by installing more hydropower plants than the rest of the world combined. There are some important issues when constructing hydropower plants and designing spillways and chute sidewalls. One of these issues is considered to be the presence of air, which can be entrained in large amounts through the free surface into the water within high-velocity flows and leads to the bulk of the flow to increase. This project aims to describe the relationship between free-surface entrapped deformation and air entrainment (entrained air in the flow) by studying uniform self-aerated flows in open-channels. The performed experiments have been carried out in a physical scaled model at the State key laboratory of Hydraulics and Mountain River Engineering at Sichuan University in Chengdu, China. A literature study has been done in order to examine the self-aerated flows in open-channels and used as the basis for the experiments in this project. The software, Free Video to JPG Converter, AUTOCAD, Photoshop, OriginPro and Motion Studio have been utilized for the evaluation of free-surface deformation curve and the process of surface deformation and air entrainment. After analysing images taken by a camera during the experiments, the result has been divided in two parts, namely, to describe the air-entrainment phenomenon using the relevant curve type and fitting the obtained curves into the known curves, and to obtain the critical radius of curvature for the air-entrainment affected by flow mean velocity and water depth. Based on the experimental data from the image analysis and one sample t-test which has been utilized to obtain a confidence interval of the adjusted R-square in order to evaluate how good fitting the relevant curves have, the Gaussian-curve type have been decided to describe the phenomenon. After the image analysis of the free-surface deformation curve type has been done, the process of surface deformation and air entrainment has been evaluated. For this evaluation, various processes with various durations have been observed. As a result, it is found that the surface achieves closure at a relatively low position at a range of 7 to 19 milliseconds and the free-surface deformation occurs when the air bubble is entrained into the flow, varies between 8 and 19,5 milliseconds for different observation. The results of evaluating the radius of curvature with respect to the time of occurrence indicate that the free-surface deformation intensity in the experiments exceeds the critical condition of curvature radius when the surface instability occurs. The results of the radius of curvature reveal that that the radius of curvature has a “bell-shape” behaviour. It is shown that the critical radius of curvature varies for different processes and it occurs after the maximum value is reached, the critical radius of curvature has a range between 0,53 and 2,97 mm for all the processes. The results show that the experiments have a good potential for evaluating the process of freesurface deformation and air entrainment. To make final conclusions, further studies and experiments in self-aerated open-channel flows should be done including non-intrusive measurements for the entrained air bubble size and the relationship between the free-surface deformation and bubble distributions